EPA-600/9-80-050



                                                                               PB81-126443
          Critique of Methods to Measure Dry Deposition. Workshop Summary
         Bruce B. Hicks, et al
         Environmental Sciences Research Laboratory
         Research Triangle Park, North Carolina
         October 1980
                                                                            AL IAB,
                             U.S. DEPARTMENT OF COMMERCE  " r r  ;   L:-'^'  -^ "°
                           National Technical Information Service                '

-------
                                               EPA-600/9-80-050
                                              October 1980
CRITIQUE OF METHODS TO MEASURE DRY DEPOSITION
               Workshop Summary
                      by

       Bruce B. Hicks/ Marvin L. Wesely
         Argonne National Laboratory
           Argonne, Illinois  60439

                     and

                Jack L. Durham
     U.S. Environmental Protection Agency
Research Triangle Park, North Carolina   27711

                IAG 78-D-X0193

               Project Officer

                Jack L. Durham
  Atmospheric Chemistry and Physics  Division
  Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina   27711
             KPfiODUCED IV
             NATIONAL TECHNICAL
             INFORMATION  SERVICE
                BA DEFARTEXfit Of COMNEKf
                  SP8l«f IUO. V* SIM
  ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
      OFFICE OF RESEARCH AND DEVELOPMENT
     U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA   27711
                                                       Agency

-------
                                   TECHNICAL REPORT DATA
                            (Please read Instructions on the reverse before completing)
   •EPORT WO,
   EPA-600/9-80-050
                                                           3. RECIPIENT'S ACCESSION NO.
                                                                   1 2644
   •TLE AND SUBTITLE
   CRITIQUE OF METHODS TO  MEASURE  DRY DEPOSITION
   Workshop Summary
                                                         5. REPORT DATE
                                                          October  1980  Issuing Date.
                                                         6 PERFORMING ORGANIZATION CODE
  AUTHOR(S)
   Bruce B. Hicks, Marvin  L.  Wesely,  and
   Jack L. Durham*
                                                           8. PERFORMING ORGANIZATION REPORT NO
PERFORMING ORGANIZATION NAME AND ADDRESS
 Argonne National Laboratory
 9700 South Cass Avenue
 Argonne, Illinois
                                                           10. PROGRAM ELEMENT NO. *"*
                                                             C1MH1E/01-0510   (FY-80)
                                                           11. CONTRACT/GRANT NO

                                                             IAG 78-D-X0193
   SPONSORING AGENCY NAME AND ADDRESS
  'Environmental Sciences Research  Laboratory  -  RTP,
   Office of Research and Development
   U.S. Environmental Protection Agency
   Research Triangle Park, North Carolina   27711
                                                    NC
13. TYPE OF RE PORT AND PERIOD COVERED
  Final
                                                          4. SPONSORING AGENCY CODE


                                                           EPA/600/09
  . SUPPLEMENTARY NOTES
   **C2QN1E/07-0582  (FY-80)
     A05A1A/06-0010  (FY-80)
 16. ABSTRACT
         At the Workshop on Dry Deposition  Methodology,  held December 4 and 5, 1979,  at
   the Argonne National Laboratory  in Argonne,  Illinois, dry deposition measurement
   techniques were assessed for routine monitoring use.   A majority opinion was  reacheu
   that commonly-used techniques such as  surrogate surfaces and collection vessels are
   not sufficiently accurate for use in networks,  because the highly varied propertie
   of the natural surfaces of interest cannot  be  simulated adequately.  Further  researcn
   was recommended on dry deposition parameters  in order to estimate dry deposition
   rates, if possible, from measurements  of atmospheric  concentrations at a single
   height, together with observations of  surface  properties and micrometeorological
   parameters.  The ability to perform such investigations in the field is critically
   dependent upon advances in chemical and  physical  capabilities to provide methods with
   standard relative errors of less than  1  percent for a single instrument on successive
   measurements, or with time responses of  less  than 1 second.   These requirements are
   not being achieved for many pollutant  species.   At present,  the most promising
   methods for monitoring are eddy  accumulation, modified Bowen ration, and variance.
alternative views are presentee in appendix L.
'7. KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Air pollution
^Deposition
*Measurement
*Meeting
<&. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
1
b. IDENTIFIERS/OPEN ENDED TERMS
Dry deposition
19. SECURITY CLASS /Tins Report/
UNCLASSIFIED
20. SECURITY CLASS (This pagej
UNCLASSIFIED
c. COSATI l-icld/Group
13B
05B
21. NO. OF PAGES
22. PRICE
EPA. farm 2220-1 (Rev. 4-77)   PREVIOUS EDITION is OBSOLETE

-------
                                  DISCLAIMER
     This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for
publication.  Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
                                       ii

-------
                                   FOREWORD
     Responsible and effective management of our nation1s ecosytems requires
a quantitative understanding of the relationships between emissions of air
pollutants from all types of sources and air quality; transport,
transformation, and removal processes are involved.  The Environmental
Sciences Research Laboratory investigates these source-air quality
relationships for the purpose of providing the technical basis for air
pollution control strategies by

     •  conducting laboratory and field investigations of the chemistry,
        physics, and meteorology of air pollutants

     »  developing techniques and instrumentation for the measurement and
        characterization of air pollutants in ambient air and in source
        emissions.

