r/EPA
            United States
            Environmental Protection
            Agency
Office of Acid Deposition,
Environmental Monitoring, and
Quality Assurance
Washington DC 20460
                         EPA/600/8-85/001
                         August 1985
            Research and Development
The Acidic Deposition
Phenomenon and
Its Effects
            Critical
            Assessment
            Document

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                       CRITICAL ASSESSMENT DOCUMENT
     The Critical Assessment Document  (CAD) is a  summary,  integration,  and
interpretation of information, by the  three authors  identified  on the title
page, of information on acid deposition.   Tt  is based  on  a compendium of
information on acid deposition, called  the Critical  Assessment  Review Papers
(CARP), based on literature published  through 1982 and prepared for EPA by
some 60 scientists.  It was published  after exhaustive review in 1984.   In
some areas CAD authors updated scientific  references through early 1985.

     The CAD is a reference iccumer.t,  intended primarily  for Agency use
although it is available thorugh the National  Technical  Information Service.
It is one in a series of ..-v. reasj ngi y  soptu sticated  assessment  documents
produced by the NationaJ Ai:ii  Precipitation Assessment Program  (NAPAP), the
Federal Interagenoy Acid Deposit ion Pesearch  Program,   The CAD  will be super-
ceded by the more complet -  198 ~_ _As Sfe_s_sm_en t, to be published late in 1985.

     The CAD is organized  , r.  a question and answer  format.  'Questions on
effects are raised and answered  in  Chapt-r II, those on atmospheric sciences,
in Chapter III, and in Chapter  IV,  linkages between  atmospheric sciences and
effects are made.  The questions raised in the CAD are in most  cases based
on the original issues identified in  1980  by  an  EPA  Steering Committee.  In
the intervening years, research  and assessment activities have  raised new
questions or dramatically  altered the  frame of reference in which a question-
must be answered.  The rapid  expansion of  research efforts has  led to much
better questions, but has  not  always provided definitive answers.  This is
the natural evolution of research—the quality of questions rapidly improves,
and, after a period of time appropriate to the individual study, the answers
follow.

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                                    EPA/600/8-85/001
                                          August 1985
     The Acidic  Deposition
Phenomenon and Its  Effects:

Critical Assessment Document
                  Prepared by
    David A. Bennett1, Robert L. Goble2, and Rick A. Linthurst1

         'Office of Acid Deposition, Environmental
           Monitoring, and Quality Assurance
               Washington, DC 20460

                2Clark University
                 Worcester, MA
             EPA Cooperative Agreement
                  CR806912
               EPA Project Officer
                David A. Bennett
U.S. Environmental Protection Age
Region V, Library
230 South Dearborn Street
Chicago, Illinois  €0604
        Office of Acid Deposition, Environmental
          Monitoring, and Quality Assurance
          Office of Research and Development
         U.S. Environmental Protection Agency
              Washington, DC 20460

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                                     NOTICE





     The information in this document has been funded wholly  or  in  part  by the



United States  Environmental  Protection   Agency   under   Cooperative  Agreement



CR806912 to  North   Carolina State  University.    It  has  been subject  to  the



Agency's peer and administrative review, and  it has  been  approved  for publica-



tion as an EPA  document.   Mention  of trade  names  or commercial products  does



not constitute endorsement or recommendation  for  use.
 U.S. Environmental rvc^cticn Agency

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                              TABLE  OF CONTENTS

                                                                         PAGE
PREFACE 	   xi i

      A.   What is this document? 	   xii
      B.   History of the documents  	  xiii
      C.   The  future 	   xiv

  I.   INTRODUCTION 	     1

      Organization 	     1
      A chemical  primer 	     3
      The  evolving point of view 	     4

 II.   EFFECTS  SCIENCES SUMMARY 	     8

      Section  A.   Ecosystem Components 	     8

      A.I   Introduction 	     8

           A.1.1   What effects of acidic  deposition  have  been
                  studied?  	     8
           A.1.2   What components of ecosystems  must be  considered
                  to understand the  effects  of acidic deposition?  	     8
           A.1.3   How do these ecosystem  components  interact? 	     8
           A.1.4   What level  of understanding will be necessary  to
                  define interactions among  ecosystem structure,
                  function,  and acidic deposition's  influence?  	    11

      A.2   What is known about the influence of  acidic deposition on
           ecosystems?	    13

           A.2.1   What are  the effects on  soil systems?   [CARP  E-2]  	    13

                  A.2.1.1  Why is knowledge  of soils important  for  under-
                           standing  the effects  of acidic  deposition?
                           [CARP  E-2.1] 	    13
                  A.2.1.2  Why is understanding/quantifying  the  effects
                           of  acidic  deposition  on soils difficult?
                           [CARP  E-2.1] 	    13
                  A.2.1.3  What are  the sources  and  fates  of sulfur,
                           nitrogen,  and  hydrogen ions in  soils?
                           [CARP  E-2.2.1]  	    16
                  A.2.1.4  What soil  processes could be affected by
                           acidic deposition?  [CARP E-2.3]  	    16
                  A.2.1.5  What evidence  is  there that soil  processes
                           have been  affected?   [CARP E-2.3] 	    17

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                                                              PAGE

                A.2.1.5.1  Is soil  pH likely to change as a
                           result of present loadings of
                           acidic deposition?
                           [CARP E-2.3] 	    17
                A.2.1.5.2  Are nutrients lost from soils at
                           an increased rate as a result of
                           acidic deposition?
                           [CARP E-2.3.3] 	    17
                A.2.1.5.3  Is nutrient renewal  in soils from
                           mineral  weathering occurring at
                           rates similar to the losses of
                           nutrients?  [CARP E-2.3.3] 	    18
                A.2.1.5.4  Are microbially-controlled soil
                           processes likely to  be affected
                           by present loadings  of acidic
                           deposition?
                           [CARP E-2.4, E-2.5]	    18
                A.2.1.5.5  Are metals more likely to be
                           mobilized as a result of  acidic
                           deposition?  [CARP E-2.3.3.3] 	    19

       A.2.1.6  What is a sensitive soil?  [CARP E-2.3.5] ...    19

                A.2.1.6.1  What are the characteristics of a
                           sensitive soil?  [CARP E-2.2.8,
                           E-2.3.5] 	    20
                A.2.1.6.2  Where are the sensitive soils in
                           the United States?  [CARP E-2.3.2,
                           E-2.3.5] 	    20

       A.2.1.7  Could observed/potential changes in  soils as a
                result of acidic deposition be  reversed?
                [CARP E-2.3.4] 	  22
       A.2.1.8  What is the time frame in which changes in
                soil chemistry could result from acidic
                deposition? [CARP E-2.3] 	    22

A.2.2  What are the effects of acidic deposition on  forests
       and crops?  [CARP E-3] 	    23

       A.2.2.1  What constituents of precipitation and the
                atmosphere influence vegetation? 	    23
       A.2.2.2  What are the primary mechanisms by which
                vegetation might be affected by acidic
                deposition?  [CARP E-3.2, E-3.4] 	    23
       A.2.2.3  Why are the effects of acidic deposition
                on vegetation difficult to discern?
                [CARP E-3.3, E-3.4]  	    23
                           IV

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                                                              PAGE

       A.2.2.4  What do we  know about acidic deposition's
                effects on  crops?   [CARP  E-3.4.2] 	    24

                A.2.2.4.1   What time frame is important in
                           defining effects of acidic
                           deposition on  crops?  [CARP
                           E-3.4.2] 	    24
                A.2.2.4.2   Is there evidence to suggest that
                           crops are responding to acidic
                           deposition? [CARP E-3.4.2.2.1] ...    24
                A.2.2.4.3   Is there evidence to suggest that
                           crop productivity has been
                           significantly  affected by acidic
                           deposition?  [CARP E-3.4.2.3] 	    25
                A.2.2.4.4   What has prevented a clear
                           quantification of the effects of
                           acidic deposition on crops?   [CARP
                           E-3.4.2.3] 	    25

       A.2.2.5  What do we  know about the effects of acidic
                deposition  on forests?  [CARP E-3.4.1]  	    25

                A.2.2.5.1   Is there evidence to suggest that
                           acidic deposition is affecting
                           forest growth?  [CARP E-3.4.1] ...    26
                A.2.2.5.2   Is there evidence to suggest that
                           a regional decline of forests is
                           occurring in Europe or North
                           America? [CARP E-3.4.1] 	    26
                A.2.2.5.3   What hypotheses have been proposed
                           to explain recent regional forest
                           declines? 	    26

A.2.3  What are the effects on aquatic chemistry?  [E-4] ....    27

       A.2.3.1  Why are surface and groundwaters an important
                consideration in studies  of acidic deposition
                effects? 	    27
       A.2.3.2  What surface water chemical characteristics
                may be influenced by acidic deposition?
                [CARP E-4,  E-4.2,  E-4.3]  	    27
       A.2.3.3  What atmospheric chemical inputs influence
                chemical  characteristics  of aquatic systems?
                [CARP E-4.3.1] 	    27
       A.2.3.4  Why is deposition of sulfur compounds
                particularly important to aquatic chemistry?
                [CARP E-4.3.1.5] 	    28
       A.2.3.5  How dependent on atmospheric deposition are
                surface water sulfate values?
                [CARP E-4.3.1.5.2] 	    29

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                                                              PAGE

       A.2.3.6  What factors "control" surface water
                chemistry? [E-4.3.2]	   32
       A.2.3.7  Is there evidence to suggest that acidic
                deposition has altered surface water
                chemistry?  [CARP E-4.4.3] 	   35
       A.2.3.8  What time frame is important in acidic
                deposition's effects on aquatic chemistry?
                [CARP E-4.4.2, E-4.4.3] 	   36
       A.2.3.9  What options are available to counteract
                surface water acidification?  [CARP E-4.7] ..   36

A.2.4  What are the effects on aquatic biota?  [CARP E-5] ...   38

       A.2.4.1  What potential effects of acidic deposition
                are of concern?  [CARP E-5.5, E-5.6] 	   38
       A.2.4.2  How may changing water chemistry influence
                the fish populations of surface waters?
                [CARP E-5.6] 	   39
       A.2.4.3  What are the characteristics of those surface
                waters where changes in fish populations
                might occur?  [CARP E-5.2, E-5.6] 	   40
       A.2.4.4  What evidence is there that changing water
                chemistry has affected fish populations?
                [CARP E-5.6] 	   41
       A.2.4.6  What options are available to maintain fish
                populations?  [CARP E-5.9] 	   42

A.2.5  What are the effects of acidic deposition on human
       health?  [CARP E-6] 	   42

       A.2.5.1  How could acidic deposition affect human
                health [CARP E-6] 	   42
       A.2.5.2  What evidence exists to suggest human health
                is being affected?  [CARP E-6.2, E-6.3] 	   43
       A.2.5.3  What options are available to minimize the
                risk of indirect health effects due to acidic
                deposition?  [CARP E-6.3] 	   43

A.2.6  What are the effects on materials?  [CARP E-7] 	   44

       A.2.6.1  What effects on materials may occur as a
                result of atmospheric deposition or
                atmospheric pollution?  [CARP E-7.1] 	   44
       A.2.6.2  What is the role of acidic deposition in
                degradation of materials?  [CARP E-7.1.1] ...   44
       A.2.6.3  What components of acidic deposition are
                most important in materials degradation
                processes?  [CARP E-7.1.1] 	   44

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                                                                         PAGE

      Section B.   Ecosystem Interactions  	    46

      B.I  Introduction  	    46

      B.2  What explanations have  been  proposed  for  observed
           regional  declines of forests?  	    46

           B.2.1   What role could  acidic  deposition  play in  the
                  proposed  explanations?	    46
           B.2.2   Is there  evidence to  support these two hypotheses?  ...    47
           B.2.3   What forest regions  of  the United  States would
                  most likely be affected by acidic  deposition  if
                  the hypotheses were  correct? 	    49

      B.3  What hypotheses  have been proposed to explain changes in
           surface water chemistry? 	    49

           B.3.1   What role does acidic deposition play in the
                  acidification of lakes  and streams?  	    51
           B.3.2   What conclusions can  be drawn  from the available
                  evidence? 	    52

      B.4  What hypotheses  have been proposed to suggest future
           changes in water chemistry  will or will not occur? 	    52

           B.4.1   What data are needed to test these hypotheses? 	    54
           B.4.2   If increases in  surface water  acidity were  to occur,
                  what locations in the United States  would  be  at highest
                  risk?  	    55
           B.4.3   What is the time frame  in which changes might be
                  observed? 	    57

III.   ATMOSPHERIC SCIENCES  SUMMARY 	    58

      Section A.   Atmospheric Processes 	    58

      A.I  Introduction  	    58

           A.1.1   What are  the relevant questions concerning  the
                  emissions and processing of acidifying substances?  ...    58
           A.1.2   What are  the most important substances that are
                  emitted and deposited?   What spatial  scales and
                  temporal  scales  are  most important?   Where  are the
                  most sensitive areas?  [CARP E-3,  E-4] 	    58
           A.1.3   What is the causal structure relating emission to
                  deposition? 	    59
           A.1.4   What are  the issues  in  relating emissions  to
                  deposition? 	    61
                                      VI1

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                                                                   PAGE

A.2  What is known about the steps in the source-receptor path? ..   61

     A.2.1  What amounts of acidifying substances are wet and dry
            deposited?  [CARP A-8] 	   61

            A.2.1.1  How is wet deposition of sulfur and nitrogen
                     compounds and hydrogen ions measured?  [CARP
                     A-8.2.3] 	   61
            A.2.1.2  What important collections of data have been
                     made and are ongoing? 	   65
            A.2.1.3  What are the patterns for wet deposition of
                     sulfate, nitrate and hydrogen ions?  [CARP
                     A-8.4.1] 	   69
            A.2.1.4  What is the spatial and temporal variability
                     of the wet deposition patterns? 	   69
            A.2.1.5  What historical trends can be seen in wet
                     deposition data?  [CARP A-8.4.3] 	   74
            A.2.1.6  How is dry deposition of acidifying
                     substances measured?  [CARP A-8.3.2] 	   77
            A.2.1.7  What can be concluded about dry deposition
                     rates from the data available?  	   78
            A.2.1.8  What is the quality of the data for wet and
                     dry deposition? 	   79

     A.2.2  What are the ambient concentrations of substances
            important  in acid deposition?  [CARP A-5] 	   80

            A.2.2.1  How are concentrations measured? 	   80
            A.2.2.2  What collections of data have been made?  ....   81
            A.2.2.3  What concentrations of important substances
                     have been observed? 	   81
            A.2.2.4  What is the spatial and temporal variability
                     of  the data?  What  is the  quality of the
                     data?  	   84

     A.2.3  What  is  known about dry deposition  processes?
            [CARP  A-7]  	   84

            A.2.3.1  What are the important mechanisms in dry
                     deposition?  [CARP  A-7.2]  	   84
            A.2.3.2  How do dry deposition rates depend on
                     substance, ambient  concentration,
                     meteorological conditions, and  surface
                     characteristics? 	   85
            A.2.3.3  What are typical deposition velocities?
                     [CARP  A-7.4]  	   86

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                                                              PAGE

A.2.4  What is known about wet deposition processes?
       [CARP A-6] 	   86

       A.2.4.1  What are the important mechanisms in wet
                deposition?  [CARP A-6-2] 	   86
       A.2.4.2  How does wet deposition depend on substance,
                ambient concentration, amount of rainfall,
                and storm type?  [CARP A-6.5] 	   89
       A.2.4.3  What fraction of the ambient pollution is wet
                deposited?  [CARP A-6.3] 	   91
       A.2.4.4  What is the spatial and temporal variability
                of wet deposition rates? 	   91
       A.2.4.5  What generalizations are possible for amounts
                wet deposited by season or region? 	   93

A.2.5  What is known about chemical changes of acidifying
       substances in the atmosphere?  [CARP A-4] 	   93

       A.2.5.1  What are the important processes leading to
                S02 oxidation?  [CARP A-4.2, A.3.5] 	     93
       A.2.5.2  What are typical rates of oxidation:   how do
                they depend on time of day, season, S02
                concentration, concentration of oxidants,
                meteorological conditions?  [CARP A-4.4.4,
                4.4.5] 	   96
       A.2.5.3  What is known about oxidation of nitrogen
                compounds?  [CARP A-4.2, 4.3.4]  	   96
       A.2,5.4  What is known about neutralization of
                acidifying materials?  [CARP A-4.3.6] 	   97

A.2.6  What is known about atmospheric transport?  [CARP A-3]   97

       A.2.6.1  What are the important mechanisms in
                transport?  [CARP A-3.2, 3.3] 	   97
       A.2.6.2  What meteorological information  is needed
                to characterize transport over various
                spatial/temporal scales?	  100
       A.2.6.3  To what extent is the needed meteorological
                information routinely collected?  What does
                it show?  [CARP A-3.5] 	  103

A.2.7  What are the sources of substances important to
       acidic deposition?   [CARP A-2] 	  103

       A.2.7.1  What are the natural sources of  these
                substances?  [CARP A-2.2]	  103
       A.2.7.2  What amounts are emitted by natural
                sources?  How are they distributed over
                space and  time?  [CARP A-2.2] 	  104

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                                                                   PAGE

            A.2.7.3  What are the anthropogenic sources?  [CARP
                     A-2.3] 	  106
            A.2.7.4  What are the anthropogenic emission rates?
                     How are they distributed in space and time?    109
            A.2.7.5  How do natural  and anthropogenic emissions
                     of acidifying substances compare? 	  117
            A.2.7.6  How well known  are emission rates? 	  120
            A.2.7.7  What are projected future emissions of
                     acidifying substances? 	  122

Section B.  Relationships Between the Emission and Deposition of
            Acidifying Substances 	  123

B.I  Are some sources more important than others? 	  123

     B.I.I  What source/receptor relationships are of interest? ..  123
     B.I.2  Are source/receptor relationships expected to
            be linear?  [CARP A-4.4.3] 	  125
     B.I.3  How are models for long-range transport and
            deposition useful?  [CARP A-9] 	  128
     B.I.4  What is the relative importance of distant and
            short-range sources to deposition in sensitive
            regions?  [CARP A-3.5] 	  133
     B.I.5  How do the deposition patterns produced by tall
            stacks differ from those produced by low level
            (urban) releases?  [CARP A-3.4] 	  135
     B.I.6  How important is the emission of primary sulfate? ....  136
     B.I.7  How do source/receptor relations for nitrogen
            oxides compare with those for sulfur oxides? 	  136
     B.I.8  Has the installation of particulate controls on
            power plants and other sources contributed to
            acidification? 	  137
     B.I.9  How well can acidity be predicted, knowing
            emissions? 	  137

B.2  What are the overall budgets for acidifying substances?  	  137

     B.2.1  What are the best estimates of sulfur and nitrogen
            oxide budgets for the eastern United States? 	  137
     B.2.2  What is the relative importance of natural and
            anthropogenic sources to deposition in sensitive
            regions? 	  141
     B.2.3  What can be concluded about deposition trends from
            emissions trends? 	  142
     B.2.4  How predictable are reductions in deposition resulting
            from reductions in emissions? 	  143

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                                                                        PAGE

IV.   INTEGRATION  AND  SUMMARY  	   144

     1.   Are  the  sulfur  and nitrogen  compounds  found  in  the  air,  in
         soils, and  in water  primarily  from anthropogenic  sources?  ....   144
     2.   Have there  been adverse  effects  that can  reliably be
         attributed  to acidic deposition? 	   145
     3.   Are  there potentially serious  but not  demonstrated  adverse
         effects  of  acid deposition?  	   145
     4.   Where are the areas  within  the United  States in which  adverse
         effects  are  occurring or may occur? 	   146
     5.   Is  it feasible  to  identify  sources responsible  for  the
         deposition  that produces adverse effects?	   149
     6.   What effects can be  expected from continuing present trends
         in sulfur and nitrogen emissions? 	   152


     APPENDICES


     APPENDIX A—Steering Committee  	   A-l

     APPENDIX B—-Authors of Critical  Assessment Review Papers  	   B-l

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                                    PREFACE
A.  What is this document?

This  document,  The  Acidic  Deposition  Phenomenon  and Its  Effects:  Critical
Assessment Document, hereafter referred to as CAD, is a summary, Integration,
arid   interpretation  of   the   current  scientific  understanding  of  acidic
deposition.   It is  firmly  based  upon  The  Acidic Deposition  Phenomenon and
Its Effects:  Critical  Assessment Review Papers described below, augmented by
additional  scientific  references  listed at the end.   The  interpretations in
the CAD  are solely  the  responsibility  of its authors, although  much  of the
basic  information  is  taken  from  Critical  Assessment  Review  Papers,  whose
authors and steering committee have provided valuable comments and guidance.

The   Critical   Assessment  Review   Papers   (CARP)   is   a   mul ti-authored,
comprehensive,  critical  review of  the  published  scientific literature  of the
atmospheric phenomena and effects of acidic deposition.  Literature published
later  than  December 1982 generally  is not  cited.    References  published as
recently as early 1984 may have been reviewed by individual authors, however,
as the authors  responded  to public comments received  in July and August  1983.

The original  charge  to the  editors was  to  produce  "a comprehensive document
which  lays out  the state  of our knowledge with regard to precursor emissions,
pollutant transformation  to acidic  compounds,  pollutant transport, pollutant
deposition  and  the  effects  (both   measured   and  potential)  of  acidic
deposition."  This charge is met  upon  completion  of final  drafts of both the
Critical Assessment  Document  and  the Critical Assessment  Review Papers.  To
secure  success  in  meeting  the  charge,  the  authors  and  editors  of  the
Critical Assessment  Review Papers have  adhered to the following  guidelines:

      1.  Contributions  are  to  be  written  for  scientists and  informed lay
         persons.

      2.  Statements  are  to  be  explained and supported by references; i.e.,  a
         textbook type of approach, in  an objective  style.

      3.  Literature  referenced  is  to  be  of  high  quality  and  not   every
         reference available is to be  included.

      4.  Emphasis is to  be  placed on  North  American  systems with  concentrated
         effort on U.S.  data.

      5.  Overlap  between this document and the SOX  Criteria  Document  is  to
         be minimized.

      6.  Potential  vs  known  processes  or effects are to be clearly noted  to
         avoid  misinterpretation.
                                     xn

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     7.  The certainty of our knowledge should be quantified, when possible.

     8.  Conclusions are to be drawn from scientific evidence only.

     9.  Extrapolation beyond the available data should be avoided.

    10.  Scientific  knowledge  is  to  be  included  without regard  to policy
         implications.

    11.  Policy-related  options  or recommendations  are  beyond  the  scope of
         this document and are not to be included.

The authors of  this integrative  summary have  also  attempted to follow these
guidelines, consciously  trying to  write for the informed  lay  person, and to
draw  together  and  interpret  evidence  from disparate  sections  of  the CARP.
Reference is made, where possible, to CARP sections  providing information for
the CAD  analyses;  citations are  made  to additional references.  Quotations
and paraphrases of conclusions and observations from the CARP provide much of
the material in the CAD.  As such, they represent the conclusions of  the  CARP
authors interpretated by the CAD  authors.   Some additional evidence  has  been
developed by the CAD authors, and the conclusions drawn from this information
are solely  those  of  the CAD authors.    Several  of  the  authors  of  the  CARP
commented on earlier drafts of the CAD.

The documents  have been designed  to  present  the status  of  knowledge of the
acidic  deposition  phenomenon and  its  effects.   Neither  CAD  nor CARP  is  a
criteria  document.    Neither document  was  designed to  set standards  or to
suggest regulatory  policies or  recommendations.  The  literature is  reviewed
and conclusions are drawn based on the best available evidence.   Both CAD and
CARP  are  authored  documents, and as such,  the conclusions  are  those of the
authors  after  their  review  of  the  literature.    Both  documents  strive to
provide an  accurate,  comprehensive,  and impartial  evaluation  of the science
of acidic deposition.

The authors of  the  Critical  Assessment Document are indebted to  the  approxi-
mately  sixty  authors  of  the  CARP,  the  editors,   and  the  members  of  the
Steering Committee  identified in Appendix A for their dedication  and  patience
in meeting  the demanding objectives for these  documents.

B.  History of the Documents

The  idea  of  preparing  a  Critical  Assessment Document  was  first  suggested
formally in August 1980 in a letter from Dr. Sheldon Friedlander, Chairman of
the  U.S.  Environmental  Protection  Agency's   Clean  Air   Scientific   Advisory
(CASAC), to the EPA  Administrator  recommending "a separate document  that can
recognize and  incorporate   the  new information  on  causes,  effects  and  data
bases  for  all  of  the various  pollutants  relevant  to acidic  deposition ...
addressing  'Acidic Deposition' in a complete sense  ..."

Soon  thereafter, the  Federal  Interagency  Task  Force on Acid Precipitation in
its draft National  Assessment Plan called  for an  assessment document to aid
in setting  the national  research agenda.  The  CAD was designated  to  satisfy

                                   xi ii

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that need.   By  the  end of  1980  a steering  committee was formed  and began
identifying issues for  analysis in the  document.   Editorial  and  production
responsibility for the document was awarded to the Acid Precipitation Program
at North Carolina State University in February  1981.   It was  soon  determined
that a comprehensive  critical review of  the  scientific literature  (the CARP)
was needed before an assessment (the CAD) could be written.  Since  that time,
authors with  the desired  technical expertise have  been  identified;  chapters
written, reviewed, and  revised;  public  reviews held  (11/82);  public comment
periods called (6/83  to 7/83);  and revisions  of the CARP written.    In short,
every  chapter  has been exhaustively reviewed,  and improved,  several  times.
The  final  version  of  the  CARP  is available  from  the  National  Technical
Information Service  (NTIS)  in  two volumes:    Volume   I  Atmospheric  Sciences
(PB85-100030)   and Volume  II Effects  Sciences  (PB85-100048).    The  Critical
Assessment Document is based upon the  final revision of the CARP and has Deen
reviewed by a small number of scientists and the Steering Committee.

C.  The Future

These  documents  are  the first of  a succession  of increasingly sophisticated
assessment  documents  to   be  produced  by  the  National   Acid  Precipitation
Assessment Program  (NAPAP).   The next document,  a  NAPAP report,  will be the
1985 Assessment,  an analysis of the impacts of acidic deposition, atmospheric
source-receptor  relationships,  emissions  and  deposition  data, and control
technologies,   to  be  published  late in  1985.   Succeeding assessments in 1987
and  1989  will  seek   to integrate  knowledge  within  a framework  of source-
receptor,  cause-and-effect,  cost-benefit, and control-mitigation analyses.
                                       xiv

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                               I.  INTRODUCTION
Organization:

The acidic deposition phenomenon, although often cited for its complexity, is
represented in a straightforward manner in Figure I.I.  Substances emitted to
the atmosphere  by  human activities or by  natural  processes  are transported,
transformed,  and  deposited from  the  atmosphere  in  precipitation or  as dry
gases  or  particles.   The  deposited  materials  then  act,  directly  or after
additional  transformations, on  various components of  aquatic or terrestrial
ecosystems.  The emitted  substances  of interest are  those,  or  their precur-
sors,  that  may  beneficially  or  adversely affect ecosystems.   If impacts can
be  quantitatively  related  to  the depositional  'loading',  a 'dose-response
relationship' may  be determined.   Such a  relationship  may  be  difficult to
separate  from  the  effects  of  other  natural   processes  occurring   in  the
ecosystem.

The  atmospheric  components of  the picture  must  be  considered in  order to
apportion the sources  of deposition  (the  'source-receptor  relationship') or,
more  importantly,   to  predict  the  impact  of  changes  in  emission  sources.
Causality must run from emission to deposition to impacts,  but attribution of
effects  proceeds  from  identifying  effects  to  identifying amounts  of depo-
sition  to  identifying  sources.    The  latter  approach,  upon  which  this
document's organization  is based,  allows  the  reader  to ignore  parts  of the
greater complexity by focusing only on those steps important in determining a
given  impact in  a  specific  region.    Later  chapters will  expand on  this
approach.

The Critical Assessment Document (CAD), is organized in a question and answer
formalQuestions on effects are raised and answered in Chapter II and those
on  atmospheric  sciences,  in  Chapter  III.    Linkages  between  atmospheric
sciences  and  effects are made in Chapter  IV.   Chapters II and  III  are each
organized into  two  sections,  A  and B.   Section  A in each of  these  chapters
has  subsections corresponding  to chapters in the  Critical  Assessment Review
Papers  (CARP),  upon  which  the CAD is  based.   Section B in each  chapter,  in
answering the questions raised,  integrates information found in more than one
CARP chapter.  Section B of Chapter II considers interactions among ecosystem
components, selecting for discussion  only impacts or potential impacts on the
chemistry of aquatic ecosystems  and forests.  This very substantial narrowing
of focus from the  discussion in  Section A results from the  following logic:

0    These  are  the  sensitive ecosystems  where  significant  impacts  may  be
    occurring.

°    Understanding   impacts   in   these  ecosystems  requires  information  from
    several effects  chapters  in  the  CAD and  corresponding   sections  of the
    CARP.

    Direct (pre-depositional)  effects on human health from  inhalation of acid
    precursors or  sulfate particulates are not discussed in the CARP and have
    been  dealt  with  previously  in U.S.  EPA criteria  documents.   Indirect

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   ACIDIC  DEPOSITION
Figure I.I  A schematic representation of  the acidic deposition phenomenon and its consequences,
           Adapted from The Interagency Task Force on Acid Precipitation (1982).

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     (post-depositional) effects from ingestion of food or drinking water have
     not been documented.

     Impacts  on  aquatic  biota  (e.g.,  fish)  follow  directly  and immediately
     from changes in aquatic chemistry.

     Impacts  of  acidic  deposition  on agricultural crops  appear  small  and are
     not manifested through  the  soil.   Sufficient discussion of this is found
     in  Section  A.   Impacts  of gaseous  pollutants (ozone,  sulfur dioxide,
     nitrogen oxides)  are significant,  but  are extensively discussed  in EPA
     Criteria Documents.

     Effects  on  materials,  found mostly in  urban  areas,  are significant, but
     they are at this  time inextricably  entwined with  ambient  air quality,
     which  in urban  areas  is  determined  largely  by  emissions  from  nearby
     sources.
The  questions  raised  in  the  CAD  are  in  most  cases  based on  the original
issues identified by the  Steering  Committee  and  peer reviewers late in 1980.
In the intervening four years, research and assessment activities have raised
new  questions  or  dramatically altered  the  frame  of reference  in  which  a
question must  be  answered.   The rapid  expansion of research efforts has led
to much  better questions,  but has  not always  provided  definitive answers.
This  is  the  natural  evolution of  research—the  quality  of questions rapidly
improves and, after a period of time appropriate to the individual study, the
answers  follow.   Certain  physical  and  chemical  principles are often cited or
assumed  in discussions  in this document and are  introduced  in the  'chemical
primer1 below.

A chemical primer:

The emissions  shown  in  Figure I.I include sulfur  dioxide ($03)  and nitrogen
oxides  (NOX);  respectively,  they may  be oxidized to  sulfate   (S042~)  and
nitrate  (N03~)  either  in   the  atmosphere  or  after  deposition.    In  the
atmosphere,  the oxidizing agents  include hydrogen  peroxide  and  ozone,  whose
concentrations are in turn  affected  by  emissions of hydrocarbons.  Reactions
that   generate   S0^~   or    N03~   also   produce   hydrogen   ion   (H+),   a
component present in or generated by all acids.

     0  All pure substances are electrically  neutral.

Whether in a rain droplet,  as  a  solid  material,  or in soils, vegetation or a
lake, a charged species (an ion) must be accompanied  by  species  of equal  and
opposite   charge;  i.e.,  $042- cannot  be isolated  by itself.    In  solution
5042-  may   be   accompanied   by  H+   (sulfuric   acid,   H2S04).   or   some
other positive  ion.   Neutralization of  the  sulfuric  acid by a  base  such  as
calcium  carbonate  (CaCOs)  would  result  in  a  neutral   product,  CaS04  in
this   case.    The  ions  Ca2+  and  Mg2+  are  referred  to  as  'base  cations'
because  of   reactions  such   as this.    In   Chapter II   the  association  of
5042-  with  aluminum  (A13+)   will   be   shown  to   be  of  great  importance.
Ions  of positive charge are  called cations;  those of negative charge, anions.

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      0  All acids may donate H+ to a suitable receptor.

Those  acids  that donate  H+ (hydrogen  ion)  most readily  are  called  'strong
acids',  the  strongest  being  the  'mineral  acids',  H2S04,  HNOs,  and  HC1.
Some  organic  compounds and Al3+  hydrated in  aqueous  solution act as  weak
acids; that is, they incompletely dissociate giving H+.

      0  The concentration of H+ in aqueous solution may range from very
        large to very small.

A neutral  aqueous  solution at 20°C  contains  10-7 g  hydrogen  ion  per liter,
which  is one  ten-millionth  mole.   Acidic  solutions  have more  H+  per liter;
basic  solutions  have  less. Because  natural  waters  may  have  hydrogen  ion
concentrations that  vary  by at least a  factor  of 108  (one  hundred  million)
a more convenient measure  of H+  concentration, [H+],  has  been  developed.
This measure is "pH."

          pH = -log [H+]

The  range   of  10~2   to  10~10  moles  per  liter,  for  example,  is  simply
expressed as pH 2  to pH 10.   Basic  solutions  (low acidity)  have  pH > 7.   A
change in  pH  of  one  unit (e.g.,  pH  5.5 to 4.5)  corresponds to a  change  in
[H+]  by  a  factor  of ten.   Small  changes  in  pH represent  large  changes  in
concentration; H+ is the important factor in solution.

The evolving point of view:

The acidic deposition phenomenon has been  recognized  in  some form for over a
century,  and  research  to  understand   the many  aspects  of  the  phenomenon
accelerated beginning in the 1950's  (Cowling 1982).   By  the early 1970's the
phenomenon  had  become  an  issue   of  much  public  debate.   In  this  temporal
context the authors  of  the  CAD,  with a mean experience  in acidic deposition
of  five  years,  are neophytes.   None the  less,  we feel  that  there existed a
characterization  of  acidic  deposition,  part   scientific  and  part popular
public  perception,  when  this  document was  begun in  1980.    The  scientific
basis  for   this  view  can  readily  be  found   in  papers  presented  at  the
international  conference  in Sandefjord,  Norway, in 1980  (Drablrfs  and Tollan
1980) and  in  other  important reports  (Likens  1976,  MAS 1981).   Significant
aspects of  the characterization include:

    A focus on hydrogen  ion  concentration or pH  of rain and 'acid'  rain of pH
    less than  5.6

0   The prospect of a continuing  'titration' and  acidification of  lakes,  and
    loss of fish,  in  locations  receiving acid  rain

0    A concern  for  accelerated leaching  of nutrients  from soils  because  of
    acid rain  with adverse effects on forest and crop plant nutrition

0   A  view  of  sources  of acidity  to  the  atmosphere  hundreds  of  kilometers
    from affected  receptors.

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This characterization is admittedly an extreme oversimplification.  There was
much healthy scientific debate and a much broader scientific view.  [See, for
example,  Rosenqvist's  view  of  acidification  of   surface  water  (1978)  and
Hepting's discussions of  the occurrence  and large  number of potential causes
of widespread forest declines (1971).]  The local air pollution episodes such
as that in Donora, Pennsylvania, had not been forgotten.

We further  feel  that the  research  of hundreds  of  scientists  over  the  last
several years  has resulted  in  a rapid  evolution  of the questions,  and the
definition of the acidic  deposition  'problem'.   We  attempt  below to give the
reader our sense of this evolution.

The  focus  has   shifted  from  hydrogen  ion  concentration   (or  pH)  as  the
"culprit" to  hydrogen ion as an  indicator.  This  reflects an  attention  to
biological  activity  and  soil  chemistry,  the  flows  of sulfur  and  nitrogen
through   soils   and   their  biological  or  physical   storage.     Important
conclusions include:

0   The sensitivity of  a  component  of the  terrestrial  ecosystem (e.g.,  soil
    or soil water) to acidification is  determined  both by  concentrations  of
    chemical substances entering or within the system (intensity factors) and
    by  the  amounts  of   substances   or  storage   of  substances  (capacity
    factors).

0   Capacity can  be regenerated  by  chemical or  physical  processes;  the  rate
    of regeneration is thus very important.

°   The effects  on ecosystems  from  deposited substances  other  than hydrogen
    (e.g., sulfur and nitrogen compounds)  may be more significant than those
    of hydrogen.

°   The deposited  material's  form seldom  makes  a   difference;  dry  deposited
    sulfur and nitrogen compounds may  be as  important in quantity and effect
    as those deposited in  precipitation.

°   Nitrogen,  usually a  beneficial  nutrient  to  unmanaged ecosystems,  may
    create adverse effects at  high amounts  of  deposition—ecosystems become
    saturated; nitrogen concentrations in  water following  snowmelt  may  also
    be large.

0   The past  history  of  an  ecosystem, even  over   'geological   time1  is  very
    important in determining effects of  current  deposition  and  the  potential
    for future  effects.   Thus,  regional  analyses  (e.g.,  of  the  glaciated
    Northeast  or  the   unglaciated  Southeast)   are  both  necessary   and
    appropriate.

    Future increases  in  aquatic  acidity in  the northeastern  United States and
    Canada  may  be  less  than  was  predicted;  future  acidification  may  be
    greater  in the Southeast.

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0   Lakes are  not acidifying rapidly;  rates  of change  in  aquatic chemistry
    and  fish  populations  are  unknown,   but  dramatic  acidification   over  a
    period  of  just  a  few years  has not  been  observed; some  reductions in
    acidity may have been observed.

0   Soil pH's are not likely to change rapidly, even in  unmanaged ecosystems.
    Acidic soils become acidic over centuries of weathering.

    Acidity production within  the  aquatic/terrestrial  ecosystem is a  natural
    process.  One must not  look  solely  to atmospheric  deposition for  sources
    of  acids.     Nitrification,   the  oxidation of  NH4+   to   N03~,   is  one
    example  of a  major  natural   source  of  acids  in  most  soils in humid
    regions.

The  above  conclusions  take  into  account  the  importance  of  knowing total
deposition, particularly of sulfur, and perhaps of nitrogen.  In addition:

0   Dry  materials collected  in  buckets  or  in  most  air  quality  samplers
    represent an unknown proportion of materials deposited on trees, grasses,
    etc.

0   Concentrations of  substances  other  than  hydrogen  ion  in  solution  will
    determine the pH.  These  concentrations must  be  known  both to understand
    the  causes of  acidity  and   to  evaluate  the quality  of  measurements,
    Depending  on  the substances  present,  the  'natural'  pH of  rainwater in
    pristine areas remote from man's activities may be  greater or less  than
    5.6, perhaps ranging from 4.8 to 6.5.

The observations  about  effects  above have implications  for what information
is needed about atmospheric phenomena:

°   Knowing the  specific  chemical  or physical  forms  of deposited materials
    may be less critical  than was previously thought.

0   Pollutants  deposited from both distant and  relatively  nearby sources are
    important.    Understanding both  long-range and  short-  to  intermediate-
    range  transport,  therefore,   is  necessary.    This  requires  knowledge of
    specific chemical or physical forms in the atmosphere.

Certain observations on effects on ecosystem components may also be made:

°   Loss of fish populations may often  be  from  reproductive failure (lack of
    'recruitment') rather than deaths of adult fish.

0   Adverse effects of acidic deposition  on crops are  small to none;   impacts
    of other air pollutants  are greater.

0   Widespread  sustained decreases in the  growth of  certain coniferous trees
    have  been   observed   in  the  eastern  United  States,  but  the cause  is
    unknown.   To attribute decline to acidic deposition is premature,   perhaps
    unwarranted; symptoms are similar to  those  reported  for declines   in  many
    different species and regions over the years.

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0   Dramatic changes  in  soil  chemistry,  although requisite only  a  few years
    ago in models of effects of acidic deposition on  forests,  are not always
    found where dieback is occurring.  Soil-mediated change is not ruled out,
    however.

The statements above represent the authors'  perspective and are elaborated in
the following chapters.

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                         II.  EFFECTS SCIENCES SUMMARY

                       SECTION A.  ECOSYSTEM COMPONENTS
A.I   INTRODUCTION
The acidic  deposition  phenomenon  is complex and our  understanding  of it and
its  effects is  limited.    This  complexity,  plus   the  uncertainties  in  our
knowledge of natural processes and  cycles,  makes difficult the definition of
effects  of  acidic  deposition,  isolated  from  other  manmade   or  naturally-
induced stresses.  In addition, understanding the behavior of precursor gases
that  contribute  to  acidic  deposition,   their  reactions,  transport,  and
ultimate fate,  and  determining which  deposited  chemicals are  of  concern to
receptor  systems  are  not  any  simpler.    Therefore,  answers  to  acidic
deposition  related  issues  are  likely  to  be  equally  complex,  somewhat
uncertain,  and  subject  to  change  as  the  quality   of  available  evidence
improves.   There is  no doubt, however,  that  effects have occurred  and  are
occurring due to acidic deposition.  Future research will better quantify the
magnitude and extent of these effects.

A.1.1   WHAT EFFECTS OF ACIDIC DEPOSITION HAVE BEEN STUDIED?

Effects research has focused on soils,  vegetation,  aquatic chemistry, aquatic
biota,  structural  and  cultural  materials,  and human  health.    Effects  are
described by changes  in these components and are  determined  by interactions
in the ecosystems in which the components are found.

A.1.2  WHAT COMPONENTS  OF ECOSYSTEMS MUST BE CONSIDERED TO UNDERSTAND THE
       EFFECTS OF ACIDIC DEPOSITION?

Since the number of possible  interactions  in ecosystems  staggers  the human
mind, one  direct approach  is  to  track  the  individual  chemical  components
throughout  the ecosystem, a  type  of bookkeeping.   The cycling of sulfur and
nitrogen  is of  particular  interest in  understanding the effects  of acidic
deposition.     The  sulfur  and  nitrogen   cycles,   Figures   II.1  and  II.2
respectively, are  comprised of  interdependent  pathways; alterations  of  any
part of  the cycle will  effect  change  in other parts of the cycle.  Since one
component depends  on  another,  and  nitrogen and  sulfur compounds interact in
air,  soil,  and  water,  any  attempt to  quantify  effects  on  the  cycles  and
ecosystems, or to predict subsequent changes adequately,  is difficult.

A.1.3  HOW DO THESE ECOSYSTEM COMPONENTS INTERACT?

Reviewing the sulfur  and  nitrogen  cycles reveals that the vegetation,  soils,
animals, water, and air all  interact.   While the acidic deposition phenomenon
begins  with  an  alteration  of  air   chemistry,   it   is   the   deposition  of
substances  on  the  land, water, and vegetation  that  causes  effects.   Acidic
deposition, in  wet  form,  is a dilute  solution  of  acid,  i.e.,  hydrogen  ions
and  associated  anions.   This  solution  is  easily   altered,  chemically, upon
contact  with any exposed  surface.   If  the  interacting  surface is basic,  the

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                     atmosphere
organic
sulfur
compounds
                                   Iwchlnj
Figure I I.I   The sulfur cycl,

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pH increases.  If the surface is acidic, the pH may decrease.  The anions may
exchange  with  other anions at the  surface.   The same surfaces  that come in
contact with precipitation also collect  dry-deposited  substances,  including
those  dry   substances  that  are  nitrogen and  sulfur  rich.   Therefore,  when
precipitation, which falls an average  of  less than 10 percent of  the time in
the eastern  United  States, contacts a  surface, it  dissolves  the dry-deposited
nitrogen  and sulfur  compounds (in addition  to those compounds already in the
precipitation),  often  making the  total of  these  substances  in  the aqueous
solution  quite high.   Because  basic   substances  are  also  deposited  in dry
form,  the pH  of the  precipitation may  or may  not  change.   Undisputably,
however,  cumulative  loading  of  sulfur  and nitrogen compounds to the surfaces
below  continues to  increase.

Figure II.3  is a representation  of a  stream  ecosystem,  the  stream and its
watershed, including  soil  and  vegetation.  The  material  from the atmosphere
is deposited on  soil,  vegetation,  or  stream.    The deposited substances may
remain  for  some  time  within  soil  and  on vegetation  before entering the
stream.   At  any  point  along  the pathway  the  materials  can  change chemically
and/or have an effect on another ecosystem component.

A.1.4  WHAT LEVEL OF UNDERSTANDING WILL BE NECESSARY TO DEFINE INTERACTIONS
       AMONG ECOSYSTEM STRUCTURE, FUNCTION, AND ACIDIC DEPOSITION  INFLUENCE?

The level  of understanding ultimately  needed  to attribute effects  to acidic
deposition will be  determined  by the  purpose  for  which  the understanding is
needed.   If  society  perceives that  acidic  deposition  is  a highly  detrimental
phenomenon,  correlative evidence  is  likely  to  be sufficient for recommending
change.   From  a purely  scientific  perspective, the  data base  required for
determining cause-and-effect relationships will be quite  detailed.

The  information  needed  to  suggest  that  acidic   deposition is,   in   fact,
affecting ecosystems comes in two pieces.   The first  is  correlations between
observed  ecosystem  changes and deposition  levels.   Such  correlative evidence
has driven  the recent  concerns  and such  evidence continues  to  accumulate.
These  correlations are not sufficient  to  prove that acidic deposition causes
effects;  statistical correlation is never sufficient proof of causality.  The
second  piece  of  evidence  to determine the  effects of acidic  deposition is
controlled experiments  that  demonstrate ambient levels of acidic deposition
cause  changes  not  expected  if  the  chemistry  of  deposition  were  less
'polluted1.

