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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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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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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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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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
-------
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
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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
-------
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
-------
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
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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.
-------
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).
-------
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).
-------
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).
-------
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
-------
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
-------
Figure III.18 Climatology of air stagnation advisories issued over a ten-
year period. Adapted from Lyons (1975).
102
-------
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"! )
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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
-------
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
-------
>+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 >
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— ' ui
05
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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
-------
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.
-------
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.
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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).
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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.
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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:
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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
126
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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.
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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
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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.
-------
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,
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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
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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
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Figure IV.3 pH contour lines (Figure III.7) and high-elevation
forests in the United States (Figure II.9).
150
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<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.
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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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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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