     During the past several years, the public has become aware of the acid
rain problem and the subsequent acidification of aquatic systems.  Scientists
suspect that acidification results from the long-range transport/transfor-
mation and deposition of acidic gases and aerosols.  Wet and dry deposition
pathways of acids are presently thought to be comparable, but serious
reservations exist in accepting dry deposition estimates inferred from air
quality measurements on the basis' of measurements by dry deposition monitors
presently in use.  For these reasons, the U.S. Environmental Protection Agency
sponsored a workshop for technical experts on dry deposition phenomena to
critique current techniques and to recommend research and development.
                                    Dr° Alfred H. Ellison
                                    Director
                                    Environmental Sciences Research Laboratory
                                     iii

-------
                                   PREFACE
     Within the last decade, there have been improvements in urban air
quality, but deterioration has occurred on the regional scale.  Contributing
factors are the shifting of electrical power generation to rural areas and
the use of tall stacks.  There has been a recent awareness of the decline of
visual quality in the eastern United States, and a serious concern for what
now appears to be a significant trend of increasing cruantities of strong acids
removed from the atmosphere by both wet and dry deposition.

     Continued industrial growth is of great importance to us as Americans
wishing to preserve and expand our personal economic well-being.  However,
protection of atmospheric quality is also an important consideration, and has
been expressed through the Clean Air Act and its amendments.  The conflict
between these two important concerns can rationally be resolved through an
environmental air quality management program.  However, to be effective and
acceptable politically, air pollution control strategies need to be devised
that are based upon defensible procedures.  Two important methods providing a
basis for air pollution control programs are predictive modeling and
environmental monitoring; these two activities provide the mechanism for
selective control of those emissions that give rise to unacceptable loss of
air quality and undesirable ecological effects.

     Knowledge of dry deposition rates is necessary for the successful
application of predictive models.  Of equal importance is the routine or
semi-routine monitoring of dry deposition of pollutants, especially the strong
mineral acids.  Presently, our knowledge of rates is too limited, and our
ability to monitor in networks is severely restricted.

     There has been a continuing interest in dry deposition for many years.
Theory, methodology, and experimental results have been reviewed in an Energy
Research and Development Agency symposium (1974) and in many  subsequent
papers.  However, it was not the intent of this workshop to attempt to provide
an updated summary of the status of this research field.  Instead, the U.S.
Environmental Protection Agency (EPA) wished to focus its attention on two
principal items:

     1.  fhe identification of methods that might lead to the
     development of a routine technique for measuring dry deposition
     at monitoring stations, and

     2.  Fresh  or improved approaches to research grade measurement
     of dry deposition rates.
                                       iv

-------
Because of the shortness of time and limited resources, goals can be met only
by adopting a narrow sense of purpose and by rejeering extraneous, yet
important, topics.  It is important to understand what the purpose of this
workshop was as well as what it was not.

     The purpose wass

     1„  To identify the advantages and disadvantages of current
     methods for measurement of dry deposition of air pollutants*

     2.  To speculate on the potential of other methods.

     3.  To make recommendations to EPA for research and develop-
     ment in regard to (a) research methods,, instruments, field
     measurements, and tests, and (b) methods for evaluating and
     monitoring dry deposition on a routine or semi-routine "basis.

     The purpose was not:

     1.  To certify the methods and results obtained by the
     participants or others, or to lobby for a favorite method.

     2.  To address ecological effects.

     3.  To address transport modeling.

     4.  To obtain a consensus on everything*

     Regrettably, in the 'interest of obtaining a manageable level of verbal
exchange and a timely pace, it was not possible to invite all who are active
in this field or others with an interest in attending.  Our intent was to
select only one scientist to serve as a "representative" expert on a special
method.  There is a critical limit above which efficiency declines.  I take
the responsibility for defining it and attempting to keep the group small.

     Some of the participants may have wondered why they were invited.  The
reason is of interest, as it is in itself a reflection of the state-of-the-
art.  As more and more becomes known about the proccesses involved in air-
surface interaction, it is becoming increasingly clear that the critical con-
sideration might not be physical but chemical or biological.  For this reason,
we have recognized the need to look carefully at what is known in the closely
related fields of agrometeorology, forest methorology, etc.  We could not hope
to represent all of those related fields at this workshop.  However, we
certainly can take some first steps toward improving relationships.

     There are other fields of endeavor that were certainly poorly represented
here.  We apologize and suggest that all we could do was represent these areas
as best we could and boldly state our limitations.