Evidence of both sorts is accumulating.   The  evidence  is  as yet insufficient
to provide  a  complete  picture   of  the effects  of acidic deposition.   The
correlations are confounded  by   changes in several anthropogenic pollutants
that   have   occurred  simultaneously   with  suspected   changes   in  acidic
deposition.  These  changes include increases in ozone, metals deposition, and
sulfate aerosols.    All  have been  linked  to  the  burning of fossil  fuels.
Research  has  demonstrated that  these   pollutants,   as  well  as  precursors of
acidic  deposition   (sulfur   and   nitrogen  oxides),   can   cause  effects.
Definitive   results  from  controlled   experiments  are   few.     Laboratory
experiments  may  lack important  ecosystem components; ecosystem   experiments
are difficult to control.


                                     11

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                                  INPUTS
GASEOUS
 OUTPUT
                     DRYFALL       WETFALL
   ROOT
   TURNOVER
          LEACHING
        (biological export)
                                  GEOCHEMICAL EXPORT
Figure 11.3  A conceptual diagram of wet  and  dry deposition pathways  in an
              ecosystem context.   From Johnson et al.  (1982).
                                         12

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A.2  WHAT  IS KNOWN ABOUT THE  INFLUENCE  OF ACIDIC  DEPOSITION  ON  ECOSYSTEMS?

This  section  reviews  the  state of  scientific  knowledge   described  in the
Critical  Assessment  Review Papers about effects  on each ecosystem  component
afnclmaterials.  HJe  will   follow  a  progression  beginning  with  terrestrial
effects   (soils  and   vegetation)   through   aquatic  effects  (chemistry  and
biology)  to  indirect effects  on  human health and  effects on  materials.

A.2.1  WHAT ARE THE EFFECTS ON SOIL SYSTEMS? [CARP  E-2]

A.2.1.1.   WHY  IS KNOWLEDGE  OF SOILS IMPORTANT FOR UNDERSTANDING  THE  EFFECTS
           OF ACIDIC DEPOSITION?  [CARP E-2.1]

Soils  play a  key  role  in ecosystems.   They are one  of   the  most  stable
ecosystem  components and,  together with climate,  they  determine  a  terrestrial
system's  productivity.   Because  much   of the  water that  enters streams and
lakes flows first through  soils,  soil properties, particularly  soil  chemistry
and pore  structure, can greatly  influence aquatic systems.   Thus,  significant
changes in soils could have serious ecosystem implications.

As soils  change  so  do associated aquatic ecosystems.   Acidic deposition may
directly  influence  the transfer of substances  from  terrestrial  to aquatic
systems, as when material   deposited from the atmosphere flows rapidly over or
through the  soil with  little  interaction,  or it  may act indirectly, as when
deposited  materials   cause changes  in  soil  processes  such  as weathering,
leaching,  and/or  organic   matter  decomposition.   In  either  case,  substances
produced  and/or  deposited  in  soils are transported to the aquatic system.
[CARP E-2.1.1]

"Soil provides the  physical  support  and most  of  the water,  nutrients, and
oxygen needed  by plant roots for normal growth and  development.  Well over 95
percent of our food  and much of our  fiber  come  directly  or indirectly from
terrestrial  plants"  [CARP  E-2.1.2].  Soil  properties  limit the productivity
of terrestrial ecosystems and changes in the soil properties, whether natural
or man-induced, may  alter  the productivity  of  the  terrestrial  system.  [CARP
E-2.1.2]

A.2.1.2  WHY IS UNDERSTANDING/QUANTIFYING THE EFFECTS OF ACIDIC  DEPOSITION ON
         SOILS DIFFICULT?    [CARP E-2.1]

Soils in  the  United  States  can be divided  into  approximately  12,000 soil
series,  each  with  a  unique  combination  of properties  (see  Figure  II.4).
Because   of  this  diversity,  generalizations  about  the  effects  of  acidic
deposition  on  soils  are   difficult.    Furthermore,  changes  in  soils  are
normally measured in decades,  not days, weeks,  or years.

Hydrogen ion deposition is  expected  to  have a  minor influence on most soils.
Sulfate  and  nitrate  deposition  are  potentially more  significant.   Soils are
complex   chemical,  physical,  and  biological  systems;  thoroughly  assessing
effects  of atmospheric deposition on elements transferred from terrestrial to
aquatic  systems requires  extensive  measurement and quantification of soils'
inputs,  internal  processes, and  outflows.   Our  knowledge  of  these various


                                     13

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Figure 11.4  Generalized  soil map of the United  States  (Soil  Survey Staff  1975)  show-
             ing  regions  dominated by  suborders  or  groups of  suborders.   The  most
             common  suborder  is named.  Many other  suborders  exist  within the bound-
             aries of  each  area.
             Alfisols
               AT   Aqual f s
               A2   Boralfs
               A3   Udalfs
               A4   Ustalfs
               A5   Xeralfs

             Aridisols
               Dl   Argids
               D2   Orthids

             Entisols
               El   Aquents
               E2   Orthents
               E3   Psamments

             Histosols
               HI   Hemists
               H2   Hemists and  Saprists
               H3   Fibrists,  Hemists, and Saprists

             Inceptisols
               II   Andepts
               12   Aquepts
               13   Ochrepts
               14   Umbrepts
Mol1i sols
  Ml  Aquolls
  M2  Borolls
  M3  Udolls
  M4  Ustolls
  M5  Xerolls

Spodosols
  SI  Aquods
  S2  Orthods

Ultisols
  Ul  Aquults
  U2  Humults
  U3  Udults

Vertisols
  VI  Uderts
  V2  Usterts

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GENERAL SOIL  MAP OF THE UNITED STATES
»od comcmTio* atnvict

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processes  is  incomplete,  even  in  unperturbed  systems.    As  a  result,
understanding  the  effect  of  acidic  deposition  on  soils   is  an  ongoing
challenge.   The  changes  that  may  occur  within  days,  weeks, or  years are
likely small, and their quantification nearly impossible.

Chemical,  physical,  and microbial  properties  are some  of  the major factors
making one soil distinct  from another.   Soil  chemical properties may provide
the  protective  mechanisms  to  prevent  alterations   in  soil  water  chemistry
resulting from acidic deposition.  A soil's resistance to chemical changes is
measured  by  its  buffering   capacity,  nitrogen  status,  carbon   to  nitrogen
ratio,  initial  pH,  sulfate  adsorption  capacity,  and  amount  and type  of
weatherable minerals.   The  physical properties control  water flow,  the most
important  variable   determining  the  potential  for   deposition  and soil  to
interact.   Soils  having high surface  runoff rates,  such  as  those  with low
porosities  on  steep  slopes, allow  precipitation   to  flow  through  rapidly
without changing its chemical composition.  Such systems would provide little
protection  for  aquatic  systems,  which  would  receive  the  precipitation
unbuffered by the terrestrial system.   The biological  component  of the soil
contributes  some  of  the  means  of  resistance  and/or  recovery.    Microbial
processes in the soil may consume or generate acidity.  [CARP  E-2.1.3]

Each of  the above factors must  be considered when  one  attempts  to determine
whether  acidic  deposition is influencing soils,  vegetation,  and associated
surface water chemistry.

A.2.1.3  WHAT ARE THE SOURCES AND FATES OF SULFUR, NITROGEN,  AND  HYDROGEN
         IONS IN SOILS?  [CARP E-2.2.1]

The sources of these ions are site specific.  Sulfur  in ecosystems comes from
either  sulfur-bearing  minerals  or   deposition  (natural  and  anthropogenic).
The primary sources  of  nitrogen  are  the atmosphere,  biological fixation, and
deposition.  The sources of hydrogen are  biological  processes  and deposition.
Deposited  sulfur, nitrogen,  and  hydrogen ions may  be retained in,  or passed
through, the soil  component of an ecosystem.

In  the  soils,  the  ions may  be  chemically  transformed through  physical  or
biological processes,  physically adsorbed  to  soil   particles, or removed by
leaching  into  surface  or groundwaters,   uptake by  plants,  or conversion to
gases.   These  processes and their names  are  illustrated in Figures I I.I and
II.2.

A.2.1.4  WHAT SOIL PROCESSES COULD BE AFFECTED BY ACIDIC DEPOSITION?
         [CARP E-2.3]

Five  types  of  soil   processes could be affected  by  the deposition  of acidic
and acidifying substances:

     1.    Leaching of cations (base  cations or aluminum)
     2.    Weathering (solubilization) of  minerals
     3.   Adsorption of anions
     4.   Mobilization  of metals
     5.    Microbial  processes


                                      16

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 Any   of   these  processes  may  result  in  impacts  on  the  productivity  of
 vegetation  or on  surface  and  groundwater  chemistry.

 A.2.1.5   WHAT EVIDENCE IS THERE  THAT SOIL PROCESSES  HAVE BEEN AFFECTED?
          [CARP  E-2.3]

 Historical  field  data in  the  United States  are not  available to  indicate
 definitively  that  soils   are  changing at rates  faster  than  expected  from
 natural  processes.   Both  field and  laboratory studies  using  simulated  acidic
 precipitation suggest  pH  changes do occur in soils, aluminum is  more rapidly
 mobilized,  basic  cations  are  lost at  a  more  rapid  rate,  and  microbial
 processes are affected by simulated acid precipitation.   In all  instances,
 however,  these  studies involve either application  rates  far exceeding natural
 precipitation  or  application   of  concentrated  acid.     Neither   of   these
 techniques  allows  for   the  normal  influences  of  mineral   weathering  and
 vegetative  interaction associated  with  natural  nutrient cycling.    "Soils
 exposed   to   concentrated   acids   over  short  periods  undergo  reactions  and
 changes  that would  never  occur  with more  dilute  acid  inputs  over  longer
 periods"  [CARP  E-2.3.1].   Therefore, our  understanding  of effects  of  acidic
 deposition  on soils must  rely  heavily on  basic  soil  chemistry and theoretical
 calculations, rather than  on experimental  results.

 A.2.1.5.1   IS SOIL pH  LIKELY TO CHANGE AS  A RESULT OF PRESENT LOADINGS  OF
           ACIDIC DEPOSITION?  [CARP E-2.3]

 No.   Changes  in  soil  pH, at  current  amounts  of  acidic deposition  observed
 regionally  in the United States,  are  unlikely.   "Soil  acidification is a
 natural process in humid  regions.   It is  obvious that atmospheric deposition
 contributes  to  this   process;  however,  at current  levels  it  is  a  minor
 contribution" [CARP E-2.7].  Most  soils that are easily  acidified are already
 acid,  having  changed over  the  centuries.   Soils likely  to become perceptibly
 more  acid due  to  current  levels of  deposition are  limited in  number and
 geographic  extent.   In fact,  very few soils  in the United  States  meet the
 acidification criteria discussed in Section A.2.1.6.1 below.

 A.2.1.5.2  ARE NUTRIENTS LOST FROM SOILS AT AN INCREASED RATE AS  A RESULT OF
           ACIDIC DEPOSITION?  [CARP E-2.3.3]

 Yes.   "There  is  little  doubt  that acid deposition  can  accelerate  cation
 leaching  rates,  but  the   magnitudes of  these   increases  must  be  evaluated
within the context of  natural  internal  leaching processes" [CARP E-2.3.3.1].
 Data  on  basic cation   leaching due to acidic inputs  are  inconsistent.   The
 long-term effect  (decades  or  centuries)   of acidic  deposition,  however,  is
 likely to be  increased removal of  cations from soils.  Unfortunately,  it is
not  clear  whether this   will   reduce  available  cations  and   enhance  the
acidification  of soils.   Cation  leaching  rates,  although increased by acidic
deposition,  may  remain insignificant relative  to  total  soil  cation  supplies
and plant growth requirements.
                                     17

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A.2.1.5.3  IS NUTRIENT RENEWAL IN SOILS FROM MINERAL WEATHERING OCCURRING AT
           RATES SIMILAR TO THE LOSSES OF NUTRIENTS?  [CARP E-2.3.3]

Maybe.   Exchangeable  cations lost  as a  result of  leaching  and  uptake  by
vegetation are  replenished by  the  weathering  of  primary minerals.   Acidic
deposition is expected to increase the weathering rates of minerals.  If this
replenishment rate were equivalent to  the  rate at which cations are lost, no
change in soil  nutrient status would be  expected.   Studies  of  the most acid
lake  waters  (Wright in Johnson  et  al.  in  press)  indicate  that  base  cation
neutralization  inputs may not have kept up with acid sulfate inputs, at least
along the major water flow-paths through soils.

The rate at which minerals weather naturally can be estimated from the amount
of  hydrogen ion  that can  be  assimilated  by soils without  a  decrease in soil
pH.   It is expected  that mineral  weathering  rates  for soils  in  the  United
States naturally range from  20  to  2000 meq m~2.   Most  soils have weathering
rates  in  excess of  200  meq  m~2.   Even in  those  soils thought  to  have the
lowest  mineral   weathering  rates  (Spodosols),  20  to  200 meq   m~2  of  H+
could be assimilated.   Therefore,  only in  areas  where  H+ inputs exceed this
range would  a  loss  of nutrients  be  expected to  be  significant.    In  the
northeastern  quadrant  of   the United States,  H+ deposition  is approximately
40  to  160  meq m~2 (estimated wet  plus dry).   The soils  in  this  region that
have  H+ assimilation  rates  below 40  to  160  meq  m~2,  assuming  no increase
in  mineral weathering,  would be vulnerable  to change.    However,  if mineral
weathering rates were to double (hypothetical, actual unknown-see below) as a
result  of  acidic deposition  in  this  same  region,  no  significant  change  in
base  cation   status  would  be anticipated,  even  in soils  with  the  lowest
weathering rates.   [CARP  E-2.3.3.1,  Table 2-5;  Seip  in  Johnson et  al.  in
press]

At  present, no  changes in mineral  weathering  rates  due to acidic deposition
inputs  have  been computed from  field or  laboratory  research.   Studies are
presently  underway  to  quantify these  rates as  influenced  by  acidic deposi-
tion.   Until  these  studies  are  complete,  it is  not  possible  to evaluate
accurately  the  influence   of  mineral weathering in  preventing  soil acidifi-
cation  or  significant base cation  losses  in  natural systems.    The lack  of
this  necessary  data  base is  one  of  the  most  obvious deficiencies  in our
present  knowledge  about  current  or  future effects  of acidic  deposition  on
soils  or surface  and  groundwaters.   As  is   pointed  out by  Johnson,   "soil
weathering rate  remains one  of  the  least  understood  of the master  variables
controlling  soil  acidification  even  after  many  years'  recognition  of its
great importance"  (Johnson et al. in  press).

A.2.1.5.4  ARE  MICROBIALLY-CONTROLLED  SOIL PROCESSES LIKELY TO  BE  AFFECTED
           BY PRESENT LOADINGS OF ACIDIC DEPOSITION?  [CARP E-2.4, E-2.5]

Not significantly.  No evidence exists that suggests current rates  of acidic
deposition  in  the  United States   will  cause  a   decrease  in  microcrobial
activity  over the long  term.  Although  alterations in  microbial  processes
have been documented in short-term simulated exposures, longer-term  exposures
have  demonstrated  that such  processes can  recover.    It  is recognized that
certain  microbial  species  are  quite sensitive  to  changes  in  pH, changes


                                     18

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 similar to  those  that  might occur  during  a  single  rain  event.    However,
 microbial  populations in soils are quite adaptable; species  shifts  occur  and
 processes  continue when  microbial  populations  are  impacted.   Possible effects
 of acidic  deposition on soil  microbial  activity in natural  systems  have  not
 been ruled out, but  important effects  have  not been demonstrated at ambient
 levels  under  field conditions.   [CARP E-2.4]

 An important biologically-mediated process that has been extensively studied
 is organic matter  decomposition.  It appears  this  process  in forests will  be
 only slightly inhibited  over  the  long-term by  acidic deposition,  i.e., a less
 than 2  percent decrease  in decomposition  rates at  pH above  3.0.   Thus, unless
 average precipitation pH falls to 3.0 or  below,  significant impacts  of acidic
 deposition on  litter  decomposition  in  natural  systems  are  not  expected.
 [CARP E-2.5]

 A.2.1.5.5   ARE METALS MORE LIKELY  TO BE MOBILIZED  AS A RESULT OF  ACIDIC
            DEPOSITION?   [CARP  E-2.3.3.3,  E-2.6]

 Yes.   Metals  are more readily mobilized with increased soil water  acidity.
 The  increased mobility of  aluminum in uncultivated, unamended acid  soils  has
 the  greatest potential for adverse impacts  on both terrestrial  plant growth
 and  surface water chemistry.   Aluminum  is the third most abundant element  on
 Earth  and  a  major structural  component of soils.   When pH drops below 5.0,
 because of  either   natural  or  anthropogenic  influences,   aluminum  becomes
 soluble in soil  water solution.   The  presence of dissolved organic  matter  in
 solution may  reduce aluminum  solubility.  Caution must be exercised,  however,
 in  attributing  all  aluminum  movement  into  soil   water solution  to acidic
 deposition.   Naturally acid soils with high free aluminum concentrations will
 likely  contribute aluminum to  soil water  independent of  rainfall  pH.

 The  introduction of  a mobile  anion  to  an acid  soil  will  lower  the pH  of a
 soil  solution.   A cation-anion balance  in solution is required,  and most of
 the exchangeable cations in acid soils are hydrogen ion and aluminum.  Due to
 cation  exchange  processes  in  the  maintenance  of  cation-anion   balance,
 increased  concentration  of an  anion such  as  sulfate in an acid soil  solution
 causes  increased  hydrogen   ion   and  aluminum  concentration  regardless  of
whether the anion is  introduced  as a  neutral  salt  or an acid.  Field studies
 have  confirmed that   this  mechanism  operates  in many  soil   systems.   Thus,
 there remains some  question whether,  in  the eastern United States where soils
are already acid, acidity of deposition,  per_ ^e_, is an  influential  factor in
aluminum mobility.    It is  important to recognize,  however,  that introducing
 sulfur,  in any  form,  can  increase the  potential   for  aluminum  movement  in
 highly acid soils (pH <_ 4.0).

A.2.1.6  WHAT IS A  SENSITIVE  SOIL?  [CARP E-2.3.5]

Two  views  of  sensitivity must be considered.   Until  recently,  a  sensitive
soil  was   thought of  as one  whose pH  was  likely  to change due  to acidic
deposition.  Soil sensitivity, or potential  effects of  acidic deposition,  can
be  separated  into  two categories:   "1)  changes   related  to  soil  pH/basic
cation  changes,  which include any direct losses of nutrients  or changes  in
processes  or  availability, and 2) changes  in  soil solution  and/or  leachate
chemistry  that might  affect aquatic  systems  or  be  toxic to plant roots,  for


                                     19

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which  the  primary concern  is  change in  (available)  aluminum concentration"
[CARP  E-2.3.5.2].  Within each of these sensitivity categories are distinctly
different soils.  Therefore, the term 'sensitivity1 must be used carefully to
express  sensitivity  of  the  soil  to  one or  the other of  the  above potential
effects.

A.2.1.6.1  WHAT ARE THE CHARACTERISTICS OF A SENSITIVE SOIL?  [CARP E-2.2.8,
           E-2.3.5]

Many soil classification schemes have been  proposed.   Each has serious weak-
nesses  when  studied  in  detail,  but all  generally  agree  when  viewed  on  a
national  scale.   Soils  in  which pH changes  and  basic  cation  losses  might
occur  have the following characteristics:   1)  the  cation status of the soils
is not renewed by flooding, 2) they  have no buffering carbonates to depths in
excess  of  1.0  m, 3)  they  have  a low cation exchange  capacity,  but present-
ly have  a  pH greater  than  5.5,  and 4)  they  have  a low  sulfate  adsorption
capacity.  There are  very few  soils  in  the  United  States with high enough pH
and low enough reserves of  exchangeable cations  that they could be acidified
by current amounts of acidic deposition.

The soils in which  acidic  deposition is  likely  to  increase aluminum in soil
solution are those that are already  extremely  acid, i.e.,  pH  less than 4.0.
The buffering capacity of these  soils  is  largely controlled by aluminum min-
eral chemistry.   Increased  acidic  inputs  can  therefore  increase  the  rate of
aluminum release and the aluminum concentration in soil solution or leachate.
This is most likely  to occur  in  coarse-textured  acid soils, again, where the
cation exchange  capacity is not  renewed by  flooding,  and the soils are free
of  buffering  carbonates  and  have  a  low  sulfate adsorption  capacity  (SAC).
Most soils of the eastern United  States meet all  but the low SAC criterion.

Knowledge of sulfate  adsorption  capacity  of soils  is  a  primary missing link
to understanding  the acidic deposition effects.   The  presence of  a mobile
anion  is necessary  for  leaching  of   cations  to occur.  The dominant anion in
atmospheric  deposition  is  sulfate.    Therefore,  the  reaction  of  sulfate,
especially its adsorption or  free  movement, is an  important  soil  character-
istic.   Soils  containing  large quantities  of  iron and aluminum  oxides have
the  capacity  to  adsorb  sulfate.    These  metals  are common  to  all  soils.
Aluminum  is  a  major  structural  component  of  soils, but  the oxide  form is
common  only  in  highly-weathered, acid soils,  similar to  the  soils  found in
the eastern  United States.   Sulfate-adsorbing  soils  are  believed  to  delay
cation leaching effects of  dilute sulfuric acid inputs until a point when the
adsorption capacity  is exceeded  down through the  soil  zones  of interest.
Thus,  SAC  determines the rate  or  time of  release  of  metals  to  soil  water
solution and/or aquatic systems  but  does  not  necessarily  prevent  it.  [CARP
E-2.2.8]

A.2.1.6.2  WHERE ARE THE SENSITIVE  SOILS  IN THE UNITED STATES?  [CARP
           E-2.3.3, E-2.3.5]

Figure  11.5  identifies regions  of  the United  States  where acidic deposition
is most  likely to  have an  impact on  soil  or surface waters.  It should first
be noted  that  it is  unlikely that  acidic  deposition will  adversely affect


                                     20

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      Spodosols

      Ultisols

      Inceptisols

      Select Alfisols

   A|  Mountainous Regions
Figure II.5   Regions  with  soils  potentially sensitive to mobilization
             of aluminum or with limited  hydrogen  ion neutralization
             capacity.  Areas  in black, Spodosols  are of immediate
             concern, due  to their  potential to contribute aluminum
             to the  soil water solution.   Ultisols and  Inceptisols
             may contribute aluminum  after saturation of sulfate
             adsorption capacity.   Mountain regions with thin soils
             and steep  slopes  have  limited hydrogen ion neutralizing
             capacity.
                                 21

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 cultivated  soils.   Not  only do  many management  practices  result  in  acid
 production  much greater than  that  expected from acidic deposition,  but  good
 agricultural  practice also  requires  controlling pH  within a  range  that  is
 favorable  for plant  growth.   Routine additions  of nitrogen  fertilizers may
 result in  the  release  of  between  one  and  two  orders  of  magnitude  more
 hydrogen  ion  than will  be derived  annually from acidic deposition.  Because
 impacts  of   these  additions  are  routinely  counteracted  by  liming,   soil
 scientists  have  little  interest  in the negative effects of  acidic  deposition
 on  cultivated  soils.    Deposition  of  sulfur   has  helped  alleviate  sulfur
 deficiencies  in  soils in the  Southeast and  Midwest.  As a  result, the concern
 over   acidic   deposition   effects  is  focused  primarily   on   uncultivated,
 unamended soils,  which are  found mostly  in forested and grassland  regions  of
 the United States.

 Those  regions of the United States dominated  by Ultisol,  Spodosol, and  some
 of  the  Inceptisol  soil  orders  are  predicted   to  be  sensitive  to  the
 mobilization  of  aluminum  by  acidic  deposition.    Because   Ultisols  and
 Inceptisols have a relatively high SAC,  soil  or surface waters  in  regions
 dominated by  these soil  orders  should respond  slowly  to  changes  in sulfate
 deposition.   Only  the Spodosol  regions of  the  United  States are expected  to
 respond  quickly to  changes  in  sulfate  deposition.   In  the  regions  of the
 United  States  that  have  very thin  soils,  in  particular,  steeply  sloped,
 mountainous regions, surface water quality will respond quickly  to  changes  in
 acid  deposition.  Rapid  transfer of  precipitation directly  to the  aquatic
 system in regions of very thin soils prevents neutralization.

 It  must be  recognized   that  mapping  efforts at  any  scale  above  the  most
 detailed (e.g.,  county soil  maps)  will by  necessity  include a wide range  of
 conditions within any map  unit.   For  this reason, the associated maps should
 be used with  some caution.

 A.2.1.7  COULD OBSERVED/POTENTIAL CHANGES IN SOILS AS A RESULT OF ACIDIC
         DEPOSITION BE REVERSED?  [CARP E-2.3.4]

The natural  trend in soil pH is toward greater acidity, independent of acidic
 deposition.    Decreasing the loading of acidic deposition will  not change  this
 process.   However, decreasing anion  deposition, sulfate  for  example,  would
 likely  decrease  the  rate at which  aluminum will  be  mobilized.  The   time
 required to observe a significant change  in aluminum mobility after decreased
sulfate deposition is unknown.

A.2.1.8  WHAT IS THE TIME FRAME IN WHICH  CHANGES IN SOIL CHEMISTRY  COULD
         RESULT FROM ACIDIC DEPOSITION? [CARP E-2.3]

Decades or centuries are the  most  likely  time frames in which soil chemistry
 changes could result from acidic deposition.  The time frame is  soil and  site
 dependent.
                                     22

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A.2.2   WHAT ARE THE EFFECTS OF ACIDIC DEPOSITION ON FORESTS AND CROPS?
        [CARP E-3]

A.2.2.1   WHAT CONSTITUENTS OF  PRECIPITATION AND THE ATMOSPHERE  INFLUENCE
          VEGETATION?

Plants  need  16 elements or essential  nutrients,  in appropriate proportions,
for optimal growth.  Precipitation supplies all of these nutrients in various
quantities and  for  some  elements,  provides enough to replenish that taken  up
by the plants during a growing season.

Plant  growth  can  be  limited  by  the scarcity  or absence  of any  element.
Nitrogen  is usually  the  limiting nutrient  in  unmanaged  systems.    Infre-
quently,  sulfur is  a  limiting  nutrient.   The  amount  of sulfur  or  nitrogen
needed  for  optimal   plant  growth  is  species  specific.  The  constituents  of
precipitation   that  are  most  important   in   determing  plant  response  are
nitrogen, sulfur, and  trace  metals.    Hydrogen  ion  is  also important because
it influences the availability of  nutrients to  the  plant.   Many of the trace
metals,  along  with  sulfur,  nitrogen, and hydrogen, are  essential elements.
The  concern,   however,  is  related   to  the  quantities  of  these  elements,
particularly quantities sufficiently  large to induce phytotoxicity.

A.2.2.2  WHAT ARE THE PRIMARY MECHANISMS BY WHICH VEGETATION MIGHT BE
         AFFECTED BY ACIDIC DEPOSITION?  [CARP E-3.2, E-3.4]

Both  indirect  and   direct  mechanisms  are known.    Changes   in   soils,  and
resultant  changes   in  productivity,   are   the  potential  indirect  effects  of
acidic  deposition  on vegetation.  The discussion  of  soils in  the  previous
section  recognized  that  soil  properties  play  a  major  role  in  controlling
plant growth.   Soil pH, £er_ se_,  is  unlikely  to change and  nutrient cycling
and biologically-mediated processes  in soils  should remain  relatively stable
at current regional  average amounts  of acidic deposition.   However,  nutrient
and aluminum concentrations can  change  in  soil  water  solution  and eventually
be taken up by  plants or delivered to ground or surface waters.

The following potential direct effects of acidic deposition on vegetation are
hypothesized:  leaching  of  nutrients   from  foliage;  increased  permeability  of
leaf  surfaces   to   toxic  substances,  water,  and  disease  agents;  altered
reproductive  processes;   altered  rhizosphere  relationships;   erosion    of
protective wax  surfaces;  chlorophyll  degradation; premature  senescence;  and
general  physiological  alterations.    Only  the first of these  direct effects
has been observed in the field under  ambient conditions [CARP  E-3.4.1.1],  but
all six have been reported in simulated rain experiments.

A.2.2.3.  WHY  ARE THE EFFECTS OF ACIDIC DEPOSITION ON VEGETATION DIFFICULT TO
          DISCERN?  [CARP E-3.3, E-3.4]

Because  plants  exist  in  a  complex,   often  stressful, environment  and  are
themselves complex  biological  systems,  acidic deposition is only  one  factor
that may  alter  their response.   Isolating acidic deposition's effects  from
the effects  due to  other stresses and natural  variability  is quite difficult.
As a  result,  our  knowledge  and  ability   to  discern  the  effects of  acidic


                                     23

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deposition on plants  is  developing  slowly.  The  effect  of acidic deposition
or any other anthropogenic stress, on  plants depends  upon the concentration,
duration,  frequency,  and  combination  of  stresses  to  which  the  plant  is
exposed.    In addition,  many  natural   environmental  factors  influence  plant
performance.   These  include  the availability  of  nutrients,  proper  soil  pH,
light, temperature, adequate water, etc.   Each  factor  alone,  if dramatically
altered,  can induce both positive and negative  plant effects.   Furthermore, a
plant must interact  with  its  biological  environment—disease,  competition
between  plants,   insects,  and  other   organism  interactions.    Finally,  the
genetic makeup,  species,  and/or life  stage of the  plant must be considered in
determining  the   effects  of  any given  pollutant on  plant  response.    The
response to  combined  effects  of these natural  factors and of anthropogenic
stresses determines how well  a plant  will fare  in  a particular environment.

A.2.2.4  WHAT DO  WE KNOW  ABOUT ACIDIC  DEPOSITION'S EFFECTS ON  CROPS?
         [CARP E-3.4.2]

A number of  studies  on  the effects of acidic  deposition  on  crops  have been
published  in the  last five years.  Limitations  in research designs, however,
restrict  the  usefulness  and   applicability  of  many  of  the  experimental
conclusions.   In  most of these studies, only large differences in crop yields
would  be  considered  statistically  significant.    Results  from  different
studies are often  inconsistent  and difficult to compare  because of  important
differences in methodologies.   Consequently,  we currently  know  little  about
the effects of acidic deposition on agricultural  crops.

A.2.2.4.1  WHAT TIME FRAME IS  IMPORTANT IN DEFINING EFFECTS OF ACIDIC
           DEPOSITION ON  CROPS?  [CARP E-3.4.2]

The time frame in  which  acidic  deposition might affect crops  depends  on  the
crop,  the  acidity of  deposition,  and the frequency  of  exposure.   A crop
likely responds  to every rain  event.   It is not known,  however,  whether a
single  rain   event is  sufficient  to  induce  a  beneficial  or  detrimental
longer-term response,  e.g., an increase or decrease in productivity.

At present, weighted mean  pH  during  the growing season has been  used  as  the
dose  parameter  for annual  crop effects.   Perennial  plants  are exposed  to
acidic deposition  for years;  longer-term average  and/or  total  exposures  are
considered most  important, primarily  because   no  data indicate  significant
effects resulting from a  few extreme  exposures.

Any conclusions drawn relative  to  the  important time  frame need  to be plant
family, if not species or cultivar,  specific.

A.2.2.4.2  IS THERE EVIDENCE TO SUGGEST THAT CROPS ARE RESPONDING TO ACIDIC
           DEPOSITION?  [CARP  E-3.4.2.2.1]

Yes.  Thirty-four  crop varieties  (28  species)  have been exposed to simulated
acidic precipitation  in  controlled-environment experiments.   Of  the  34,   six
exhibited  a  decreased yield,  eight exhibited  increased yield,  17  showed  no
effect,  and   three species  exhibited  both  increased and  decreased  yield
depending  on the  hydrogen  ion  concentration  or  conditions  of  exposure.


                                     24

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Fourteen crop cultivars (nine species) have been exposed in field studies and
only  one  (garden  beet)  showed consistently  decreased  yield at  all  experi-
mental acidity levels.  Three cultivars were negatively affected and six were
positively  affected by at  least one  of the  acidity levels.   Most  of the
cultivars  studied  in the field  and in controlled  environments exhibited no
effect  on  growth  or  yield as  a  result  of  exposure  to  simulated  acidic
precipitation.

A.2.2.4.3   IS THERE EVIDENCE TO SUGGEST THAT CROP PRODUCTIVITY  HAS BEEN
            SIGNIFICANTLY AFFECTED BY ACIDIC DEPOSITION?  [CARP  E-3.4.2.3]

There is very little evidence.   Field  studies are the most appropriate means
of  estimating effects  because  the  experimental  plants are grown under normal
environmental conditions, especially when  common agricultural  practices are
used.   Only  'Amsoy'  soybeans  have  consistently  shown  significant decreased
productivity as a result of exposure to simulated acidic precipitation at the
ambient loadings currently observed  in the eastern United States.  Studies on
this  soybean  variety  have  provided the  most convincing evidence  that  crop
productivity might be affected by acidic deposition.

"Available  experimental results  do not appear to indicate  that the negative
effects of  acidic  precipitation  outweigh  the  positive effects;  however,  many
crops and agricultural systems  have  not  been  properly or adequately studied"
[CARP E-3.5].  In comparison to  the  effects of  natural  stresses and of other
gaseous pollutants, the effects of acidic deposition on crop productivity are
likely to be small.

A.2.2.4.4   WHAT HAS PREVENTED A CLEAR QUANTIFICATION OF THE EFFECTS OF ACIDIC
            DEPOSITION ON CROPS?  [CARP E-3.4.2.3]

Crop  response to  acidic  deposition is  measured  by  changes   in  growth and
yield, or productivity.   This  productivity is influenced  by  all  other envi-
ronmental  conditions  and  cultivation practices, as  well  as by the positive
effects of  sulfur  and nitrogen  fertilization  and  the  negative  effects  of
acidity.    Quantifying  the net  effects   of  one  specific  factor,  acidic
deposition, is complicated by interactions among all of these variables.

A.2.2.5  WHAT DO WE KNOW ABOUT THE EFFECTS OF ACIDIC DEPOSITION ON FORESTS?
         [CARP E-3.4.1]

We  know that  within the  last 25 years changes  without obvious  natural cause
have  taken  place  in  the  growth  and  development  of  forests   especially  in
Europe but  also in  the eastern  United  States.   We also know that changes are
occurring  in the amounts and patterns  of  emissions  of atmospheric pollutants
and  the  exposure  of  forests  in  Europe  and  North  America  to  gaseous
pollutants, toxic metals,  and acidic deposition.   But we do not  know  if the
latter  have caused  the  former.    The  complex  chemical  nature  of  combined
pollutant  exposures,  the  fact  that both sulfur  and  nitrogen  are  essential
nutrients,  and the potential of  these  pollutants  to  have  both  direct effects
on  vegetation  and  indirect effects  (through soil-mediated  impacts)  makes
quantifying effects on forests  particularly challenging.
                                     25

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A.2.2.5.1  IS THERE EVIDENCE TO SUGGEST THAT ACIDIC DEPOSITION IS AFFECTING
           FOREST GROWTH?  [CARP E-3.4.1]

No.  Although we cannot yet conclude that acidic deposition has not, and will
not, affect forest growth, we  have  no  direct evidence that acidic deposition
per se  currently limits, or  has  limited forest growth  in North  America  or
Europe.   Experimental  data  from  irrigation  studies  on  seedlings  and  young
trees have not shown cause for  immediate alarm  but  the data  are  difficult to
interpret because of treatment artifacts.  Growth response of forest trees is
influenced by many variables such as genetic diversity,  competition, climate,
and site  factors.   The  additional  possible effects  of  acidic  deposition,
gaseous pollutants,  trace metals,  and  interactions between  pollutants,  and
the lack  of appropriate  control  sites  for  comparison  increase  further  the
problem of detecting responses to any one factor.

"Although the  task  of  assessing potential  impacts  on forest productivity is
assuredly difficult,  the potential  economic and ecological  consequences  of
even  subtle  changes in  forest growth  over  large  regions dictates  that  it
should be attempted" [CARP E-3.4.1.7].   Such studies are  now under way.

A.2.2.5.2  IS THERE EVIDENCE TO SUGGEST THAT A REGIONAL  DECLINE OF FORESTS
           IS OCCURRING IN EUROPE OR NORTH AMERICA? [CARP E-3.4.1]

Yes.  Researchers in Europe, particularly in the Federal  Republic of Germany,
have  observed  obvious,  but unexplained, large-scale  regional  changes in  the
growth and  behavior  of forests containing  Norway  spruce, silver  fir,  Scots
pine,  European beech and certain other broad-leaved and  needle-bearing trees.
These  decline  phenomena,  called   'Waldsterben',   include  three  types   of
symptoms:   growth   decreases,  abnormal  growth,  and water  stress  (Schutt  and
Cowling  1984).    In the  United  States,  a  decline  of  red  spruce at  high
elevations  has  been observed  in  New  York  and  New  Hampshire  and  has  been
quantitatively documented in the Green Mountains of Vermont,  where widespread
mortality was preceded by decreased annual  growth.   Between 1965 and 1979, an
overall reduction of approximately  50  percent in tree basal  area  and forest
density was observed  in  the Green Mountains  (Johnson and  Siccama  1984).   In
the pine  barrens  of New  Jersey,  pitch,  short-leaf, and loblolly  pines  have
shown decreased diameter growth.

A.2.2.5.3  WHAT HYPOTHESES HAVE BEEN PROPOSED TO EXPLAIN  RECENT REGIONAL
           FOREST DECLINES?

Forest  scientists  have proposed  several major  hypotheses to explain  one or
more parts of  the declines.   These hypotheses have been  developed largely to
explain  observations  in  the  German   forest environment.    Since  decline
symptoms  in high-elevation spruce  forests  in the United  States resemble  some
of  those  found  in  Germany,  the hypotheses deserve  careful  consideration.   A
report  expected from  a   joint U.S.-German  scientific  group  that carefully
studied German  and  U.S.  forests  in the  spring and summer  of  1984 should
provide  an  excellent  comparison  of  symptoms  and  evaluation  of  damage
hypotheses.  One classification of  'Waldsterben1 hypotheses has recently been
provided by Schutt and Cowling  (1984):
                                     26

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0   Acidification - aluminum toxicity: changes in soil from acidic deposition

0   Gaseous pollutants causing direct damage to foliage

0   Magnesium-deficiency  resulting  from leaching  by materials  deposited on
    foliage

    Excess-nutrient,  especially  excess  nitrogen,   introduced   to  the   soil
    system

0   Air transport of growth-altering organic substances which may be taken up
    either through foliage or from the soil.


These  five  hypotheses include  both  direct effects  on foliage  and indirect
effects, changes in soil which  cause  changes  in  growth.   All  of these assume
further that the trees are subject to

0   General stress  resulting  from climatic effects  (fluctuation in rainfall
    and  temperature),  and  exposure  to  biotic  pathogens  such as  viruses,
    fungi, or insects.

A.2.3  WHAT ARE THE EFFECTS ON AQUATIC CHEMISTRY?  [E-4]

A.2.3.1  WHY ARE SURFACE AND GROUNDWATERS AN IMPORTANT CONSIDERATION IN
         STUDIES OF ACIDIC DEPOSITION EFFECTS?

Surface waters  are important for human  consumption,  recreation and wildlife
habitat.   The  most  critical  use of  surface  and groundwater  is as drinking
water.   Detrimental  effects  on  drinking  water  would have  serious ramifi-
cations.  To date, there is no  evidence  to warrant concern about groundwaters
in  the  United States  being  affected  by acidic deposition.  However, evidence
that  some  surface waters  are  being acidified,  for  whatever  reason,   has
resulted in  focusing  research  on these  systems.   The  discussions  to follow
are limited to  changes in lakes and streams.

A.2.3.2  WHAT SURFACE WATER CHEMICAL CHARACTERISTICS MAY  BE INFLUENCED BY
         ACIDIC DEPOSITION?  [CARP E-4.2, E-4.3]

The status  of   hydrogen ions,  sulfate,  base cations,  aluminum,  nitrate,  and
organic carbon  in aquatic systems may be affected by acidic deposition.   Acid
neutralizing  capacity  (ANC),   ion  exchange  and  mineral weathering  rates,
nutrient  and  organic  carbon  availability  in   the  watershed  may  all  be
affected, depending on  the  aquatic  system  and the amount and composition of
deposition it receives.

A.2.3.3  WHAT ATMOSPHERIC CHEMICAL INPUTS INFLUENCE CHEMICAL CHARACTERISTICS
         OF AQUATIC SYSTEMS?  [CARP E-4.3.1]

Hydrogen ion additions  in  aquatic systems  were originally  the  focus of  most
concern  because  increases  in  hydrogen ion,  by  definition,  increases  the
acidity of  surface waters.   Whether  the hydrogen enters  the  aquatic system


                                     27

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from the  terrestrial  system  or directly from acidic  deposition  makes little
difference.

More recently, however,  other  deposition  components have been recognized  as
being  important.   In  addition to H+  deposition,  atmospheric deposition  of
sulfur   ($042-   and   S02),    nitrogen   (as  N03~   and   NH4+),   and   base
cations  (e.g.,  Ca2+  and  Mg2+)   can  greatly influence  the  chemistry  of  a
system.   Phosphorus and  organic  carbon deposition  may  also be  important  in
some systems.

Significant  internal   production   of   ANC  may  occur  due  to  chemical  and
biological  transformations  of  NOs"  in  either  the  terrestrial  or  aquatic
system.    Under  conditions  where nitrate  is  not rapidly  immobilized  and
metabolized, it may serve as a mobile  anion,  carrying base  or  acidic cations
from the  terrestrial  to  the aquatic  system.   Nitrate mobility  is  not  often
observed, however,  except during  periods of rapid  snowmelt.

Because  sulfur  is  especially  problematic,  its  influence will  be  treated
separately in the following Sections  A.2.3.4 and A.2.3.5.

Deposited  ammonium   ion,  NH4+   is   consumed  chemically   or  biologically,
often  resulting  in decreased  ANC.   NH4+  deposition  can  be  a major source
of  the  net acid  input to some systems, e.g., about 25  percent at Harp  Lake,
Ontario.   Clearly,  the impacts of  acidic  deposition  cannot be  assessed  based
upon free acid (H+) measurements  alone.  [CARP E-4.3.1.1]

Deposition  of base   cations  must  also  be  considered  in  calculating  ion
exchange,  mineral  weathering,  acid  neutralization  and   net  loss  of  base
cations  from  the system.   The  total  deposition of  other  cations  and anions,
particularly  nitrogen and phosphorus,  may  contribute  substantially to  the
available  nutrients of inland  freshwaters.   These atmospheric inputs may  be
an  important nutrient source for  aquatic  organisms  in nutrient-poor  systems,
e.g., watersheds with  granitic substrates and a large water  surface area  to
drainage area ratio.   Gaseous  exchange of nitrogenous compounds  may  also  be
an  important influence on lake  chemistry but it is poorly  understood.

Precipitation inputs of phosphorus and nitrogen  may account for about half of
the  concentration  of  these elements   in  oligotrophic  lakes.   This  input
becomes  much  less  influencial  on  the  total  budget of  an  aquatic  ecosystem
when runoff  from  land-use activities  increases (i.e.,  agriculture,  urbani-
zation).   Systems dominated by terrestrial  inputs  of  phosphorus  and nitrogen
are  usually much  more  biologically  productive,   if  not  eutrophic.    [CARP
E-4.3.1.5.1]

Precipitation inputs  of  organic  carbon  may be ecologically  significant  for
some aquatic ecosytems, particularly oligotrophic lakes, based on preliminary
data.   Mean concentrations  in precipitation averaged about  6 mg C  £~1  and
accounted  for  28  percent  of  the  total  organic  carbon  inputs  for  a  small
oligotrophic  lake  in  New  Hampshire.    Data  are not sufficient,  however,  to
extrapolate  the   importance  of   atmospheric  inputs  of  organic  carbon  to
oligotrophic lakes  in general.   [CARP  E-4.3.1.5.1]
                                     28

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A.2.3.4.  WHY IS DEPOSITION OF SULFUR COMPOUNDS PARTICULARLY IMPORTANT
          TO AQUATIC CHEMISTRY?  [CARP E-4.3.1.5]

Increased sulfate deposition has a great effect on aquatic chemistry because,
on an equivalent basis, the sulfate increase in the waterbody must be matched
by  an increase  in  cations,  either  protolytic  (proton-donating,  e.g.,  H+,
Ain+)  or  non-protolytic  (e.g.,  Ca2+, Mg2+, etc.).   Increasing the former
will  result  in  loss  of  alkalinity   (acidification)   of  the  waterbody.
Increasing the  latter  will  result in basic cation  loss  from the terrestrial
system but no alkalinity loss in the aquatic system.  The proportion of these
two processes in  any  given watershed is a  matter  of research investigation.

Sulfate may  be  stored  in  watersheds through  sulfate adsorption, a  process
that  may also  generate ANC if  the  sulfate is reduced  or if strong  acid is
neutralized.  Sulfate reduction in lakes will  also generate ANC although only
the net production of  ANC,  i.e.,  net reduction of nitrate  or  sulfate,  on an
annual  basis is  important.    In  some  systems,  sulfate  moves through  the
watershed as a conservative substance.

Sulfur dioxide and sulfate, whether  wet or dry deposited, move along similar
pathways through  terrestrial  and  aquatic systems;  therefore, sulfur's effect
on aquatic systems does not  depend  on  the chemical  form  or physical  form of
deposition.

A.2.3.5.  HOW DEPENDENT ON ATMOSPHERIC DEPOSITION ARE SURFACE WATER SULFATE
          VALUES?  [CARP E-4.3.1.5.2]

As with soils,  rock weathering  and atmospheric deposition provide  sulfur for
surface waters.   In  the absence of  reactive  sulfur  sources in  bedrock or in
decaying  organic  matter,  atmospheric  deposition  is the  primary  source  of
sulfur.  This is  especially  true  in  areas that receive acidic deposition but
do not have significant sources of reactive sulfur in the watershed.  In such
areas sulfate still  can become the dominant  anion  in low alkalinity waters.

Plotting the  mean and  range  of excess  sulfate  (above that  supplied  by sea
salt  cycling) export from  watersheds across northern North America on a line
that  transects  the region  of  large  atmospheric deposition of sulfate (Figure
II.6)  shows  a  positive  relationship between  excess sulfate  deposition and
sulfate  in the  runoff,  although sulfate  export  exceeds deposition  in the
areas of highest  deposition.   The  wet deposition of  excess  sulfate  is shown
at  each  location,  with estimated  total  sulfate  deposition  shown  at  four
locations.   Dry deposition of  sulfate  and S02 maY  account for most  of the
greater  sulfate  export  from  watersheds compared  to wet-deposited  sulfate
entering watersheds.   In areas still  accumulating sulfate in watersheds, this
positive relationship will  not hold.

The  influence  of atmospheric  sulfate  deposition  on surface  water  sulfate
values  is  also  suggested  by  statistically  significant  correlation  between
sulfate  concentrations  in  surface  waters   and   sulfate  concentrations  in
precipitation, over a wide range of  concentrations,  as  illustrated in Figure
11.7.    Southeastern   Canada   and  the   northeastern  United  States  receive
precipitation with  high  concentrations  of   sulfate  and  also   have  surface


                                     29

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                                  C71 C\J
                                  CM CVJ
                                                      LABRADOR
                                                       ISLAND OF
                                                       NEWFOUNDLAND
HALIFAX


NEW BRUNSWICK


LAFLAMME


MAURICIE


ADIRONDACK


N. OF OTTAWA


ALGONQUIN


HAL IBURTON


SUDBURY


ALGOMA


THUNDER BAY


QUETICO


ELA
                                 _
                                   ui baui
Figure II.6  Mean  and  range of basin specific yield of excess  sulfate
             ( —®—)  compared with atmospheric excess sulfate  deposition
             (--•—)  in precipitation  for  1980 (Thompson and Button
             1981,  1982) and the range of  estimated wet deposition for
             1977-80  from the CANSAP precipitation network  (Barrie and
             Sirois 1982).   Also shown are the ranges of wet plus  dry
             deposition of sulfate (—) calculated from the 1980
             measurements of SOX in the  atmosphere at 4 Canadian Acid
             Precipitation Network Stations  (Barrie 1982).  Adapted
             from  U.S./Canada (1983).
                                    30

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       180
       160
       140
       120
   ~  100
   a-
   OJ
        80
        60
        40
        20
                             E. Ontario
                     Connecticut
                       Adirondack*
                        Maine
            K  Florida  •
                                  Laurentian
                                        Mts.
                          Nova Scotia
                                     New Hampshire
-  W. Ontario
.Labrador /•  Kereke
             • Newfoundland
          Quebec

          > Rocky Mts.

          Labrador
                    20
                 40
60
80
100
                      2-
                   SO/"  PRECIPITATION (yeq
             Y « 1.92X -f  14.08     R  «  0.86    P <_ 0.001

Figure  II.7  Mean concentration of  864*  (excess SO^-, over and above
            that supplied  by sea salt cycling) for 15 lake groups in
            North America  and mean 864* in wet deposition at nearby
            deposition monitoring  stations.  Adapted from Wright (1983)

                                   31

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waters with high  concentrations  of  sulfate.   Similarly,  the  areas receiving
precipitation with  low  concentrations  of sulfate  are also  those with  low
concentrations  of  sulfate  in  surface  waters   (northern  Quebec,  Labrador,
Colorado,  Rocky  Mountains).   Based on  this data,  background  sulfate  con-
centrations are  estimated  at  20 to  40 fxeq  j?~l for  North  American  lakes.
Sulfate  concentration  values  of  100  to  160  pieq (-1  in  lakes  in areas  of
eastern North America  receiving  acidic  deposition  suggest that anthropogenic
atmospheric  deposition  accounts  for  80  to   120  neq  f"1  (average  100
f*eq  j?~l)  of   sulfate.     A   relatively  large  region  of   eastern   North
America is included in this  estimate,  with some  areas relatively distant from
sulfate sources.

"Those waterbodies  in  areas  closer  to  sulfur  emission  sources will  have
larger increases in sulfate  concentrations.   For example,  lakes near Sudbury,
Ontario   have  ~ 400  ^eq   ^ "1  of   sulfate  from  atmospheric  deposition
while lakes east of the Rhine-Ruhr industrial region of Germany can have   >
1000 Meq j0-l of sulfate from atmospheric deposition" [CARP E-4.3.1.5.2].

A.2.3.6  WHAT FACTORS "CONTROL" SURFACE WATER CHEMISTRY?  [CARP E-4.3.2]

Each  of  the  following  may  play  a  significant  role  in  contributing   to  or
neutralizing  system acidity:     vegetative  canopy,   soils,  bedrock,  hydrol-
ogy/residence time  of  water,  presence  of and processes in  wetlands,  and the
surface  water system  itself.    Each  of  several  components   of  aquatic  or
terrestrial systems may assimilate some or all acidic deposition falling on a
watershed, depending on the site.  On a regional basis, some variables play a
much more  important role than others.   The components are  linked; atmospheric
deposition  may  affect  one component  directly  with  effects  subsequently
propagated to others.   The  pathways  of  water, and  its chemical constituents,
through  the  system  from  first  interception  of  precipitation  to  final
appearance in surface waters determine which components are  affected.

Water that flows through the vegetative canopy contains higher concentrations
of most elements  than  incident  precipitation  because the  chemical content of
precipitation  changes   as   it  washes  off  deposited  particles  and  collects
leachates  from  the  vegetation.    Particle  washdown  by  precipitation  is
independent of  any ability of  the  canopy  to assimilate deposited  chemical
constituents.     However,  cation  leaching  from  the  canopy  may  represent
significant acid-neutralizing  capacity.   The relative  importance  of  each of
these processes is not well  understood.  [CARP E-4.3.2.1]

Soils  assimilate   acidic   deposition   through   mineral   weathering,   cation
exchange,  sulfate  adsorption,   and  biologic  processes.    Generally,  soils
containing carbonate materials  can assimilate acidic deposition to an  almost
unlimited  extent  because  of abundant exchangeable  bases.    Soils that do not
contain  carbonate  materials   can  also  assimilate  some  acidic  deposition
because  of cation  exchange reactions  and mineral  weathering.   Assimilation
ability  is affected  by  the  soil   chemical  properties  (especially  cation
exchange  capacity,  base  saturation,  and  sulfate  adsorption  capacity),  the
permeability at each layer, the  surface area of the  soil particles,  and the
depth of soil in the watershed.  [CARP E-4.3.2.2]
                                      32

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 The effective surface area of massive, impermeable silicate bedrock for chem-
 ical reaction is minimal.   Bedrock  that is jointed  or  fractured has greater
 surface area for reaction, but  complete  assimilation will  occur only at con-
 siderable depth, potentially  affecting  the groundwater  chemistry  but having
 little effect on stream and lake chemistry.  Silicate bedrock having a porous
 nature, e.g., weakly cemented sandstone and  bedrock of  carbonate minerals,
 will have the greatest capacity  for surface reactions.  [CARP E-4.3.2.3]

 Hydrology, specifically flow  paths  through terrestrial  systems  and residence
 times of water in lakes, can determine the  extent of reactions between strong
 acid components  of  deposition and  each  component the water  contacts.   Soil
 physical  properties  are  major determinants of  water flow  and  thus, of  the
 interaction  of  soil with  acid  rain.   Water  running rapidly  through  soils
 (e.g.,   on  steep slopes with  low  porosities  or well-drained,  highly  porous
 soils)   may   have  little  opportunity  to  interact and  may  be  changed  only
 slightly in  composition [CARP E-2.1.3.1].  A generalized view of  the  flow  of
 water through a  terrestrial  ecosystem  is  shown  in Figure 11.8.  Water striking
 the surface  may  infiltrate  the  soil  or  flow  across  the  surface.   About  75
 percent of precipitation enters  the soil in temperate climates.   The type  of
 forest   floor  or its  disturbance,  the  presence of  large   channels  ('macro-
 pores')  from  burrowing animals  and  decomposition   of  tree  roots,  and  the
 degree   of water saturation  of  the  soil  at  the time  of   precipitation are
 examples of the many  factors  affecting  soil  hydrology.   Considerable  scien-
 tific debate has arisen from  the  suggestion that during heavy rain or  rapid
 snowmelt a greater  proportion of  flowing  water will contact the  most  acid,
 humus-rich soils high  up  in  the  watershed, resulting in  'natural1  acidifi-
 cation.   [CARP E-2.1.4,  E-4.3.2.4]

 The role of  wetlands in assimilating acidic deposition  is  generally  unknown.
 Alkalinity present  in  the aqueous  component  of  the wetland can  neutralize
 acidity, and other  processes  including  reduction and ion exchange  reactions
 may contribute to assimilation.   The biogeochemistry of wetlands is poorly
 understood;  these  systems  are considered by some to  be extremely  vulnerable
 to  acid  deposition  while  others  view   them  as  potential  contributors   to
 acidity  and  acidification  because  of their  often   natural  low pH.    [CARP
 E-4.3.2.5]

 Whatever the  factors  controlling  surface  water  chemistry,   it  appears  that
 alkalinity  in the surface water  is  the best  single  measure of the combined
 acid-neutralizing processes in the watershed.  Although alkalinity  represents
 only  one  component,  it is  a  result  of  continuing   terrestrial  and aquatic
 interactions.  Undisturbed surface waters of high alkalinity are likely to be
 contained  within   watersheds   with   high   neutralizing   capacity.    [CARP
 t.~^r»O« ^ • D • ij

 "Alkalinity or acid-neutralizing capacity (ANC)  determines  a  lake's instan-
 taneous  ability  to  assimilate acidic  deposition, but the  ANC  renewal  rate
depends  upon  the ANC  supply  rate  from the watershed.   In addition, internal
production of alkalinity is important, especially  in  lakes  with  low alkalin-
 ity.  Because biological processes can alter the relative amounts of acidity
and  alkalinity  within  a  body of water,  nutrient   status  is  important  in
determining the sensitivity of a  lake to  acidification" [CARP E-4.8],


                                      33

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                                 ACIDIC DEPOSITION
                                                       I
                                               DIRECT DEPOSITION
                                                       I
INTERCEPTION
                     THROUGHFALL
                                                SURFACE FLOJj
                                                 Minimum to
                                                 Moderate  soil
                                                 interaction
                                          CHANNELIZED  FLOW
                                      Minimum soil  interaction
                      GROUNDWATER FLOW
                  DIFFUSION  FLOW
              Maximum soil interaction
                                                     ofrV^
Figure  II.8  Flow paths of precipitation through a terrestrial  system.
                                 34

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A.2.3.7   IS THERE  EVIDENCE TO  SUGGEST THAT  ACIDIC  DEPOSITION  HAS  ALTERED
         SURFACE WATER CHEMISTRY?  [CARP E-4.4.3]

Yes.  Many studies have been conducted  to  examine  temporal  changes in stream
and lake chemistry in relation to  the  chemical  composition  of precipitation.
A consistent drawback of studies of  trends is a lack of  clear documentation
of  the  "historic" data  used.   Often these  crucial  data  have  not been proven
unbiased, either in  the sampling  or analytical procedures used.

In each case listed below,  viewed  by  the  authors of the  CARP  as being 'most
reliable1, the scientists who performed the  studies concluded that pH and/or
alkalinity decreased in at  least some  of  the surface waters  studied.   [CARP
E-4.4.3.1.3]

     0   La Cloche Mt. region,  Ontario
         Halifax, Nova Scotia
     0   New England;  Maine, New  Hampshire, Vermont
     °   Adirondack  region,  New York
     0   New Jersey, Pine Barrens
         Sierra Nevada Mts., California
         USGS Hydro!ogic Bench-Mark Stations

"In every case reviewed the  scientists  who performed  these  studies concluded
that  changes  in  surface water chemistry  reflected, at  least partly,  either
(1)  trends  in  regional   emissions   of  S0£,   or   (2)  changes   in  chemical
composition  of  incident  precipitation.   This  reviewer finds  the  body  of
evidence  presented  ...  convincing. Particularly noteworthy  by its absence is
any  body of  data  indicating  consistent  decreases  in  alkalinity  or  pH  of
surface  waters  at  otherwise  undisturbed  sites not  receiving  acidic depo-
sition.   Furthermore,  this  reviewer  is unaware of any  natural  process that
would cause decreases in pH and/or alkalinity at the rates indicated by these
studies.   Until  appropriate evidence  is  presented in  support  of  some such
natural process or until some better  explanation of the data presented ... is
put forth, the only logical  conclusion is that  acidic  deposition (of either
remot or  local  origin)  at  these  sites  has  caused,  or  is now  causing,  acidi-
fication of some surface waters.    It is only  reasonable  to  assume that other
surface waters of similar  sensitivity  that receive similar levels  of acidic
deposition have become, or  are  now being,  acidified."   [CARP E-4.4.3.1.3]

The concentration of  sulfate in clearwater  lakes  and  streams  has increased
due to  atmospheric  deposition  in some systems  and  may  be decreasing  in some
regions with decreasing acidic  components  in deposition [CARP  E-4.4.3.1.2.3].
Measures  of  sulfate,   bicarbonate, hydrogen  ion,   and  base  cations   are  an
appropriate means to evaluate site-by-site changes.

In addition to examining available historical  records,  scientists have begun
to  analyze  the  record contained in  lake  sediments.    Paleolimnological
techniques, including the dating of sediments,  have been  used to reconstruct
chronological   sequences of pollution  inputs   to  lakes   (e.g.,  lead)  and
responses of  the lake  biota  (e.g.,  plankton).    Knowledge  of  pH  relation-
ships  between  water  and  diatoms  for  present-day   diatom assemblages  allows
researchers to calibrate the sedimentary  diatom record and estimate  past pH


                                      35

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of  lake waters  to  produce  a  dated  record  of  lake  acidification.   [CARP
E-4.4.3.2]

Detailed  paleolimnological  analyses  for  pH change  over  the past  300 years
have  been  completed for  15  acidic  lakes in  the  northeastern  United States.
Analysis of  diatoms in the sediment  cores  indicated that nine  of  the lakes
have experienced pH decreases in recent years (9 to 80 years);  at least three
of these declines  may  represent,  in part,  recovery from earlier pH increases
caused  by disturbances such as lumbering in the watershed.  [CARP E-4.4.3.2]

A.2.3.8  WHAT TIME FRAME  IS IMPORTANT IN ACIDIC DEPOSITION'S EFFECTS ON
         AQUATIC CHEMISTRY?  [CARP E-4.4.2,  E-4.4.3]

Both long-term acidification  (over years  or decades) and  short-term (days or
weeks;  episodic) acidification  from release of  accumulated   chemicals during
snowmelt or  deposition from heavy rains  are of  concern  to the  extent that
biota,  particularly   fish,   may  be  affected.     Evidence   for  long-term
acidification has been cited above,  but little evidence of the  time course of
acidification is available.   The sediment  records  cited  above  indicate that
there  may  be  a  lag  of  decades between   the  first  evidence   of  increased
deposition and  significant  increase  in acidity  of  the  lake.    The  acidity
change of the lake may then occur over a period of a few years.   (Davis et al.
1983).    Section  B of   this  chapter  further  discusses  rates of  aquatic
response.

To predict the importance of  episodic events to aquatic ecosystems, one must
be able to  evaluate the  probability  of chemical  (pH,  alkalinity,  aluminum,
etc.)  change of  specific magnitude  in a  lake  or  stream  for  a  specified
duration.

Episodic events have resulted in  pH decreases  of  more than  one pH unit.  Not
all  aquatic   systems  within  areas   receiving  acidic  deposition  experience
significant  pH decreases, however,  and even  simple  dilution  by 'non-acidic1
rainwater  can result  in  decreased  pH  and  alkalinity.    Studies  to  date
indicate that pH during  spring  snowmelt or  heavy  rain  may  reach 4.5 to 5.0,
the same range observed in long-term acidification (see Table II.1.).  During
episodic acidification, aquatic  systems with pH's as high as 7.0 can reach pH
£  5.0;  long-term  acidification  to pH  < 5.0 is  generally  not  observed  in
aquatic systems with apparent initial  pH^s > 6.5.   Much of the  water reaching
a stream during a storm event may pass through the upper layers  of soils that
are often dominated  by organic  acidity.  Nitrification  in  the  soils,  during
drought periods in  the summer or under  the snowpack, may  generate acidity,
resulting  in  pH depression and increased nitrate flux during episodes.  [CARP
E-4.4.2]

A.2.3.9  WHAT OPTIONS ARE AVAILABLE  TO COUNTERACT SURFACE WATER
         ACIDIFICATION?   [CARP E-4.7]

Although the  most effective control  of acidification is to control the acidic
and acidifying  inputs, another   option  involves  treating  acidic waters with
acid-neutralizing  chemicals.    Lime   [CaO,  Ca(OH)2]  and  limestone   (CaOs)
are   two   such   neutralizing  agents   that  have   been  added  to  aquatic


                                     36

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   TABLE  II.1  MAGNITUDE OF  pH AND  ALKALINITY  (yeq  JT1) DECREASES IN  LAKES
AND  STREAMS DURING SPRING SNOWMELT OR HEAVY RAINFALL.  SURFACE  ALKALINITIES
                   IN THESE AREAS ARE  GENERALLY  < 200  yeq i
                                                                  -1
Location
Adirondack!, NY
Panther Lake, 1979a
Sagamore Lake, 1979a
Woods Lake, 1979a
Little Moose Lake, outlet.
Mew Hampshire
The Bowl -upstream, 1973C
The Bowl -down stream, 1973°
South-Central Ontario''
Harp Lake fl, 19/b
Paint Lake 11, 1978
Dickie Lake #10, 1978
Southern Blue Ridge Province
White Oak Run, VA, I9606
Raven Fork, NC, 198U
EMoe Creek, MC, 1981 f
West Prong of the Little
Pigeon River, 19789
Southwestern Ontario'1
Speckled Trout Creek, 1981
Barrett River, 1981
Quebec^
Ste. -Marguerite River, 1981
Minnesota-!
TTTsoiTcTeek, 1977
Washington
Ben Canyon Creek k
Idaho
ITTver Creek*
Approximate annual
sulfate loading
(kg ha"1 yr'1)
38


1977b
38


30



27





25


22

17

<20

£20


prior to episode
pH Alkalinity

6.6 162
6.1 29
4.8 -39
7.0

5.6
6.2

6.6 108
5.5 61
4.8 -16

6.0
5.7 20
5.9 60
6.3 40


6.7
6.6

6.7 76

6.6

7.0

6.1

Hater Chemistry

During episode
PH

4.8
4.9
4.5
4.9

5.0
5.8

5.4
5.0
4.5

5.7
4.4
5.5
5.8


5.1
5.0

5.9

5.5

5.8

5.7
Alkalinity

-18
-17
-42





8
8
-32


<20
<20
10





70






A PH

1.8
1.2
0.3
2.1

0.6
0.4

1.2
0.5
0.3

0.3
1.3
0.4
0.5


U6
1.6

0.7

1.1

1.2

0.4

Change
A Alkalinity

180
46
4





100
53
16




30





6






 «Galloway et al.  1980
 bSchofield 1977
 CMartin 1979
 ^Jeffries et al.  1979
 ^Shaffer and  Galloway  1982
 'Jones et al. 1983
 9Silsbee and Larson 1982
 hKeller 1983
 ifirouard et al. 1982
 iSiegel 1981
 *Lefohn and Klock 1983
                                          37

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systems.   Base  additions  have  been  made  both  directly  to  lakes  and  to
watersheds or streams,  but  the  relative  effectiveness  and total  consequences
of each of these approaches have not been fully evaluated.

Base additions  are intended  to change  the  chemistry  (e.g.,  pH  and calcium
levels, trace metal concentrations)  of the aquatic system  to  produce a more
hospitable environment  for  certain aquatic biota, particularly  fish.  Nega-
tive consequences may include pH  shock associated with dramatic  increases in
pH,  the  problems  associated with aluminum  hydrolysis  at the stream-neutral-
ized lake interface, and the potential  for lake reacidification.

An alternative option is fertilization of surface waters by adding phosphorus
to  increase   algal  productivity  and  generate  acid   neutralizing  capacity.
While the chemical  costs associated  with phosphorus  addition are low, appli-
cations may not be  efficient, particularly  in  view of  potential  interactions
with aluminum.  In  the  few  studies conducted,  the benefits  to the ecosystem
have not been evaluated.  [CARP  E-4.7.2]

Mitigative options as described  above have several major drawbacks:
     0  The costs are generally unknown.
     0  The applications must continue over time.
     0  The  area  of  coverage  would  need  to  be  large   to  be  regionally
        effective.
     0  Effects of mitigative options are not fully understood.

A.2.4  WHAT ARE THE EFFECTS ON AQUATIC BIOTA? [CARP E-5]

A.2.4.1  WHAT POTENTIAL EFFECTS OF ACIDIC DEPOSITION ARE OF CONCERN?
         [CARP E-5.5, E-5.6]

The  potential  effects  of acidic deposition on  aquatic  biota  include changes
in  plankton  and  algal  populations  and productivity,  and  losses  of  fish
populations.  Acidification  results  in  a shift in the structure and  function
of  the  plankton  community.   However,  the many  chemical,  biological,  and
physical interactions involved make  it difficult to predict potential changes
in  phytoplankton  and  zooplankton  communities  and the  subsequent  impact on
higher tropic levels.   Acidification eliminates sensitive  algal  species, may
decrease  phosphorus and  inorganic  carbon  concentrations,  and  may depress
nutrient cycling rates.  These changes tend to  decrease  phytoplankton biomass
and  productivity.   Acidification  also  may increase  water clarity,  allowing
light  to penetrate into  deeper  waters,  where  nutrient  levels are  generally
higher.    This would   tend  to  increase productivity.    Evidence  for  both
productivity changes has  been gained in  field  studies.   Productivity may not
be affected despite changes  in the community structure:  plankton biomass was
unaffected  in one field experiment.  [CARP E-5.5]

Aluminum and  hydrogen   ions  interact to cause  fish  mortality.   This inter-
action  may  be  most  important  during  short  time   periods   (e.g.,  spring
snowmelt).  Results  of  laboratory  experiments  suggest that fish growth rates
decrease  in acidified   waters  [CARP  E-5.6.4.1.3], yet  increased fish growth
has  often  been  observed  in  the  field.   The  reason  for  this   apparent


                                     38

-------
inconsistency may be that the greater abundance of forage organisms available
to a  dwindling  fish population outweighs increased metabolic  demands  at low
pH.

Recruitment  failure  evident  by   reduced  or  missing  age  groups  in  fish
populations—not  decreased  growth, loss of  food  items,  or adult mortality
—appears  responsible  for many fish  extinctions.  This failure  may  result
either  from  acid-induced mortality  of  fish  eggs and/or  larvae  or  from
reductions in the number of eggs spawned.  [CARP E-5.6.2.2]

A.2.4.2  HOW MAY CHANGING WATER CHEMISTRY INFLUENCE THE FISH POPULATIONS
         OF SURFACE WATERS? [CARP  E-5.6]

Fish  reproduction and  survival  depends on water  chemistry,  both  pH and the
concentrations of metal  ions.   Increases in  certain metal  concentrations are
associated  with  decreasing  pH  levels  in   acidified  surface  waters  [CARP
E-4.6],    Declines  in  fish  populations as  a  result  of acidification  may,
therefore, be a function of both low pH levels and elevated concentrations of
some metals, especially aluminum.

Critical  values  for fish survival,  if  developed only on the  basis  of water
quality  in laboratory  experiments or  at  one  location  in  a  lake,   may  be
misleading.   Water  quality may vary  substantially in different  areas of an
aquatic system.   Behavioral  responses,  such  as avoidance of low  pH  regions,
may offset, in part, the  effects of  acidification.  The  presence of  'refuge1
areas and  behavioral adaptations must be considered in our  assessment  of the
impacts of acidification.

Physiological   toxicity  of  low pH  waters  is  generally  believed  to   be  the
result of  impaired body salt regulation, due  to  the interference  of  elevated
hydrogen  ion   levels   with   osmoregulatory  mechanisms.    Through  the  gill
epithelium, freshwater fish normally actively exchange sodium  from the water
for  hydrogen  or  aluminum  ions, and  chloride for  bicarbonate,  in order  to
maintain  higher salt  concentrations  in  their tissues  than  is in  the  water.
This  active  uptake  of  sodium  may be  disrupted  by  increased hydrogen  ion
concentrations in  the  water.   "Brown  trout surviving  in  the Tovdal  River,
Norway,  collected immediately  following a   fish  kill  (apparently  resulting
from an acid  episode),  had significantly reduced  plasma chloride  and  sodium
levels  (Leivestad  and  Muniz 1976;  CARP E-5.6.2.4).   The plasma  content of
potassium, calcium,  and magnesium was not affected.  Therefore, impairment of
the active  transport  mechanism for sodium and/or chloride ions  through the
gill   epithelium was suggested as  the  primary cause of  fish  death.    Severe
internal  ionic  imbalance  would affect  fundamental physiological  processes
such as nerve conductions and enzymatic reactions" [CARP  E-5.6.4.1.5J.

High metal concentrations can  also  be  toxic  to fish and are  associated with
decreased  pH levels in waters.  Aluminum, manganese,  and zinc concentrations
increase  in  acidic surface  waters  apparently  as a  result  of  increased
solubility  at  lower pH  levels.   Concentrations  of  cadmium,  copper,  lead,
nickel,  and  other   metals  can also   increase   due   to  direct  atmospheric
deposition, and acidification  may  increase  their  availability  and subsequent
toxicity.   At  present,  measurements  of zinc,  manganese,  cadmium,  copper,


                                     39

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lead, and  nickel  in  surface  waters of eastern North  America  and Scandinavia
are  below  toxic  concentrations (and/or maximum acceptable limits),  unless  a
local emissions source exists.  [CARP E-5.6.4.2]

Aluminum,  however, has  been  found to be toxic to fish  at  a  level  within the
range   of   concentrations   measured  in   acidic  surface   waters,   i.e.,
concentrations  as  low  as  0.1   to  0.2  mg  _£-!.     "Total  aluminum  levels
measured  range  up  to  1.4   mg   H~^  in  the  Adirondack  region,   New  York
(Schofield  1976),  0.76   mg  ^-1  in   southwestern   Sweden   (Dickson  1975,
Wenblad  and  Johansson  1980), 0.6  mg ^-1 in  southern  Norway  (Wright  et al.
1980),  and 0.8  mg jf"!  in   the  Pine  Barrens  of New  Jersey  (Budd  et al.
1981)"  [CARP  E-5.6.4.2].    In  addition,   for brook  trout  stocked  in  53
Adirondack lakes, aluminum was found to be  the primary chemical  factor, of 12
water  quality  parameters  measured,  most   highly   correlated   with  trout
survival.

Aluminum  toxicity in  fish appears  to  result  from   the  combined effect  of
impaired ion  exchange  and of respiratory distress caused  by  mucous  clogging
of the gills.  Brown trout exposed to  aluminum concentrations as low as 0.19
mg &~^  at pH  5.0 rapidly lost  sodium and  chloride  from the  blood.   Mod-
erate to  severe  gill  damage  was  noted at  aluminum  levels of 0.5  and 1.0  mg
£'*•  at  pH 4.4  and higher.    At  pH levels  5.2   to  5.4,  aluminum was  par-
ticularly  toxic  in supersaturated  solutions.  Complexation  of aluminum  by
organic  chemicals appears  to reduce  toxicity and   enhance  fish  survival.
[CARP E-5.6.4.2]

A  recent  study  using  controlled  experiments  in a  continuously  monitored
Norwegian  river  illustrates  a relationship bewteen fish mortality and river
chemistry and hydrology (Henriksen et al. 1984).   During a two-week period in
the  winter of 1983,  four episodes of  pH-drops from  pH 5.9 to  5.1 coincided
with  increased  water  flow from  rainfall and  snowmelt, dilution  of calcium
ion, and increase of aluminum species.   Concurrent observations of health and
behavior of Atlantic salmon at three stages of development (eggs, summer old,
one-year old)  during  the  episodes  showed  changes in behavior  indicative  of
physiological  stress  in  both  ages of fish  and  death  of  four  one-year old
fish; stress  and  mortality were  not found in a chemically protected control
group.

"It  should be kept  in mind   that acidification of  freshwaters  is a complex
process  that  involves  more  than  merely  increases in acidity.    Other well-
documented changes  include increased concentrations  of  metal  ions, increased
water clairty  the  accumulation of periphyton  (microflora  attached  to bottom
substrates) and detritus,  and  changes  in trophic  interactions (e.g., loss of
fish as  top predators).   The  response of aquatic systems to acidic deposition
must be  viewed in  terms of   all  these  changes that  together constitute the
acidification process" [CARP  E-5.1].

A.2.4.3  WHAT ARE THE CHARACTERISTICS OF THOSE SURFACE  WATERS WHERE CHANGES
         IN FISH POPULATIONS  MIGHT OCCUR? [CARP E-5.2,  E-5.6]

Fish population changes  attributable to  acidic deposition  are most likely to
occur in those surface waters  that would exhibit long-term changes in aquatic


                                     40

-------
 chemistry or substantial  short-term  fluctuations.   Either fish  mortality  or
 recruitment  failure  could   occur.    Chemical  changes  can  also  alter  an
 ecosystem's habitat  suitability  for some  fish species.   Ul tra-oligotrophic
 (nutrient poor) lakes and streams are  the  most likely to  be  affected.  Lakes
 and  streams  of this  type occur  in  large  areas  of  eastern  Canada and  the
 northeastern United States,  as well  as  in some sections of the western  United
 States and northern Florida.

 Ultra-oligotrophic   waters  are  especially   common  where  glaciation  removed
 younger  calcareous  deposits  and exposed  weather-resistant  granitic and
 siliceous bedrock.   The  absence  of carbonate rocks  in  the drainage  basin
 results in lakes with little  carbonate-bicarbonate  buffering  capacity;  hence,
 such  lakes  are very vulnerable  to  pH  changes.    In  areas of  low  acidic
 deposition,  such lakes often  have  pH's  in the 5.5 to  6.5 range (thus they are
 naturally acidic)   with  most  of  the acidity due   to  carbonic acid  (^03).
 These lakes  tend to be small and  have  low  concentrations  of  dissolved  ions.
 [CARP E-5.2.1]

 Simply stated,   the  characteristics  of  the surface waters where changes  in
 fish populations might occur  are  those  characteristics which  cause an aquatic
 system to be  susceptible  to chemistry changes.  The extent of  effects on  fish
 populations  is  largely  determined by  the  system's   capacity  to assimilate
 increased chemical  inputs.   The more severe  the changes in aquatic  chemistry
 as  a result  of  those inputs,  the more likely the aquatic  biota,  particularly
 fish,  will be subsequently affected.  The determining  factors  are part  of the
 specific  system itself; the same deposition amount  has the  potential  to  cause
 different degrees of  change,  ranging from no  change to acidification, in  dif-
 ferent systems.

 A.2.4.4   WHAT EVIDENCE IS THERE THAT CHANGING WATER CHEMISTRY  HAS AFFECTED
          FISH POPULATIONS?  [CARP E-5.6]

 Effects   of  acidification  on  aquatic  biota,  independent   of  cause,  are
 reasonably well  documented.    Evidence  is clear from  field,  laboratory, and
 whole  system studies  that acidification of  sufficient magnitude affects fish,
 other  aquatic organisms,  and  aquatic system  structure.   Having demonstrated
 that change  in  the acidity of  waters  is,  in some  areas,  a  result of acidic
 deposition  is   sufficent  evidence  to  suggest  that  concommitant  changes  in
 biota  followed.   Observation of mortality  of  individual  fish is not suffi-
 cient  to  explain population loss.   The  absence of fish in  an acidic aquatic
 system  is not sufficient evidence  that  such loss  is  the  result of acidifi-
 cation.    Attribution of  fish  population   loss  to   acidification  requires
 records of changes  in both aquatic chemistry and fish populations.

Few  reliable  long-term  records of changes  in fish  populations  exist in the
United States.  The  best  evidence  for concommitant  acidification and loss of
fish  populations is  for  the  Adirondack   region  of   New  York  State.    The
presence  today  of  fish  in  Adirondack  lakes  and  streams  is  inversely
correlated with  pH  levels.   Loss  of fish  populations since  the  1930's has
been documented  for  about 180 Adirondack lakes (out  of approximately 2877).
Historical  records   are   not  available,  however,   to   relate   each   loss
specifically to  acidification.   In other  regions  of  the  United  States,  no


                                     41

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adverse effects  of  acidic  deposition  or acidification  on  fish  have  been
definitively identified.  [ CARP E-5.6.2.1.1]

A.2.4.6  WHAT OPTIONS ARE AVAILABLE TO MAINTAIN FISH POPULATIONS?
         [CARP E-5.9]

Selection for  tolerance  to  acidity is the only general  strategy,  other than
direct alteration  of  the  water  chemistry,  that might  maintain  fish  popu-
lations.    Genetic  screening,   selective  breeding,  and  acclimation are  all
potential   mechanisms  to  maintain  fish  resources  in   acidified  systems.
Although   each  has  merit,   it  is doubtful   that  they  could  be  used  to
reestablish naturally-reproducing  fish  populations, and  they do not address
the  problem  of restoring  other  biotic  components.    Because metal  concen-
trations  will  remain  high  in  acidified  waters,  any  tolerant  fish species
introduced would  need to  be  monitored for  metal  concentrations  that  might
harm the  health of those who consume the fish.

Regular restocking of  some  acidic systems is also an  option.   Acidification
appears first  to affect  recruitment of fish.  Adult  fish  can tolerate  lower
pH's  than  fish fry.   Thus, although  the population  might  not be able  to
maintain  itself, restocking would be a viable strategy in some systems.

A.2.5  WHAT ARE THE EFFECTS OF ACIDIC  DEPOSITION  ON  HUMAN HEALTH? [CARP  E-6]

A.2.5.1  HOW COULD ACIDIC DEPOSITION AFFECT HUMAN  HEALTH?  [CARP E-6]

Acidic deposition  or its precursors could affect human health either directly
by inhalation or indirectly by  ingestion  of  affected  food  or drinking water.
Direct effects on  human  health  have  been  studied extensively in  U.S.  EPA
criteria  documents and will not be discussed here.

Indirect  (post-depositional)   effects  on  human  health  causally  related  to
acidic  deposition   have  not  been  demonstrated.    Human  exposure  to  toxic
substances may be  influenced  by  acidic  deposition through bioaccumulations
along  food  chains  and  drinking  water   contamination.    The substances  of
concern are methyl mercury, which can  accumulate  in aquatic food chains, and
lead,  a  potential  drinking water contaminant.   In  addition,  high  aluminum
concentrations  in  water used  in  dialysis therapy are  a  potential  cause  of
brain  damage.    Other  elements and  chemicals of  concern  include  arsenic,
asbestos,  cadmium,  copper,  and   nickel,  but  data  on   these   elements  are
limited.   [CARP E-6.1]

"Bioaccumulation of methyl  mercury  in  fish is  the  main if not sole  source of
human exposure, barring episodes  of accidental discharge on misuse of manmade
methyl mercury  compounds"  [CARP E-6.2.3],   Pike  and trout are among the most
likely species  to be affected  by  acidic deposition and have the  highest human
consumption  figures  and  average  methyl   mercury  concentrations.    Elevated
methyl mercury  concentrations  in  fish muscle (most notably  of pike and perch)
have  been  statistically  associated with higher acid concentrations  in water.
However,  changes  in acidity  may  also coincide with  changes  in a  number  of
variables  that affect mercury concentrations  in fish:    the  available data
does  not establish  increasing  acidity as  the causal factor.


                                     42

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 Assessments of the  impacts  of  acidic deposition  on drinking  water quality are
 tentative and based on limited data.  Drinking water quality  is  dependent on
 the source and the  management  of  the water supply.  Sources  of drinking water
 include  both  surface  and  groundwaters  as  well  as  direct  precipitation.
 Management practices  that  can  greatly  affect water quality include  storage,
 treatment, and distribution of water prior to use.  [CARP  E-6.3.1]

 Increasing  corrosivity  due   to  increasing  acidity  is  probably  the  most
 significant potential  impact of acidic  deposition  on drinking  water  supplies.
 The risk of exposure  to higher concentrations of  toxicants (e.g.,  corrosion
 products such as  lead  and  possibly  cadmium) is greater where the  water  supply
 is not treated for  corrosivity, and where  water  storage  facilities are  small,
 necessitating the direct use of  raw water during  storm flow  periods.   Water
 systems of concern  include  surface-water and  roof-catchment  cisterns.

 Very  few  data  on the  impacts  of  atmospheric  deposition on  drinking  water
 quality exist,  however,  increases in metal concentrations due to  acidity  are
 a  potential  adverse effect  that may increase  the risks  to human  health.   Any
 increases  in lead concentrations  in  drinking  water are  an additional  burden
 of lead to the body,  especially in  children where  many already have elevated
 blood  and  bone lead concentrations.   [CARP E-6.3.1, E-6.3.2]

 In  general,  the  smaller the water  supply,  the greater  the risk,  with  small,
 privately-owned surface  water  systems serving a  single  dwelling at greatest
 risk.   Groundwaters  in  the United  States  do not  appear  affected by  acidic
 deposition  although reports from Scandinavia suggest  there  is  a potential
 effect.   [CARP E-6.3.1.3]

 A.2.5.2  WHAT  EVIDENCE EXISTS TO SUGGEST HUMAN HEALTH IS BEING AFFECTED?
         [CARP E-6.2, E-6.3]

 No  adverse human  health effects, either  from fish consumption  or  drinking
 water,  have  been  documented as being a  consequence of  metal  mobilization by
 acidic  deposition.  The extent of human exposure appears small.   [CARP  E-6.5]

 A.2.5.3  WHAT OPTIONS ARE AVAILABLE TO MINIMIZE THE RISK OF INDIRECT HEALTH
         EFFECTS DUE TO ACIDIC  DEPOSITION? [CARP E-6.3]

 The primary  mechanism  by which health effects could be  minimized is through
 treatment  of  drinking  water  to  decrease  corrosivity.    This  is a   common
 practice  in  municipal  drinking water supplies  of  low alkalinity and/or pH.
 Those  populations  not supplied  drinking  water  by  major  water  treatment
 facilities  could  be  encouraged  to  monitor  their  drinking  water  supplies
 periodically to ensure the  water  supply  is  safe.   Small-scale  technology for
mitigating corrosivity is available, if  needed.
                                     43

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A.2.6  WHAT ARE THE EFFECTS ON MATERIALS? [CARP E-7]

A.2.6.1  WHAT EFFECTS ON MATERIALS MAY OCCUR AS A RESULT OF ATMOSPHERIC
         DEPOSITION OR ATMOSPHERIC POLLUTION?  [CARP E-7.1]

Damage to  materials  from atmospheric  deposition may include the corrosion of
metals, erosion and discoloration of paints, decay of building stone, and the
weakening  and fading  of  textiles.   All of  these  effects  occur under natural
environmental conditions as a result  of  moisture,  sunlight,  carbon dioxide,
atmospheric  oxygen,   temperature   fluctuations,   and  the  action  of  micro-
organisms.   Quantifying  the  relative amount  of damage  caused by  specific
manmade air pollutants, and by specific pollutant transformations and contact
processes  (e.g.,  acid  precipitation)  is  extremely  difficult.    Table  II.2
summarizes potentially damaging effects  on  materials  generally attributed to
air pollutants and other environmental  factors.

A.2.6.2  WHAT IS THE ROLE OF ACIDIC DEPOSITION IN DEGRADATION OF MATERIALS?
         [CARP E-7.1.1]

The percentage of materials degradation  occurring from  acidic deposition, in
contrast to that caused by other natural  and human factors, is not known.  In
general,    distinguishing   between   the  effects  of  gaseous   S02,   sulfate
aerosol,  and  wet deposited substances is  difficult.   If effects  of  acidic
deposition are  defined  to  include all  the mechanisms by which  acidic and
acidifying pollutants may  contact  and  damage  surfaces, a  considerable   body
of  experimental  evidence  for damage  to materials exists.    Sulfur oxides,
other  acidic gases,  and  particulates are  important,  potentially  damaging
pollutants; moisture (atmospheric humidity and wetness of surfaces) is a very
important factor.

A.2.6.3  WHAT COMPONENTS OF ACIDIC DEPOSITION ARE MOST IMPORTANT IN
         MATERIALS DEGRADATION PROCESSES? [CARP E-7.1.1]

The primary factor  in  materials  degradation due  to acidic deposition  is the
corrosivity  of  the  acids  themselves,  i.e.,  the nitric  and  sulfuric  acids
formed from  sulfur  and  nitrogen  oxide   transformation.   Although  evidence
suggests that acid  precipitation  can  cause material  degradation,  it appears
that more  extensive  degradation occurs from gaseous  impaction of  S02  or dry
deposition onto moist  surfaces  or  in  a  very  humid  environment.   Under  these
conditions the acids  may become  highly concentrated  and  do  the  most damage.
Information  on   sulfur  dioxide   concentrations,  duration of wetness,  and
oxidation-reduction  rates  is  needed  to  understand   this problem.     [CARP
E-7.1.1]
                                     44

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                       TABLE 11.2  POTENTIAL EFFECTS OF AIR POLLUTION ON MATERIALS
tn
Materials
Metals



Building
Stone






Ceramics
and Glass


Paints and
Organic
Coatings



Paper




Photo-
graphic
Materials
Textiles




Textile
Dyes

Leather



Rubber


Type of
impact
Corrosion,
tarnishing


Surface erosion,
soiling, black
crust formation





Surface erosion,
surface crust
formation

Surface erosion
discoloration,
soiling



Embrittlement,
discoloration



Microblemishes


Reduced tensile
strength,
soiling


Fading, color
change

Weakening,
powdered surface


Cracking


Principal air
pollutants
Sulfur oxides
and other acid
gases

Sulfur oxides
and other acid
gases





Acid gases,
especially
fluoride-
containing
Sulfur oxides,
hydrogen
sulfide, ozone



Sulfur oxides




Sulfur oxides


Sulfur and
nitrogen
oxides


Nitrogen
oxides and
ozone
Sulfur oxides



Ozone


Other
environmental
factors
Moisture, air,
salt, particulate
matter

Mechanical ero-
sion, particulate
matter, moisture,
temperature
fluctuations,
salt, vibration,
CO?, micro-
organisms
Moisture



Moisture,
sunlight,
particulate
matter, mechan-
ical erosion,
microorganisms
Moisture, phys-
ical wear,
acidic materi-
als introduced
in manufacture
Particulate
matter.
moisture
Particulate
matter,
moisture,
light, physical
wear, washing
Light,
temperature

Physical wear,
residual acids
introduced in
manufacture
Sunlight,
physical wear

Methods of measurement
Weight loss after removal of
corrosion products, reduced
physical strength, change in
surface characteristics
Weight loss of sample, surface
reflectivity, measurement of
dimensional changes, chemical
analysis




Loss in surface reflectivity
and light transmission, change
in thickness, chemical
analysis
Weight loss of exposed painted
panels, surface reflectivity,
thickness loss



Decreased folding endurance,
pH change, molecular weight
measurement, tensile strength


Visual and microscopic
examination

Reduced tensile strength,
chemical analysis (e.g.,
molecular weight) surface
reflectivity

Reflectance and color value
measurements

Loss in tensile strength,
chemical analysis


Loss in elasticity and
strength, measurement of crack
frequency and depth
Mitigation measures
Surface plating or coating,
replacement with corrosion-
resistant material , removal to
controlled environment.
Cleaning, impregnation with
resins, removal to controlled
environment.





Protective coatings,
replacement with more
resistant material , removal to
controlled atmosphere.
Repainting, replacement with
more resistant material




Synthetic coatings, storing
in controlled environment,
deacidification, encapsula-
tion, impregnation with
organic polymers.
Removal to controlled
atmosphere

Replacement, use of substi-
tute materials, impregnation
with polymers


Replacements, use of
substitute materials, removal
to controlled environment.
Removal to controlled
environment, consolidated with
polymers, or replacement

Add antioxidants to
formulation, replace with more
resistant materials

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                       SECTION  B.   ECOSYSTEM  INTERACTIONS


 B.I   INTRODUCTION

 This  section  of  the  effects  summary  answers questions  about  interactions
 among   ecosystem  components—bedrock,   soils,  soil  water,  microorganisms,
 vegetation,  animals,  surface  and groundwaters.    Of   potential   vegetative
 effects,  only  those  on forests are considered.  Impacts of acidic  deposition
 on crops  are most certainly  overwhelmed  by the  additions of fertilizer,  lime,
 and  other amendments  in  common  agronomic and  horticultural  practices.   The
 impacts  of the gaseous  precursors of acidic  deposition  and ozone on  crops
 have  been discussed  in criteria  documents prepared for  rule-making under the
 Clean  Air Act.   Analysis of  impacts upon  perennial  vegetation  other  than
 forests is not sufficiently  developed in  the CARP  to allow discussion  here.


 B.2  WHAT EXPLANATIONS HAVE  BEEN PROPOSED FOR OBSERVED REGIONAL DECLINES
     OF FORESTS?

 Explanations proposed  for forest declines in the United States and  Europe can
 be broadly described:

     1.   Climatic changes such as  drought have  induced the declines.

     2.   Biotic pathogens such as  viruses, fungi,  or insects have induced the
         declines.

     3.  Air and  precipitation  quality  (acidic deposition,  its  precursors,
         oxidants, or  trace  metals) have directly  induced the declines.

     4.  Air and  precipitation quality  have indirectly  induced  the declines
          through changes in  soil  chemical characteristics.

     5.  Complex combinations of the above.

The hypotheses  for the cause of 'Waldsterben1 raised in  Section A.2.2.5.3 are
encompassed in  explanations  3 and  4.  Explanations 1 and  2 encompass general
 stress, also described  earlier.   Below we discuss potential  direct (foliar)
and indirect (soil-mediated) roles of acidic deposition  or its precursors.

 B.2.1  WHAT ROLE COULD ACIDIC DEPOSITION PLAY IN THE PROPOSED EXPLANATIONS?

Acidic  deposition has been  proposed  by some  scientists  as  a   causal  or
 contributing factor  in  forest  growth  declines.  Two general  ideas  have  been
proposed:

     1.  Acidic deposition,  its precursors,  or both  directly affect vege-
         tation and  induce decline.   One  hypothetical mechanism  of  action is
         as follows:
                                     46

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          (a)   Acidic deposition,  including  fog or  cloud  water at  high  ele-
               vations,  directly  impacts  the aerial  portion of  a  tree  such
               that increased leaching of  nutrients,  particularly Mg, occurs.

          (b)   The root  system  is unable  to replenish  nutrient losses at a
               rate sufficient to maintain optimal  nutrient status  and growth.

          (c)   The  trees  with  limited nutrients  support  the  newest,  most
               actively   growing  parts of  the  tree;   older  leaves/needles
               prematurely die.

          (d)   Decreased  aerial  growth  is   accompanied  by  decreased  root
               growth, continued leaching  losses, and  further  decline.

          (e)   Ultimately,  the  tree  is weakened and  secondary  factors begin
               playing  a  role  (e.g.,  pathogenic organisms  and  environmental
               stress) eventually  causing  death.

      2.   Acidic  deposition,  its precursors, or both indirectly affect vege-
           tation  and induce  decline.   One hypothetical  mechanism of  action is
          as  follows:


           (a)  Acidic  deposition  directly  changes   the  chemistry   of  soil
               systems,  i.e., increases acidity and aluminum  concentrations.

          (b)  The  changing  conditions in  the  soil,  perhaps during  changes
               from  warm  and  dry to wet  periods,  cause a  direct toxicity to
               the roots, reducing the  uptake of water  and nutrients.

          (c)  The  reduced root  system  is  no  longer  able  to  support  the
               aerial biomass.

          (d)  Older leaves or  needles  prematurely die  and drop  from  the tree
               as  the newer  growth receives most  of  the available water and
               nutrients.

          (e)  Decreased  aerial and  root production  over time  weakens  the
               tree, making it  susceptible to secondary  stress factors.

Detailed  studies  should  allow  evaluation  of these and  other  hypotheses over
the next several  years.

B.2.2  IS THERE EVIDENCE TO SUPPORT THESE  TWO HYPOTHESES?

Limited  evidence   is  available  to  support  each  of  them.    This  evidence
includes  increased  leaching  of  basic  substances  from  foilage  and  soil,
increased sulfate  concentrations  in  soil,  and,  in   at least  one  instance,
increased aluminum  concentrations  in  soil.  Unfortunately,  trees  respond to
stress in a  limited  number of  ways.   As  a  result,  many  factors  that cause
declines result in quite  similar  symptoms.   Identifying any  single factor as
a causal  agent in  a  forest system may  require  years of investigation usinq
both field and laboratory data.


                                     47

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The recent  regional  declines of red  spruce,  pitch pine, and  shortleaf pine
all began  in the late  1950's  or early  1960's.   In  many  areas, this  was  a
severe  drought  period.    Some  scientists   believe  that  the  drought  was
sufficient to cause growth  losses during  that period  and possibly predispose
the  trees  to   other  environmental   stresses.   But  not all  areas  showing
declines were subjected to drought.   If there was  some other  general,  for
example climatological, change during  this period  of  sufficient magnitude to
stress forests,  it has not yet been  identified.

Oxidants and other  gaseous  pollutants can cause decreases  in  growth similar
to those expected from  acidic deposition, and these  pollutants cannot now be
excluded as causal factors.  Furthermore, it  might be expected that oxidants
and acidic  deposition  interact  to  cause an  effect.    Ozone  for example is
known  to  cause  membrane  damage  in   cells,  making  them more  susceptible to
nutrient  leaching  losses.   Such mechanisms  would be  quite  significant in
hypothesis 1 above.

Several highly speculative relationships are  described below.

Coniferous  forests,   such  as  those  experiencing  recent  regional  decline,
normally are quite tolerant of acidic  soil conditions,  with many of the tree
species growing on soils with pH's  below 4.0.   The dilute acidity contributed
by acidic deposition  probably would not have  caused  changes  in  soil  pH.   As
will  be  discussed   in  the following  aquatics  section,  however,  sulfate
deposition  can  increase soil water acidity   in  already acid  soils,  without
accompanying changes  in  soil pH.   This acidity is  derived  from the aluminum
exchange  process.    If  the soils of concern   in  these  systems  were mineral
soils, the  hypothesis  that  root damage occurs could  be plausible.   However,
many  soils  being  studied are organic  soils,   and aluminum  would be complexed
by organics, making biologically-toxic forms  of aluminum minimally available.

A metal-induced toxicity is also plausible (Friedland et al. 1984).  Measure-
ments  of  heavy  metals in the areas  of  reported  decline confirm increases in
metal   concentrations  in  the soils  and  trees.    As  acidic  deposition  has
changed over time,  so  has  the deposition of metals,  although  metals  depo-
sition is  rarely measured.

Organic matter  often  tends  to  accumulate in   sites  showing  the  most dramatic
declines.    This could  indicate  that microbial processes have  been affected
and  needed  nutrients  are  accumulating  in   the  litter  and   are  no  longer
available   for   plant  growth.    A  nutrient  deficiency  would  result,  again
showing symptomatology similar to that being observed.

Recently,   it has been  proposed  that nitrogen may  be playing a  role  in  the
observed declines.  Nitrogen inputs  can detrimentally affect the relationship
between tree roots and  beneficial fungi  that  symbiotically exist in the soil.
Nitrogen,   usually  considered a limiting nutrient  in  most forest ecosystems,
could  now be in excess  of amounts  required by some  tree species.   Friedland
et  al. (1985)  found  a  predisposition of red spruce  to  winter  damage  and
suggest that it may be  a  component of  decline.   As one  of  three suggested
causes,  they  propose  a  testable  hypothesis  that  nitrogen  alters  growth
processes and interferes with winter-hardening or cuticle formation.


                                     48

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In summary, it has been confirmed that most of the recently described decline
of forests  in  the eastern United States began  in  the  late 1950's  and  early
1960's.   Drought is  the  one  obvious  climatological  change  that  has  been
examined  that  might explain  these declines,  but  it  has not  proven to  be
sufficient  explanation.     Therefore,   anthropogenic  causes  are  suspected,
although  other  climatic  factors such as  early or  late  frost  frequency  and
severity,  or  winter  temperature   extremes  merit  consideration.    Decline
appears  to  have  occurred during  the  same  period when  regional  emissions
increases were most evident.  As a  result,  effects of  substances emitted from
fossil-fuel   burning  are  the  focal  point of  research.    Research  has  not
demonstrated  that  acidic  deposition,  or   any   component   of  atmospheric
deposition,  is  the  primary cause  of  decline or  that  it  is  even  playing  a
role.  If it were,  it  is  unclear  whether  sulfur  or  nitrogen  in  deposition
would  be implicated  as  the  element  of  most concern.    However,  current
hypotheses suggest nitrogen deserves more attention in  forest growth studies.

8.2.3.  WHAT FOREST REGIONS OF THE  UNITED STATES WOULD  MOST LIKELY  BE
        AFFECTED BY ACIDIC DEPOSITION  IF THE  HYPOTHESES WERE  CORRECT?

Because  published data have  shown  only recent  declines  of  coniferous  tree
species,  those regions of  the United  States  vegetated  by  such species are of
primary concern.  These areas  are shown  in Figure 11.9.   The  areas  at highest
risk  would  be  those coniferous  forests  at high elevations where  deposition
and  cloud water  acidity  tend  to  be  greatest,  air pollution  episodes  may
occur,  and  natural   environmental  stress   is  great.    This  includes  the
mountainous  areas of the  eastern  and  western  United  States.   Since it  is
mainly  the  eastern United  States   that is  receiving  regional  acidic  depo-
sition,  eastern  mountain  regions would be  considered  the most  susceptible
areas;  future  surveys of  western   forests  are not expected to  demonstrate
general forest decline attributable to  acidic deposition  except in  localized
areas receiving high levels of pollution.

B.3   WHAT HYPOTHESES  HAVE  BEEN  PROPOSED TO  EXPLAIN CHANGES  IN  SURFACE  WATER
     CHEMISTRY?
Hypotheses have been proposed to explain observed  changes  in  pH,  alkalinity,
or  sulfate  concentrations in  surface waters.   Each  is  a plausible  expla-
nation, supported  by experimental  evidence,  for  acidification at  selected
sites.  None applies universally to all  surface  waters; no single  explanation
for change will likely  ever  suffice for all  sites.   Six hypotheses  to  con-
sider for  clear water lakes and  streams  follow:

     1.  A decrease  in  normally  high,  anaerobic,  groundwater table  levels
         creates high hydrogen ion production and  accelerated  loss of  cation
         through oxidation of reduced  chemicals.

     2.  Logging,  fires,  landslides,  or  other disturbances have  resulted  in
        accelerated cation  leaching  from  watersheds,  due  to  exposure  of
         previously unweathered  material  to chemical  weathering,  leading  to
         increased   pH   in  water.     As   the  watershed   recovers from   the
        disturbance,  the  pH  of  the  system  may  decrease,  returning  to
         'natural'  steady  state.


                                      49

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Figure II.9  Regions  of the  United  States  with  high-elevation  forests
             (boreal, sub-alpine, and  montane;  mixed  boreal  and
             deciduous; mixed  boreal,  lake,  and deciduous).  Adapted
             from Eyre (1963).
                               50

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      3.   Long-term  trends  in  the  net  production  of  forest  biomass  affect soil
          and soil  water  chemistry.    A  rapidly  growing  forest will  acidify
          soils.    A decrease  in  forest growth  rate,  such  as  in an  aging
          forest,  or  accumulation of  organic  matter  on  the  forest  floor,
          perhaps  through  decreased  decomposition  and  mineralization  rates,
          will  decrease  the rate  of acidification.

      4.   Shifts  in  vegetation  cover  type  result  in  physical  and  chemical
          changes  in soils  and  watersheds,  and  long-term  changes  in  surface
          water  chemistry.   Reforestation can cause  soil  acidification.   In
          general,  coniferous forests  and  forest species  such  as red  alder
          that  support  high nitrogen  fixation rates increase the rate  of soil
          acidification.

      5.   Acidification  of  agricultural  soils occurs from fertilizer  amend-
          ments and  is  usually counteracted  by addition  of agricultural  lime.
          Acidification  of  waters  could occur   in  regions where abandoned
          farmland  is undergoing  reforestation  and basic  inputs from  liming
          have ceased.

      6.   Acidic  deposition introduces additional  hydrogen ion  in excess  of
          what is normally  produced.

B.3.1  WHAT  ROLE DOES ACIDIC DEPOSITION  PLAY  IN THE ACIDIFICATION  OF LAKES
       AND STREAMS?

Acidic   deposition   must  contribute  to   acidification   somewhere  in  the
ecosystem.   The  deposition inputs may  be  overwhelmed  by  the  natural  acidi-
fication  processes,  however,  and  not  cause   a  measurable  change.    The
magnitude of the depositional contribution  is discussed in Section A.3.1.

The fate  of  hydrogen and nitrate  ions  in ecosystems is  difficult  to monitor;
hydrogen  ion  exchanges are  ubiquitous  and  nitrate  is  often   immediately
transformed  in various processes as illustrated in the  nitrogen cycle  (Figure
11.2).    The effects of atmospheric  deposition  on  surface waters  are best
known  from   the  study  of  the  transport and  fate  of  sulfate.    If  sulfate
increases in waters  in  areas where sulfur is  not a dominant component of the
forest  soil  minerals or  underlying  bedrock, the  increases  must  have been
derived  from atmospheric  deposition  of  sulfur:   whether  wet  or  dry,  or  in
neutral,  acidic or  acidifying  form.    Since monitoring  of  surface waters  in
the past  did not include  sulfate  as  a water quality parameter  of interest,
few historical  data are available  to  judge  the  significance of deposition  to
changes  in  aquatic  systems  or   the  number  of  systems  showing  increases or
decreases  in sulfate.    Therefore,  we  know  that  sulfur is  deposited  on
watersheds from the atmosphere,  but we do not know the number of systems that
have experienced  changes,  how  frequently deposition  leads  to  acidification,
or how soon changes in sulfate in waters follow changes  in  sulfur deposition.

If  sulfate   arrives on  land or  water  in   acidic  form,  it  unquestionably
contributes   to acidification.   However, neutral  salts  of  sulfur  introduced
into the  aquatic system from either the  atmosphere  or the  terrestrial  system
may not lead to acidification,  although sulfate concentration would increase.


                                      51

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The  terrestrial   assimilation/retention/transformation   of  sulfur  compounds
prior  to  discharge into the aquatic  system  plays the key role.   Studies of
surface waters  that account for changes  in  both  the cations  and anions over
time are needed to assess the role of changing sulfate deposition.

B.3.2  WHAT CONCLUSIONS CAN BE DRAWN FROM THE AVAILABLE  EVIDENCE?

All of the above  hypotheses can  be  supported with documentation for selected
ecosystems.  Therefore, we conclude the following:

     8   Atmospheric deposition  of acidic and acidifying substances contrib-
         utes to acidification processes.

     0   Surface water acidification can occur from natural  processes.

     0   Estimates  of  the magnitude  and  geographic  distribution  of  acidic
         deposition's  influence  on changes in surface water  acidity over the
         years are almost exclusively based on correlative evidence.

     0   Man's activities can lead to both increases and decreases in surface
         water pH.

         No single acidification hypothesis applies to all locations.

     0   Acidification  occurs  without concommitant land  use  changes.

     0   In those  studies  where land-use  changes can be demonstrably  ruled
         out as a  factor,  acidic deposition  must be considered as a  likely
         cause of observed surface water acidification.

     0   At what rate  surface  water  chemical changes occur,  with  or  without
         acidic deposition, is still  the primary question to be answered.

B.4   WHAT HYPOTHESES HAVE BEEN PROPOSED TO SUGGEST FUTURE CHANGES IN WATER
      CHEMISTRY WILL OR WILL NOT OCCUR?

Acidic deposition contributes  to the  natural  processes of acidification. Over
geologic time  (thousands  of years),  all  soil systems receiving  rainfall  in
excess  of  evapotranspiration   will   naturally  become   acidic.  It  is  also
clearly evident that some systems have become increasingly acidic as a result
of  acidic  deposition.    However, those  areas where  acidification has  been
documented are relatively simple systems where lakes are surrounded by little
vegetation, shallow soils, steep slopes,  and  acidic  deposition (<  pH  5.0) is
occurring.  These  systems are  likely  to respond relatively  quickly to acidic
deposition  (within  years  to decades).    At  the  other end  of  the  scale are
systems where soils are  highly buffered and water passes slowly  through the
soils with a maximum opportunity to be buffered.   Between these extreme cases
is  a  continuum  of  systems  with a  multitude  of  possible   combinations  of
characteristics.    Thus,  we can  expect a  continuum of  responses  by  aquatic
systems.
                                     52

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 One scientific view,  subscribed  to by  the  authors,  is that  the  terrestrial
 assimilation/retention/transformation  of sulfur compounds prior to  discharge
 into the aquatic  system plays a key role  in  determining  the  rate  of acidifi-
 cation.    This  view is  described  below but  is not  a  scientific  consensus;
 others  believe that natural organic acidification is the  determining  factor.
 Sulfate   adsorption,   mineral  weathering,   cation  exchange   processes,   and
 hydrologic retention must  be  insignificant  in a watershed for  acidification
 to occur quickly  from acidic deposition.  If  any  of  these processes  is  sig-
 nificant,  effects  are  delayed until the capacity for the  process  is reached.
 In a watershed where  sulfate inputs and  outputs  are equal  ('steady-state  )
 the question of further  aquatic  acidification will  depend  primarily on  the
 rate at  which mineral  weathering  can  replace  lost cations.   In watersheds
 where  sulfate steady  state  has  not been  reached  (sulfur  inputs are  greater
 than sulfur  outputs),  acidification will be  delayed until  sulfate adsorption
 capacity  is  exceeded and  a  steady  state  in sulfate input/output  relationships
 is reached.    Then, ion  exchange   and  mineral  weathering rates  become   the
 primary  factors determining  the onset of acidification.

 For surface  or groundwater acidification  to  occur,  the  concentration of  hy-
 drogen ion in the  entering water must  be  greater  than  that already present.
 In many  aquatic systems,  the  major input of  water arrives  from the  terres-
 trial  watershed system.    Much  acidic  precipitation  and  dry  deposition of
 acidic  and   acidifying  substances  passes  through  the   vegetation  canopy,
 through  or over soils,  and is subsequently delivered  to aquatic systems.   At
 any point  along this pathway,  processes  that  release  base  cations can  lead to
 neutralization  of  the  water's acidity.   Other processes can produce hydrogen
 ion and  increase  acidity.   In most systems,  sufficient  base  cations in  the
 terrestrial  system prevent the acidity  in precipitation  from increasing   the
 acidity  of a  water  body.

 In some  ecosystems  of  northeastern North America,  the amount of sulfur input
 to the watershed has been  shown  to approximately equal  the amount of  sulfate
 coming out of the  watershed  [CARP,  E-4.4.1,  Table 4.3].   That  is,  the eco-
 system is  saturated with  sulfur.   When the  waterbody  is  also  acidic under
 this  'steady-state1  condition, and has  little organic acidity,  it  is likely
 that acidic deposition increased the acidity  at  some  time.  However, in some
 systems where  sulfur  inputs equal  sulfur outputs, the associated  water body
 is  not currently  acidic or  is  not sufficiently acidic  to have had obvious
 impact on  biota.   In this  case, it is believed that the  hydrogen  ion asso-
 ciated with the sulfate exchanges for base cations and the sulfate  enters  the
aquatic system largely in neutral,  nonacidifying form.

How  long will  those watersheds where  sulfate inputs and  outputs  are  equal,
and  where  significant  acidification has not yet  occurred,  remain  unacidi-
fied?   Two  rate-related  hypotheses,   each  leading  to  a different  future
condition,  address  this question:

     1.   Continued  deposition will   deplete the  cation reserves  and  acidifi-
         cation  will  follow—presently,  acidification   is   not   occurring
         because sulfate is moving  to the aquatic systems  accompanied by base
                                      53

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         cations rather than hydrogen ion, but  these  cation  reserves will  be
         depleted in years to decades.

     2.  At  present levels  of  deposition,   most aquatic   systems  not  now
         affected will  not be affected  because  mineral  weathering  is replen-
         ishing base cations at  a  rate equal to  losses—cation  reserves  are
         sufficiently  large  that  changes  would  not  occur  in  decades  to
         centuries.

A  restriction  placed  on  the above  discussion  is  that  sulfate inputs  and
outputs from  the  watershed were equal.   This  steady-state  condition  is  not
universal.  In watersheds where  sulfate inputs  are greater  than  the outputs,
sulfate  is accumulating.    Most of  the  accumulation  occurs  in   the  soils
through the process  of  sulfate  adsorption.

Sulfate adsorption occurs primarily in acid,  highly weathered soils common to
the eastern and, particularly,  the  southeastern United States.  The principal
exchangeable cation  in  an acid soil  is  aluminum.   Aluminum  exchange provides
a  buffering system  that prevents rapid  changes  in soil  pH of acid soils just
as base cations buffer high  pH  soils.   An increase in  anion concentrations,
e.g., sulfate in an  acid soil low in base  cations, tends to  mobilize aluminum
as the accompanying  cation  coming  into the  system  is scavenged by  the clay
and exchanges  aluminum.   The sulfate  now forms  an aluminum sulfate complex
which upon hydrolysis produces acidity  in water.   In  acid soils,  whether  the
anion is introduced as an acid or  neutral  salt, the  same reaction occurs and
aluminum is released.  However,  the  soil  pH  must be  low  for aluminum trans-
port to occur readily.

In summary, sulfur  deposited  on  acid soils  mobilizes aluminum.   Much of  the
eastern United  States  has  soils  that are acidic  enough for  this  process  to
occur.   Many  of  these   soils,  particularly  in  the  Southeast,  are  highly
weathered, however,   and   have a characteristically  high sulfate  adsorption
capacity.  That is,  acid  soils  tend to adsorb  sulfate  to the clays so it is
not easily mobilized.    As  a  result,  aluminum,  a  cation   whose  hydrolysis
acidifies  soil  water or  surface water,  is  not easily mobilized.  There  is,
however, for a  given concentration of sulfate  input a saturation point where
no additional  sulfate can be adsorbed, and aluminum sulfate  moves through the
soil  column.   At  this  point of  'breakthrough1,  the receiving aquatic system
could become increasingly acidic.

Several  recent  publications  provide  expanded  discussions   of  rate-related
acidification  hypotheses  and the  terrestrial  phenomena  controlling surface
water  acidification.    The  authors   have  found  the  following  particularly
helpful:   Johnson and Reuss  1984,  National Academy of Sciences 1984, Schnoor
and Stumm  1984, and  Johnson et al.  1985.

B.4.1  WHAT DATA ARE NEEDED TO TEST THESE  HYPOTHESES?

Data needed to  test surface water acidification  hypotheses  must be gathered
both by survey  and  intensive research on  chemical and  physical  processes in
watersheds.
                                      54

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Surveys indicate the  current  status of surface waters,  providing  a baseline
for predicting future changes.  Important data are aquatic chemistry (partic-
ularly alkalinity, organic acidity,  and  concentrations  of sulfate, aluminum,
base cations, and hydrogen ion) and the  physical  characteristics (hydrology,
bedrock,  and soil  chemistry, depth,  and  texture)  of  the  watershed.   The
chemical  and physical  data  combined provide  a  good  indication  of  water
quality and  an  indication,  by correlations  among  factors,  of whether  the
hypotheses  provide  explanations  of  changes  in   the   past  consistent  with
current observations.

Surveys of  greater frequency  or  intensity  could provide  better  predictive
information.  Measurements of sulfate inputs and  outputs  in  many watersheds
would indicate the regional distribution  of  sulfate  steady state.   Extensive
sampling and intensive measurements of those soil   properties—cation exchange
capacity, percent  base  saturation,  sulfate adsorption  capacity,  sulfate and
nitrate retention, organic and mineral mattei—thought  to  be  most important
in determining  soil  and  surface  water chemistry  would  reduce  uncertainty in
predictions.

The  predictions  of  greatest  certainty  must  wait  for  results  of detailed
watershed studies  over a  period  of  years,  with   corroboration  in  the labo-
ratory.   Processes  in watersheds are dynamic, and  their  changes  over time
(seasons  or  years,   for  example)  will  have  a   profound effect  on  water
chemistry.   Changes  in  rates of  mineral  weathering   to  produce  acid  neu-
tralizing materials,  the reversibility of sulfate adsorption, and the cycling
of  sulfur and  nitrogen  in  biomass are  particularly   important.   To  study
changes in  rates  may  require manipulation of  parts  of  watersheds  (simulated
rain or exclusion of rain) or  studying the  responses of materials taken from
watersheds.

B.4.2  IF INCREASES IN SURFACE WATER ACIDITY WERE TO OCCUR, WHAT LOCATIONS IN
       THE UNITED STATES WOULD BE  AT HIGHEST RISK?

The best  single indicator of  sensitivity  of  surface  waters to increased acid
inputs  is alkalinity.  Alkalinity  in an  unperturbed,   surface  water  system
reflects  a  watershed's characteristics.   Systems with high  alkalinity  are
likely to have high acid neutralizing capacity in  the watershed and would not
be sensitive to acidic deposition.   Systems with low alkalinity are likely to
have  poor acid  neutralizing  capacity  in  the watershed.   As  shown in Figure
11.10, areas throughout the eastern United States, the upper Midwest, and the
mountainous  western  regions  are  the  only  areas found  to  have extensive low
alkalinity systemsl.  For  a  system to increase in acidity,  however,  several
conditions must be met:

     1.  Acidic inputs must be present.
Regional  alkalinity  maps  showing   more   detail,   particularly  in  regions
 having  less  than  200 neq  ^~1  alkalinity,  are  in  preparation.    Revised
 maps based on  the  U.S.  EPA National Surface  Water  Survey will  be available
 at the end of 1985.
                                     55

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     2.   The watershed  must not be  able  to  neutralize  the acid  by base cation
         exchange  processes.

     3.   Anions (sulfate)  must be mobile.


Criterion 1  is met  only  in the eastern United States  except for local,  iso-
lated areas.  Criteria 2 and  3 are  not  known for most watersheds,  but based
on several  watershed studies and geochemical  theory,  Criterion  3 is likely to
be met often in the Northeast and seldom in the Southeast.

The region of highest risk for increasing acidity of aquatic resources in the
near future  is  likely  to  be  the southeastern United States.   This response
would be observed as sulfate  adsorption capacity  is exceeded.   Assuming  that
most  sensitive watersheds  in  the  Northeast are  sulfate-saturated  (i.e.,
satisfy Criterion 3) and that deposition  of  acid there  has been constant or
decreasing over the last decade, this region  would experience  little increase
in  acidity  in the  near  future, although  changes in  some watersheds  could
stil 1 be expected.

B.4.3  WHAT IS THE TIME FRAME IN WHICH CHANGES MIGHT BE OBSERVED?

The currently available database provides  no way  to  predict exactly the time
frame for  changes.   Additional acidification at  current input amounts could
be  expected  in  decades in some  systems not  now  satisfying  Criteria 2 and 3.
Aquatic  systems now  satisfying Criterion 3  above would  respond most rapidly
to  changes,  either  increases  or decreases,  in acidic  inputs;  of these,  only
those meeting Criterion 2 would become more acidic.
                                       57

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                      III.  ATMOSPHERIC SCIENCES SUMMARY

                      SECTION A.  ATMOSPHERIC PROCESSES
A.I  INTRODUCTION
A.1.1  WHAT ARE THE RELEVANT QUESTIONS CONCERNING THE EMISSION AND PROCESSING
       OF ACIDIFYING SUBSTANCES?

The  processes  through  which  acidic  substances  reach  the  environment  are
complex and uncertain,  and  it is important  to  ask  the right questions about
those processes to avoid being  overwhelmed  by  scientific detail.   Three sets
of considerations determine how appropriate a question is:

     1)  the nature  of  the effects that  are of most  concern; what doses of
         pollutants are producing the effects occurring at present?

     2)  the causal  relationships  that  determine  what  happens  to an  acid
         precursor when  it  is released to  the environment;  what  have doses
         been in the past and what doses are projected?

     3)  the nature  of  the  possible controls  that  could  be instituted to
         reduce the deposition of harmful  material;  what would be the dose if
         emissions changed?

Although  it is not  the  purpose of this  document to evaluate the  merits or
feasibility  of  alternative  control  strategies,  part  of  the  document's
usefulness depends on its  providing a guide to the  best  scientific estimates
and  an evaluation  of  uncertainties  in estimates of  how emissions are related
to deposition.

This  section  evaluates  the  present  scientific  capability  for  answering
questions  relating emissions  to deposition of acidifying  substances.   The
first-level questions,  those asking for a  broad  characterization  of each of
the  steps  leading  to  acidic  deposition,  are  discussed  in  Section  A.
Second-level  questions,  those  directly  concerned  with  the  relationship
between emissions and deposition, are discussed in Section B.  They fall  into
two  categories:  questions  about  whether it  is possible  to identify   some
sources as  significantly different from  or more important  than  others,  and
questions  about  the  overall  material   budgets  and  the predictability of
changes in them.

A.1.2  WHAT ARE THE MOST IMPORTANT SUBSTANCES THAT ARE EMITTED AND DEPOSITED?
       WHAT SPATIAL SCALES AND TEMPORAL SCALES  ARE MOST  IMPORTANT?  WHERE ARE
       THE MOST SENSITIVE AREAS?  [CARP E-3, E-4]

Part II concluded  that  the  primary  material  of concern  to aquatic ecosystems
is  the  sulfate  ion,   S042~,   whether  it  is   deposited as  sulfuric  acid,
H2S04,  or  as   a   neutral  salt  (ammonium  sulfate  or  calcium   sulfate,  for
instance).    Harmful  effects  can  also   be  attributed to  nitrate,     ~


                                      58

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 deposition,  and it is possible that some effects  are  associated  specifically
 with acidity—i.e.,  unneutralized  sulfate  and  nitrate  deposition.   Because
 precipitation or other moisture eventually  arrives at surfaces,  it  does  not
 matter for most effects  of concern whether the  sulfate or  nitrate  was  dry or
 wet deposited.  For  forests,  it  is not yet  clear  whether  nitrogen compounds
 or sulfur compounds are  of most concern,  or  indeed whether  either of  them  is;
 nor  is  it  known  whether  exposure to  particular  compounds  in  the air  or
 deposition to the  soil  system is more  important.

 Other substances play subsidiary roles.   Neutralizing  material,  coming  mostly
 from soils  but  also  released in combustion,  affects  whether the  sulfate  or
 nitrate  appears as acid or  salt.   It can also  affect the  rates of  chemical
 transformation  in  the  atmosphere.   Oxidizing  materials  are  important  in
 producing  chemical  change.

 Most effects on  soils,  forests, and aquatic  systems appear  to  result  from  the
 long-term  accumulation of  sulfates and  nitrates.   For  that  reason average
 deposition rates over one or  more  years are  the  most important  quantities.
 The magnitude of  effects  may depend  on  time of  year,  whether  the  material
 accumulates  during  the growing season, for  instance.  Some evidence  suggests
 that acid  deposition  episodes, sudden  introduction of high concentrations  of
 acidic materials,  may produce shocks  to  aquatic systems.   These shocks  may
 occur  from heavy rains.    In  the North these generally result, however, from
 snowmelts  releasing the material accumulated  over an entire winter  season.

 Areas  sensitive  to  acidic  deposition are   broadly  distributed  over   North
 America;  those  receiving  the highest  inputs from  deposition are  found   in
 northeastern   North   America,  while   the  southeastern   United   States  has
 experienced  the  largest recent increase  in  deposition.  Sensitive watersheds
 tend  to  be  small and  widely distributed.   Sensitive  soil  areas  and forest
 tracts are  larger  and widely distributed.    For  all  three  systems only a
 limited amount of detailed survey information exists.

 A.1.3  WHAT  IS THE CAUSAL STRUCTURE RELATING  EMISSION TO DEPOSITION?

 The  Atmospheric Sciences  volume  of  the  Critical  Assessment Review Papers
 (CARP) is  organized by chapter (A-l through  A-9) according to the sequence of
 events from emission of acid precursor (sulfur oxides and nitrogen oxides) to
 deposition.    Burning  of  fuel produces  sulfur  dioxide,   sulfate,  nitrogen
 oxides, and other materials  (A-2).   The pollutants are transported, sometimes
 for  long   distances,  by  a  variety  of atmospheric  processes  (A-3);  during
 transport  some  of  the S02 is oxidized  to  sulfate  and  some  of  the N02  to
 N03-  (A-4).    If  the polluted  air encounters  a  storm system,   some  SO?,
 SU^-,  N02,   and   HQ^~   will   be   scavenged   and  wet   deposited   (A-6).
 Otherwise  the  material is  dry deposited  (A-7) or  leaves  the  continent.   The
 result is  flows  of  compounds  in the  air  (A-5)  and in dry and  wet deposits
 (A-8).  These deposits may  affect  soils (E-2), forests  and crops  (E-3),  water
 chemistry  (E-4), fish and  other aquatic life  (E-5), human  health  (E-6), and
materials  (E-7).   The  current  capability   for  modeling  the   atmospheric
 sequence  is discussed  in Chapter A-9.  The sequence  is illustrated  in Figure
 III.l (Goble 1982).
                                     59

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                         A-3
                                                          I	I     I
                                                    A-9
                                                                              A-5
                                           A-8
Figure III.l   Atmospheric  processes  in  acidic deposition.  Each stage is
              labeled  by the  chapter  in CARP, Vol.  I.
                                    60

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The   transformation   of   S02   to  50,2-  can  take  place  directly   in   the
gas  phase, in  solution  with  water  vapor,  or  on  particles.    In  each  case
numerous  potential  contributing chemical  reactions  occur.   Such a  breakdown
is   illustrated  in  Figure  II 1.2,  which  shows  only  the  primary  chemical
interactions  with S02J  along  with each  interaction comes a  whole cycle  of
chemical  reactions.   Thus, when the  acidic  deposition problem  is  considered
in scientific detail, Figure III.l begins  to  look something like  the artist's
sketch  given  in  Figure 11.3,  where  each   line  represents  an alternative
process (Goble  1982).

A.1.4  WHAT ARE  THE ISSUES IN  RELATING  EMISSIONS TO DEPOSITION?

From  the  perspective  of  the public,  planners, or  regulators,  the purpose  of
this document must be to provide a  basis for evaluating the current state  of
affairs  regarding acid  deposition  and  for anticipating  future  trends  in
deposition with or without the  implementation  of  control  policies.   Oppor-
tunities  for  control are  limited.   Controls on  sulfur and  nitrogen oxide
emissions from  various sources have most frequently been considered; controls
on the production  of  oxidizing material, possibly by controlling hydrocarbon
emissions, and  attempts  to treat  effects  directly have also  been proposed.
Future emissions trends are tied directly  to economic activity and  to control
policies and, thus, are  somewhat easier to  gauge  than  trends  in deposition.
For  that  reason, the  important question is  how  will  changes over a spectrum
of  emissions  alter  flows and effects?   If  we attempt  to  answer  such a
question by tracing in detail  through all  of the branches of Figure III.3  we
will  learn  nothing;  uncertainties and  errors accumulate rapidly  because  of
the  large  number  of  branches  and  stages.    Instead,  we  need  summary
information that represents averages over many processes.

A.2  WHAT IS KNOWN ABOUT THE STEPS IN THE SOURCE-RECEPTOR PATH?

This  section  reviews  the state  of  scientific  knowledge  described  in   the
Critical   Assessment  Review Papers  (CARP) for  each  stage  in  the  path  from
emissiontodeposition.Just  as  we  reversed  the  causal order,  treating
effects first and atmospheric processes second,  we will begin with deposition
and  work  backward, in  Figure  III.l,  to  emissions.    The  justification  is
similar:   what  happens in  later  stages  determines  what information is needed
about earlier stages.

A.2.1  WHAT AMOUNTS OF ACIDIFYING SUBSTANCES ARE WET AND DRY DEPOSITED?
       [CARP A-8]

A.2.1.1.   HOW IS WET  DEPOSITION OF SULFUR AND NITROGEN COMPOUNDS  AND HYDROGEN
          IONS MEASURED?  [CARP  A-8.2.3]

The  idea  behind wet  deposition  measurements is  simple enough.    You  put a
bucket out in  the  rain  and then perform chemical  analyses  of  the rainwater.
Obtaining  measurements that  are both  reliable  and  representative,  however,
requires  considerable care.  Five  characteristics of data  collection  can  be
distinguished; all must  be considered  in  comparing  data from different sites
(or networks).
                                     61

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cr>
PO
             Figure  III.2   Sample processes  in  SO;?  oxidation.   This  figure illustrates how complex

                           each of the  steps  in Figure  III.l  is when examined in detail.

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Figure 111. 3
Alternative pathways leading to acidic deposition.   The
multiplicity of branching and recombining lines  represents
the multiplicity of processes present in a detailed  examina-
tion of acidic deposition.
                                    63

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Site  selection.    A  site  can  fail  to  provide  representative  information
because it is too close  to  particular  sources,  its  local  topography leads to
unusual  meteorological  patterns  (this  problem can  be acute  in  mountainous
regions or near  large  bodies of  water),  or it provides  contaminants  (blown
soil,  bird  droppings,   etc.).     Assuming   that  the  sites  are  reasonably
representative, the number  of sites  and their  spatial  distribution determine
the spatial resolution.

Collection of  samples.    Three  types of  collection methods  have  been used:
wet-only  samplesin  which  the  bucket  is exposed only while it  is  raining,
wet/dry  samples  in which one  bucket  is exposed  only during  rain while  a
second bucket  is  exposed the remaining time, and  bulk samples in  which  one
bucket is continuously exposed.    Wet-only or wet/dry sampling are definitely
preferable.  Information  from bulk  sampling cannot  be  compared reliably from
one  site  to  another.    The  samples   may   have  an  undetermined  amount  of
evaporation; contamination is more likely and,  as we shall see later (Section
A.2.1.7), the amount of dry deposited material  in the collector is an unknown
component of the  material  available for dry deposition.   While  wet-only  and
wet/dry  samples  should  provide   reliable   purely   wet  deposition  samples,
contamination  (particularly  from  failure of seals  in  the  automatic covering
mechanism)  can  be a  serious problem and  probably   limits  the  usefulness of
much of the older network data.

The  choice  of  sampling  time   is  important   in   determining  the  temporal
resolution of a network and in attempting to compare the measurements made at
one  site  with  those made at another.    Choices  that  have  been  made include
sampling  by precipitation  events  and  daily,  weekly,  or  monthly  samples.
Reliable  information  on  rainfall amounts  is   needed  if  shorter-term meas-
urements  are  averaged  for  comparison  with longer-term  data.   Researchers
attempting  to  characterize  what  happens  during a  rainstorm  have  made  a  few
very short duration measurements.

Sample  handling.   While  field  analysis of   samples  is possible  and has been
done, any large-scale network operating over a reasonable length of  time will
collect  samples  and  deliver  them  to  a  central   laboratory for   analysis.
Storage and  handling can  lead to  chemical  change  due to biological  activity,
contamination, or ongoing chemical processes.  Varying the techniques used to
prevent  such changes  can make  data from one  set  of  samples not comparable
with  those  from another.   Biological  activity in  particular can affect the
amount  of organic acid  present  so  that,  except for  pH  lower  than 5.0, pH
measurements may  indicate manmade acidification poorly.  [CARP A-8.4.2]

Chemical analysis.  Standard chemical techniques exist for measuring the most
important  ions  that appear  in  rainwater: anions (sulfate, nitrate,  chloride)
and  cations  (calcium, magnesium,  ammonium,  sodium,   potassium, and hydrogen).
Another  anion,  bicarbonate, is not readily measured.   Its  concentration is
usually  calculated  by assuming equilibrium with  atmospheric carbon dioxide.
Bicarbonate  is  unimportant  for  rainfall  with a  pH less than  5.0.   Because the
anion  sum should  equal  the  cation  sum  (in the absence  of measurement errors),
an  analytic  check  is   available:   measured pH  should  agree with  the  pH
calculated  from  the  amounts  of  the  other ions  listed.    Because the  ion
concentrations  in most  samples  are quite  low,  great  care  must  be  taken in


                                     64

-------
measuring   them;   even   direct   pH   measurement   at  typical   rainwater
concentrations is susceptible to instrument calibration failures.

Quality assurance.   The  preceding discussion makes  clear  that major efforts
at  quality  control  for  each  of the  four  steps are  necessary  for long-term
network data  to be  reliable.   Equally  important  is  documentation  of quality
control efforts so information  from different networks  can  be compared.  One
problem  deserving special  attention  is  rare  extreme  events.    Statistical
outliers  may  result  from contamination  or  measurement error,  but  a  small
number  of  high  concentration  events may,  in  fact,  provide  a significant
portion of  the annual  chemical  wet deposition at a site.  An important check
on  the  comparability  of  procedures from one contemporary  network  to another
involves  co-located  sites.   At present, uncertainties  about quality control
have  made all comparisons  of historical  data  controversial and  have  meant
that  reliable information  about  deposition trends  is  difficult  or perhaps
impossible to extract.

A.2.1.2   WHAT IMPORTANT  COLLECTIONS OF  DATA HAVE BEEN  MADE  AND  ARE ONGOING?
          [CARP A-8.2.4]

The  wet  deposition  data  bases   available  for  North  America  have  been
summarized  by many  authors.  Miller  (1981)  points  out that the  history  of
precipitation chemistry measurements  in  North America has  been  very erratic,
with networks  being  established and  disbanded  without  thought  of  long-term
considerations.  Miller suggests one possible time grouping  of network data:

     1.  1875-1955,   the  period   when   agricultural   researchers   measured
         nutrients  in  precipitation  to determine  the  input  to  the  soil
         system;
     2.   1955-1975,  the period when atmospheric chemists were measuring the
         major ions  in precipitation to  better  understand chemical  cycles in
         the atmosphere;  and
     3.  1975-present,  the   period when  network  measurements  were  often
         primarily to evaluate ecological effects.

Table III.l by Miller  summarizes  the  "agricultural data bases"  taken from a
review by Eriksson (1952).

Table  III.2  summarizes   some  regional-  and national-scale wet  deposition
networks  in  Canada  and  the United States that  have begun  operation  since
1955.   These  networks  were  generally  not established to monitor  acidic
precipitation.   The first  two  are no  longer  operating.   The  PHS/NCAR  and
EML-DOE networks  include  sites  influenced  by large  urban  areas,  thus are not
as  useful  in addressing regional acidic precipitation  issues.    All  the
networks  followed the pattern  of  the Junge network in measuring  the  major
inorganic ions  that account  for  most  of  sample  conductance.   Sulfate  was
measured in all the  networks;  pH was not measured in the Junge network.

In  addition  to  regional- and national-scale  wet deposition  networks,  local
sites  and  networks  have  provided  data  that  may  be  useful  either  in
interpreting  time trends of  chemical concentrations in precipitation  or  in
studying characteristics  of  urban  or power plant plumes.


                                     65

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             TABLE 11 I.I.   AGRICULTURAL  DATA BASES  (1875-1955)
 Period
Number of studies
Locations of sites
1875 - 1895

1895 - 1915



1915 - 1935



1935 - 1955
        3

        7
Missouri, Kansas, Utah

Ottawa, Iowa, Tennessee,
Wisconsin, Illinois, New York
Kansas

Kentucky, Oklahoma,  New York,
Illinois, Texas, Virginia,
Tennessee

Alabama, Georgia, Indiana,
Minnesota, Mississippi,
Tennessee, Massachusetts
                                      66

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                         TABLE 111.2.   SOME  NORTH  AMERICAN  WET  DEPOSITION  DATA  BASES  (1955-PRESENT)
en

NETWORK
National
Junge
PHS/NCARb
WMO/EPA/NOAAC
CANSAPd
NADP6

PERIOD
1955-1956
1959-1966
1972-Present
1977-Present
1978-Present
APPROXIMATE
NUMBER OF
SITES
60
35
17
54
115

SAMPLING
MODE3
W-M
W
W
W
W-D

SAMPLING
INTERVAL
Daily (with monthly
compositing)
Monthly




Monthly (weekly after joining
NADP in 1980)
Daily (with monthly
ing) (monthly before
Weekly
composit-
1980)

            Regional

            US Geological
            Survey Eastern
            (USGS)

            Canadian Centre
            for Inland
            Waters (CCIW)

            Tennessee Valley
            Authority (TVA)

            MAP3Sf
1964-Present       18



1969-Present       15



1971-Present        9


1976-Present        9
 W



W-D


 W
Monthly



Monthly



Biweekly


Daily

-------
                                                  TABLE  111.2  CONTINUED
NETWORK
Canadian APN9
EML-DOEh
EPRIjSURE1
UAPSJ
U.S. EPAk
Great Lakes
PERIOD
1978-Present
1977-Present
1978-1981
1981-Present
1977-Present
NUMBER OF
SITES
8
7
9
20
30
SAMPLING SAMPLING
MODE9 INTERVAL
W Daily
B, W-D Monthly
W Daily
W Daily
B, W Monthly and Weekly
CTl
CO
 B for bulk, W for wet-only with automatically opening device,  W-M for wet-only via manual
 operation, W-D for wet-dry with automatic  device.
 U.S. Public Health Service/National  Center for Atmospheric Research.
cWorld Meteorological  Organization/U.S.  Environmental  Protection Agency/National  and Oceanic
 and Atmospheric Administration.  These  sites  are now  part of NADP.
 Canadian Network for Sampling Acid Precipitation.
eNational Atmospheric Deposition Program.   There were  115  operating  sites  on 1  July 1983  and
 the network was growing rapidly.   In 1983, many of the NADP sites were also named as sites  for
 inclusion in the National  Trends Network  (NTN).
 Multistate Atmospheric Power Production Pollution  Study.
^Canadian Air and Precipitation Network.
Electric Power Research Institute-Sulfate  Regional  Experiment.
 Environmental Measurements Laboratory of  the  U.S.  Department of Energy.
^Utility Acid Precipitation Study.   This was preceded  at some of the same  sites and with  the
 same central laboratory by the 9 site,  wet-only, daily sampling EPRI/SURE network.
!/
 United States Environmental  Protection  Agency.

-------
 The  largest  U.S.  network  now  in  continuous  operation  is  the  National
 Atmospheric Deposition  Program  (NADP).   Its 170  sites  give  it an  average
 spatial  resolution of  about  one  site per 21,300 sq  mi  (54,400 sq km).   The
 density  of sites in  the  eastern  United States  is  somewhat higher, one  site
 per 10,000 sq  mi  (25,000 sq km).

 A.2.1.3   WHAT  ARE  THE PATTERNS FOR  WET  DEPOSITION OF  SULFATE,  NITRATE  AND
          HYDROGEN  IONS? [CARP A-8.4.1]

 One choice has to be  made immediately  in  presenting  data on  wet  deposition:
 that is,  whether to  present  total  amounts deposited  per year per area  or to
 present   amounts  deposited  per   amount of   rainfall.    Because  amounts  of
 rainfall  differ  in  different  locations,  the  patterns  differ  and  either choice
 could be  appropriate  depending  on  the  types of  effects considered.   Because
 the qualitative features  of  either  pattern are similar  and  because we  are
 primarily interested  in  presenting  information  about the  total  loadings  of
 the important  ions, we  show in Figures  III.4,  III.5,  and  III.6,  the  total  wet
 deposition in  1980 for sulfate,  nitrate,  and H+,  respectively, measured  in
 the NADP, the CANSAP,  and other  networks.   The  patterns for  all three  ions
 are similar, with  the  highest  deposition rates  roughly centered  on  a  line
 drawn  from the upper  Ohio Valley  to northern New York, with  high  deposition
 areas  extending  northeast and southwest.  For comparison,  we  show in  Figure
 III.7  the 1980 average  hydrogen  ion concentration per  amount of rainwater,
 measured  as  pH;  it shows  a similar spatial  pattern.   The  amount of  sulfate
 wet deposited  ranges  from 15  to  45  kg  ha-1 yr~l for  most of North  America
 east  of   the Mississippi  River.    [Note:   1  hectare  (ha)  equals  about  2.5
 acres.]    In the  West,  amounts  deposited  range  from 2 to  10 kg ha~l yr-1.
 The amount of  nitrate  wet deposited ranges  from  10  to  30 kg  ha~l  yr"l  in
 the East   and  2  to 10  kg ha'l  yr-1  in the  West.    Hydrogen  ion deposition
 ranges  from 0.2  to  0.8  kg  ha~l  in  the  East  and  0.0005 to  0.1  kg  ha'1
 yr-l  in   the West.  The pH of  rainfall ranges  from  4.0  to 4.8  in  the  East
 and from  5.0 to 6.0 in  the West.

 The reader should  note  the choice  of unit for presenting  these  data:   kg  of
 sulfate,   nitrate,  or  hydrogen.   Other  choices are  possible and often appear
 in  the literature, hence the potential for confusion.   In particular, data  on
 sulfate and  nitrate deposition  are often  presented in  terms of kg of sulfur
 or  nitrogen  (because  they are the  conserved species).    Because  sulfate  has
 the  chemical  formula   S04,  while  nitrate  has the formula  N03,  the  mass  of
 sulfur deposited is about  1/3 that  of sulfate, and  that of nitrogen is about
 2/9 that of nitrate.  Thus, in the eastern United States,  deposition rates  of
 15  to 45  kg  ha'1  of sulfate correspond  to deposition rates of 5  to  15 kg S
 ha-i;   deposition  rates  of  10  to  30 kg   ha'l  of   nitrate  correspond  to
 deposition rates of 2  to 6 kg  N  ha-1.

A.2.1.4  WHAT IS THE SPATIAL AND  TEMPORAL VARIABILITY  OF THE WET DEPOSITION
         PATTERNS?  [CARP A-8.4]

Spatial  variation.  A close look at Figures  III.4  to  III.7, along with other
analyses  of deposition  patterns,  yields two observations:   1)  great spatial
variability exists in  the  eastern data  on  a  small  spatial  scale (neighboring
network stations distant from  major sources  can  differ  in  annual  amounts wet


                                      69

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CANADA
  • CANSAP
  • APN
  AOME
UNITED STATES
  • NADP
  • MAP3S
                                                                             ,0
                                                         1980  pH
    Figure  III.7   pH from weighted-average-hydrogen ion concentration  for
                  1980, for wet deposition samples.  Adapted from Barrie
                  et al. (1982).
                                    73

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deposited by  as  much as a  factor  of 2),  so  the observed  patterns  are  only
broad averages;  and 2) the resolution of  the  network  west  of  the Mississippi
is  so  coarse  that  it  does   not   suitably  define  any  clear  pattern  of
deposition,  beyond the  observation  that total annual  deposition  of  sulfate,
nitrate, and  hydrogen  ion  per unit  area  is generally  smaller than  in  the
East.

Although the patterns of sulfate,  nitrate, and hydrogen ion deposition appear
superficially  similar,   it  is interesting   to  try  to  compare  them  more
quantitatively.    Figure III.8 shows  the  molar  ratio  of sulfate  to nitrate
ions [#  of  sulfate ions/# of  nitrate  ions  or (62/96) x mass  of sulfate wet
deposited/mass of  nitrate wet  deposited]  (NAS   1983).   If  the  patterns  of
sulfate and nitrate deposition were exactly the  same,  then  the ratio would be
approximately constant  over the region studied; this  is  what  is  observed.
The  ratio   tends  to  be slightly  higher near  the Ohio Valley  and  in  the
southern Appalachians and in remote parts of Maine  and eastern Canada.

Temporal variation.   The year to  year  variation  in total  wet  deposition  of
sulfate,nitrate,or  hydrogen  ions as  observed  in 1978-80  for  the  NADP
network is  at least  30  percent.   Individual  sites  show yearly  variation  of
more  than  50 percent.   The  variation  is  somewhat less for  wet deposition
concentrations;  apparently variability  in annual  rainfall amounts contributes
significantly to the  variability in  amounts wet  deposited  (NAS 1983).  [CARP
A-8.4.1]

Seasonal variation  in sulfate wet  deposition is substantial,  and  even  more
variability occurs  in sulfate  and  nitrate deposited  from  one rainfall event
to  another,  as  can  be  seen  from  Figure  III.9.    Variation  in  ion concen-
trations during  a single storm can be as high as 1000 percent.

The  seasonal  variation  in  sulfate  wet deposition and  the absence of  seasonal
variation in  nitrate  deposition, shown  in  Figure III.9 are characteristic of
sites  across most of  the  eastern  United  States  (although  some   seasonal
variations  have  been seen  in  the  southeast;  Bowersox  and  Stensland 1981).
Thus a marked seasonal dependence of the molar ratio of  sulfate wet  deposited
to  nitrate wet deposited occurs.  The ratio is roughly  0.7 in winter and 1.4
in  summer (NAS 1983).

A.2.1.5  WHAT HISTORICAL TRENDS CAN BE  SEEN IN WET  DEPOSITION DATA?  [CARP
         A-8.4.3]

Because no  continuously operating  wet  deposition monitoring  network existed
over  a  substantial  period  of  time  in  North America,  all  attempts  to
reconstruct  historical  trends in deposition  have  been  controversial.   What
consensus exists  may  be summarized by  the  following  conclusions quoted from
Section A-8.6 of the CARP:

     a.  "On  the  broad  scale,  nitrate  in  U.S.   precipitation  has likely
         increased  since  the  1950's,  in  conjunction with  NOX   emissions
         increases."
                                      74

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Figure III.8  Average molar  ratio of sulfate to nitrate in precipitation
              in  eastern  North America in 1980.  Adapted from NAS  (1983).

-------
       150
        100
        50
                             TOTAL  SULFUR
                         •    •
                                                      %  •

   o
   o
        150
        100
        50
          0
                                         NITRATE
                 *••
 •   •
 •    .
A   • ••
                                                 *
      I
                                              J_
           0     0.5     1.0     1.5    2.0    2.5    3.0     3.5    4.0

                            YEARS SINCE JULY 1, 1976


Figure III.9   Sulfate  and  nitrate  concentration  data for event
              precipitation  samples collected at Penn State University,
              PA.   Lines are least-squares of linear and periodic
              functions  (MAP3S/RAINE  1932).
                                  76

-------
     b.  "Calcium measured  in  U.S. precipitation  has  decreased,  perhaps due
         to lack of extreme  drought recently as compared  to  the  1950's, but
         more  certainly  due  to  improved  sampling  procedures."    (Other
         possible contributing reasons are a  decrease  in unpaved  roads-and a
         decrease in particulate emissions, shown in Figure III.19.)

     c.  "A combination of drought effects and  the mixing of urban data with
         more  regionally  representative  data,  the mixing  of  bulk  data and
         lower quality  wet-only  data with higher  quality  wet-only data, has
         led  to  statements  concerning  increasing  acidity  of  precipitation
         which are difficult to support.   In  general, it appears difficult to
         use  historical  U.S. network data to  discern  the  precipitation  pH
         time  trend as related to the acid precursor emissions."

     d.  "The most reliable  long-term trends  for precipitation  chemistry are
         available for the Hubbard Brook Forest site in New Hampshire  (record
         continuous since 1964).   The nitrate data record suggests an erratic
         trend of increasing nitrate  from 1964 to about  1971,  followed  by a
         leveling off or  slight  decrease  from 1971  to  1981.   Wet sulfate at
         the  site  declined  by about  33  percent from  1965-66  to  1979-80.
         Emissions  of  NOX  and   SOX  are  generally  consistent   with   these
         observations for wet sulfate and  nitrate....  From 1964-77 there was
         no  statistically  significant   trend  in  precipitation   pH  at the
         Hubbard Brook site."

Broadly distributed data for estimating  a trend in sulfate deposition are not
presently available.   The  basic problem  in estimating  trends is that neither
a single year  (1956-6: the Junge network), nor a single site (Hubbard Brook),
can be expected to provide representative data for a period of years or for a
broad  area.    A  closer  look at  other local  data  may  provide some  further
information.   Future trends,  at  least,   should  be measured by  the  National
Trends  Network  (NTN)  of the National Acid Precipitation  Assessment Program
(NAPAP) and NADP network.

Polar and glacial  studies until  now have  provided some  evidence  for transport
of anthropogenic emissions to remote northern regions and provide  some histo-
rical  evidence for the variability  of natural  background deposition  in those
remote areas,  but they do not provide clear  information  about global  trends.
[CARP A-8.5]

A.2.1.6  HOW IS DRY DEPOSITION  OF ACIDIFYING  SUBSTANCES MEASURED?   [CARP
         A-8.3.2]

The settling  and  dry deposition  of large particles (20 ^m  in  diameter  or
larger) depends principally on  gravity.   Their dry-deposition rates are quite
well  measured  by  sampling  buckets  or  other  sampling  surfaces.    However,
deposition from small  particles  and gases,  which pretty much  move  with the
rest of the atmosphere, is  very  difficult to measure  unambiguously.   Unfor-
tunately,  most dry  deposition  of  sulfate   and  nitrate  comes   from  small
particles and  gases.   Field measurements  of dry deposition of small particles
and gases  have been made  by various techniques,  such as  comparing  concen-
trations of the pollutant  in air  during updrafts  with  concentrations during


                                      77

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downdrafts,  or  by careful  measurements of  vertical  differences  in  concen-
tration.  These  techniques  are,  as yet,  not suitable for  long-term  network
use.   Current estimates  of  dry  deposition  are  made  in  a  two-step  process:
(1) concentrations of  the pollutant in  question are  measured  or  estimated;
(2) a deposition velocity that depends  on  that  pollutant and the surrounding
surface characteristics is estimated based on field studies.   The  deposition
rate is then given by the product,

            deposition rate  = concentration x deposition  velocity.

A.2.1.7  WHAT CAN BE  CONCLUDED ABOUT DRY DEPOSITION RATES FROM THE  DATA
         AVAILABLE? [CARP A-8.3]

The major  conclusion  is  that with  present data  the amount  of  acidifying
material  dry-deposited  over  a   region   can   be   only   roughly  estimated.
Uncertainties  of  a   factor  or   two   or  three   are  to   be  expected  and
uncertainties of an order of magnitude  are quite possible.

The most  important  limitations  for calculation  of dry  deposition are  the
following [CARP A-8.3.3]:

     o   No monitoring program in the United States reports air concentration
         of pollutants in a manner  such  that dry-deposition fluxes of  acidic
         and acidifying pollutants  can  be readily evaluated, although several
         networks  offering  suitable  information  have  operated for  limited
         periods.  Such networks are operating  in  Scandinavia and  in Canada.

     °   Deposition  velocities  vary by at  least  two   orders  of  magnitude
         depending on  surface characteristics  and vegetation,  time of day,
         season,   and   meteorological    conditions;   deposition   velocities
         representative of a  broad region can,  with the  present field data,
         be only approximate.

     0   Concentrations  of   the   important  pollutants    S02,   $04^,   N02,
         and  N03~  at a  particular location also  vary  substantially  (up  to
         an order of  magnitude) with time of day, season, and meteorology.  It
         is not  likely  to be true   that  (average  deposition rate)  = (average
         concentration) x (average  deposition  velocity).   Instead,  correla-
         tions between concentration and  deposition velocities must be taken
         into account.
In spite of all  the  uncertainties  and  limitations  cited above, a few further
conclusions,  based  on  information  on  atmospheric  concentrations  to  be
developed in the following subsection, are pertinent.

     °   Sulfate concentrations  in rural eastern North  America  are somewhat
         lower  than  S0£  concentrations  (Section   A.2.2).     S0£  deposition
         velocities  are  typically  2  to  10  times  greater  than  sulfate
         deposition  velocities  (Section A.2.3).  Hence,  the  bulk  of  sulfur
         dry deposition comes from the gaseous S02
                                      78

-------
         Concentrations of N0£  are  typically  five  times  those  for  N03  (includ-
         ing nitric acid),  and  HN03 deposition velocities,  though  rapid,  are
         not likely to exceed those  for  N02  by  a  compensatory amount.  Conse-
         quently, the  bulk  of  nitrate  deposition  probably  comes   from
     0   Based on typical  deposition  velocities  (0.3  to  0.7  cm  s'1)  and typical
         rural concentrations   of  SO?  (10  to  40ygm~li),  dry  deposition   of
         sulfur compounds  is  5 to  40  kg ha'1,  or close to the amount of  sulfur
         wet deposited in eastern North America.   (Note the units:   deposition
         here is measured  in  kg of sulfur).

     0   Similarly, the  amounts of   dry-deposited  nitrates  are   close  to   the
         amounts wet deposited.   A  concentration  of 10 y g m    of  N02  and  a
         deposition velocity  of 0.5 cm s'1  would  lead to an annual  deposition
         rate of 5 kg  N ha"1.   This  number  should  only  be treated as an order
         of magnitude estimate, however.

A. 2. 1.8  WHAT IS THE QUALITY  OF THE DATA FOR WET AND  DRY DEPOSITION?
         [CARP A-8.3]

Estimates of  current  annual  amounts  of wet-deposited sulfate  or nitrate over
a broad  region  in  the eastern  United  States may  be correct  to within  +_  30
percent.  Estimates of the amount expected  to be  wet deposited in the West  or
in a  small  area of  the  East  or  over a  short time period will  be substan-
tially less  certain.  Estimates  of   current   annual  amounts   of  sulfate   or
nitrate dry deposited  are uncertain  by  at least  a  factor of  two or  three.

What are the  most   important  sources  of  uncertainty?  The answer  depends   on
whether wet or dry  deposition  is at issue.

For the  case  of wet  deposition we  have observed  that  numerous problems with
sampling, sample handling, and analysis cause significant problems  in problems
in comparing measurements from  one network  with those from another.  The best
remedies for  these problems   are  formal  quality  assurance programs  for each
network and the establishment  of overlapping sites for different networks.   We
have also  noted that  data   from  bulk  sampling  are difficult  to  interpret.

A second class  of  problems in interpreting  wet-deposition data comes from  the
intrinsic variability  in  the  data.   Substantial  variation occurs  in   annual
deposition both from  one  year  to  another,  and from one  location to another,
i.e., adjacent  network  sites  whose  locations  may  be as  near  as  one  or  two
hundred miles.  To determine  the representativeness  of  yearly data  and   to
identify trends, significantly  longer  records  from  particular  networks will
be needed.  To  obtain  more  detailed  spatial  patterns and to  obtain an  annual
deposition budget  with higher confidence,  better  spatial resolution   (more
sites) will  be  needed.   These,  rather   than  the  limitation   in   measurement
techniques, appear  to  be  the  most significant  deficiencies in  wet-deposition
measurements.
                                      79

-------
In  contrast,   for  the  case  of  dry-deposition,  the  present limitation  is
primariy the lack of  data.   Routinely  collected network information on rural
concentrations of the  important  acidifying species  is  the  most  urgent need.
In addition to such a network, more measurements giving deposition velocities
for a  broader  range  of surfaces  and meteorological  conditions  would be very
useful.

A.2.2  WHAT ARE THE AMBIENT CONCENTRATIONS OF SUBSTANCES IMPORTANT IN ACID
       DEPOSITION?  [CARP A-5]

A.2.2.1  HOW ARE CONCENTRATIONS MEASURED?

S02 concentrations  have been  measured routinely  through  the use  of chemi-
cally-impregnated filters;  short-term  measurement can  be  made  using  a  wide
range  of  techniques  including ultraviolet absorption,  flame  photometry, and
laser  techniques.   It  is  important to  note  that many  of the  network  S02
measurements  have  been  near  the  limit  of  detectability  for   the  sampling
equipment used.  [CARP A-5.2.2]

Sulfate concentrations have been  measured through collection on filters, with
or without sorting by  size.   There is  some debate over the losses from vari-
ous sampling means, and  added amounts  of  sulfate  detected  are  also possible
from  the  conversion  of S02 to sulfate on  the  filter.   In the eastern United
States, a large fraction of  light  scattering  is due  to sulfate  particles, so
light  scattering  and  visibility  mesurements are  potentially  useful  as indi-
rect means of  measuring sulfate concentrations  and trends.  [CARP A-5.2.3]

Nitrogen  oxides  include   nitric   oxide  (NO),  and  nitrogen  dioxide   (N02)-
Nitric oxide  is  the  principal oxide of  nitrogen  produced in combustion  (see
Section  2.7);  however,  it  is  fairly  rapidly oxidized  to  N02  so  that in
urban  areas NO represents  30 to  50 percent of  gaseous nitrogen oxides, while
in rural areas NO is  10 to 30  percent  of the oxides. Independent measurements
of  NO and N02 are  not usually made.  Rather, NOa  is  reduced  to  NO, and NO
is  detected by chemiluminescence.   Problems,  particularly in measurements at
low  concentration,  have arisen  because  of  uncertainties  in the  amount of
N02 that  is  converted to NO,  and  because  other  nitrogen  compounds may also
be converted.  [CARP A-5.3.2]

The important  nitrates found  in  the  atmosphere  include  nitric  acid  (HN03),
ammonium  nitrate  (NH4N03),  and  peroxyacetyl  nitrates  (PAN).    Nitric  acid
is  usually  measured  by continuous  coulometry or by  infrared  spectrometry;
both  techniques are suitable  only  for  short  time  periods.  PAN is  identified
by  electron-capture   gas   chromatography,   another  short-time   technique.
Nitrate  particles  are collected on  filters,  but  serious  problems  both with
losses  and additions  have  been  identified,   so  most  reported  particulate
nitrate  measurements  are  questionable,  and  only recent  measurements using
diffusion-denuder tubes appear reliable.    [CARP A-5.3.6, A-5.3.8]

Ammonia  (NH3)  is  the gaseous  precursor  of  the  ammonium cation   (NH4+).
It  has been  measured in the  atmosphere  by various  techniques  that are  best
suited for short time  periods.  [CARP  A-5.3.5]
                                       80

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Several   oxidants  are  important  to  the  conversion  of  S02  to  S0~  (see
Section  2.5).   These  include ozone  (03),  which is  an  important air  pollu-
tant  in  its  own  right,  hydrogen peroxide  (^02), and  the  short-lived  OH
radical.    Most ozone measurements  since  1970  have used a  chemi luminescent
ozone analyzer,  a  technique  considered quite  reliable.   Hydrogen  peroxide
measurements have been made both in air and  in  rainwater, using  a variety of
techniques.   One  technique  using chemiluminescent oxidation of  luminol  is
suitable  for  continuous monitoring.    However,  questions have   been  raised
about the  validity  of  measurements  of H202  in air  because of generation
of  H202  in  aqueous  solution.    The  high   variability  of  measurements  in
rainwater  raises  questions  about  how representative  these  values are  of
conditions in clouds or at cloud level.  [CARP A-5.4, A-5.5]

A. 2. 2. 2   WHAT COLLECTIONS OF DATA HAVE BEEN  MADE?

The  National  Air Sampling  Network  was established in the  1950's,  but  S02
was  measured  only  after  the  early  1960's.    This network   is  concentrated
almost entirely  in  urban  areas  and has only  six  rural  sampling  areas.   The
Electric  Power Research Institute's (EPRI)  Sulfate Regional  Experiment (SURE)
provides   the  only  presently available network  data  on  nonurban  S02  and
sulfate   concentration  in  the eastern United States.   It operated  for  five
months between August 1977 and  October 1978 with 54 stations  in  the eastern
United States and  continued through  1979   with  9  stations.    In  addition,
various  measurements of concentrations  of the different  substances  have  been
made at one or several sites for relatively  short periods;  these comprise the
remaining  field  information  on  concentrations in the eastern  United States.
The  Air  and Precipitation Monitoring  Network  (APN) in Canada  has  six  rural
sites measuring S02 and SO^" east of Manitoba.
A. 2. 2. 3  WHAT CONCENTRATIONS OF IMPORTANT SUBSTANCES HAVE BEEN OBSERVED?
         [CARP A-5.9]

Table I II. 3  summarizes  the  ranges of concentrations observed  in  rural  areas
by  the  measurements  described  in Section  2.2.1.   The  table  distinguishes
between eastern  and western U.S. measurements  and gives annual  averages in
the case of S02 and sulfate, for which substantial amounts of data exist.

For  sulfate  and  S02 the  SURE  network  provided  sufficient information  to
indicate  spatial  patterns  of  concentration.   These  patterns  are  shown  in
Figure II 1. 10.  The patterns are, generally speaking, consistent with the wet
deposition pattern  of Figure  III. 4,  with  the  highest concentrations  in  an
oval running southwest to northeast,  centered on  a  line  from the  Ohio valley
to  northern  New  York.  The  fall  off with  distance  from  the highest concen-
trations is steeper for S02 than for sulfate.

The  six  rural  stations of  the  NASN  network (ME,  NH,  MD,   VA,  NY,  IN)  are
insufficient  to  provide  information  on  spatial  distribution,  but  some
comparisons and observations on seasonal  dependence and trends are noteworthy
[CARP A-5.9]:

     °   Sulfur dioxide concentrations have been high in urban areas but have
         decreased from the 1960's through the 1970' s.


                                      81

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        TABLE  III.3.   CONCENTRATIONS  OF SULFUR,  NITROGEN,  AND CHLORINE
         COMPOUNDS  AT  RURAL  SITES  IN   THE  UNITED STATES  IN THE 1970'ST
Compound
                                                      Range of
                                          Average concentrations,  jig m
   -3
                                        East
West
Sulfur dioxide
Sulfur aerosols (as sulfate)
Nitrogen dioxide
Nitrate aerosols
Nitric acid
Peroxyacyl nitrates
Ammonia
Hydrogen chloride
Chloride aerosols
Maritime
Inland
Ozone
10-40a
5-159
5 -2 Ob
1C
0.3-3C
0.5-3C
0.5-2C
NA

1-10C
1 lc
40-200
NA
l-3a
< 2c
NA
11°
0.1-1°
0.5-2°
1-10°

1-10C
< 1°
40-200
aAnnual average.

^Summer months:  August to December averages.

cLimited number of measurements.

NA=Not available.

"•"Modified  from CARP  A-5, Table  5-13.   Information  from  Tables  5-1,  5-2.
 5-4, 5-6, 5-9, 5-13 and Section 5.4.1.
                                      82

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                                                        1-HOUR
                                                      S02  (ppb)
                                                       24-HOUR
                                                        (yg m"
Figure III.10  Sulfur dioxide  (arithmetic  mean)  and  sulfate  (geometric
               mean)  concentrations.   Data obtained  during 5 months
               between August  1977  and July 1978.  Adapted from  Hi 1st
               et al. (1981).   Note:   10 ppb S02 is  approximately
               28 ug  m-3 S02-   If this were all  oxidized  to  $042-,  it
               would  form approximately 37 yg m-3  5042-.

                                   83

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         Sulfur dioxide concentrations in rural  areas have fallen very little
         since the 1960's.  They are  now  approximately  one-third the average
         urban levels.

     0   Sulfate concentrations in eastern  urban  areas  have  decreased except
         in the third quarter  of the  year.

     °   Sulfate  concentrations  as  annual  averages  have not   decreased  in
         rural areas, and they have increased in summer months.

     0   Sulfate  aerosols  can  contribute one-third  to  one-half  the  sulfur
         budget (sulfur dioxide plus  sulfate)  in rural areas in the summer,
         but a smaller portion in  the winter.

     °   Sulfate  aerosols  occur  predominantly  in  the  small  particle  size
         range, with most  of  the  mass concentrated in  particles between 0.1
         and 1 urn in diameter.

     °   Sulfur dioxide and sulfate  concentrations  in eastern  North American
         rural sites are a  factor of  10  to 100  greater  than  concentrations
         measured in remote areas.

A.2.2.4  WHAT IS THE SPATIAL AND TEMPORAL VARIABILITY OF THE DATA?  WHAT IS
         THE QUALITY OF THE DATA?

There  is  great  variation  over time  in  concentration  measurements.   Daily
average values  of S02 and  sulfate  concentrations can vary  over a factor of
fifty  (within  the  5  to 95 percent frequency range)  from one day  to another
[CARP A-5.2.3.3].  The variation  from one year  to another in yearly averages
appears to be 30 to 60 percent from the NASN network.  Much of this variation
appears statistical;  however,  strong  seasonal  effects can  be  observed also
Rural sulfate concentrations appear greater  in  the  summer than  in the winter
by  up  to  a factor of  3  or 4  [CARP A-5.2.3.2], while  S0£ concentrations are
greater in the winter than in  the  summer by similar factors [CARP A-5.2.2.2].
There  is  limited evidence   that  winter  rural  concentrations   of  N0£  are
greater than  summer  values  [CARP  A-5.3.2.4].   It  is  likely  that substantial
variation of average concentration occurs from one rural  site to another over
distances of  a  few hundred miles  or  less occurs.   Even more than  with wet
deposition network data, the  limitation  on  the  use of existing  concentration
data  for  S02, sulfate,  N02,  and  nitrate is the  absence of any reasonably
dense network operations over an extended time.

Certain concentration  measurements have  specific  problems.   Until recently,
nitrate measurements have been questionable.  Debate about the interpretation
of  hydrogen peroxide measurements continues.

A.2.3  WHAT IS KNOWN ABOUT DRY DEPOSITION PROCESSES?  [CARP A-7]

A.2.3.1  WHAT ARE THE  IMPORTANT MECHANISMS  IN DRY DEPOSITION?  [CARP A-7.2]

Large  particles   (of   radius  greater   than   10  n m)   settle   out  of   the
atmosphere.  Typical  settling  velocities (which  depend  on  the  density and


                                      84

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shape  of  the  particle)  are 1.0  cm s"1  for a  10  ^m spherical  particle of
density  1 g  cm"3 and  40  cm   s"1  for  a  50  urn  particle.    Because   these
settling  velocities  are  much  smaller  than  the  vertical  velocities  of
turbulent  wind  fluctuations,   the  particles  will   follow  a   complex   path;
however, on  the   average  the  net flow out of the atmosphere will be  given by
the settling velocity, and the deposition rate will  be given by

            deposition rate = concentration  x settling velocity.

In  the case  of  trace  gases  and  small   particles  for  which  the  settling
velocity  becomes very small  (0.01  cm  s~l  for  a  1 urn  particle),  two  kinds
of processes are  important in dry deposition.  One kind of  processes  involves
the effectiveness  of  the  surface together with  the  layer of air immediately
adjacent  to  the  surface  in capturing the  material.   The second  kind is the
mixing of air  that has been depleted of  material  (from  its contact with the
surface)  with  less  depleted air  farther  up;   this  mixing  replenishes the
supply of material near the surface.  In an analogy with the case of  settling
particles, we  can still  summarize these  complicated  processes  by defining a
deposition velocity such that

           deposition rate = deposition velocity x concentration.

One other dry-deposition process, deposition by fog and cloud nuclei, must be
mentioned.    This  might  be  thought  of  as  wet  deposition  because  the
processes, formation  of water droplets,  and  reaction of acidifying materials
are determined  by water droplet  properties  similar  to  rain.   However, the
motion and capture of fine water droplets is analogous to dry deposition, and
networks do  not  generally  collect and measure such  deposits.   Such  deposits
occasionally  contribute  significantly  to  total deposition,   especially in
high-elevation forests.

A.2.3.2  HOW DO DRY DEPOSITION RATES DEPEND ON SUBSTANCE, AMBIENT
         CONCENTRATION, METEOROLOGICAL CONDITIONS, AND SURFACE
         CHARACTERISTICS? [CARP A-7.2]

There  are two  ways  in  which  deposition rates  depend  on  substance   [CARP
A-7.2].  One is  the aerodynamic properties of the material;  the second is its
chemical and physical  interaction  with the  surface.   We  have  already  noted
that  large particles  fall  faster than small ones or gases.  We will discuss
the substance/surface interaction when we discuss types of  surfaces.

The  dependence  on concentration  is  generally  linear.    Deposition   rates
usually are proportional to the concentration of material in the air; that is
the justification  for defining a deposition  velocity.

The dependence on  meteorology  is  at least threefold.  Atmospheric  turbulence
increases  mixing  and  helps  replenish  material  near  the  surface.    This
turbulence  is  greatest  in  sunny  weather,  thus  making  summer months and
daytime of greater importance.   Finally, the amount of  water vapor can also
strongly affect dry deposition rates.  Moisture, such as from rain or dew, on
surfaces may  increase the deposition velocity,  especially  for  S02  which is
highly soluble, and deposition by fog can be important in its own right.


                                      85

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Surface characteristics  can  affect adjacent  air  and also  the  likelihood of
capture [CARP A-7.5].  In general, increased surface roughness increases dep-
osition.   For S02  gas,  uptake by  plants  is  largely  by stomata  during the
daytime, with about 25 percent direct collection on leaf surfaces.   At night,
uptake through the stomata will decrease substantially (as the stomata close)
while leaf  surface  deposition will be  unaltered.   The  gas N02 is  slightly
less easily taken up  by  plants  and  is less soluble in water; however, nitric
acid is very  soluble  and has a high  affinity for  moist surfaces.    For most
surfaces,   HN03  has   the  highest  deposition  velocity;     SCL  has  signifi-
cantly lower  deposition  velocity; N0£  appears  to  be similar  to  SOg  with  a
slightly smaller deposition velocity,  the effective rate of deposition may be
even smaller if  some  deposition occurs  through oxidation of  the surface and
re-emission of NO.   Small  particles,  sulfates, and  nitrates  other than HN03
are not as reactive as the gases and are taken up more slowly by vegetation.

A.2.3.3  WHAT ARE TYPICAL DEPOSITION VELOCITIES?  [CARP A-7.4]

Deposition velocities from a  large number of laboratory and field studies are
summarized  in  Table  III.4.    The  numbers  in the  table  should be  taken as
representative only.   Where ranges are  given,  they  correspond to an  unscien-
tific selection of more than  one measurement; equivalent uncertainties are to
be expected even when no range  is  given.   Noteworthy in Table  111.4 are the
following:   S0£  deposition  tends to  be  more rapid than deposition  of sul-
fate or  nitrate  particles;  deposition  velocities  vary  greatly from  day to
night, partly due to more rapid atmospheric mixing in daytime and,  equally or
more  important,  changes  in  foliage   characteristics.    For  these  reasons
substantial  seasonal   variation   in   deposition  velocities  is  likely,  with
winter  deposition much  slower  on  the average than summer.   A  great deal of
variability  occurs  from surface  to  surface.    A  great  deal  of  residual
variability occurs, so deposition  velocities for a particular type  of surface
are not well determined.  Taken together, these uncertainties probably amount
to well over a factor of two, so present day science only provides  deposition
velocities to within an order of magnitude.

A.2.4  WHAT IS KNOWN ABOUT WET DEPOSITION PROCESSES?  [CARP A-6]

A.2.4.1  WHAT ARE THE IMPORTANT MECHANISMS  IN WET DEPOSITION?  [CARP A-6-2]

In  a  very  general  sense  pollutant  material  may  participate  in  four major
events prior to its wet removal from  the  atmosphere.   These events are shown
in Figure III.11.

     1-2  The pollutant and the condensed atmospheric water (cloud, rain,
          snow) must intermix.

     2-3  The pollutant must attach to  the  condensed water elements.

     3-4  The pollutant may react physically and/or chemically within  the
          aqueous phase.
                                     86

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    TABLE 111.4.  SUMMARY OF TYPICAL FIELD AND LABORATORY MEASUREMENTS OF
                      DEPOSITION VELOCITIES (cm s~l)a
Substance
S02





N02
Small
particles





Surface
soil
grass
wheat
soybean
pines
snow
soybean
soil
grass
grain
pines
deciduous
forest (winter)
snow
Deposition
velocity
(day)
0.5-1.0
1.0-1.3



0.1-1.0
0.6

0.3-0.7
0.4-0.8




Deposition
velocity
(night)
0.3-0.5
0.3-0.7




0.06







Deposition
velocity
(24-hr ave)


0.4-0.7
1.3
0.1-0.6


small

0.2
0.7

0
0.1-0.2
aFrom CARP A-7, Tables 7-5 and 7-6.
                                     87

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  MIXING
  1
               UNREACTED POLLUTANT
               REACTED  POLLUTANT
               CONDENSED WATER
PRECIPITATION
Figure III.11 Steps in the scavenging sequence:  Pictorial representation.

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     3-5 or 4-5  The pollutant-laden water elements must be delivered to
          the Earth's surface as precipitation.

It  is  important  to  note  that  although  Figure  II 1.11  proceeds  forward  to
deposition,  with   the   exception   of   precipitation,   the   processes  are
reversible.   The water  droplet  the  pollutant has  attached  itself to  may
evaporate, for  instance.   So a  particular pollutant molecule  may  experience
numerous cycles before being deposited.

Another  noteworthy  feature  is  that  precipitation  scavenging  of  pollutant
materials  from  the atmosphere  is  intimately  linked with  the  precipitation
scavenging of water.  If we  replace the word "pollutant"  with  'water  vapor"
all  steps  (except 3-4)   provide  a  general description  of  the  precipitation
process.   In view  of  this  intimate  relationship,  it is not surprising that
pollutant  wet-removal   behavior   tends   to  mimic  that  of  precipitation.
Pollutant  scavenging  efficiencies  of  storms  are  often  similar  to  water
extraction  efficiencies,  a  relationship  useful   in  practically  estimating
scavenging rates.

A.2.4.2  HOW DOES WET DEPOSITION DEPEND  ON SUBSTANCE, AMBIENT CONCENTRATION,
         AMOUNT OF RAINFALL, AND STORM TYPE?  [CARP A-6.5]

It  is  convenient  to  characterize  wet  deposition by  a few key  parameters.
Figure III.12 provides assistance  in  doing this by illustrating the material
balances for water and pollutant entering and leaving a storm.   We  can  define
certain efficiencies of  scavenging:

 Efficiency for water removal  ep = W/Win = precipitation out/water vapor in

   Efficiency for pollutant removal  &= F/Fin = scavenged pollutant/pollutant
in.  We can further define  a scavenging  ratio

   £  = concentration of scavenged pollutant in rainwater,
                  concentration of pollutant in air

If water  extraction and  pollutant scavenging occurred  with  equal  efficiency
then the scavenging ratio,  £, would be

    £ =          density of water               « 105-io6.
           concentration of water vapor in air

Experimental measurements  often give  scavenging ratios  in  this  range,  though
wide variation  is found.   Scavenging  appears to  be more effective  within the
cloud than below it.

What accounts  for the  variability?  The  substance  scavenged  is  important.
S02  and  N0£  are  apparently less  efficiently  scavenged  than sulfate  and
nitrate.   The  concentration  of  S0£ affects the scavenging efficiency;  lower
concentration favors  scavenging,  since  aqueous  phase reactions are important
in  preventing   loss   of  S02  from  droplets,  and  the  amount  of  S02  that
reacts is limited by the availability of oxidant.
                                      89

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               CONDENSATION,
         PRECIPITATION FORMATION,
            POLLUTANT ATTACHMENT
                        FLOW RATE OF WATER  VAPOR OUT  « w
                                                       out
                        FLOW RATE OF POLLUTANT OUT
                                                                          out
FLOW RATE OF WATER VAPOR IN
FLOW RATE OF POLLUTANT IN - f.
                            in
'OR IN - w1n \\\W\ \MV\V\0\\\v
•iN-f.nin     \V^\\\^
                                              FLOW RATE OF PRECIPITATION OUT = W
                                         FLOW RATE OF SCAVENGED POLLUTANT OUT = F
            DEFINITIONS OF EFFICIENCIES:
                     WATER REMOVAL
                        £P * W/win
                             POLLUTANT REMOVAL
                                    F/f,
                                      in
      Figure III.12  Schematic of a typical  macroscopic material balance.
                                     90

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The  nature  of the storm  affects  the scavenging  efficiency  considerably.  A
decrease in  the  rate  of  precipitation  over  an order  of  magnitude increases
the  scavenging ratio  an order of magnitude  or less depending on storm type.
This suggests that often  a  storm  may extract  almost  all  of  the accompanying
pollutant.   Convective  storms  tend  to be  efficient scavengers;  they  also
inject some  material  into the free troposphere,  where  it loses contact  with
normal  deposition mechanisms.    Warm  front  storms  can   be  expected  to be
effective scavengers  of  pollution  originating  from within the warm air mass.
Scavenging  of  pollutants from the  underlying  cold air mass  will  take place
below cloud level where scavenging is usually  less effective.

A.2.4.3  WHAT FRACTION OF THE AMBIENT POLLUTION IS WET DEPOSITED? [CARP
         A-6.3]

One  other  parameter  characterizes  precipitation  scavenging—the  scavenging
coefficient which tells  how rapidly  the pollutant is  depleted.  A scavenging
coefficient  of  (1 hr~l)  means  that  in an hour  the  concentration  will  have
fallen  to  1/e  or roughly  37  percent  of  its   initial  value.   Scavenging
coefficients  typically  range  between  (1  hr~l)  and   (0.1 hr~l).    Hence,  a
storm of long duration can remove most  of  the  pollution within it.

The other important determinant of whether a pollutant will  be wet deposited
is  how  likely it is  to encounter  a  storm  system.    In the  Northeast  for
instance, precipitation  is  occurring  roughly 10  percent  of  the  time  both in
winter  and  in  summer,   but  the  characteristics  of  the  storms  are   very
different.   In  winter,  fewer storms  occur  but  they  are of significantly
longer duration.    Hence,  the  pollutant  is more  likely to spend several  days
without encountering precipitation in winter than in summer.

A.2.4.4  WHAT IS  THE SPATIAL AND TEMPORAL  VARIABILITY OF  WET DEPOSITION
         RATES?  [CARP A-6.3]

Unlike dry  deposition  which,  at a single  location,  is a  continuous process,
most of the  material  wet  deposited  is deposited  in comparatively few events.
Statistically  this means  that  fluctuations  are  likely  to  be  significant
compared  to average  amounts.   This  is  particularly  true   because  of  the
dynamics of  storm systems.   Within  an individual  storm in  the amount of
material  scavenged and  deposited   varies  enormously,   so  places  a  short
distance apart can experience large  differences  in the amount deposited; it
should  be   noted  that  the  characteristic  spatial   scale over  which   this
variability  exists  is  much  smaller  than  the  separation of  wet  deposition
network stations.

Patterns of  storm systems  introduce  their  own  spatial  and  temporal  varia-
bility  [CARP  A-6.3.5].   Figure   III.13 shows average  storm  tracks  for  low
pressure (cyclonic) centers across the  United States.   These  are  long-term
composite averages: there is marked  seasonal  variability; the  pattern  from
one year to  another may be substantially  different;  there is  good  evidence
that  long-term   trends  shift  these   patterns;  finally,  the   flow  processes
within a storm mean that the  paths feeding pollutants  into the storm  are  not
the same as  the  trajectory  of the cyclonic  center.   Despite  these complexi-
ties,  from   Figure  II 1.13  we  can  conclude  that  important  precipitation


                                      91

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Figure 111.13
Major climatological storm tracks for the North American
continent.  Adapted from Haurwitz and Austin (1944).  Dashed
lines denote tropical cyclone centers, and solid lines denote
those of extratropical cyclones.
                                     92

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events  have  spatial  structure and  flow patterns  that will not  be  properly
accounted for if  only  average wind  flow  patterns  without  regard  to  precip-
itation are used  in modeling  or  other  data  interpretation.   The fluctuations
also imply that data over substantial  periods  (years)  are  needed  to discover
representative wet deposition loadings.

A.2.4.5  WHAT GENERALIZATIONS ARE POSSIBLE FOR AMOUNTS WET DEPOSITED BY
         SEASON OR REGION?  [CARP A-6.2, A-6.3]

The close  connection  of the  precipitation  scavenging of pollutant  with the
precipitation scavenging of water means  that,  as a very rough approximation,
amounts wet deposited  in  a  region will  be  proportional to average  rainfall
and to the average concentration of pollutant present.  This rule of thumb is
consistent with the wet-deposition network  results and the  very limited data
on  concentrations.  Thus,  the eastern United  States,  with  both high concen-
tration and  high  rainfall,  receives much more acidifying  material  than the
western  United  States.   Figure 111.14,  long-term monthly  average  rainfall
statistics by weather  station, shows  the substantial variation of rainfall by
season and region.

A.2.5  WHAT IS KNOWN ABOUT CHEMICAL CHANGES  OF ACIDIFYING SUBSTANCES IN THE
       ATMOSPHERE? [CARP A-4]

The chemical  changes  of concern are  (1)  those in which S02 is  oxidized to
sulfate,  including  sulfuric  acid:     we   have  already  observed  that  the
chemical/physical   form  of   S02  gas  compared  with  SQ$  aerosol  strongly
affects both wet  and dry  deposition  rates;  (2) those in which NO is oxidized
to  N02  and  N02   is oxidized  to  nitrate,   especially  nitric  acid,  and  (3)
those in which sulfuric acid or nitric acid  is made into a neutral salt.  The
first two of  these  chemical  changes  can take  place either  while  the  S02 or
N02 is  a gas  (gas-phase  reaction)   or  after  scavenging into  water droplets
(aqueous reaction).  The  third takes place  only  in water.    These possibil-
ities and the important reacting substances  are illustrated in Figure 111.15.

A.2.5.1  WHAT ARE THE  IMPORTANT PROCESSES LEADING TO S02 OXIDATION? [CARP
         A-4.2, 4.3.5]

Both gas-phase and aqueous-phase reactions are important to the production of
sulfate.  In addition  reactions  on the surfaces  of airborne particulates may
contribute significantly in  special  circumstances.   In much of the West where
water droplets are less frequent, it is likely that gas-phase transformations
are most important; in  much  of  the  East  it is  likely that  aqueous-phase
reactions dominate, though  gas-phase transformations may be  as important or
more important in the  summer.

The most  important  initiating reaction  for  gas-phase S02  oxidation  is with
the HO radical:

                               HO + S02 = HOS02.

The  HOS02  then  is  coverted  to  sulfuric  acid,  H^SO^,   via  an  as  yet
uncertain chain of reactions.  An important  characteristic  of  the HO radical


                                     93

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                                      NORMAL MONTHLY TOTAL PRECIPITATION (Inches)
Figure III.14  CTimatological Summary of U.S.  Precipitation.  From  U.S.  C1imatological Atlas (1968).

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             GAS PHASE
GASEOUS OXIDE
     S02
NO; N02;
                                            OXIDATION
                                          HO, H02,
                  AQUEOUS OXIDE
   AEROSOL,         (WEAK ACID)
CLOUD DROPLET,                    .
    RAIN        S02(aq.)= H+,  HS03
                 HN02 = H+ + N02"
                    OXIDATION
                                                   GASEOUS ACID
                                                       H2S04
                                                       HN03
                                                            AQUEOUS    NEUTRALIZATION
                                                           STRONG ACID
                                                           2H+, S042'   NH3;MO; MC03
AQUEOUS or
 DRY  SALT
       ;  MSO,
                                            02 +  Fe, Fin...   H+t  N03
Figure III.15  Schematic representation  of pathways  for atmospheric formation of sulfate and nitrate.
               Adapted from Schwartz  (1982).

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is  that its  concentration appears  to  be  principally  determined  by  other
photochemical  reactions,   including  those  involving  nitrogen  oxides.    Its
concentrations do  not  appear  sensitive to  pollutant, and  reactions  with S02
at normal atmospheric concentrations are not likely to deplete the HO.

The  initiating  reactions   for   S02  oxidation  in  water   depend   on  the
concentrations  of   the  substances  present.    Most  important   in   the  more
polluted air of the eastern United States is reaction with hydrogen peroxide,
H202.    The  conversion  rate,   determined  by  both  hydrogen   ion  and  gas
solubility,  is  independent  of   pH,  unlike  reactions with  dissolved  ozone
(03) or dissolved  oxygen  (02)  in the  presence  of  metallic catalysts.   For
"polluted" clouds  with  low pH,   H202  reactions will  dominate.    However,  at
higher pHs, reaction with  ozone  or catalyzed reaction with oxygen can be more
important.  The relative  importance  of the various reactions depends  on the
concentration  of   H202  or  03 or metals  as  well  as  pH;  as   we   noted  in
Section  A.2.2,  little  is known  about  H202  concentration  in   clouds.    At
plausibly  estimated  concentrations   for  urban  polluted  air,   the   H20?
reaction will produce acid quite  quickly, and  since  the  reactions are not pH
limited  (above pH  1.8), only  acid neutralization prevents pHs  from  dropping
to values lower than are typically seen.

A.2.5.2  WHAT ARE  TYPICAL  RATES  OF OXIDATION:   HOW DO THEY DEPEND ON  TIME OF
         DAY, SEASONS,  S02 CONCENTRATION, CONCENTRATION  OF OXIDANTS,
         METEOROLOGICAL CONDITIONS?  [CARP A-4.4.4,  4.4.5]
uu ou pert-eni,  nr *.   average daytime conversion rates seem
to  5 percent  hr~l,  while   there  is  evidence suggesting
nighttime conversion  rates  are  less than  1  percent  hr~l.
S02  oxidation  rates measured  in  urban and  power  plant plumes  range  from 0
to 30  percent  hr"^.   Average  daytime  conversion rates  seem to lie between 3
                                                             that winter  and
                                                               Most  of these
studies,  particularly  the power  plant plume studies,  are  probably weighted
toward gas-phase oxidation.  Observed  liquid-phase  oxidation rates range from
0 to 100  percent per hour  and  depend  on  the extent of pollutant contact with
water  vapor  as well as  as  on chemical  reaction  rates.    The  somewhat high
oxidation  rates  observed  in urban  plumes  suggest  that, as expected,  higher
concentrations of  oxidants or possibly  catalysts  in the urban  polluted  air
produce increased  oxidation  rates.   The wide  scatter in the  data,  however,
with  the   limited  measurements  of  oxidant,  has  not  given clear  results.
Atmospheric  mixing  rates  and the  amounts  of water  vapor  are  certainly
important as well,  but we  lack quantitative verification of  their role.

A.2.5.3   WHAT  IS KNOWN ABOUT  OXIDATION  OF  NITROGEN  COMPOUNDS?  [CARP  A-4.2,
         4.3.4]

The  atmospheric  chemical   cycles  for  nitrogen  oxides, which   involve  more
species,   are  both  more  complicated  and  less  well   studied  than  those  for
sulfur oxides.   Probably the  reaction  of N02 with the  hydroxyl  radical,  HO,
is most  important,  and,   in  contrast  with  S02,  aqueous-phase  reactions  are
essentially unimportant.    N02  to  nitrate  conversion  rates   are  3 to  10 times
the  rates  for  gas-phase  S02 oxidation;  they also  peak  at  midday  and  in  the
summer.    A  significant   fraction  of N02  is  converted   to  N90c  and   to
peroxyacetyl  nitrate (PAN).  Little  is known about  the ultimate fate of these
compounds.


                                     96

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A. 2. 5. 4  WHAT IS KNOWN ABOUT NEUTRALIZATION OF ACIDIFYING MATERIALS? [CARP
         A-4.3.6]

Neutralization  can  take  place  before  or  after  deposition.    Furthermore,
neutralization need not be  permanent.   For instance,  as discussed in Section
II. A. 2. 1.3, the  biological  oxidation  of ammonium  ions  generates  acidity,  so
deposition of neutral  salts such as ammonium sulfate and ammonium nitrate may
lead to acidification in ecosystems.

Probably the  most  important neutralization process in  the  atmosphere  is the
absorption  (or   hydration)  of  ammonia   (NH3)  by  acid  aerosols  and  hydro-
meteors (cloud drops and  rain).   The  preeminence  of  this  process  is because
NH3  is  the  only  basic   gas  of wide-spread   common  occurrence  in  the
atmosphere.

The hydration and dissociation of NH3,

                                        NH4OH -NH4+ + OH~,
occurs very rapidly, much  more  rapidly than any of  the  oxidizing reactions.
For that  reason  it is  possible  to calculate  the  neutralizing  capability of
ammonia as a simple function of its concentration.

The widespread occurrence of calcium cations  in  water  has  led to suggestions
that calcium carbonate and dolomite from soils and perhaps even calcium oxide
from  fly  ash  play  a  role  in neutralizing  sulfuric  acid.   The  prototypic
reaction is
                   CaC03 + H2S04  -> CaSOq + H20 + C02

and since hydrogen ion  is  lost  from  the system,  the substitution is neutral-
izing.   The interesting  point  about  these  minerals is  that  they  have  low
solubility  in  neutral  water,  while  the  solubility increases  with acidity.
Thus,  as they dissolve they act as buffers.  The amount of material available
for such buffering is highly variable.

A. 2. 6  WHAT IS KNOWN ABOUT ATMOSPHERIC TRANSPORT?  [CARP A-3]

A. 2. 6.1  WHAT ARE THE IMPORTANT MECHANISMS IN TRANSPORT?  [CARP A-3. 2, A-3. 3]

Four  meteorological  variables  are particularly  significant  in  the transport
and dispersion  of air  pollution:   the  mixing  height  below  which  air  and
pollutants  mix  freely,  and the wind,  temperature,  and  moisture  within  this
layer.   The Earth's  atmosphere  is about 100 km deep.   Most  anthropogenic
pollutants  are   confined  and  transported within  the  daily maximum  mixing
height  of  the  atmosphere,  typically  2 km  in summer and  somewhat  lower  in
winter.  The  layer  below  the mixing  height  is  called  the planetary boundary
layer.   The wind within  this  layer  is driven  by  the  flow  of  air  above  it
combined  with  the  influence of  the  surface below. The  result  is  complex
patterns of flow which  depend on  time,  on location, and on  the  height above
the surface.    The   dispersion  of pollutants results  from  the  spatial  and
temporal inhomogeneities of  the  winds.   The  dispersive  capacity  is strongly


                                      97

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influenced  by  temperature  distribution,  particularly  the  variation  of  air
temperature  with  height,  which  in  turn  depends  on  the  amount  of   solar
radiation and  the  nature of ground  surface.   Upward vertical motions  in  the
planetary boundary  layer enable  moisture, transported from upwind as well  as
local evaporation  of  surface water, to condense  to form clouds and  precipi-
tation.

Meteorological behavior  has  different characteristics depending  on  the  spa-
tial ^scale  over  which it is evaluated.   Typically the scales are classified
as micro, meso,  synoptic,  and  global.  The meteorological  microscale  is  set
by a  typical  maximum  mixing height.   This  distance is 1  to 2 km; a typical
time  associated  with  such  distances  is  10  minutes  (the  approximate time  it
takes  for  a  plume to   spread  over that vertical  distance).  The  important
microscale  phenomena  include  convection—vertical  air  motion  driven   by
differences  in  temperature  and,  linked  to  convection,  turbulence—random
fluctuations  of  the  wind  speed and  direction.    These  are  responsible  for
mixing.

The meteorological   mesoscale extends  out  to about 500 km, and the associated
time  is about a day,  the approximate time needed  for mean transport over  that
distance.   Mesoscale  transport is affected by  the  daily  variation in  mixing
height  and  by  the  vertical  variation of  the wind  below  it.    It  is  also
strongly  influenced  by  surface  terrain,  by   heat,  and  by  moisture fluxes.
Within the range of the  mesoscale a plume from a  power plant  or an urban  area
will  lose its  identity  by  mixing with other  plumes and  by diluting into  the
background.   Transport  over microscale and mesoscale distances  is  commonly
referred to as short  and intermediate range transport, respectively.

Beyond  the  mesoscale is  the   synoptic   scale,  the scale  of weather  maps,
characterized  by  horizontal  dimensions  of  1000  to 2000  km;  the associated
transport times  are  1 to 5  days.   Characteristic  of  the  synoptic scale  are
major weather  patterns.  Beyond the synoptic  scale is the  global  scale  (or
better,  hemispheric scale)  that  includes  trans-hemispheric  as well as  inter-
continential transport.

The  height  of the  mixing  layer  changes  continuously;  it grows  during  the
daytime,  typically  to heights  of 1  to 2  km,  due  to  thermal  convection,  and
subsides at night to  heights ranging from zero to a few hundred meters.   When
the mixing height subsides  at night, the  pollutant  that was mixed  through  the
daytime  mixing layer  does  not  subside with  it;  it is transported by  a  wind
field that has lost contact with the ground and is  characterized by much  less
turbulent mixing.   We can  thus define a  transport layer for  any  day as  the
layer between  the  surface  and  the peak mixing  height  of  that day.  When  the
mixing height is low,  emissions from  tall stacks  will  produce a  plume  in  the
transport layer above  the mixing height (the effective height of release  of a
power plant plume is  significantly above  the  top  of the  stack because  of  the
buoyancy of  the plume).   The plume can then spend a substantial fraction  of a
day  decoupled  from the  ground.   This  situation  is  illustrated in   Figure
III.16 [CARP A-3.4],  On this occasion there was considerable cloud formation
(and presumably aqueous-phase chemical reactions).   Note  at the  far right  of
this  figure  the  subsiding    of the mixing  layer,  with S02  distributed  more
or less  uniformly through the transport layer.


                                      98

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                 1500
^

Q
z:
=3
O
ct
O
              o
              ca
                 1000 -
                        CUMBERLAND PLUME
                         AUGUST 23,1978
                       (BNL and EMI DATA)
                            0800
                                       1000
                                     1200

                                     TIME OF DAY
1400
1600
1800
                                                t—i    _<   DOWNWIND DISTANCE
                                               80 km  110 km    AT SAMPLING
                                                                160 km
Figure  III.16
  The physical  behavior of  a  tall-stack plume  on a rather  typical  summer day.   The plume
  shown  is  the  reconstruction  of the Lagrangian  transport  of  the 0700 release  on 23 August
  1978 from the 305 m tall  stacks of the 2600  MWe Cumberland  Stream Plant  in northwestern
  Tennessee.  The reconstruction is based on aircraft sampling,  ground-based lidar returns.
  and tetroon transport data  (Gillani and Wilson 1983).

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Much of the complexity of atmospheric  transport arises  because  the wind  speed
and  direction  vary with  height through  the  transport  layer;  the  variation
depends on mixing height, since the air that  is above  the mixing  height  is  no
longer  subject  to  the  same   frictional  drag  caused  by  the  surface.     An
illustration  of  this,  Figure III.17,  shows  the  average variation  of  wind
direction  with  height for  day and night and winter and summer  at  a single
location.  Wind  speeds also show a comparable variation  in magnitude.  In the
summer situation, after the sharp  shear of  wind  direction in the first  50  m,
which affects only a small  fraction of the  pollutant transported,  the  daytime
wind direction is relatively  constant.  Nighttime winds, however,  continue  to
vary in direction with  height; this  means that material at  different  heights
can be widely separated after  a night's transport.   This separation, with the
mixing  that  takes  place  during  the next day,  accounts   for most  of  the
dispersal of pollutants for mesoscale and farther distances.

Other meteorological  complexities  can also be  important in  transport.    Two
examples follow:

Special  types of weather patterns.   Certain weather  conditions  are especially
important  to  deposition  patterns.    We  noted  in Section  A.2.4  that  storm
systems  have  characteristic   wind  patterns  (see  Figure  III.13).     It  is
probably  appropriate   to  separate  wind  field   data  for  precipitating and
non-precipitating  conditions, because  the use  of  average  wind  data  based
mostly on  non-precipitating conditions could give an  unrealistic picture  of
transport  leading  to  wet deposition.   Another special  set  of conditions  is
stagnation in which high concentrations of  acidifying  substances  can build  up
over a  broad region.    Figure 111.18  shows the  distribution of frequency  of
such conditions.   They too might merit special treatment.  A third example  is
highly convective air  motion, as takes place  in  thunderstorms.  Such motion
may  vent  significant amounts  of acidifying  substances above  the  planetary
boundary layer, where they  can spend a long time decoupled from the  ground.

Shorelines  and  complex   terrains.    The  special  temperature and  moisture
patternsproducedatashoreline   can   strongly   affect   wind   motion and
precipitation  patterns,  both  on a local   scale  and  for  large-scale weather
systems.

Urban areas  can  provide  local elevation  of the  mixing height  at  night   as  a
result of  the  heat  island effect.   Hills  and mountains can  also alter  local
and mesoscale flow.

A.2.6.2  WHAT METEOROLOGICAL  INFORMATION  IS NEEDED TO  CHARACTERIZE TRANSPORT
         OVER VARIOUS SPATIAL/TEMPORAL SCALES?  [CARP A-3.2, A-3.3]

Three  items   of  meteorological   information   are   crucial   to   transport
calculations.  One  is  the  height of  the mixing layer  and its  variation  over
time.  The second is wind velocities as a function of  height up to the top  of
the transport  layer  (2 km).   The  third  is the  synoptically derived weather
conditions—precipitation and  cloud formation.
                                     100

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           ST. LOUIS 1976
    1000
 E



 O
 O
 o:
 o
 CO
 g  1500
 LU
 rr:
                       SHEAR IN  WIND DIRECTION (deg)

                Relative to Wind  Direction  at Ground  Level.
Figure II1.17  Monthly-average absolute change in wind direction with
               height relative to wind direction at ground level.  Data
               are for July 1976 near St.  Louis, MO.
                                  101

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Figure III.18  Climatology of air stagnation advisories issued over a ten-
               year period.  Adapted from Lyons (1975).
                                   102

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A.2.6.3  TO WHAT EXTENT IS THE NEEDED METEOROLOGICAL INFORMATION ROUTINELY
         COLLECTED?  WHAT DOES IT SHOW?  [CARP A-3.5]

Weather conditions,  such  as precipitation, cloud  cover,  surface  winds,  are
measured at more  than  two hundred stations.   Although  this does not provide
resolution the  size  of many  smaller  storm systems,  the  density  is  greater
than  that  for  any  of  the  acid deposition  networks.    Mixing   height  is
routinely  inferred  at only  a  small  number  of  stations.   Fortunately  the
heights appear  to  be relatively uniform over  broad  regions.   Upper air wind
velocity measurements  are also  only  made  at about 50  locations over  the
United  States,  giving  a  spatial  resolution of  about  100,000  km2.    The
measurements are made only twice a day, noon and  midnight EST.

The first conclusion to be drawn from the limited amount of upper air data is
that  neither  the  spatial  nor  temporal  resolution  of the  upper air measure-
ments  is   good  enough  to  support  calculations  of  actual  (as  opposed  to
"representative")  patterns of flow.   However,  some  further conclusions can be
drawn from upper air measurements.  The  seasonal  flow tends to  be  from west
to  east,  with also a significant flow  north  from  the Gulf of  Mexico to the
Great Lakes.  Wind speeds in summer  are lower than  in winter.   In particular,
the  Southeast  has  quite  low mean  velocities  in   summer;  this means  that
average transport velocities will be lower in the summer,  and the Southest is
particularly  susceptible  to  stagnation episodes.    The  Midwest  has strong
nighttime wind shears,  which are likely to lead to  enhanced dispersion.

A.2.7  WHAT ARE THE SOURCES OF SUBSTANCES IMPORTANT TO ACIDIC DEPOSITION?
       [CARP A-2]

A.2.7.1  WHAT ARE THE NATURAL SOURCES OF THESE SUBSTANCES? [CARP A-2.2]

Sulfur compounds.    Sulfur is a  common  trace  element in  soil  and water, and,
as  we have seen,  is  found  in  the  atmosphere even in  remote  areas.   The
natural sources of  sulfur  compounds  emitted include both  biological activity
and  the  geophysical  processes of volcanism and  sea  spray.   For  land areas
probably the  most  important natural  source is biological  activity  in soils.
The emission rate  from soil sources  increases with  ambient temperature and is
highest for coastal  wetlands.   The most common sulfur compounds produced are
hydrogen  sulfide   (H2S),  carbonyl   sulfide,  (COS),  and  carbon  disulfide
(CS£).    These  compounds  are  converted  in   the  atmosphere  to S02  and/or
sulfate.   Volcanic activity produces  both  S02 and  H2S and emits  a  more  or
less  continuous component,  with occasional major  releases during eruptions.
Sea spray produces mostly neutral sulfate aerosols.  [CARP A-2.2.1]

Nitrogen  Oxides.    Most  natural nitrogen  oxide  production  occurs   in  the
terrestrial biosphere;  however,  lightning  and  oceans  are also significant
sources.    Biological  production  comes mainly  from  soils and appears  to
decrease with decreasing ambient temperature.  [CARP A-2.2.2]

Ammonia.   The identification of  a  biogenic source  for ammonia and ammonium
compounds  exclusive  of agricultural  activities  is  more or  less circumstan-
tial.    Dawson  (1977)  summarizes  the  evidence  from  which   biogenic  land
                                    103

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emission of ammonia  can  be inferred.   First,  ammonia  is found in relatively
high concentration in rainwater and,  since  there  are no apparent atmospheric
sources, a  surface  source must be  inferred.    Second,  concentrations of NH3
in the air are  directly  related to  pH of the  underlying soil; they  increase
with soil temperature and  are  higher over  land than over water.  Furthermore
the concentrations decrease with altitude,  and during an inversion a  build up
of concentration can be observed.   [CARP A-2.2.2.7]

Calcium  and  Magnesium Compounds.    The  principal  natural  source  for  these
substances is dust from soils.[CARP A-2.2.4]

Oxidants.  Most oxidant production—ozone,  hydrogen  peroxide,  the HO  radical,
anolothers—occurs  as  the  result   of  complicated  chains  of photochemical
reactions  in   the  atmosphere.    The rates  of  production  may   be   strongly
influenced by  sunlight and  natural  and anthropogenic  emissions  of  nitrogen
oxides  and volatile  organic  hydrocarbons,  though,  as  noted   earlier,  HO
concentrations appear largely independent of pollutant  concentration.

A.2.7.2  WHAT AMOUNTS ARE EMITTED  BY NATURAL SOURCES?  HOW ARE THEY
         DISTRIBUTED OVER SPACE AND  TIME? [CARP A-2.2]

An attempt to measure the  average biogenic  emission of sulfur in the eastern
United States  was  part of  the  EPRI-SURE experiment.   The results,  based on
soil   type  are  summarized  in  Table   III.5  (Adams  et al.  1981).   The annual
average, weighted by  land area,  is  about  0.03 g  S nr2.   Multiplied by the
land  area  east of  the  Mississippi,  2.2  x  1012  m2,  this yields  an annual
emission  rate  for  the  eastern United  States  of 0.07  x  1012  g S  yr-1 or
0.07  Tg.   (1012 g  equals  one  teragram,  abbreviated Tg,  or  one  million
metric  tons.    0.07  Tg  is  approximately  the  annual  sulfur  emission of one
large coal-fired power plant.)  The  same emission rate for the 48 contiguous
states  yields  a total  emission rate of 0.23  Tg  yr-1.  This is probably an
overestimate since  arid  lands are  likely  to  have  lower  emission  rates and
since there is a lower percentage of coastal wetland area.

Volcanic emissions are not  considered large  for the United States except for
an occasional  major  eruption.   The  estimated  sulfur emissions  from  Mt. St.
Helens from March 1980 to  March 1981,  which included the  two  major eruptions
in May and June, were  about 0.17  Tg S,  about  twice  the annual emission of a
major  coal-fired  power  plant.  The  Pacific Ocean and  Gulf  of Mexico may be
significant sources.   Crude  estimates  give  0.36  Tg yr~l  and 0.24  Tg yr
of sulfur respectively for these two sources.  The Atlantic Ocean is  expected
to contribute much less since the prevailing winds are  offshore.

No comparable  set of field measurements of  biogenic nitrogen oxide  emission
has  been developed,  so  the  estimates  have been  based  on  material balances,
either  globally or  locally,  using  the  gradient  of  concentration.   These
methods  are  quite uncertain;  aside  from  the  statistical   problem  that  they
depend on taking differences of large quantities,  they are also  sensitive to
the  remote  background concentration  of N02,  which is  not  well  determined.
The  estimates   range  from  0.04  to  1.5 Tg  N  yr'1 for  the  eastern  United
States  and  from 0.15  to 5.3  Tg  N   yr~l for  the contiguous  48 states [CARP
                                     104

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        TABLE 111.5.   SUMMARY OF ANNUAL SULFUR FLUX BY SOIL GROUPINGS
           WITHIN THE STUDY AREA (ADAPTED FROM ADAMS ET AL. 1981 )a
Soil grouping
Coastal wetlands
Inland high organic
Inland mineral
Total
Sulfur fluxb
Tg S yr~l
0.05
0.01
0.06
0.1
Land area
m2
2.56 x 1011
6.85 x IQll
27.26 x lOH
36.7 x lOH
Emission density
g S m~2 yr~!
0.2
0.02
0.02
0.03
aAdapted with rounding.
bEquals 1012 g s yr~l.
                                      105

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 A-2,  Table  2.3].   Values toward the lower end of  the  range are  based  on more
 recent measurements of N0£ levels and are preferred.

 Natural  biogenic  emissions of  ammonia  are similarly uncertain; the estimates
 give  ranges of 0.3 to  1.2  Tg N yr~l  for the eastern  United States and 1.1
 to 4.3 Tg N yr"1  for the 48 contiguous states.

 The estimated  calcium and magnesium emission rates are 3  Tg  yr~l and 0.5 Tg
 yr~l,  respectively,  for the eastern  United  States.   Estimates  were made by
 assuming that  calcium and magnesium appear  in about the same proportion that
 they  do in  the  Earth's  crust (3.6  percent and  2.1  percent respectively).
 Dust  emissions are estimated from the concentrations of "coarse particles" (2
 to 10 nm in  diameter).  Clearly  there  is  considerable uncertainty in these
 estimates,  the  more so  because the  dust concentration measurements are based
 on only  12 stations.  No representative data on dust emission for  the western
 United States are available.

 All estimates  of natural material  emissions  are  too  crude  to  permit much
 estimation  of  temporal  variation;  biogenic  emissions  will  be  strongly
 weighted toward the summer.

 A.2.7.3  WHAT ARE THE ANTHROPOGENIC SOURCES?  [CARP A-2.3]

 Sulfur Compounds.   Emission of  sulfur compounds  by man-made  sources comes
 largely from burning fossil  fuels to produce heat for industrial  processes or
 for space heating or  to generate electricity.   In addition  the  smelting  of
 sulfur-containing  ores   produces   significant additional  sulfur  emissions.
 Most emissions are in the form of  S02»  though some  primary emission  of sul-
 fate  occurs.   Table III.6 lists  estimated  percentages that  primary sulfate
emissions are of total  sulfur emissions for  various categories of sources.

 Nitrogen Oxides.  Unlike sulfur oxides,  which are produced from the burning
 or sulfur in  fuel,  nitrogen  oxides are produced from  the  combination of the
 nitrogen and oxygen in  the  air at  the  high temperatures  during combustion.
The type of fuel  being  burned  does  not  matter,  but the nature of the combus-
 tion  process as it affects temperatures  matters considerably to  the  rate  of
 nitrogen  oxide  formation.     In  particular,  internal  combustion  engines,
 because  of  high  temperatures  associated with  the  explosion in  the  engine
 cylinder, are high emitters  of nitrogen oxides when compared  to an industrial
 boiler on a per amount  of fuel  burned basis.  Most  nitrogen  oxide emissions
 are in the form of NO,  which, as previously  noted,  is rapidly  oxidized in the
atmosphere to N02-

Ammonia.   Anthropogenic sources of  ammonia are  principally livestock wastes,
 fossil fuel  combustion,  and  agricultural  fertilizer use.

Calcium and  Magnesium  Compounds.   The principal  anthropogenic  sources of cal-
cium  and magnesium  compounds in the  air  are fly  ash  from coal  burning  and
dust  from dirt roads.
                                     106

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 TABLE III.6.  SULFATE EMISSIONS FACTORS FOR SOURCE CATEGORIES
          AND FUELS (after SHANNON ET AL. 1980)a
   Source category                Sulfate emissions factor
                                              (Z)
Coal point sources                           1.5

Residual oil—utility and                    7.0
  industrial

Residual oil—commercial and                13.4
  residential

Distillate oil                               3.0

Mobile sources                               3.0

Smelters                                     1-2

Miscellaneous                                5.0
aSulfate emissions factor is the percentage of total sulfur
 emissions released directly as sulfur in $04.

Estimated similar to coal point sources.
                             107

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                                        801


                                   TOTAL  EMISSIONS  (  1012gyr"!  )
                                                            CO
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UD O
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CO CO
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3> DJ
Q. -S
Cu ,-f
"a —'•
r+ O
ro c
Q- —•
   CU
-h n-
-s ro
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3 3
   Cu
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co ro
   -s
  co
  O
  IN3
00
  CU
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  o
  -5
  ro
  rt>
  a.

  CO
  H-
  Cu
  -5
  O
                  ID
                  4^
                  o
                 CD
                 en
                 o
                 to
                 CTl
                 O
                  10
                  ~J
                  o
                 CO
                 o
                 CD
                 00
                 CO

-------
A.2.7.4  WHAT ARE THE ANTHROPOGENIC EMISSION RATES?  HOW ARE THEY DISTRIBUTED
         IN SPACE AND TIME?  [CARP A-2.3, UPDATED]

Annual  emissions  for  the United  States  from  1940  through  1983  for  S02,
nitrogen oxides, and  total  suspended particulates (TSP) are  shown  in Figure
III. 19.  The trends are interesting:   TSP  remained relatively constant until
the seventies  and  then declined  as a  result  of  emission  controls.   Sulfur
dioxide  emission   increased  about  50  percent  from  1940   until  the  early
seventies  but  has  declined  about  25  percent  since  then.    Nitrogen  oxide
emission  has   increased  nearly  a  factor  of  three  as a   result  of  major
increases  in motor  vehicle  use,  as well as  increased electricity generation
and other combustion uses  as  the economy has expanded.

We now look, using  another source with greater detail but lacking TSP, at the
types   of   sources   and   their  spatial   distributions   for   this   period
(Gschwandtner et al. 1985).  The  study  area  considered is  that considered in
the CAD, based  on  Gschwandtner et al.  1981,  but  the improved emissions data
of Gschwandtner et al. 1985 are used here  to  respond to peer reviewers' sug-
gestions.  Analyses of the 48  contiguous states  based on this reference will
be made in the 1985 Assessment.  The area  covered  is the eastern half of the
United States from  Minnesota  eastward, plus Texas, as shown in Figure III.20.

Sulfur  Dioxide.   Historical  emissions of S0£ for the  study  region  by type
of source are  shown in Figure  III.21.   It is noteworthy that by 1980,  about
2/3  of  S02  emissions  came  from  the  generation  of  electricity,   and  the
growth of S02 emissions from  1950 to 1980 can almost entirely be attributed
to electric  utility  growth.    The  spatial  patterns  of  S02  emission  are also
interesting;  Figure II1.22 shows emission rates  by state.   Highest emissions
stretch along on an east-west band centered on the Ohio Valley.

Two  changes  in  S02 emission  patterns  in recent  years deserve  attention.
One change  is  the  spatial distribution  of emissions.   In  Figure  II1.23 we
show  changes  in   S02  emissions  from  1970  to  1980  by  state.    Recently,
increases  in  S02  emissions  have  been   almost  entirely  in  the  South.
Northeastern S02 emissions have generally declined.

A  second change  is the height of smokestacks.  As noted in Section 2.6, the
height  at  which S02   is  released  can affect significantly  its  subsequent
fate.    The  increase  in S02  emissions  from  electricity  generation  has
included a  substantial  shift  to  tall   stack  emission.  This shift  is  most
pronounced  in   the  highest  density S02  emission  region,   the Ohio Valley
area,  and is illustrated in Figure  III.24.  The figure shows trends in total
S02 emissions  for   three  heights of  emission,  for  all  the  power  plants of
capacity greater than  50  MW,  located  in a  two-county row  on either  bank of
the  Ohio  River  in  Illinois,  Indiana,  Ohio,  Kentucky,   West Virgina  and
Pennsylvania.  These plants account for  about 15  percent of the total annual
U.S. emission of S02-

Sulfate.  That primary sulfate emission  is a  comparatively  small fraction of
the total  emission  of  sulfur  compounds is  illustrated  in  Table  III.6.   The
historical   pattern  is shown  in  Figure  III.25.    The sharp  rise  in  primary
sulfate emissions  to  1970 results from  fuel  switching  to  residual  oil; the


                                     109

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Figure III.20  Map showing the study  area  included  for emissions  density
               calculations.   Adapted from Gschwandtner et al.  (1981).
                                       110

-------
      25
      20
 CD

CM
i—I
 O
 oo
 OO
  
-------
               ^ 50  x  106  kg yr"1
               > 50  2 250 x  10   kgyr"1
               > 250 < 1000 x 106 kgyr"1
               > 1000 x  106  kgyr"1
Figure III.22  Annual  1980 emissions of S02 by state.   Data are from
               Toothman et al.  (1984).
                                 112

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    >+250  x  106 kgyr"1

       0 to 250 x  106 kgyr"1

       0 to -250 x  106kgyr"1

    <-250  x  106kgyr"1
       Not  Studied
Figure III.23   Map  showing changes in SO? emissions from 1970 to 1980
               for  each  state in the study area.  Data are from
               Gschwandtner et al. (1985).
                              113

-------
           oo
           z
           o
           OO
           oo
           CM
           o
           oo
                   1950
                                                               m
                                                       0 - 100
                                                               m
1960
1970
1980
                                       YEAR
Figure III.24  Trend in emissions  of S02 from 62 study power plants in
               the Ohio River Valley:
               (A) Total  tonnage;
               (B) Tonnage breakdown according to specified physical
               stack height intervals.
               Adapted  from Koerber (1982).

                                     114

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                                                        EMISSIONS  ( 1012  g  )
  LQ
   -5
   03
   ro
   en
o >
ro a>

oo -••
»   co
   CO
— I -••
cu o
cr 3
— ' ui
05
   O
ro -h
i
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   <-+
   (D
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   a>
   -s
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   -5
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-------
      25
      20
CM
t— i
 o
 I/O
 z
 o
 LO
 oo
15
      10
             D  Other

             Q  Highway Vehicles

             |U  Pipelines

             (§3  Commercial/Residential

             0  Industrial

             I  Electric Utilities
        1950
           1955
1960
1965

 YEAR
1970
1975 1978 1980
   Figure  111.26
            Historical  trends of nitrogen oxide emissions by
            source  category for the study area.  Data are
            from  Gschwandtner et al.  (1985).
                                116

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sharp  drop  to  1980,  from  the  subsequent  shift  away  from  residual  oil  in
reaction to the oil  embargo and OPEC price  rises.   Comparison  of the spatial
distribution  of sulfate  emissions  to  that  for  S0£  (Figure   III.22)  would
show that the high density regions are  shifted north and east,  where residual
oil use is most extensive.

Nitrogen Oxides.  Historical  trends for nitrogen oxide  emission,  by type of
source, are shown in Figure III.26.  The largest  increases  from 1950 to 1980
were  in  motor  vehicle  emission and in  electricity generation.   Unlike the
case  of  S02 emission,  NOX  emission continued  to  rise  through the 1970's.
The   spatial  distribution  of  NOX  emission  reflects  the  importance  of
electricity  generation  and  motor  vehicles.   Figure  III.27 shows  that high
emission  areas  are  more  broadly  spread  over  the  eastern United States,
including both the Ohio Valley area and the population centers  of the East.

Canadian  Emission  of  S02  and  NOX.     Historical  emissions  of  $03  and
NOX by  source  category  are  shown  irr*Table  III.7.   Several  observations from
this  table  are  pertinent.   One is  the  high  emission  of S02 from  copper and
nickel  smelting; these  sources  account  for  about  half   of   Canada's  S0£
emissions;  they  are mostly located  in eastern  Canada, especially Ontario.
This  is  the most striking  instance  of a more  general  observation, that the
mix of  important sources  for  S02  is quite  different for Canada  compared to
the  United  States.     Canadian  S02 emissions  are a  little   less than  20
percent of  U.S.  emissions.   Canadian emissions from  electricity generators,
however,  are  about 4  percent  of  U.S.   utility   emissions  while  Canadian
emissions from non-ferrous smelting are more  than half  again as large  as the
U.S.  emissions  from this  source  category.   Canadian emissions  of nitrogen
oxides are  less  than 10 percent those  of the United  States, a  significantly
smaller  fraction than  S02  emissions.   The  distribution  of   NOX  emissions
by  source  category  is  closer  to  the  U.S.  distribution, though electricity
generation  gives a  significantly  smaller  fraction  of the   total.    Most
emissions occur  within  200 miles  (300  km)  of  the  U.S. border;  the highest
emission rates occur in southeastern Ontario and southern Quebec.  The nickel
smelter in  Sudbury,  Ontario,  is  the largest single  source  of   S02  in North
America.

Ammonia.  Anthropogenically-derived emissions of  ammonia are estimated to be
about  3  Tg  yr"1.   The major  source  is  domestic  animal  wastes.   There  is
considerable  uncertainty   in   this  estimate  and  little   information  on
historical trends or spatial distribution of emissions.

A.2.7.5  HOW DO NATURAL AND ANTHROPOGENIC EMISSIONS OF ACIDIFYING SUBSTANCES
         COMPARE?  [CARP A-2.2, A-2.3]

Sulfur  Oxides.   Natural  emissions  of   sulfur  compounds are estimated  to be
about  0.3  Tg   yr"1  of  sulfur  for the  eastern   United  States   (including
organic sources)  and  less than 0.9 Tg  yr"1 of sulfur  for  the  48 contiguous
states  (Section  A.2.7.2).   Anthropogenic emissions are  about  11 Tg yr"1 of
sulfur in the eastern United States  (note  that  emissions expressed as sulfur
are  1/2 those  expressed  as of  S02 and  1/3 those as  S042")  and  about 13
Tg yr"1 for the 48  states.   Thus,  in  the  East  natural  emissions are roughly
3 percent of anthropogenic emissions,  while in the West natural  emissions may


                                     117

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        < 100  x  106  kgyr"1
{      ] > 100  < 400  x  10°  kg yr
                              -1
        > 400  s  800  x  106  kgyr"1

        > 800  x  106  kg yr"1
  Figure  III.27   Annual  1980  emissions  of NOX  by  state.
                 Toothman  et  al.  (1984).
Data are from
                                   118

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                      TABLE 111.7.  HISTORICAL EMISSIONS OF S02  AND NOX -  CANADA
                          (U.S./CANADA WORK GROUP 38 DRAFT REPORT 1982)
                                        (103 kg yr-1)
Sector
Cu-Ni smeltersb
Power plants
Other combustion0
Transportation
Iron ore processing
Others
TOTAL
1955
S02 N0xa
2,887,420
56,246 10,335
1,210,108 227,837
83,474 323,785
109,732
189,876 68,065
4,536,856 630,022
1965
SOg N0xa
3,901,950
261,837 57,402
1,129,548 247,323
48,669 511,868
155,832
1,095,341 33,778
6,593,177 850,371
1976
S02
2,604,637
614,323
884,867
77,793
175,829
954,215
5,311,664
N0xa
-
206,454
445,315
1,017,936
-
190,327
1,860,032
aNOx expressed as N02.
"Includes emissions from pyrrhotite roasting operations.
clncludes residential,  commercial,  industrial,  and fuelwood combustion.   Industrial  fuel
 combustion also includes fuel  combustion  emissions from  petroleum refining and natural  gas
 processing.

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be as much 25 percent of  anthropogenic,  but probably are significantly less.
On  a  continental  or  global  scale,  estimations   of  the  natural  biogenic
emissions from all land areas range  over more  than  an order of magnitude but
appear  comparable  to the  global  anthropogenic  emission  of  about  60 Tg  S
yr"1.   Oceanic  emissions are  also  of  that  order  of  magnitude,  (Eriksson
estimated  44 Tg  S  yr"1) but  do  not  make  a  large   contribution   to  the
terrestrial sulfur cycle.   [CARP A-2.2.1.6]

Nitrogen oxides.  Natural  emissions of nitrogen have been variously estimated
at 0.04  to  1.5  Tg N yr"1  for the  eastern United States  and  0.15 to  5  Tg N
yr""1 for  the  48 contiguous states; the  lower  estimates  are to be preferred.
Current anthropogenic emissions are  estimated  to be  about 3.5 Tg N  yr"1 in
the  eastern  United  States and  6  Tg N yr"1  in the  48 states.   It is likely
that natural  sources contribute less  than 10 percent  of  the  NOX emissions
in eastern North America.  In the  West the amounts  may be more nearly compa-
rable  (10 to 50  percent).   On a  global  scale,  natural  and  anthropogenic
emissions of  nitrogen oxides appear  roughly comparable,  with, however, great
uncertainties in the estimate of natural  emissions.   [CARP A-2.2.2.2]

Ammonia.  For the United  States,  anthropogenic emissions of ammonia (princi-
pally  from  domestic  animal wastes) appear  to  be  roughly three times  natural
emissions,  but   uncertainties  of  a  factor of  three  or  so  exist   in  the
estimates.  [CARP A-2.2.2.9]

A.2.7.6   HOW WELL KNOWN ARE EMISSION RATES?  [CARP A-2.2, A-2.3]

We  have already observed  that  there are  very large  uncertainties,  up to an
order  of  magnitude, in   the  estimates  of  natural   emission  rates.    The
uncertainties in  the estimates  of anthropogenic emission rates are smaller,
but  still  significant,   if  the  estimates are  to   be  used   in   calculating
quantitative  materials  budgets.   The principal  sources  of  uncertainty are
different for different sources.

Sulfur  emissions.  Sulfur  emissions result  from the burning of  sulfur  in  fuel
(and ore).   ETfimates  of sulfur  emissions are made  by  obtaining records of
fuel consumption (and ore  processed) and  source of fuel,  and  estimates of the
sulfur    content  of  the  fuel,  and multiplying the  two  quantities together.
For  the  case  of  major   sources  that   have  installed  S02  scrubbers,  an
estimate  of  the effect of the  scrubber  must also  be included.  The greatest
limitation  in this  procedure is  the estimate  of  average  sulfur  content by
fuel  source;  it contributes an  uncertainty  of  20 percent at least.

Nitrogen  Oxide  Emissions.  Because most of the  nitrogen emitted  as nitrogen
oxides  comes  from the air  rather than fuel, estimates  of  nitrogen  oxides  must
be  based  on  records  of  fuel  consumption  for various  categories  of  combustion,
and  estimates based on  measurements  of  average  emission factors  telling  the
amount of NOX emitted per amount  of fuel consumed  for  various categories of
sources.   Because  a  major emission source  is  motor vehicles  and  because NOX
emissions vary  enormously from one  car  to another, or for even  the same car
depending on how well  tuned  it is,  substantial uncertainty  in the emission
estimates,  probably  30  percent  or  more is likely.
                                      120

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     TABLE  III.8.   NATIONAL  U.S. CURRENT  AND  PROJECTED  S02  AND  NOX
                            EMISSIONS  (Tg  yr~l)a


1.
2.

3.
4.
5.


Source category
Electric utilities
Industrial boilers and
process heaters
Nonferrous smelters
Residential /commercial
Other industrial
processes
6. Transportation
TOTALS
Current
1980
S02 NOX
15.0 5.6
2.4 3.5

1.4
0.8 0.7
2.9 0.7

0.8 8.5
24.1 19.0
Projected
1990
S02 N0x
15.9 7.2
3.4 3.0

0.5
1.0 0.7
1.2 0.8

0.8 7.8
22.8 19.5
Projected
2000
S02
16.2
6.5

0.5
0.9
1.5

1.0
26.6
NOX
8.7
4.0


0.6
1.1

9.7
24.1
aSummarized from U.S./Canada Work Group 3B Draft Report (1982).
                                     121

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A.2.7.7  WHAT ARE PROJECTED FUTURE EMISSIONS OF ACIDIFYING SUBSTANCES?

The uncertainties in  estimating  current emission levels will  also  appear in
predictions  of  future levels.   Such  predictions  may be  valid as  relative
predictions,  however,  and even  if  they  turn out  to be  wrong they may be
useful as  pointing  to trends  in  the  current  development of  technology  and
regulation processes.   In Table III.8 we show one set of predictions for U.S.
emissions  of S02 and NOX  in  1980,   1990  and 2000.   The  projections  are
about a  10 percent  increase  in  S02  emissions and about a 25  percent growth
of NOX emission  by  the year  2000.   More interesting  are  the  predictions  for
1990, which are for  essentially constant emissions.
                                      122

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     SECTION B.  RELATIONSHIPS BETWEEN THE EMISSION AND  DEPOSITION  OF
                            ACIDIFYING SUBSTANCES


The  processes  of  atmospheric  transport,  transformation,  and  wet  and  dry
deposition, which we  have treated separately up  to  now, are closely  linked.
The amount of sulfur  transported over some distance, for  instance,  depends on
the  amount of  S02  and  sulfate  emitted,  the rate  at  which  S02  1S   trans-
formed  into  sulfate,   the  rates  of dry  deposition for  S02  and  sulfate,  and
the  likelihood  of  encountering a  rainstorm  or  snowstorm, and the  efficiency
of  scavenging.   The   direction  of transport  of  material,  even,  depends  on
deposition and  transformation  rates:   the vertical  distribution of material
depends on deposition  rates, which in turn depend on the  chemical form  of  the
substance; and  the frequent  pronounced  shear in  wind direction, particularly
at  night,  means that  material at different  heights  can travel  to different
regions.

In  this  part,  the  linkages  between  the  different atmospheric  processes  are
examined as we  attempt to  answer  questions  of two sorts:  Do certain charac-
teristics distinguish  one  kind of  source from another?   What  is known  about
the relationship between emission and deposition?

As  we   have  learned   from  the effects   section,  we  are  concerned  with  the
answers to these questions for sulfur compounds,  for  nitrogen  compounds  and
for the production  of acidity.   The linkages are better  understood  for  sulfur
compounds at present;  so,  our  approach will   be  to present information  first
for sulfur, then to  compare nitrogen with sulfur,  and finally  to attempt to
draw conclusions about acidity.

B.I  ARE SOME SOURCES MORE IMPORTANT THAN OTHERS?

"Important" has to be  defined, of  course.   For the purposes of  this section,
we mean, can we distinguish one kind of source from another  in the  fractional
amount of acidifying material likely to be wet or dry deposited  in  previously
specified sensitive areas?  We shall  return to this question, and the problem
of defining "sensitive" in the concluding sections  of  this document.    In  the
next  three  subsections,  we  discuss the  general  characteristics  of  source/
receptor relationships and how they may  be  simulated  with computer models  or
inferred  from  data  analysis.   In the  remaining subsections we compare  the
deposition patterns predicted for various classes of sources.

B.I.I  WHAT SOURCE/RECEPTOR RELATIONSHIPS ARE OF INTEREST?

There is a surprising  amount of confusion about how deposition (either  wet or
dry) at a  receptor  site may be related  to  emission from  a  source.   Some  of
the confusion  reflects the complexity of the processes,  but some results from
a failure to specify exactly what relationships between  sources and receptors
are at issue.    The following need  to  be specified if  the relationship is  to
be unambiguous:
                                     123

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     0   The  emitted  substance  of  concern  and  the   amounts   of  relevant
         co-emitted  species,  (i.e.  species which  might participate  in  the
         production or depletion of oxidants).

     0   The  background  levels  of  the  emitted substance and other relevant
         species (oxidants).

     °   The meteorological conditions affecting  transport and deposition.

     °   The nature of the deposition of concern, wet, dry, or total.

In  very  general  terms,  for  typical  sources  and  receptors  in  the eastern
United States,  the dependence on  background levels  and on meteorology may  be
summarized by a simple classification of  source/receptor  relationships  into
four types  based on whether  one  or many  sources are  considered and whether
one  or  many  receptor sites  (localized  within about  10   to   30  km)  are
considered.

Type I.    One  Source/One  Receptor:   Except for relatively  short distances
(usually less than 50  km even for short periods of time), the contribution  of
a  particular emission source,  even  a  large  power   plant,  to background
(ambient) levels is  small.   Consequently,  in  a  type  I  source/receptor  rela-
tionship  it  is   appropriate  to  specify  background  levels  independently  from
specifying  characteristics  of the  source.   The  actual contribution  of  the
source to  deposition at the  chosen receptor  site will  be  very   sensitive  to
the  detailed  specification of meteorological  conditions.   This is particu-
larly  true  for  wet deposition,  where the  variation  in contribution from  one
year  to  another  may  be an order  of magnitude  or more.    The  variability
results  from  two effects:   most  of the deposition will  occur  during  only  a
few rain storms, and they  are likely  to  have considerable local   variation  in
duration  and  intensity;   the actual  path taken  by  the   pollutants   on  a
particular occasion will be sensitive to small variations in wind direction.

Type  11.    One   Source/Many  Receptors:     Again,  because there   is  only  one
source,  the   background  in a  type  II  source/receptor  relationship  can  be
specified  independently  from  the  source  specification.    In  this   case,
however,   it  can be  hoped  that  much  of  the dependence  on meteorological
variability will be  averaged  away when considering  the average  contribution
over a substantial  region.   The averaging  should smooth out the  fluctuation
in  the path  the pollutants follow  and  the  local variability of  rainstorms.
Only broad-scale variation in annual rainfall or annual storm   paths should
remain.

Type III.  Many  Sources/One Receptor:   Once we are  considering  many sources,
the  contribution of the  sources  to  background  cannot  be  neglected even  at
large  distances.   In fact,  for  a  large enough group  of sources,  the sources
may  be responsible  for  almost  all  of  the  "background".   Consequently,  any
dependence of oxidation rates, etc.,  on  the levels  of emitted substances  and
relevant   co-emitted   species  must  be   included   in  the  source/receptor
relationship.    Local  variability  in  meteorology at  the receptor  site  may
still  be  important,  particularly  variation in rainfall intensity,  duration,
and season of occurrence.


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Type  IV.   Many  Sources/Many  Receptors:    For evaluation of  acid deposition
policies in the eastern United States, these  relationships  are probably most
important.  As  with type  III  relationships,  background cannot  be  specified
independent of the source specification.   As with type II relationships, some
but not all meteorological variation will  be  averaged away  on a year-to-year
basis.

Only  for  the  case  of type III and  type  IV relationships are  empirical  data
presently  available.   Since  deposition  measurements  combine   the  contri-
butions from  all  sources, single  sources  contribute  too  little to  average
deposition at a receptor  site  to  provide  an unambiguous  signal.   Unfortu-
nately, as we have  noted  in  Section  A.2.1,  there  are not enough  long term
data  to detect with  confidence the  effects of even changes  in emissions. Nor
are emissions well   enough specified  (see  Section  A.2.7.6) for  the  current
data  to  be  adequate to provide  more  than qualitative estimates  of mass for
wet deposition, while amounts dry deposited are still largely unmeasured.

B.I.2  ARE SOURCE/RECEPTOR RELATIONSHIPS  EXPECTED TO BE LINEAR?  [CARP
       A-4.4.3]

As  is  described below,  some  non-linearity  is  to  be expected in relationships
between deposition at a receptor  site  and emission  from a  source.   However,
for most  type IV  source/receptor  relationships over  periods of years,  the
authors believe  the non-linearities are  likely to  be small,  i.e.,  smaller
than  other uncertainties.  In practical terms, a source/receptor relationship
is  useful  if  it can be used to predict  a change  in  the  deposition at the
receptor  site resulting from  a  change in  the  emission at  the  source  site.
Until   very recently  most  modeling  of  acid emissions  and deposition and most
policy  analyses  have explicitly  or implicitly  assumed that  the connection
between emissions and deposition, however it depends on location and time, is
linear:   that is  a change  of  X percent  in  the  emission  of sulfur from  a
source site or sites will  result  in a  change  of X  percent  in  the deposition
of  sulfur  attributable  to the source(s)  at a receptor site  or  sites.   This
assumption has  generated  a considerable  amount  of  controversy  often,  as we
shall  see,  based  on disagreements over  what are  the appropriate  source/
receptor relations and what is assumed to be held constant when a change of x
percent in emissions is proposed.

After  all, any sulfur  that  is emitted will be deposited some place  and the
deposition processes appear  to be  largely linear.   However, there  are two
species  of   sulfur  compounds,  S02  and  $04,  that  can  be  deposited  at
different  rates;  if the  transformation   of S0£  to  sulfate  is  not  propor-
tional  to  the amount of S02 present,  then the spatial pattern of deposition
and the relative amounts of sulfur  wet and  dry deposited may be altered.  In
particular, if  a  reduction in S02  emissions  leads to a smaller than propor-
tionate reduction in sulfate production  (as  seems  likely)  the result will be
a greater than proportionate reduction in dry deposition of sulfur  (this will
be  more  noticeable  nearer  the   source)   and  a  smaller  than proportionate
reduction in  wet deposition of sulfur and  the amount of sulfur carried across
boundaries.

How significant  could  such  non-linearity  be  to deposition  patterns.   Three
considerations  are  important:   how  far   from  linear is  the transformation


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from S02  to $04; what is assumed  to be  held  fixed  in  the  source/receptor
relationship under  consideration;  and,  what  is  the  relative  importance  of
SC>2 to $04 in deposition at the sites.

The three  most important processes  for  oxidation of SC>2  (see  CARP  A-4, and
CAD, Section III.A.2.5; the discussion here is  based  on  Durham and Demerjian
1984)  are gas-phase  reaction with  the OH radical, aqueous-phase reactions at
low  pH  with   H202,  and  aqueous-phase  reaction  at   high  pH  with  03  and
other  oxidants.   Each of  these  processes has a  different  dependence  on the
amount of S02  present.   Concentrations  of the  OH radical  in  the atmosphere
appear  to  be  quite  insensitive to  concentrations of  the other pollutants
present.  Hence  gas-phase  oxidation  of  S02 will  be  essentially proportional
to  the  amount of  S02  present  and  thus  linear,  independent  even  of the
amount  of  co-emitted species  or  background  pollutants.    It  will,  however,
exhibit  a  marked dependence  on  time  of day, season, latitude,  and meteor-
ological conditions.

Aqueous-phase  reactions  with  H202  proceed rapidly,   independent  of the pH,
until   either  the   S02  or   H202   oxidant  present   is   substantially  con-
sumed.    If there   is  enough  H202  present  for the  reaction  to  consume
S02,  then  the amount of  S02  oxidized  by this  mechanism is  determined  by
the proportion of the S02 that contacts  with the aqueous  solution, and the
reaction  is again  linear.   If,  however, insufficient  H202  is  present  to
oxidize  all  of the   dissolved  S02>  the  relative  amount  of S02  oxidized can
depend  on   the  amount  of  H202  present.  This   possibility,   called oxidant
limitation, is illustrated in Figure  111.28.  The figure shows a hypothetical
case 1,  the  "present situation"  in  which the  amount of sulfate is determined
by  the  amount  of  oxidant  ^02)  initially   present  and  the  available
oxidant  is  substantially  consumed  by  the transformation  process.    In the
hypothetical case 2,  the  initial amount  of S02  is decreased  but the initial
amount  of  oxidant  is the same.  By  assumption  the initial amount of oxidant
determines the amount of sulfate produced, so this will  be the same for case
1  and  case  2, and  the reduction  in the  initial  amount  of  S02 translates
only into a reduction in the final  amount  of  S02.

Because  the   rates  for  aqueous-phase  reactions  involving  03  and   other
oxidants depend  on  pH,  they  too  can exhibit  non-linearity.  As the  dissolved
S02  is oxidized  to sulfuric  acid,   the  pH   will  decline, and  the  reaction
will proceed more slowly.

It  is  likely  that  aqueous-phase   reactions  with  H202  are   most  important
for oxidation  in  the eastern  United  States,  with gas phase reactions  produ-
cing a  significant fraction of the sulfate also.  Enormous  variability  occurs
in  the amounts  of  H202 measured  and in  the aqueous phase  reaction   rates.
It  is  likely,  therefore, that oxidant limitation  is sometimes  significant and
sometimes  not, but  there  is  little  quantitative information  available,  at
present, to  specify  for what fraction of the  time, and for  what regions.

When  considering many  sources, Type  III  or Type  IV source/receptor relations,
there  may  sometimes  be  changes  in  the  emission  of  nitrogen   oxides and
hydrocarbons  which   can  affect  the  background   level  of  oxidants  (H202 and
03,  not  OH).    Hence,  to estimate  the  amount of  S02  converted  to $04


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                       CASE 1

                 Limited Oxidant
                                         \
                                         /
     CASE  2

Limited  Oxidant
                                                            Reduced  SO
                                                                         S°
                        \
                         >
Figure  III.28  Illustration of  the possibility that a reduction in S02 may  not proportionately
              reduce 804.

-------
after  a  substantial  reduction  in  502  emissions,   one  must  also  specify
whether the reduction affected other species.

Finally, we turn  to  the question,  what  is  the effect of  non-linearities in
oxidation on deposition.  Over most of the eastern United States, the mass of
sulfur  in  the  form of  S02  is greater  than  the mass  of sulfur  in  sulfate.
S02  is  significantly more  efficiently  dry  deposited than  sulfate;  sulfate
appears  to  be more  efficiently wet  deposited  than  S02,  but  the  relative
efficiencies are not well known.   Because oxidant limitation appears signif-
icant at  some  times  and at  some  locations,  the effect  of a reduction  of X
percent  in  S02 emissions  will be  S02  concentrations  in  the  air  that are
reduced  somewhat  more  than  X percent  and  sulfate  concentrations  that are
reduced  somewhat  less  than  X percent.   The  result will  be  a  greater  than X
percent reduction of dry deposition  (for most sites)  and a  reduction  in wet
deposition  that might  be  somewhat  more or  somewhat less  than X  percent
depending  on  the  relative   concentrations  of  S02 and  $04 above   the  site
and  on  the  relative scavenging efficiencies.   For most sites  the  change in
total deposition resulting from a  broad  scale change  in emissions  is  likely
to be close to proportional.

The  case of nitrogen compounds is somewhat different.   Here the oxidized form
HN03  is  dry deposited  more  readily.    The  basic  oxidizing  reaction appears
to be in  the  gas  phase  with  the  OH radical,  so one might expect linearity in
HN03  production; however, the existence  of  numerous  nitrogen  oxides and the
reversibility  of   many  of  the production   reactions  makes this  assumption
dubious.  There is  no  particular  reason  to  expect strong  non-linearities in
deposition, however.

B.I.3   HOW ARE MODELS FOR LONG-RANGE TRANSPORT AND DEPOSITION USEFUL?
        [CARP A-9]

A  constant  theme  of  this section has  been  that the  processes  of transport,
transformation, and  deposition are complicated  and  inextricably  linked in
acid  deposition.   The  complexity  and linkages have spurred the  development
of many numerical  models using  computers which simulate  the  processes  from
emission  to deposition.  Such models provide an explicit framework to account
for  the  complexities   in the processes  and  the  links  between  the various
stages.

Models  are  very useful  in  learning how  significant  one aspect  of  the  acid
deposition  problem  is  to  another  aspect.  Models  can provide answers to such
questions as, how does  the location of a  source affect the  amount of material
deposited at  a  receptor site?  What difference exists between  the deposition
pattern  produced  by  tall  stack  and  short  stack  emissions?    What  is  the
seasonal  variation  to  be expected  in deposition?  How  large  are the  uncer-
tainties  introduced  by  inadequate  data  on  dry deposition  rates,   oxidation
rates,  concentrations,  or amounts dry deposited?

How  well  can present  day models  answer  such  questions?   They can  do  pretty
well,  in  that they provide  useful  information otherwise difficult  to  obtain
or   information   corroborating   other   answers   that  also   have  serious
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uncertainties.    It is  important,  however, not  to have  unreasonable expec-
tations of model calculations.

It is  unreasonable  to expect any  of  the currently used models  to serve as a
master model that  can numerically simulate  all  aspects  of  acid deposition;
such  a model  would be  very   complex,  and  we  are  only   gradually   learning
through the current research and model development process the most important
intermediate processes  and  how to  characterize  them.   As yet,  no  adequate
database  exists  either  to verify   model   conclusions   or   to   provide  the
necessary internal  parameters.   Hence, a  large  number of models,  each with
different  assumptions  and  different  approaches   to calculations   are  now
available.   Each model  has been  designed to be  appropriate  for  answering
certain sorts of questions, and, as  a consequence, each  is inappropriate for
other questions.

It  is unreasonable  to   expect different  models   to  give exactly  the  same
answers to questions;  in particular,  it is unreasonable to expect a model for
which  the question  is   inappropriate  to  agree  with one  for  which  it  is
appropriate.

It is unreasonable to expect quantitative answers  to questions to be  accurate
to more  than one significant  figure;  indeed,  for many questions answers  to
within a  factor of  two  or four  are all  that  should be  expected  even from
appropriate models.  No amount of clever numerical simulation can make up for
the  lack  of  empirical data on key  processes, the lack  of  data for  testing
model  conclusions,  or the lack of  relevant input conditions  on emissions and
meteorology.  An interesting example of what models can  do in answering hard
questions  has  been described  by Husar  (1983),  based on the  Memorandum  of
Intent comparison  of models.    The  questions  are 1)   what  is  the  relative
importance of  four broad source  regions  to  total sulfur deposition in the
Adirondacks? and, 2) what are   the amounts dry and wet deposited?  The answers
come  from three models,  (AES,   the Atmospheric  Environment Service Long-Range
Transport Model,  Olson  et al.  1979;  ASTRAP,  Advanced Statistical Trajectory
Regional   Air  Pollution  Model,  Shannon  1981;  and  CAPITA,   Center  for  Air
Pollution  Impact and Trends Analysis,  Monte Carlo  Model, Patterson et al.
1981).  The answers provided by  the  three models  to  question  1  are  shown  in
Figure 111.29;  agreement among  the models on the  percentages for  the  separate
regions is within  a factor  of  two.   The answers  to  question  2  are  shown  in
Table  III.9.   Agreement among  the models is better  than  within a  factor  of
two.   According to Husar  (one of  the developers  of the CAPITA Model)  the
models reproduce  the  sulfate  aerosol  concentration  in  the  Adirondacks  to
about  a  factor  of two;  he judges  the relative  contributions   of the four
source regions  to be correctly  specified to a factor of two to four.  Because
these  models, and  others,  have their  parameters chosen  to reproduce  current
emission  and  wet  deposition   data,  their   quantitative  predictions  using
hypothetical  emissions may be  even less correctly  specified.

How  can  models  be  tested and   improved?    What   are   the  prospects  for
improvement?   Obviously  a  great need  exits  for  a  much   larger database  on
concentrations   of  pollutants,  on   dry  deposition,   and  on  the  processes
described  in Part  111.A  of this review.   Very  important also  is continued
elaboration of models and the  testing of one against another  to  determine the


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                        Dry+Wet Deposition Estimates by Three MOI  Models


                           % contribution of  each source  region to Adirondacks
CO
o
                    AES
ASTRAP
CAPITA (MCARLO)
                                      NORTHEAST



                                      CANADA
             REST OF U.S



             OH  + WV
   Figure III.29  Source attribution of dry plus wet  deposition  in the Adirondacks by three MOI models.

-------
                 TABLE II 1.9  DRY AND WET DEPOSITION ESTIMATES
                    AT THE ADIRONDACKS BY THREE MO I MODELS
Model
Dry Deposition
Wet Deposition
Total Deposition
AES          1.39 g irT2 yr'l
             51%

ASTRAP       1.72 g m~2 yr-l
             44%

MCARLO       2.42 g m~2 yr"l
             69%
                       1.44 g m"2 yr~l
                       49%

                       1.35 g m-2 yr-l
                       56%

                       1.08 g m-2 yr-l
                       31%
                       2.83 g m-2 yr'1
                       100%

                       3.07 g m-2 yr~l
                       100%

                       3.5 g m-2 yr'1
                       100%
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sensitivity of results  to  the  choice of parameters,  assumptions,  and calcu-
lational   methods.   Thus,  deposition pattern  and  amounts  deposited  may  be
quite sensitive to day/night changes in  dry deposition velocities;  yet yearly
average deposition may  be  as well or as poorly  reproduced  by a model which
does  not  incorporate  such  changes, and  the added  complexity in  the model
might not be relevant to some other question in another context.

Air  quality  models  can  be  classified  by  their  treatment of  the  following:
[CARP A-9.1]

     0  Frame of Reference
     °  Temporal  and spatial scale
     0  Treatment of turbulent wind motion
     0  Transport
     0  Chemical  transformation
     0  Removal mechanisms

Frame  of  Reference.   Most long  range  transport  models  are  written either
using a coordinate  system  (frame of reference)  that is  fixed on  the  surface
of the Earth—such  models  are  called Eulerian models—or using a  coordinate
system fixed on  the moving air parcel  as  it moves  downwind  from a source or
as  its  history  is  traced  upwind  from  a  receptoi—these  models   are called
Lagrangian models.  Lagrangian models at the  present stage  of model  develop-
ment  and   verification  are  more  practical  for  estimating   single   source/
multiple  receptor  (type II) and  multiple   source/single  receptor  (type III)
source/receptor  relationships.   Eulerian  models require  much more extensive
computation;  however,   once  the  difficulties  of  extracting   source/receptor
information  from these  models  has been  resolved,  they  may ultimately  be
better  suited  for  providing these  answers because  they  provide  a   natural
framework  for  taking into account non-linearities and  topography,  and  because
the  input meteorological  information is necessarily  measured in  the  Earth's
reference frame.

Spatial/Temporal  Scales.    The spatial   scales   in  acid  deposition  are con-
ventionally  defined  as  short-range (less  than  100  km),  intermediate range
(100  km to 500 km)  and  long-range  (greater  than 500 km), although different
authors often  choose different boundaries.   Models developed  specifically  for
application  to acid  deposition generally cover  deposition over  the  scale of
interest  in  this problem,  namely  both  intermediate and long-range transport.
Occasionally  models developed for  other   purposes  may  be  used   to answer
specific  questions.   Thus,  short-range  transport models extended to inter-
mediate   distances  may  provide  information   about effects  of   detailed
meteorology  or complexity  of  terrain.   Continental  or global-scale  climato-
logical  models  may  provide  information   about  the  effect  of  large scale
weather patterns.   Both short duration  (one day  to  one  week) and long-term
average  deposition  patterns are  of  interest,   and  models  have  been speci-
fically designed for each, or both.  Questions  about long-term averages  may
be  more  interesting  for effects; short-term  predictions  may  be  more  readily
tested experimentally.

Turbulence.  Random fluctuations  in wind  speed and direction  are  responsible
for  dilution and mixing of  pollutants in air.  The  amount of  turbulence to be


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expected will  depend on  season  and  time  of day,  on terrain,  and on other
meteorological  variables.    Models  may  account  for  fluctuations  either
implicitly  through  defining a well-mixed  volume  or  through  one  of  various
parameterizations of turbulent diffusion.

Transport.   Pollutants  travel  with  the  wind.   The  available information  on
wind speed and direction is the network of ground level weather  stations plus
a sparse network  of  upper  air  measuring  stations.  The  models differ in how
they  cope   with   the  problem  of  estimating  wind  shear  (variation  of  wind
direction  and speed  with  height)   and  how  they  interpolate  between  wind
observations.   An  important  difference  is  the  role  of  the  weather  data
themselves  (this  applies  to other aspects, wet  deposition  especially):   one
use  of  models is to  simulate  actual weather conditions  over some period  of
time (for  which  there  might be  deposition data); another  possibility is  to
use  synthetic weather, based on  historical  weather measurements, to estimate
the deposition to be expected  on  the average  or  to determine  the sensitivity
of  deposition  patterns  to  variation  in  weather  conditions  for  a  particular
year.

Chemical Transformations.   In  the simplest models,  the  complexity described
Tn  Section A.2.5  fs boiled  down  to a  single  number,  an  average  rate  of
conversion  of  S02 to  sulfate.   A  more elaborate  treatment  allows  for  the
dependence of this rate on  time of day, and on meteorological  conditions such
as  cloud cover.   In  principle  the role of  nitrogen  oxides  and other  species
can  also  be taken into  account.    In  practice,  even  the modeling  of simple
nitrogen oxide transformations has  lagged, and treatment of  complex  systems
is  practically nonexistent.

Removal  Mechanisms.   Dry  deposition is most  simply  parameterized by  average
deposition  velocities  for  S02  and  sulfate.   Complexities  such  as variation
with time  of day  or  season and  dependence on  region or  terrain  have  been
modeled.    None  of  these   approaches  can  resolve  the   large  uncertainties
discussed in Sections A.2.1.7 and A.2.3.

Wet  deposition  may  be  parameterized  by  either  a  scavenging rate and/or a
washout ratio—a single parameter relating the amount of pollutant  in  rain  to
the concentration in the air.  A major problem in the use of  such parameteri-
zations, aside from the fact that the parameters are  very  poorly known   (see
Section 2.4), is that the amount scavenged depends on the nature of the storm
and that storm type  is a  function of other variables  such  as wind affecting
transport,  and season and cloud cover affecting oxidation rates.

B.I.4  WHAT IS THE RELATIVE IMPORTANCE OF DISTANT AND SHORT-RANGE SOURCES
       TO DEPOSITION IN SENSITIVE REGIONS?  [CARP A-3.5]

For the effects of most  concern—harm to  forests  or  to aquatic ecosystems--
most of  the sensitive areas,  forest stands or lakes  and  streams are spread
out in remote areas.   Consequently,  local  sources at  distances  less  than  50
km  are  not significant except in  isolated  cases.    Answering the question,
which are  more important,  intermediate range  sources  (50-500  km  away)   or
distant  sources  (>500 km),  is more difficult and  requires  further  speci-
fication.   When  considering a representative source  in the  eastern United


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States, for  most  locations,  we know  the  source  can contribute to deposition
at sensitive receptor areas  that are  both at distant and intermediate range.
Contributions in  these  two  categories, when averaged  over  many sources, are
likely to  be  comparable since a typical  net travel distance  for  an emitted
sulfur atom  is  roughly 500  km [CARP  A-3.3.3].   Of the  three  broad regions
identified  as  of  most concern,  the  Adirondack and  New  England  Mountains
receive roughly  equal   amounts  from distant and intermediate-range  sources,
the  southern Appalachians  receive  most  deposition  from intermediate-range
sources,  and  Maine and eastern Canada receive  most  deposition from distant
sources.

It is  interesting to  look  at  the  basis  for  these conclusions.   Thus, the
following  items need to be considered:

Relative  Density of Sources-If there are practically no sources within 500 km
of  the sensitive  region,  only distant  sources  will  be   important.    The
emissions  map  (Figure  III.22)  shows  substantial   concentrations  of  sources
centered  around  the Ohio Valley and around  the   mid-Atlantic  urban areas.
Close  to  those regions  these  sources  surely  dominate;  downwind more  than 500
km, they  will still be important.

Geometry  and Plume  Dispersion-If one considers  a single  receptor  point in  a
sensitiveregion(typeIII   source/receptor  relationship),  then a nearby
source will  contribute  more  than  a  distant source  of  the  same   emission
strength,  just because  the  nearby plume will encounter the receptor  point in
a less depleted state.  That  does not mean,  however,  that the distant source
necessarily  contributes less  to all  sensitive  areas (type II source/receptor
relationship); it may contribute smaller amounts to each of more areas.

Dry Deposition-The crucial parameters  determining how much of the sulfur from
a  source  is deposited  relatively  close  to  the  source  are the rate of dry
deposition  of  S02,  the  rate  for  the  competing  process   of  oxidation  to
sulfate,  and the amount of  vertical  mixing.   As  we  have seen these rates are
highly variable  and not  well  known.   If most  of  the  plume   sulfur  is dry
deposited  within  the   first  24  h   then   there  won't  be  very much sulfur
available  for  transport distances   greater  than  500 km.   Model calculations
for a  St.  Louis  Power Plant Plume  [CARP A-3.4] suggest  that "perhaps more
than half of the sulfur released from a 200 m stack may be deposited, wet and
dry, within  500  km of  the source in  the  summer."   In  the  Ohio River Valley
with less  frequent nocturnal  jets and generally  lighter winds, the effective
transport  ranges  are   likely  to  be   shorter.   Winter  residence  times and
distances  traveled are  likely to  be  substantially  greater  than summer ones.

The numerical values in these calculations  depend  on  the specific nature of
the model  and  the  choice of  dry deposition and oxidation  rate parameters.
The qualitative result  that  long-range and  intermediate-range deposition are
roughly comparable on an  annual basis for emissions from a typical source is
not sensitive to most typical  choices  of  the model  and its parameters.  This
result is  also consistent with  the  estimate  (Section B.2.3) that the average
atmospheric residence time for a sulfur atom is  1 to 3 days.
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The effect of  the  actual  distribution of emissions strengths can  be  included
in model calculations as well.  The results for the ASTRAP model  [CARP A-3.5)
using current  emission  data  and  summer 1980 meteorological  data are  that  the
boundary  where  distant  and  intermediate-range  sources  contribute   equal
amounts  to  total  sulfur deposition is a  line  running roughly southeast from
the northwest  corner  of New England.   The precise  location  of  the  boundary
line  is  very sensitive  to  the  choice  of model  and model  parameters;  however,
the  qualitative  conclusion  that  over  much   of  the  Northeast  distant  and
intermediate-range  sources  contribute comparable  amounts  seems  reasonably
secure.   Of the  sensitive  regions,  distant  and  intermediate-range  sources
appear  to be  of  comparable importance  to the  Adirondack  and  New  England
mountain  areas;  distant sources,  more important to  remote  areas  in  eastern
Canada  and  Maine;  and  intermediate   range  sources,   more  important  to  the
southern Appalachians.

B.I.5  HOW DO  THE DEPOSITION PATTERNS  PRODUCED BY TALL STACKS DIFFER  FROM
       THOSE PRODUCED BY LOW LEVEL (URBAN) RELEASES?   [CARP A-3.4]

The single  source  modeling discussed   on  the previous page was for the  power
plant emissions from a 200 m stack located in  St. Louis.  One striking change
in  the   nature of  U.S.  S02  emissions has  been  the  large  increase  in  the
proportion  of  emissions released  from stacks  200 m  or higher  (see Figure
III.24).    What is  the  effect  of  stack  height on the  amount  of sulfur
transported long distances?

It seems  surprising that a  difference of a few  hundred  meters  in  the height
of  release  could make  a  difference  when we  are  considering  transport over
more  than one  hundred  or  even  one  thousand kilometers.   However,  there is a
mechanism  that  can  separate  emissions   from  short and   tall   stacks—the
variation in height of the mixing  layer,  the lower layer  of the atmosphere  in
which there is essentially unlimited  mixing of  substances  in  the air.  On a
summer day the mixing layer  is one to  two kilometers  thick; air mixes  rapidly
within that layer, so even plumes  released several hundred meters  up will  not
differ  significantly  in their  contact with the  ground  from  plumes  emitted
lower.   In  winter,  however,  the mixing  layer  may be considerably  shallower;
much  or  all  of the release  from  a tall  stack may  be above that layer  and
travel  many  hundreds  of  kilometers   before   coming  into  contact with  the
ground.    In  addition,  shallow nocturnal  inversions   may  completely decouple
elevated plumes from surfaces, so  there will be essentially no dry  deposition
until  the inversion is  dissipated  the next  day.   Even  though  nighttime  and
winter  oxidation  rates  are  lower  than those  on  a  summer day,  the ratio  of
sulfate  produced to S02 dry deposited  can  be  appreciably greater since  the
limited  contact  of  the plume with the ground  means  there will  be much less
dry deposition.  This  mechanism is confirmed  in field  observation of  power
plant plumes.

To estimate the  magnitude of  this effect,  the amount of additional   sulfate
that  will  be   deposited  and  the   reduction  in S02  dry   deposition  as  stack
height is increased, it is necessary  to  know:   1) the distribution of mixing
heights  over time and space,  2)  oxidation rates over  time and  space,  and  3)
dry deposition rates over  time and space.  There is  a considerable amount  of
data on the temporal distribution  of mixing heights.   Oxidation rates and  dry


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deposition rates, largely uncertain, are parameterized in the models.  Infor-
mation for  the  St.  Louis area  has  been  used  to model  tall  stacks  and  low
level  (urban)  plumes  over the  year.   The  prediction  is that  the  amount of
sulfur carried long distances  by  tall  stack emissions is enhanced  nearly 25
to  50 percent  compared  to  urban  plumes.    Modeling of  Ohio River Valley
sources gives similar results.  [CARP A-3.4]

B.I.6  HOW IMPORTANT IS THE EMISSION OF PRIMARY SULFATE?

It  is unimportant.    The  previous  discussion  (Sections  B.I.1.2,  B.I.1.4,
B.I.1.5) has  indicated  that  sulfate has a  privileged  role  in the long-range
transport of  sulfur compounds  because  it is less readily dry deposited near
its source.   Typical model calculations  [CARP  A-3.4]  suggest that on average
only  20  to 40  percent of S02  is oxidized  to sulfate.   This has  prompted
suggestions  that even   comparatively small  amounts  of primary  sulfate emis-
sions  might  be  important in  acidic  deposition, especially  since  for  some
oil-fired boilers, as much as 15 percent of the sulfur is released as primary
sulfate, and  that is a percentage comparable  to  the  percentage of secondary
sulfate produced from the S02 emissions of these sources.

The amounts  of  primary sulfate released, however,  are  too  small  for primary
sulfate to  merit special  attention.   Sulfate  does  not  predominate in short-
er  intermediate-range   deposition;  SO^  is  more  important.    For long-range
transport  it is  necessary  to  consider  many  sources;  primary   sulfates
represent  less  than   3  percent  of sulfur  emission  and  are  thus  a  small
fraction of a regional   sulfate budget.

B.I.7  HOW DO SOURCE/RECEPTOR RELATIONS FOR NITROGEN OXIDES COMPARE WITH
       THOSE  FOR SULFUR OXIDES?

Several important differences affect the analysis of  the long range  transport
of  nitrogen oxides.  Most  important  is  that much less is known about the key
parameters  for  nitrogen oxides.  Emissions,  for  which  motor vehicles are an
important  component, are  less  well  determined.  Wet deposition has been  less
extensively  studied.    Dry deposition  rates are at least as  poorly  known as
those  for  S02 and  sulfate,  and the chemical  reactions  in  the atmosphere and
their dependence  on  secondary  species  are  both  more   complex   and   less
thoroughly  studied than  those  that  lead  to  the oxidation of S0£.

Even  amidst this ignorance a  few generalizations  are  possible.   A  substan-
tially higher percentage of NOX  emissions  are from low-level  urban  sources,
especially  motor  vehicles  (see  Section   A.2.7.3).    Oxidation  of  N02 is
faster than  oxidation  of  S02.    Dry  deposition   velocities  for  HN03  are
higher than  those for  S02 and N02,  which are  probably roughly  comparable.
The combined effect of  these  two  differences is  likely to  be  that  source/
receptor  relationships for  nitrogen oxides are likely to be  similar  to those
for sulfur  oxides  but  to have  a higher  percentage of  short-range  deposition.
                                      136

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B.I.8  HAS THE INSTALLATION OF PARTICIPATE CONTROLS ON POWER PLANTS AND
       OTHER SOURCES CONTRIBUTED TO ACIDIFICATION?

No.   It  has been suggested that  fly  ash from coal burning  is  a significant
source of  neutralizing  material.    Calcium  oxide, in  particular,  can form a
significant fraction  of  the fly ash  content.  It is now  becoming  routine to
install  particulate  controls  that capture most  of  those  emissions.   Even if
such  controls  were  not  in place,  the  amount of  CaO and  other potentially
neutralizing  compounds   transported  significant distances  from  the  source
would  be  small,  since most of  the mass  is  in  large  particles  with  a short
atmospheric residence time.  Those  compounds are not sufficiently reactive to
scavenge an appreciable  fraction  of the emitted  sulfur while  they remain in
the plume.

B.I.9.  HOW WELL CAN ACIDITY BE PREDICTED, KNOWING EMISSIONS?

Not very well.   The acidity is even more difficult to predict than sulfate or
nitrate  concentrations in  precipitation.  The  acidity depends  on the amounts
of  both  sulfate  and nitrate ions  present.   It also  depends  on  the amount of
neutralizing material present, and  it is clear from field studies of aqueous-
phase  oxidation  that neutralization  plays  an  important  role  in determining
acidity.    Neutralizing   substances  have  different   sources  and  transport
properties  from  sulfate  and nitrate.   At present,  the patterns for acidity,
sulfate  deposition, and  nitrate   deposition  appear  quite  similar  (compare
Figures  III.4,  III.5,  III.7)  in the  eastern United  States while they appear
largely uncorrelated in  the West.

B.2  WHAT ARE THE OVERALL  BUDGETS  FOR ACIDIFYING SUBSTANCES?

To  complete our  review   of the  atmospheric  sciences  aspects  of  the  acid
deposition  problem  we  summarize in the  next subsection  what  is known about
the total amounts of sulfur and nitrogen emitted, carried in the air, and wet
or  dry deposited.    Based on  these totals  and  the  information  developed in
parts A and B.I, we  go on  to  consider how present deposition amounts compare
with  the  deposition expected  from natural   sources,  with deposition  in  the
recent past,  and with other  deposition  changes.   Information  on emissions
provides  insight  for determining  what constitutes a  significant increase or
decrease  in  emissions and  for  determining what source  categories  are  most
important.  Information  on atmospheric concentrations can be used to estimate
atmospheric residence times for the substances of interests.  Residence times
are  directly   linked to   the  important  processes  of deposition,  chemical
transformations, and  transport  from  one  region  to another.   Information on
amounts deposited  is  essential  for establishing what  is  a significant input
to  a  "sensitive"  system,  and  for determining  appropriate  time  scales  for
changes in affected systems.

B.2.1  WHAT ARE  THE BEST  ESTIMATES OF SULFUR  AND NITROGEN OXIDE BUDGETS FOR
       THE EASTERN UNITED STATES?

Sulfur.  Three  fates  are possible  for a  sulfur  atom emitted in  the eastern
United States.   It  can be  wet  deposited  in  the eastern United States; it can
be dry deposited  in the  eastern United  States:  or  it can be blown  across a


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boundary, the Mississippi River,  the  Canadian  Border,  the Atlantic Ocean, or
the Gulf of Mexico, and subsequently  wet  or  dry  deposited.   At present it is
not  possible  to  say  that any  of  these three  fates  is  more  likely   than
another, and  various  models  estimate each contribution  to  be  between 15 and
50 percent of the sulfur emitted.

Even with limited  data,  some further generalizations  are  possible.   Because
there is no  significant long-term  build up of sulfur  in the atmosphere, the
average amount  of  sulfur  emitted in  the  eastern  United States per day will,
in general,  equal  the sum of  the average  amounts  of sulfur wet deposited and
dry  deposited in  the  eastern United States and  the  net  average  amount of
sulfur exported across boundaries.   Note  that because measurements of sulfur
atoms deposited or in the air  do  not  include labels  locating  the source of
the  sulfur,  to have  the  sum come  out  right, we must  use the  net amount
transported,  the amount  of  eastern U.S.  sulfur exported minus the amount of
non-eastern U.S. sulfur imported.   Since  emissions from the states just west
of the Mississippi, and Canadian emissions across  the eastern U.S. border are
each only about 20 percent of Eastern emissions, and since  the concentrations
across  these borders  are lower  than  the concentrations  over  much  of the
eastern United States, the  net  transport  summed over  all  borders appears to
be dominated by eastern U.S.  exports.

The  equality between  emissions  and the sum  of  deposition  and  transport
provides a  simple  picture  of  the  sulfur  budget, shown  in  Figure  III.30.
Sulfur  is released  into  the  atmosphere,  spends an average amount of time in
the  2 km of  air directly  above  the eastern United States  and  then is wet or
dry  deposited or carried  across  a  boundary.   Reasonably certain  (_+ 30%)  data
are  available  to  document emissions  and   wet  deposition;  poor data  (_+  300%
uncertainty)  exist  for  concentrations;  and no monitoring  data  exist for dry
deposition and  net transport.   Although   scant direct  information on cross-
boundary  transport  exists,   observations that  the  average  transport  wind
velocity (the average distance per  day  an air  parcel  travels)  is roughly 500
km per  day  and that  no  concentration build ups occur  at  borders imply  that
net  transboundary  transport  per  day  from the  eastern  United  States  is  less
than  1/3  the  mass  of  sulfur  in   the  atmosphere  above the  eastern United
States.   (Note:   The eastern United  States  is very approximately a square,
1500  km to  a  side.    Thus, 500   km/day/1500  km  » 1  per 3  days.)    This
information  has been used to  fill in  the  amounts  shown in Figure II1.30. For
a  more  detailed budget  for  the  eastern  North  American  continent  based on
additional   assumptions,   including   a  closer  look  at transport  into  the
Atlantic, see Galloway  et al.  (1984).    Since the average residence time in
the  air of  a given sulfur atom  is  1 to  3 days,  average amounts  emitted and
deposited per day  are convenient for considering atmospheric processes, while
for effects, yearly amounts are  probably  more  interesting.  In Table II1.10,
we show the  conversion from  amounts per day to amounts  per yeai—thus 100 gm
S per ha per day corresponds approximately to 40 kg S per ha per year.

Further  information  on sulfur  concentrations in  the   atmosphere  offers the
possibility  of  refining   these  estimates   considerably  and should  be a  high
priority for  future research.   If,  for  example,  the average concentration of
sulfur  in air turns out  to be at  the  low end of  the  observed range, near  5
ng S m-3,  then the  average  mass  of sulfur  above the  Earth's  surface  will


                                      138

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                                     MASS OF SULFUR
                                      ~ 100 - 300 gm S ha
                                  (Concentration « 5-15 ;jgm S m~  )
                                                            -3
             AVERAGE
            * 100 gm S
EMISSIONS
ha"  per day
                                       DRY
                                   DEPOSITION
CO
UD
       IMPORTED  S
    «rf^•—i
    Net Cross-Boundary
Transport < 1/3 x Mass per day
                                                                           EXPORTED S
                                          WET
                                      DEPOSITION
                                      *25  gm S ha"
            AVERAGE  EMISSIONS PER  DAY = AVERAGE  [MASS IN  AIR/RESIDENCE  TIME] =
           AVERAGE [DRY DEPOSITION + WET DEPOSITION + NET CROSS-BOUNDARY] PER DAY
   Figure  III. 30  Illustration of sulfur mass balance for the Eastern  United States.

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TABLE 111.10  COMPARISON OF DAILY AND ANNUAL
    EMISSIONS AND DEPOSITION ESTIMATES

S Emissions
S Wet Deposition
N Emissions
N Wet Deposition
Eastern U.S. Total Annual
9 Tg +_ 30%
2.2 Tg + 30%
4 Tg +_ 40%
1 Tg +_ 30%
Annual ha~l
40 kg +_ 30%
10 kg +_ 30%
18 kg _+ 40%
4.5 kg +_ 30%
Daily ha'1
100 g _+ 30%
25 g ^ 30%
40 g +_ 40%
10 g +_ 90%
                    140

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 be  approximately 100 g  S  ha~l.   Transboundary  transport can account for  at
 most  a  third  of the sulfur emitted;  therefore,  dry  deposition would have  to
 account for at  least a third of  the  sulfur.

 We may  call  this Case I.  The mass  balance equation  then  is

 Case I:  Average Emissions = Average [Mass in Air/Residence  Time]
                            = Average [Dry Dep.  + Wet Dep. +  Net  Cross Bds]

          100 g S/day =  [100 g  S/l day]  = [>30 g + 20-35  g +  <35  g]  per  day.

 If  concentrations  of S  in  air  are  higher at the upper  end  of  the  observed
 range,  say  15  ng  S m-3  or 300  g  S ha-1, which  we  call Case  II,   the mass
 balance  becomes

 Case  II:

          100 g S/day =  [300 g  S/3 days] = [>0 + 20-35 g  + <100 g] per day.

 In  Case II,  no  immediate  generalization about  the   relative  amounts of dry
 deposition  and  net  transboundary  transport   can  be  made.     The  two
 observations, (1) the average time between rainstorms is  greater  than 3  days,
 and  (2) wet  deposition  accounts for  at least 20  percent   of  the emitted
 sulfur,  provide  a lower  limit for  the average concentration  of S in  air of 3
 M9  m-3,   independent  of   the   (limited)   direct  measurements  of  concen-
 tration.

 Nitrogen Oxides.   Budget estimates for  nitrogen oxides  are  even cruder than
 those  for  sulfur oxides.   The  amounts  emitted  are  more  uncertain,  and the
 concentrations  in   air   are  even  more   poorly   determined  than  sulfur  con-
 centrations;  in particular no  counterpart  to Figure  111.10  exists.   Annual
 emissions  of  nitrogen oxides  for  the  eastern  United States  in  1980  were
 approximately  4 +_ 1.5  Tg N  yr-1  or  18  kg   ha~l  yr-1, or 50  g N  ha~l
 dayl.   The average  amount wet  deposited in  1980 was  approximately  5 _+ 2  kg
 N  ha-1.   Since  the ratio  of  nitrogen   in nitrate  to sulfur  in  sulfate wet
 deposited appears similar  to the  ratio  of  emitted  nitrogen to emitted sulfur
 (See  Figure  III.8),  it  is  likely  that the  proportions dry  deposited and
 carried across  boundaries will  also  be  similar.   However, higher proportions
 of  sulfur  at  remote  areas suggest  that dry deposition  may be  a   slightly
 larger fraction of the total for nitrogen oxides.

 It  is  important to recognize that the   apparent similarity  between  S  and  N
 emission and  deposition  ratios   holds only  for  annual  averages.   Pronounced
 seasonal  differences,  previously  illustrated  in   Figure  II 1.9,   must  be
 accounted for in any more detailed analysis.

 B.2.2  WHAT IS THE  RELATIVE IMPORTANCE OF NATURAL AND ANTHROPOGENIC
       SOURCES TO DEPOSITION IN  SENSITIVE REGIONS?

 Sulfur.   Most of the sulfur oxides deposited in eastern North America have an
anthropogenic  origin.  Anthropogenic  sources  of  sulfur oxides exceed natural
 sources in  eastern  North  America by roughly a  factor  of 20 (Section A.2.7.5).


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While  there  may not be  a  strict linear  relationship  between emitted sulfur
dioxide  and  distant  deposition,  most emitted  sulfur  is  transported  sub-
stantial  distances and  is  deposited  in  broad  regional  patterns  (Sections
A.2.3-A.2.6).

The  contribution  of  various  sources  to  regional   deposition  is  roughly
proportional  to the  amount  of  sulfur  emitted   (though  location  and  stack
height matter);  thus electric  utility  power plant emissions probably account
for slightly more  than half of eastern sulfur deposition.  Other major source
categories are  industrial boilers and metal  smelting and processing.

In  the  western  United States,  anthropogenic emissions are  at least 3  to  5
times natural  sulfur emissions and  contribute proportionately to deposition.

Nitrogen.  Similar  conclusions  may  be   drawn   for  nitrogen  oxides,  with,
however, greater uncertainties in the estimated level of natural  emissions.

Acidity.  Sulfate  and  nitrate account for  practically all  of the  anions in
precipitation in eastern North America.   Since  production  of these ions from
their precursors, almost entirely derived from anthropogenic sources, results
in production of hydrogen  ions,  the acidity in  precipitation must be largely
attributable to  human activities.

B.2.3  WHAT CAN  BE CONCLUDED ABOUT  DEPOSITION TRENDS FROM  EMISSIONS TRENDS?

We have  seen that  there are not enough network data  to support an estimate of
historical  trends   in  acid  deposition.     Although  emissions estimates  are
markedly uncertain, the uncertainty  in the  relative amounts  of emissions from
one  year to  another  should  be  substantially less  than  the  uncertainty in
overall  emission  amounts.    Furthermore,  projections  of  future  emissions
trends   can  be  plausibly   based  on  economic   projections,   fuel  mix,  and
assumptions  about  regulatory impact.  The question,  then,  concerns  to what
extent it  is  feasible  to extrapolate  from  current deposition information to
estimate  deposition  under   different emissions    conditions.  There  are  two
problems:   one  is establishing what current deposition  is; the  second is
making the extrapolation.

As  discussed  in Section A.2.1,  the  observed year-to-year  variation  in  wet
deposition  rates appears  to  be  as  much  as  30  percent,  so  several  years of
network  data will  be needed to provide an  adequate  base estimate  of current
wet deposition  rates.   Also, dry deposition rates are very poorly known.

Although  the  relationship   between  emission  and deposition  appears  roughly
linear,  provided  background   oxidant  levels are   kept   unchanged,  several
changes  in historical emissions  cast doubt on the assumption that deposition
in the past was  the same percentage  of emissions  that  it is at present.   One
is  the  trend toward the use  of  tall stacks  (see  Section  A.2.7.4)  which  has
probably  increased  the  proportion   of  sulfate and  the amount of  long  range
transport.  A second is the increase in nitrogen oxide emission which has had
an  undetermined effect  on  the  availability of  oxidants.   A third is  the
change in the relative emission rates of different regions.
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Emission  of  sulfur  oxides  has  increased  substantially  in  the southeastern
United States;  it  has  declined  in  the northeastern United States (see Figure
III.23).   At  current levels  of  uncertainty in deposition amounts,  it appears
justifiable  to extrapolate  emissions trends  in a  broad region  upwind,  in
estimating  deposition  trends;  but  high  confidence  should  not  be  placed  in
such extrapolation,  and it is likely  that  some non-linearity will appear  when
(and if) deposition  data are known with greater  precision.

B.2.4  HOW PREDICTABLE ARE REDUCTIONS IN DEPOSITION RESULTING FROM  REDUCTIONS
       IN EMISSIONS?

This question  cannot be answered generally  with confidence.   One  can begin
with a constrained  version of  this  question  and afterwards consider relaxing
various constraints.

Simple case.  A uniform X percent reduction  of sulfur emissions over a broad
region  upwind of  a receptor area  will  lead  to approximately  an  X percent
reduction  in  total  deposition  over  the  receptor area,  provided  that  the
deposition  amounts  compared  are  averages  over  several  years  and over  a
moderately  large  spatial  area  and  provided  that the amounts  of  co-emitted
oxidants are not appreciably altered.

We can now try to relax each of the constraining assumptions.

Temporal  averaging.  The year to year variation in wet deposition  at a point
is  substantial,  perhaps  30  percent,  so  several  years'  averaging will   be
needed to be certain a reduction has occurred.

Spatial   averaging.    The averaging  over  the  receptor  region probably  is
unimportant  provided that the  temporal  averaging has  been  done,  that  the
region  is  relatively   homogenous,  and  that  it  is  small   compared to  the
distance from most of the  sources.

Co-emitted oxidants.  Changes in the amounts of co-emitted oxidants may alter
the predicted equality between  the  percentage  reduction  in  emissions and the
percentage reduction in average deposition.  However, a substantial reduction
in emissions will  still lead to substantial reduction in average deposition.

Uniformity of  reduction.   A  sulfur atom  deposited at a  receptor  site will,
typically,  have traveled  a  substantial   distance and  will   arrive at  the
receptor  site  in  an air  parcel containing  sulfur atoms from  many sources,
many quite  distant  from  the source  of the  first atom.    When considering
averages  over  several  years,  then,   we   would  see  a  reduction   in  sulfur
emissions at one source is likely to  be very  approximately  equivalent to the
same size  of  reduction at another  source.  As  with  the case  of   co-emitted
species  one  cannot expect strict  proportionality:   an  X  percent  emissions
reduction that changes the mix  of tall-stack  and low  sources or that changes
the mix  of intermediate and  distant sources  may not produce  an  X  percent
reduction in average deposition at  a  site.   However,  substantial  reductions
in emissions will  lead  to  substantial  reduction in deposition.
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                         IV.  INTEGRATION AND SUMMARY


1.  ARE THE SULFUR AND 'NITROGEN COMPOUNDS FOUND IN THE AIR, IN SOILS, AND IN
    WATER PRIMARILY FROM ANTHROPOGENIC SOURCES?

Yes, for air.  Perhaps, for soils and water.   For the eastern United States,
anthropogenic sources account for at least 90 percent of the sulfur compounds
found  in air  and  at least 80  percent  of  the nitrogen compounds (ammonia and
its salts, and nitrogen oxides).   In soil and water  systems,  both anthropo-
genically derived  and  naturally derived  sulfur compounds  are  important; the
percentages cannot be readily established.  Biological production of nitrogen
compounds may about equal  amounts from anthropogenic sources in many soil and
water systems, however.

For concentrations of  sulfur  and  nitrogen compounds in air,  the argument is
simple [see Sections III.A.2.7.5 and III.B.2.2.2].  No appreciable storage of
these compounds occurs in the atmosphere  (molecules remain in the air at most
a  few  days),  so  the relative  proportion  of  anthropogenically  derived sulfur
in  the  air  directly reflects  the  proportion  of  current emissions  into the
air.

Aquatic and  terrestrial sulfur flows will reflect atmospheric proportions of
anthropogenic to non-anthropogenic sources,  but because  sulfur  can be stored
in  these  systems  for  substantial  periods  of time,  it  is necessary  to ask
whether the proportion more closely approximates that of the present, that of
a  decade  or  so ago or  that  of a  century or  more  ago.   Sulfur  enters a soil
system  through several  pathways:    mineral  weathering,   precipitation,  dry
deposition on  the  soil, washout of  material  dry  deposited on other surfaces
(the  forest  canopy,  for  instance),   and   the   fall  and  decomposition  of
biological material that has taken up sulfur either from the soil or the air.
Two mechanisms of  storage exist:    storage  in vegetation,  (just alluded to)
and  storage  of  sulfur in  the  soil  (adsorption  of  sulfur to  soil  particle
surfaces or chemically combined in organic matter).  Biological   storage  times
are  relatively short,  a  few years  to  perhaps decades.    Adsorption of most
sulfate deposited on soils can continue as long as several decades especially
in  the Ultisols  of  the Southeast.   Much of the  organically-bound sulfur in
soils  has  accumulated  over the  centuries.    Since by  1950  in  the eastern
United  States  sulfur  compounds in  the air  were  already  at least 80 percent
anthropogenically  derived,  even in  soils with  high adsorption  of sulfate,
there  will  be a substantial  excess  flow of  sulfur  over  that to be expected
with only naturally derived inputs.

Sulfur enters  aquatic  systems  through  all  the same pathways it  enters soils;
in  addition,  water passing through  soils may account for much  of the sulfur
entering an aquatic system.  Like the  soil system, the reservoir of  water and
sediments  can also  store  sulfur.   Because the  average  residence  time for
water  is  seldom  longer  than  a  decade,  in  most  lakes  only  the  sediments
provide significant sulfur  storage.

The  case  of nitrogen  compounds  is  in  one  respect simpler, because  nitrogen
adsorption  does  not  appear   significant in  soils.    Complications  arise,


                                     144

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however,  because  there  are  two  important  families  of  nitrogen  compounds,
ammonia with its salts, and  nitrogen  oxides.   Biological  activity  can affect
either  family   and  convert  between  them,  and  nitrogen  is  frequently  the
limiting  nutrient for  many  ecosystems.   Furthermore, the  biological  process
of nitrogen  fixation  of  nitrogen  gas from  the  air  can  act as another source
of nitrogen  compounds  for  soil  and aquatic systems.  Deposition  of nitrogen
compounds  from  the atmosphere  (primarily  anthropogenically  derived  in  the
eastern United States) dominates biological nitrogen fixation.

2.  HAVE THERE BEEN ADVERSE EFFECTS THAT CAN RELIABLY BE ATTRIBUTED TO
    ACIDIC DEPOSITION?

Yes.   Some lakes and  streams have been made  sufficiently  acidic  that their
fish populations have  been lost.

The earliest concerns  about acid  deposition  in Europe  and in  North  America
were  about harmful  effects on aquatic  systems.   Although  numerous diffi-
culties deter obtaining reliable historical data on aquatic chemistry, enough
studies  have been  done  at enough  different locations  to provide  a  clear
scientific  consensus.    pH  or  alkalinity  declines  have  occurred  in  some
surface waters  over broadly  distributed  regions in  Europe and North America;
the only  plausible explanation  for  these  changes  is  acid  deposition  from
anthropogenic sources.   The changes in aquatic  chemistry  have  in  some cases
led  to  those   in   fish  populations;  historical  field  evidence  from  the
Adirondacks, Canada, and Scandinavia, as well as confirmation from laboratory
and field  studies show mechanisms  through  which changes in aquatic chemistry
can harm both adult fish and fish  reproduction.

Sufficient  data  do  not exist to  support a  consensus  on how many  lakes  and
streams have been significantly altered  or how many will  change,  for better
or worse for fish,  at current deposition levels.

Harmful effects of  acidic  deposition or its  gaseous  precursors  on materials
exposed to the air are well  documented.   How much damage can be attributed to
broad  regional  background  levels  of acidifying  substances  (the  acidic depo-
sition phenomena that  are the subject of this report),  and  how  much is to be
viewed as the result of local urban air pollution is still in question.

3.  ARE THERE OTHER POTENTIALLY SERIOUS BUT NOT DEMONSTRATED ADVERSE EFFECTS
    OF ACID DEPOSITION?

Yes.    Acidic deposition might  be  implicated  in recently  reported regional
forest declines.

Over broad areas of the eastern United States and northern Europe substantial
declines in coniferous forest growth  and diebacks of forest  areas  have  been
observed  (Section  III.A.2.2.5).   The  declines or dieback  appeared approxi-
mately  25 years ago,  a  period  of  time  when  emissions  of  acid  precursors
increased  substantially  (Section  III.A.2.7).    A  number of  mechanisms  have
been proposed relating  forest declines  to  acidic deposition;  however,  more
detailed observations  attempting to establish the connection between declines
and deposition  have provided mixed evidence.   Some support but  also  some


                                     145

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contrary  evidence  exists for each  mechanism.   The  evidence  for and against
other  mechanisms for  forest decline,  not  involving  acidic  deposition,  is
similarly mixed  (Section II.B.2).

4.  WHERE ARE THE AREAS WITHIN THE UNITED STATES IN  WHICH ADVERSE EFFECTS ARE
    OCCURRING OR MAY OCCUR?

For acid  deposition  to cause adverse effects  it  is necessary  both  that the
environmental  system  of  concern  be  sensitive  to  deposition  and   that  it
actually receive substantial amounts of deposition.  Except for comparatively
small  areas  it  appears  that the  combination  of  sensitivity  and  high depo-
sition  is found only  in  the  northeastern  and  southeastern  United States,
especially in mountainous areas.

The  environmental  systems  of  most  concern are  aquatic  systems—lakes  and
streams—and  forests.   An aquatic  system appears  to  be vulnerable  to acid
deposition if  it can  provide only  a limited amount of  basic  cations and if
the  terrestrial  system within  the watershed  passes sulfur and/or  nitrogen
compounds through while adding  only  a  limited  amount of basic cations.  High
mountain  terrain, where  there are  steep slopes and  very little soil, passes
sulfur and  nitrogen  compounds  essentially  unaltered.   The same  is  true of
areas  where  the predominant soil  type  is Spodosol--acid  soils  that provide
limited amounts  of basic  material  and do not  adsorb sulfate.   Spodosols are
the predominant  soil  type over much of the northeastern  United States.

Other  soil  types in  which  future  effects on  aquatic  systems may  occur  are
Ultisols  together with certain  Inceptisols.   These  also do not provide many
basic cations; however, they do  adsorb  sulfate, thus slowing  the response of
the aquatic  system to  increased  acid deposition.   These soils predominate in
the  Southeast,  and  it is  quite possible  that  at  many locations  the  time
before response  would  be  between one and several  decades.   Since deposition
in the Southeast probably increased  one  to  two decades  ago (based on changes
in emissions; see Gschwandtner  et  al.  1985),  these soil regions might be the
locations where  new  adverse  effects would  be seen  in the  relatively  near
future.

In  Figure II.5, we  showed  the  regions whose  terrain   and  soil  type—moun-
tainous terrain  and  Spodosols—give  most  concern  for prompt response to acid
deposition;  and Ultisols and Inceptisols for delayed responses.  Using the pH
isopleths of wet deposition  from Figure  II1.7  as  representative contours for
high deposition  areas, we show  in  Figure  IV.1  the  deposition  contours super-
imposed upon  the terrain and soil  regions of  concern.   Finally,  we  show in
Figure IV.2 the deposition contours superimposed upon regions where extensive
areas  of  low surface  water alkalinity  are  found  (portions  of  the  regions
identified  <  200  ^eq ^~1).    If   deposition  does  not change  appreciably
in the next  decades,  the prompt response  regions,  mainly  the  Northeast and
mountainous West, should have little  change  in alkalinity.   As noted before,
the  alkalinity  in  delayed  response  regions,  mainly  southeastern  water
systems, may decrease.

Diebacks and declines  have been  observed in high elevation conifer forests in
the Northeast;  however,  this distribution may reflect  more the distribution


                                     146

-------
     Spodosols

  n  Ultisols
  M*J

  ^  Inceptisols

  H  Select Alfisols

  A]  Mountainous  Regions
Figure IV.1   pH contour lines  (Figure  III.7)  and  soil  regions  of
             concern in the  United  States  (Figure 11.5).
                                 147

-------
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-------
of observations than the actual  distribution of impacted forests.  We showjn
Figure  IV.3  deposition  contours  and  the  distribution  of  high-elevation
coniferous  forests.   To  the extent  that  acidic  deposition were  to  affect
forests through changes  in  aluminum mobilization in soils, the most sensitive
regions would  be  those  having vulnerable trees  where  Spodosols predominate,
with  future  impacts  possible  in  Ultisol  and  Inceptisol  regions.   To  the
extent  that acidic deposition directly  affects foliage,  the  most sensitive
regions would  be  found  where deposition  is heavy  and  vulnerable  species of
trees exist.  Neither effect may prove to be important.

5.  IS IT FEASIBLE TO IDENTIFY SOURCES RESPONSIBLE FOR THE DEPOSITION THAT
    PRODUCES ADVERSE EFFECTS?

All sources of sulfur and nitrogen compounds in the eastern United States and
Canada  contribute some  acidic  deposition  to  affected and  sensitive  areas.
Emissions from  these  sources go into the atmosphere,  are mixed, transformed
over  time  by  oxidizing agents,  and  eventually wet  or  dry  deposited.   A
substantial fraction  of  the  emissions will  be  transported well over  500 km
from  the  source,  and  the deposition pattern  from  any  source averaged  over a
year  or  so will  be spread  over  a very  large  area.    As noted  in  Section
II.A.2.3.7, locales in  eastern  North  America where acidic deposition appears
to have decreased the pH or alkalinity of surface waters are scattered over a
number  of broad  regions—New  England,  New  York,  New  Jersey,  Ontario,   Nova
Scotia.    If   acidic  deposition  were  found  to  affect  forests  adversely,
declines  and  diebacks would  be  expected  to  be similarly widespread, at least
over high elevations.

To  what extent  is it  feasible  to apportion among  sources  the  deposition
causing  adverse  effects?   One measure  is   the  strength of  the  source,  the
number  of  tons of sulfur and nitrogen compounds emitted per year.   This is
not  a  bad first  approximation  to  the relative  importance  of sources  over a
broad  region,  given  that  1) sensitive  areas are  widely distributed,  2)  a
substantial amount  of the  emitted  material  is transported long distances, 3)
there  is  considerable  uncertainty  in  our  knowledge  of  the  determinants of
long-range  transport.

Some  refinement  in  assessing the  relative importance  of sources is feasible.
A  source  close   to  a  sensitive   region  will  contribute  relatively  larger
amounts of  sulfur or  nitrogen  than will  a  source farther away.  Furthermore,
prevailing  weather  patterns  exist,  at  least in  broad  terms:   on the average
the wind  blows more often  from  the Southwest to the Northeast;  hence sources
upwind  will  contribute  relatively  more   to  deposition in  sensitive  regions
than  sources  downwind from them.   These  two effects can  be  seen by comparing
deposition  patterns with emission patterns.  We  show in Figure  IV.4 emissions
of  sulfur within  grids  (EPRI-1983) for  the eastern  United  States together
with  interpolated  (CARP  A-8)   deposition   lines   for  sulfur   in  rainfall.
Deposition  appears shifted  north  and east as  expected.   The reader should
note  that the  deposition  pattern  here  is   slightly  different  from  that in
Figures  IV.1,  IV.2,  IV.3  because  it is  sulfur compounds  not pH measured.
Also  only wet deposition is shown, because we  do not  have  adequate  data on
dry deposition  to determine  patterns.  One  further  refinement  is that sources
                                      149

-------
Figure IV.3  pH contour lines  (Figure  III.7)  and  high-elevation
             forests  in the  United  States  (Figure II.9).
                                150

-------
                                                 <78

                                                 78 to 391

                                                391 to 781

                                                 >781
Figure IV.4  Distribution of sulfur emissions  (calculated from $02)
             in the SURE area  (80 x 80  km  grids)  for summer 1977
             and sulfur deposition contour lines  from Figure III.4.
             Data are in g S ha-1 day-1.
                               151

-------
with tall  stacks  will  have a somewhat  greater  proportion of their emissions
transported long distances.

If, instead of  considering the  relative importance  of  sources  to deposition
at all sensitive  and affected localities,  we ask for the relative importance
of  sources to  a  specific  region  (though  still  a  broad region),  the data
summarized in Figure  IV.4, also permit  some  simple  generalizations.    We can
consider three  sensitive regions:  the  watersheds  and forests of eastern New
England and eastern  Canada, the northeastern mountains  (in  New York and New
England), and the southeastern mountains.  For each region, Figure IV.4 shows
the  density  of  emissions  from  sources  near  and  far  upwind  ("upwind"
determined from the deposition pattern).  Eastern New England is sufficiently
far from most sources  that most deposition  comes  from  sources  more than 500
km away.   The northeastern mountains probably  receive  roughly  equal  amounts
of deposition from  sources farther than 500  km  and  nearer than 500 km.  The
southeastern mountains probably receive most of their deposition from sources
within  500 km.   These  simple  observations  are supported by  more detailed
modeling (see 111.B.I.4).

The presently available  capability and  supporting data  are  insufficient  to
give reliable predictions  for the contribution  of a localized  source  region
to  a  localized  receptor  or  to  predict  impacts  of  localized  emission
reductions, when  the source is far from  the receptor.

6.  WHAT EFFECTS  CAN BE EXPECTED FROM CONTINUING PRESENT  TRENDS IN SULFUR
    AND NITROGEN  EMISSIONS?

In the absence  of new efforts at  regulating the emission of acid  precursors,
the  best  prediction  appears  to  be   that  sulfur  emissions  will   remain
relatively constant  in  the next decade, while  nitrogen  oxide emissions will
increase  slightly both  regionally and  nationally.   Total  emissions  of acid
precursors are  unlikely  to change more  than  10 percent.   The  prediction  is
based on continuing implementation of new  source performance  standards, which
will tend  gradually to reduce emissions as new  sources  replace old ones, and
a  moderate  increase  in  economic  activity,  which  will  tend  to   increase
emissions.    These  assumptions  have  operated  over  the  past   decade  and
emissions  trends  have satisfied  the  predictions, but other effects have been
important:  the enactment  and  implementation of the Clean Air Act amendments
of  1970,   which led  to  a  substantial  reduction  of   sulfur  emissions   in  the
early  1970's;  fuel  switching from  oil to  coal  in  response  to  the  energy
crisis, which  increased  sulfur  emissions during the  1970's; and  the  economic
recession  which reduced emissions  in  the years  beginning  about  1980.

If emissions  were to  remain within 10  percent  of  their  present values, then
deposition amounts  also   would,  although   there   might  be   some   regional
differences as  patterns  of emissions change.   Thus  deposition  would be more
likely  to  decline slightly in  the Northeast and to  increase slightly  in  the
Southeast,  judging  from  emissions  trends in the recent  past.   Changes of  10
percent  or less  in average  deposition are  smaller than  the year  to year
fluctuations  in  deposition  amounts   and   thus   would   not  likely   produce
noticeable changes  in the  response of either  aquatic systems  or  forests.
                                      152

-------
The real  question  is whether future  harm  would show  up  as a  result  of  the
accumulation of acidifying  substances  at present levels of deposition.   For
the case of aquatic  systems,  the most  important storage mechanism appears to
be sulfate adsorption  in  soils;  this would likely  be  important only  in  the
Southeast.  Thus,  a continuation of  deposition in today's amount  would  not
likely  change  by  very  much  the  numbers  of northeastern  lakes and  streams
adversely affected, though some  future change  in individual  lakes or streams,
perhaps as  a  result  of episodic fluctuations  in   deposition,  could  not  be
ruled out.  In the Southeast it  is  possible that more lakes and streams would
be adversely affected as  the  accumulation  of sulfate made  sulfate adsorption
less of a barrier to the passage  of sulfate into the aquatic system.

Because  the  mechanisms,  if  any,  through  which acid  deposition might  harm
forests are not understood, and,  in particular,  forest  response times are  not
known,  it is  impossible to  say at  present  whether  continued deposition would
produce any  adverse  effects.    Since  forest  growing  times are  as long  or
longer  than  the  two  decades or  so  that deposition  has  approximated  its
present values, accumulating damage would have  to be considered possible.
                                     153

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                                  REFERENCES
Adams,  D.F.,  S.O. Farwell,  E.  Robinson,  M.R.   Pack,  and W.L.  Bamesberger.
1981.    Biogenic  sulfur   source   strengths.    Environ.  Sci.  Technol.  15:
1493-1498.

Barrie, L.A.  1982.  Environment Canada's long-range transport of atmospheric
pollutants  programme:     Atmospheric   studies,   pp.   141-161.     ln_  Acid
Precipitation:   Effect  on  Ecological  Systems.   F.M.  D'ltri,  Ed.   Ann Arbor
Science, Ann Arbor, MI.

Barrie,  L.A.   and  A.  Sirois.     1982.     An  analysis  and  assessment  of
precipitation  chemistry  measurements made by CANSAP (The Canadian Network for
Sampling  Precipitation):     1977-1980  Report  AQRB-82-003-T.    Atmospheric
Environment Service,  Downsview,  Canada, 163 p.

Barrie, L.A.,  J.M. Hales,  K.G.  Anlauf, J.  Wilson, A.  Wiebe,  D.M.  Whelpdale,
G.J.  Stensland,  and  P.M.   Summers.    1982.   Preliminary  data interpretation
U.S./Canada MOI, Monitoring and Interpretation Sub-Group.   Report No. 2F-1.

Bowersox, V.C. and G.J.  Stensland.   1981.   Seasonal patterns  of sulfate and
nitrate  in  precipitation   in  the  United  States,   _^n  Proceedings of  the 7th
Annual  Meeting,  Air  Pollution Control Association,  Philadelphia,  PA.   June
21-26.

Brouard, D., M. Lachance,  G.  Shooner,  and R. Van Collie.  1982.  Sensibiite a
1'acidification de quatre  rivieres a saumons de  la Cote nord du Saint-Laurent
(Quebec).  Can. Tech. Rep.  Fish. Aquat. Sci.  1109F, 56 p.

Budd,  W.,  A.  Johnson,  J.   Huss,  and  R.   Turner.    1981.    Aluminum  in
precipitation,  streams  and   shallow   groundwater   in   the  New  Jersey  Pine
Barrens.  Water Resources  Res. 17(4):1179-1183.

Cowling, E.B.   1982.   Acid precipitation  in historical  perspective.   Environ.
Sci. Technol.  16:110A-123A.

Davis,   R.B.,   S.A.    Norton,   C.T.   Hess,   and   D.F.    Brakke.       1983.
Paleolimnological  reconstruction  of  the  effects  of atmospheric deposition of
acids and heavy  metals  on  the  chemistry  and biology of lakes in New England
and Norway.  Hydrobiologia 103:113-123.

Dawson, G.A.   1977.   Atmospheric  ammonia from undisturbed land.  J. Geophys.
Res. 82:3125-3133.

Dickson, W.  1975.  The Acidification  of Swedish Lakes.   Rept. Inst. Freshw.
Res. Drottningholm 54:8-20.

Drablrfs,  D.  and  A.  To!Ian,  eds.     1980.    Ecological   Impact  of  Acid
Precipitation,   Proceedings  of   an   International   Conference,  Sandefjord,
Norway.  SNSF Project, Oslo.
                                     154

-------
Durham, J.R.  and K.L.  Demerjian.   1984.   Atmospheric acidification chemistry:
A review.  In  Acid  Deposition:   Environmental, Economic, and  Policy Issues.
D. Adams ancTft. Page,  eds.   Plenum Press.

Electric Power Research  Institute.   1983.  The sulfate  regional  experiment:
Report of findings.   Vol. 2.  EPRI EA-1901, Final  report, March 1983.

Eriksson,  E.    1952.     Composition  of  atmospheric  precipitation.    Tellus
4:280-303.

Eyre, S.R.  1963.   Vegetation and Soils:  A World  Picture.  Aldine Publishing
Co., Chicago, IL.

Friedland, A.J., A.M.  Johnson, and T.G.  Siccama.   1984.   Trace metal content
of  the forest  floor in  the  Green Mountains of Vermont:  Spatial and temporal
patterns.  Water,  Air, and Soil  Pollut.  21:161-170.

Friedland,  A.J.,  R.A.   Gregory,  L.   Karenlampi,  and  A.H.   Johnson.   1985.
Winter damage  to  foliage as  a  factor  in  red  spruce decline.    Can. J. For.
Res. 14:963-965.

Galloway, J.N., C.L. Schofield,  G.R.  Hendrey, N.E. Peters, and A.H.  Johannes.
1980.    Sources  of  acidity  in   three   lakes  acidified during  snowmelt,  pp.
264-265.   J_n_ Ecological Impact  of  Acid  Precipitation.   Drablrfs, D.  and A.
Tollan,  eds.  Proc.  of an International  Conference, Sandefjord, Norway.  SNSF
Project, Oslo.

Galloway, J.N., D.M.  Whelpdale,  and  G.T. Wolff.   1984.  The flux  of S and  N
eastward from North America.  Atmos.  Environ. 18(12):2595-2607.

Gillani,  N.V.  and W.E.  Wilson.    1983.   Gas-to-particle  conversion  of  sulfur
in  power plant plumes:   II.  Observation  of liquid-phase conversions.  Atmos.
Environ. 17(9):1739-1752.

Goble,   R.L.    1982.     Atmospheric  processes  affecting  acid  deposition:
Assessing  the  assessments  and  suggestions for further  research.   American
Association for Advancement of Science,   Fall 1982.

Gschwandtner,  G.,   K.C.  Gschwandtner,  and  K.  Eldridge.    1985.    Historic
emissions  of  sulfur and nitrogen oxides in the  United  States  from 1900 to
1980,  Volume  I.   Results.   EPA-600/7-85-009a, U.S.  EPA,  Office of Research
and  Development, Washington, DC.

Gschwandtner, G., C.O. Mann,  B.C. Jordan, and  J.C.  Bosch.   1981.  Historical
emissions of sulfur and  nitrogen oxides  in the Eastern United States by state
and  county.    Presented  at the  74th Annual  Meeting,  Air  Pollution Control
Association, Philadelphia,  Pennsylvania,  June 21-26, 1981.  Paper 81-30.1.

Haurwitz,  B.  and  J.M. Austin.   1944.   Climatology.   McGraw-Hill,  New York,
NY.
                                     155

-------
Henriksen,  A.,  O.K.  Skogheim,  and B.O.  Rosseland.  1984.   Episodic changes in
pH and  aluminum-speciation  kill  fish  in  a Norwegian  salmon  river.   Vatten
40:225-260.

Hepting, G.H.    1971.    Diseases of  forest and  shade  trees  of  the  United
States.  U.S. Dept. of  Agriculture,  Forest Service,  Agriculture Handbook No.
386.
Hilst, G.R.,  P.K.  Mueller,  G.M. Hidy,  T.F.  Lavery, and J.G.
EPRI Sulfate Regional Experiment:  Results and  Implications.
Research Institute, Palo Alto, CA.   Report No.  EA-2165-SY-LD.

Husar,  R.B.    1983.   Possible  remedies to  "acid  rain".
Pollution  Impact and  Trend  Analysis  (CAPITA),  Washington
Louis, MO.  37 p.
                                                 Watson.
                                                 Electric
         1981.
         Power
Interagency  Task  Force  on  Acid  Precipitation.
Precipitation Assessment Plan.   June 1982, 92 p.

Jeffries, D.S., C.M. Cox, and P.J. Dillon.   1979.
and  streams  in central  Ontario  during snowmelt.
36:640-646.
                                        1982.
                                                Center  for  Air
                                                University,  St.
National  Acid
                                      Depression of pH in lakes
                                      J. Fish.  Res.  Board Can.
Johnson, A.H.
Appalachians:
67:68-72.
and T.G. Siccama.  1984.  Decline of red spruce in the northern
  Assessing  the  possible  role  of  acid  deposition.    Tappi J.
Johnson,   D.W.   and   J.O.   Reuss.     1984.
atmospherically deposited  sulphur  and nitrogen.
B.  305:383-392.
                                    Soil-mediated   effects  of
                                    Phil.  Trans.  R. Soc.  Lond.
Johnson, D.W., J. Turner, and J.M. Kelly.  1982.  The effects of acid rain in
forest nutrient status.  Water Res. Research 18:449-461.

Johnson, D.W.,  I.S.  Nilsson,  J.O. Reuss,  H.M.  Seip,  and R.S. Turner.  1985.
Predicting  Soil  and Water  Acidification,  Proceedings of a  Workshop Held in
Knoxville,   TN,   March   26-29,   1984.     ORNL/TM-9258,   Oak  Ridge  National
Laboratory,  Oak Ridge, TN.

Jones,  H.,   J.  Noggle,   R.  Young, J.  Kelly,  H.  Olem,   G.  Hyfantis,  and W.
Parkhurst.   1983.   Investigations of  the  cause of fishkills in fish-rearing
facilities  in Raven Fork watershed.  Report TVA/ONR/WR-83/9, Tennessee Valley
Authority.
Keller,  W.
tributaries.
1983.    Spring  pH and  alkalinity depressions  in  Lake  Superior
 J.  Great  Lakes Res. 9:425-429.
Koerber,  W.M.   1982.   Trends in S02  emissions  and associated release  height
for Ohio  River Valley power plants.   Paper No. 82-105, 75th Annual Meeting of
the Air Pollution Control Association, New Orleans, LA.
                                     156

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Lefohn,  A.  and  G.  Klock.    1983.    The possible  importance  of naturally
occurring  soil  processes  in  defining  short-term pH depressions  observed in
United  States'  surface waters.  Draft  Report,  U.S.  Environmental Protection
Agency, Washington, DC.
Leivestad,  H.  and  I.  Muniz.
river.  Nature 259:391-392.
           1976.   Fish kills  at low  pH  in a  Norwegian
Likens, G.E.  1976.  Acid precipitation.  Chem. Eng. News 54:29-44.
Lyons, W.A.   1975.
environments.    In
Analyses.  Amer. "Meteorol. Soc., Boston,
Turbulent diffusion and pollutant  transport in  shoreline
Lectures  on   Air  Pollution  and  Environmental  Impact
                    MA.
MAP3S/RAINE.    1982.    The  MAP3S/RAINE  precipitation  chemistry  network.
Statistical   overview  for   the   periods   1976-1980.     Atmos.   Environ.
16:1603-1631.

Martin,  C.W.     1979.     Precipitation   and  streamwater  chemistry  in  an
undisturbed forested watershed in New Hampshire.  Ecology 60:36-42.

Miller,  J.M.    1981.    Trends  in  precipitation composition  and deposition.
Work Group  2,  Report  2-14,  U.S./Canada  Memorandum of Intent on Transboundary
Air Pollution, July.  Chapter II.

National  Academy  of  Sciences.   1981.   Atmosphere-biosphere  interactions:
Toward  a  better  understanding  of the ecological  consequences  of fossil  fuel
combustion.  National Academy Press, Washington, DC.

National Academy  of Sciences.    1983.   Acid  Deposition Atmospheric  Processes
in Eastern  North America.  National Academy Press, Washington, DC.

National Academy  of Sciences.    1984.    Acid Deposition:   Processes of Lake
Acidification.     National   Research   Council,   National    Academy  Press,
Washington, DC.

Olson,  M.P.,  E.C.  Voldner,   K.K.  Oikawa,  and  A.W.  MacAfee.     1979.    A
concentration/deposition model  applied  to the  Canadian  long-range  transport
of air  pollutants  project:   A technical  description.   Report No. LRTAP79-5,
Environment Canada, Downsview,  Ontario,  Canada.

Omernik, J.M. and C.F. Powers.   1982.  Total  alkalinity of surface waters - a
national map.  EPA-600/D-82-333, Sept.  1982,  U.S. EPA, Corvallis, OR.

Patterson,  D.E.,  R.B. Husar, W.E. Wilson,  and L.F. Smith.  1981.   Monte Carlo
simulation  of  daily  regional   sulfur  distribution:    comparison with  SURE
sulfate  data  and  visual  range  observations  during  August  1977.   J.  Appl.
Meteorol.  20:404-420.

Rosenqvist, I.Th.   1978.   Acid precipitation and other  possible  sources for
acidification of  rivers and lakes.   Sci.  of the  Total Env.  10:271-272.
                                     157

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Schnoor, J.L. and  W.  Stumm.   1984.   Acidification of aquatic  and  terrestrial
systems.  In Chemical  Processes  in Lakes, W.  Stumm,  ed.   Wiley  Interscience,
27 p.

Schofield, C.L.   1976.  Lake acidification in  the Adirondack Mountains  of  New
York:  causes and  consequences.   Proc.  First  Int.  Symp.  Acid  Precipitation
and the  Forest  Ecosystem,  Columbus,  Ohio.   A.S. Dochinger  and  T.A.  Seliga,
eds.  Upper  Darby,  PA.   USDA  Forest  Serv.  Gen.  Tech.   Rep.  NE-23.   p.  477.

Schofield, C.L.    1977.    Acid  snow-melt  effects  on  water quality  and  fish
survival in  the  Adirondack  Mountains   of  New  York  State.   Res.  Proj.  Tech.
Comp. Report, Project  No.  A-072-NY, Office  of  Water Research  and  Technology,
Washington, DC.

Schutt, P.  and   E.B.  Cowling.   1985.   Waldsterben  -  A  general  decline  of
forests in  central  Europe:  Symptoms,  development, and  possible  causes of  a
beginning breakdown of forest ecosystems.  Plant Disease  69:000.

Schwartz, S.E.    1982.   Gas-aqueous  reactions of sulfur and  nitrogen  oxides in
liquid water clouds.    Presented  at  Acid Rain  Symposium, American  Chemical
Soc., Las Vegas, NV, March-April, 1982.

Shaffer, P.W. and  J.N.  Galloway.   1982.   The  effect of  acidic deposition on
the composition   of  streams  in  Shenandoah   National   Park.   Proceedings  of
Symposium on Acid  Precipitation  from  International  Symposium  on  Hydrometeor-
ology.  Denver,  CO.

Shannon, J.D.  1981.  A model  of  regional  long-term average sulfur atmospheric
pollution, surface removal,  and  net horizontal  flux.   Atmos.  Environ. 15:689-
701.

Shannon, J.D.,  J.B. Homolya, and  J.L.   Chevey.   1980.  The  relative importance
of primary  vs.   secondary  sulfate.   EPA  Project  Report   IAG-AD-89-F-1-116-0.

Siegel, D.   1981.   The  effect  of  snowmelt  on  the  water  quality  of  Filson
Creek and Omaday Lake, northeastern Minnesota.  Water Resources Res.
17:238-242.

Silsbee, D.  and  G.  Larson.   1982.  Water  quality  of   streams  in  the  Great
Smoky Mountains  National Park.  Hydrobiologia 89:97-115.

Soil Survey  Staff.   1975.   Soil  Taxonomy.   U.S.  Dept.  of  Agriculture   Soil
Conservation Service, Agriculture Handbook No.  436.

Thompson, M.E.  and M.B.  Button.   1981.    Selected  Canadian  lake—cations,  sul-
fate, pH,  PC02   relationships.   Environment  Canada,  Canada Centre  for Inland
Waters, 31 pp.
                                     158

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Thompson, M.E. and M.B.  Hutton.   1982.  Sulfate  in  lakes  of eastern Canada:
Calculated atmospheric loads compared with  measured wet deposition.  National
Water Research Institute, Environment Canada, Burlington, Ontario.

Toothman,  D.A.,   J.C.   Yates,  E.J.  Sabo.    1984.    Status report on  the
development  of  the  NAPAP  emission  inventory  for the  1980 base  year  and
summary  of  preliminary  data.   EPA-600/7-84-091, U.S.  EPA,  Office of Air and
Radiation and Office  of Research  and Development,  Washington, DC.

U.S. Climatological Atlas.  1968.  U.S. Dept. of Commerce,  Washington, DC.

U.S.  Environmental  Protection  Agency.    1978.     National  air   pollution
emissions estimates,  1940-1976.  EPA-450/1-78-003.

U.S./Canada.   1982.    Memorandum of Intent  on Transboundary Air Pollution.
Work Group 3B, Draft Report.

U.S./Canada.   1983.    Memorandum of Intent  of Transboundary Air Pollution.
Impact Assessment, Working Group  I.   Final  Report, January 1983.

U.S. Environmental Protection Agency.  1984.   National Air Pollutant Emission
Estimates, 1940-1983.   EPA-450/4-84-028, Office of Air and Radiation, OAQPS,
Research Triangle Park, NC.

Wenblad,  A.  and  A.  Johansson.   1980.   Aluminum   i  foer surade vaestsvenska
sjoear.  Vatten 36:154-157 cited  in Aquat.  Sci. Fish. Abstracts 11(1):100.

Wright,  R.,  N.   Conroy,   W.  Dickson,  R.  Harriman,  A.   Henriksen,  and  C.
Schofield.   1980.   Acidified  lake  districts of the world:   A  comparison of
water  chemistry  of lakes  in  southern  Norway,  southern  Sweden, southwestern
Scotland, the Adirondack  Mountains  of  New  York, and  southeastern Ontario, p.
377-379.   ln_ Ecological  Impact  of  Acid  Precipitation.   D. Drabltfs and A.
Tollan,  eds.  Proc. of an  International Conference, Sandefjord,  Norway.   SNSF
Project, Oslo.

Wright,  R.F.   1983.    Predicting  acidification  of  North American  lakes.
Norwegian Institute for Water Research, Oslo, Norway.  Report No. 4/1983.
                                     159

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                       APPENDIX A - Steering Committee
                      Dr. John D. Bachmann
                      U.S. EPA
                      Mail Drop 12
                      Research Triangle Park,  NC  27711

                      David A. Bennett, Project Officer
                      U.S. EPA
                      RD 676,  Room 3220H
                      401 M Street SW
                      Washington, DC  20460

                      Dr. Michael Berry
                      U.S. EPA
                      Mail Drop 52
                      Research Triangle Park,  NC  27711

                      Dr. Ellis Cowling
                      School  of Forest Resources
                      2028-F Biltmore
                      NC State University
                      Raleigh, NC  27650

                      Dr. Michael Davis
                      U.S. EPA
                      ECAO, Mail Drop 52
                      Research Triangle Park,  NC  27711

                      Dr. Ken L. Demerjian, Director
                      U.S. EPA
                      Meteorology & Assessment Division
                      Mail Drop 80
                      Research Triangle Park,  NC  27711

                      Dr. J.H.B. Garner
                      ECAO, Mail Drop 52
                      U.S. EPA
                      Research Triangle Park,  NC  27711

                      Dr. Ray Wilhour, Chief
                      Air Pollution Effects Branch
                      Corvallis Environ. Research  Lab
                      200 SW 35th Street
                      Corvallis, OR  97330
Former  members:   J. Larry Regens
                  Jeanie Austin
                                    A-l

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                         APPENDIX B  -  CARP Authors
                         Chapter A-l  Introduction


Altshuller, Aubrey  Paul,  Environmental Sciences Research Laboratory, U.S.
     Environmental  Protection Agency, MD 59, Research Triangle Park, NC,
     27711.

*Nader, John S.,  2336  New Bern Ave., Raleigh, NC  27610.

*Niemeyer, Larry  E., 4608 Huntington Ct., Raleigh, NC  27609.


           Chapter  A-2  Natural and Anthropogenic Emission Sources


Homolya, James B.,  Radian Corp., P. 0. Box 13000, Research Triangle Park,  NC
      27709.

Robinson, Elmer,  Civil  and  Environmental Engineering Dept., Washington State
     University,  Pullman, VIA, 99164.


                      Chapter A-3  Transport Processes


*Gillani, Noor V.,  Mechanical Engineering Dept., Washington University,
     Box 1185, St.  Louis, MO  63130.

Patterson, David  E.,  Mechanical Engineering Dept., Washington University,
     Box 1124, St.  Louis, MO  63130.

Shannon, Jack D., Bldg. 181, Environmental Research Div., Bldg. 181, Argonne
     National Laboraory,  Argonne,  IL  60439.


                   Chapter A-4  Transformation Processes

Gillani, Noor V., Mechanical Engineering Dept., Washington University,
     Box 1185, St.  Louis, MO  63130.

Hegg, Dean A., Atmospheric  Sciences,  AK-40, University of Washington,
     Seattle, WA   98195.

Hobbs, Peter V.,  Dept. of Atmospheric Sciences, AK-40, University of
     Washington,  Seattle, WA  98195.

*Miller, David F., Desert Research  Institute, University of Nevada, P. 0.  Box
     60220, Reno, NV   89506.


*Served as co-editor.
                                   B-l

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Whitbeck, Michael, Desert Research  Institute, University of Nevada, P. 0. Box
     60220, Reno, NV  89506.


           Chapter A-5  Atmospheric  Concentrations and Distributions
                            of Chemical  Substances


Altshuller, Aubrey Paul, Enviromental  Sciences  Research Laboratory, U.S.
     Environmental Protection  Agency,  MD 59, Research Triangle Park,
     NC  27711.
               Chapter A-6   Precipitation Scavenging Processes


Hales, Jerany M., Geosciences  Research  and Engineering, Battelle, Pacific
     Northwest Laboratories, P.  0.  Box  999, Richland, WA  99352.


                    Chapter  A-7  Dry Deposition Processes


Hicks, Bruce B., NOAA/ERL, Atmospheric  Turbulence and Diffusion Div., ARL,
     P. 0. Box E, Oak  Ridge, TNI  37830.


                    Chapter A-8  Deposition Monitoring


Hicks, Bruce B., U.S.  Dept. of Commerce, National Oceanic and Atmospheric
     Administration, Environmental  Research Laboratories, P. 0. Box E,
     Oak Ridge, TN  37830.

Lyons, William Berry,  Dept. of Earth Sciences, James Hall, University of New
     Hampshire, Durham,  NH  03824.

Mayewski, Paul  A., Dept.  of Earth Sciences, James Hall, University of New
     Hampshire, Durhan,  NH  03824.

Stensland, Gary J., Illinois State  Water Survey, 605 E. Springfield Ave.,
     P. 0. Box 5050, Station A,  Champaign, IL  61820.


                       Chapter A-9  Deposition Models


Bhumralkar, Chandrakant  M., Atmospheric Science Center, SRI International,
     333 Ravenswood Ave., Menlo Park, CA  94025.

Ruff, Ronald E., Atmospheric Science Center, SRI International, 333
     Ravenswood Ave.,  Menlo Park, CA  94025.


                                   B-2

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                         Chapter E-l  Introduction


Linthurst, Rick  A.,  Kilkelly Environmental Associates, Inc., P. 0. Box 31265,
     Raleigh,  NC  27622.


                     Chapter E-2  Effects on Soil Systems


Adans, Fred, Dept. of Agronomy  and Soils, Auburn University, Auburn, AL
     36849.

Cronan, Christopher  S., Land and Water Resources Center, 11 Coburn Hall,
     University  of Maine, Orono, ME  04469.

Firestone, Mary  K.,  Dept. Plant and  Soil  Biology, 108 Hilgard Hall,
     University  of California,  Berkeley,  CA  94720.

Foy, Charles D., U.S. Dept. of  Agriculture, Agricultural Research Service,
     Plant Stress Lab-BARC West, Beltsville, MD  20705.

Harter, Roberto., College of Life Sciences and Agriculture, James Hall,
     University  of New Hampshire, NH 03824.

Johnson, Dale W., Environmental Sciences  Div., Oak Ridge National Laboratory,
     Oak Ridge,  TN  37830.

*McFee, William  W.,  Natural Resources and Envi rormental Sciences Program,
     Purdue University, West Lafayette, IN  47907.


                     Chapter E-3   Effects on Vegetation


Chevone, Boris I., Dept.  of Plant Pathology, Virginia Polytechnic Institute
     and State University, Blacksburg, VA 24060.

Irving, Patricia M., Environmental Research Div., Bldg. 203, Argonne
     National  Laboratory, Argonne, IL  60439.

Johnson, Arthur H.,  Dept. of Geology D4,  University of Pennsylvania,
     Philadelphia, PA  19104.

*Johnson, Dale W., Environmental Sciences Div., Oak Ridge National
     Laboratory, Oak Ridge, TN  37830.

Lindberg, Steven E., Environmental Sciences Div., Bldg. 1505, Oak Ridge
     National  Laboratory, Oak  Ridge, TN   37830.

McLaughl in, Samuel B., Environmental Sciences Div., Bldg. 3107, Oak Ridge
     National  Laboratory, Oak  Ridge, TN   37830.
                                   B-3

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Raynal, Dudley J., Dept.  of Environmental  and Forest Biology, College of
     Environmental  Science and  Forestry,  State University of Mew York (SUNY),
     Syracuse, NY  13210.

Shriner, David S., Environmental  Sciences Div., Oak Ridge National
     Laboratory, Oak Ridge, TN  37830.

Sigal, Lorene L., Environmental Sciences  Div., Oak Ridge National Laboratory,
     Oak Ridge, TN  37830.

Skelly, John M., Dept. of PI ant Pathology, 211 Buckhout Laboratory,
     Pennsylvania State University,  University Park, PA  16802.

Smith, William H., School of Forestry and Environmental Studies, Yale
     University, 370 Prospect Street, New Haven, CT  06511.

Weber, Jerome B., Dept. of Crop Science,  North Carolina State University,
     Raleigh, NC  27650.


                  Chapter E-4   Effects on Aquatic Chemistry


Anderson, Dennis S., Dept. of Botany and  Plant Pathology, University of
     Maine, Orono, ME  04469.

*Baker, Joan P., NCSU Acid Deposition Progran, North Carolina State
     University, 1509 Varsity Dr., Raleigh, NC  27606.

Blank, G. B., School of Forest  Resources,  Bil tmore Hall, North Carolina State
     University, NC  27650.

Church, M. Robbins, Corvallis Environmental Research Laboratory, U.S.
     Environmental  Protection Agency, 200  SW 35th Street, Corvallis, OR
     97333.

Cronan, Christopher S., Land and  Water Resources Center, 11 Coburn Hall,
     University of Maine, Orono,  ME  04469.

Davis, Ronald B.,  Dept. of Botany and Plant Pathology, Univeristy of Maine,
     Orono, ME  04469.

Dillon, Peter J.,  Ontario Ministry of the Environment, Limnology Unit,  P. 0.
     Box 39, Dorset, Ontario, Canada, POA 1EO.

Driscoll , Charles  T., Dept. of  Civil Engineering, 150 Hinds Hall, Syracuse
     University, NY  13210.

*Galloway, James N., Dept. of Environmental Sciences, University of Virginia,
     Charlottesville, VA   22903.

Gregory, J. D., School  of Forest  Resources, Biltmore Hall, North Carolina
     State University, NC  27650.

                                    B-4

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Norton, Stephen A.,  Oept. of Geological Sciences, 110 Boardman Hall,
     University of Maine, Orono, ME  04469.

Schafran, Gary C., Dept. of Civil Engineering, 150 Hinds Hall, Syracuse
     University, Syracuse, NY  13210.


                  Chapter E-5  Effects on Aquatic Biology


Baker, Joan P., NCSU Acid Deposition Program, North Carolina State
     University, 1509  Varsity Dr., Raleigh, NC  27606.

Driscoll, Charles T.,  Dept. of Civil Engineering, 150 Hinds Hall, Syracuse
     University, Syracuse, NY  13210.

Fischer, Kathleen L.,  Canadian Wildlife Service, National Wildlife Research
     Centre, Environment Canada, 100 Gamelin Blvd., Hull, Quebec, Canada,
     K1A OE7.

Guthrie, Charles A., New York State Department of Environmental  Conservation,
     Div. of Fish and  Wildlife, Bldg. 40, SUNY-Stony Brook, Stony Brook, NY
     11790.

*Magnuson, John J.,  Laboratory of Limnology, University of Wisconsin,
     Madison, WI  53706.

Peverly, John H., Dept. of Agronomy, University of Illinois, Urbana,  IL 61801

*Rahel , Frank J., Dept. of Zoology, Ohio State University, 1735 Neil  Ave.,
     Columbus, OH 43210.

Schafran, Gary C., Dept. of Civil Engineering, 150 Hinds Hall, Syracuse
     University, Syracuse, NY  13210.

Singer, Robert, Dept.  of Civil Engineering, 150 Hinds Hall, Syracuse
     University, Syracuse, NY  13210.


                  Chapter E-6  Indirect Effects on Health


Baker, Joan P., NCSU Acid Precipitation Program, North Carolina State
     University, 1509  Varsity Dr., Raleigh, NC  27606.

Clarkson, Thomas W., University of Rochester School of Medicine, P. 0. Box
     R8B, Rochester, NY  14642.

Sharpe, William E.,  Land and Water Research Bldg., Pennsylvania State
     University, University Park, PA  16802.
                                    B-5

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                      Chapter  E-7   Effects  on Materials


Baer, Norbert S., Conservation Center  of  the Institute of Fine Arts,
     New York University,  14 East 78th Street, New York, NY  10021.

Kiraeyer, Gregory, Economic  and Engineering Services, Inc., 611 N. Columbia,
     Olympia, WA  98507.

Yocom, John E., TRC Environmental Consultants, Inc., 800 Connecticut Blvd.,
     East Hartford, CT 06108.
 U.S. Environmental Protection Agency
 Region V, IJhrary
 230 South  Dearborn Street  „**
 Chicago, Illinois  60604
                                   B-6

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