-------
     The U.S. Environmental Protection Agency extends its "thanks" to those of
you who graciously offered your services in this workshop.  I am confident
that the Agency will benefit greatly from your knowledge and experience.  In
return, your greatest compensation will have been the warm inner-feeling that
results from the performance of public service.
                                           Jack L. Durham
                                       vi

-------
                                   ABSTRACT
     At the Workshop on Dry Deposition Methodology, held December 4 and 5,
1979, at the Argonne National Laboratory in Argonne, Illinois, dry deposition
measurement techniques were assessed for routine monitoring use.  A majority
opinion was reached that commonly-used techniques such as surrogate surfaces
and collection vessels are not sufficiently accurate for use in networks,
because the highly varied properties of the natural surfaces of interest can-
not be simulated adequately„  Further research was recommended on dry deposi-
tion parameters in order to estimate dry deposition rates, if possible, from
measurements of atmospheric concentrations at a single height, together with
observations of surface properties and micrometeorological parameters.  The
ability to perform such investigations in the field is critically dependent
upon advances in chemical and physical capabilities to provide methods with
standard relative errors of less than 1 percent for a single instrument on
successive measurements, or with time responses of less than 1 second.  These
requirements are not being achieved for many pollutant species.  At present,
the most promising methods for monitoring are eddy accumulation, modified Bowen
ratio, and variance.  Regardless of the method employed, monitoring sites
should be chosen that are representative of the surrounding areas in terms of
surface properties, meteorological conditions, and pollutant characteristics.
Alternative views are presented in Appendix C; in particular, research on
certain applications of surrogate surface sampling is recommended, and concern
indicated that unrealistic requirements of chemical analysis precision or
response time have been allotted for the three "most promising methods for
monitoring."
                                     vii

-------
                                  CONTENTS
Foreword .................... 	    ill
Preface  . . <> .	     iv
Abstract ... 	 ...... 	 .....   viii
Tables	« o . . o	      x
Acknowledgments  ..<> ..<,....<,<>... „	     xi

     1 e  Keynote Address	      1
     2»  Introduction	      3
     3 o  Summary	      5
     4.  Conclusions and Recommendations ..» 	      7
     5.  Critique of Methods	     12

         A.  Estimates of accumulation	     15
               Atmospheric radioactivity as a tracer 	     15
               Mass-balance studies. «.... 	     17
         B.  Flux monitoring .......... 	     18
               Open pots . .	     18
               Flat filters  . . 	 ...........     20
               Flat plates and shallow pans	     21
               Fiber filters ...........  	     22
               Sticky films  .... 0 .«'....  	     23
         C.  Flux parameterization	     25
               Box-budget studies  	     25
               Airborne eddy correlation	     27
               Gradients	     29
               Modified Bowen ratio  	     31
               Eddy correlation	     33
               Variance.	     35
               Tracer experiments  	     37
               Eddy accumulation	„	     39
               Leaf washing	     41
               Surface snow sampling ........ 	     43
               Chamber studies	     44
               Wind tunnel studies	     46

     6.  Indirect Calculation of Deposition Rates  	     48
             Concentration monitoring and interpretation 	     48
             Research needs	     50

Appendices
     A.  List of participants	     53
     B.  List of observers	     58
     C.  Dissenting views	     60

                                      ix

-------
                                    TABLES
Number                                                                   Page

  1  Summary of Methods Given in Section 5 to Measure Dry
       Deposition	<, .	   13

  2  Summary of Research Presently Needed on Dry Deposition
       Processes	«	   51

-------
                                             I
                               ACKNOWLEDGMENT^


     We sincerely thank all the participants for their contributions without
which this dry deposition methodology overview would not have materialized.
Our appreciation is extended to Argonne National Laboratory, Argonne,
Illinois, for providing the conference facilities and on-site conference
support? Rebecca Spencer deserves special mention for overseeing the necessary
preparations at Argonne.  Our sincere appreciation is also extended to Olga
Wierbicki of Northrop Services, Inc., Research Triangle Park, North Carolina,
for coordinating the conference and editing the proceedings.
                                      xi

-------
                                  SECTION 1

                               KEYNOTE ADDRESS
     It is a particular pleasure to welcome participants in the U.S.,
Environmental Protection Agency (EPA) Workshop on Dry Deposition Methodology
to Argonne National Laboratory (ANL).  Not too many years ago,  convening an
EPA workshop at a Department of Energy (DOE) laboratory might have seemed
somewhat unusual.  Today, however, the environmental research responsibilities
of these two agencies are increasingly recognized to be complementary:   EPA is
developing environmental criteria with a view toward minimizing unnecessary
impact upon energy production, and DOE continues to maintain a policy for
developing new energy technology in an environmentally acceptable manner.

     More to the point of holding an EPA meeting at this DOE laboratory, the
EPA Office of Research and Development now supports almost half of the work
conducted by Argonne's Atmospheric Physics Section, and more than three
quarters of the work of our Ecological Sciences Section.  At the same time,
DOE continues to fund related research and assessment programs dealing with
the anticipated environmental impact of emerging energy technology in a number
of ANL Divisions.  Clearly, in ANL these two federal agencies have already
found common ground.

     Turning to the concerns of today1s workshop, it is appropriate to remind
ourselves at the outset that these deliberations on the measurement of
pollutant deposition can benefit from our knowledge of the turbulent structure
of the lower atmosphere.  A number of now-classical, micrometeorological field
experiments conducted during the last two decades have developed an extensive,
basic understanding of the turbulent exchanges of heat, moisture, and
momentum, which combine to maintain the kinetic and thermal energy budgets of
the atmospheric boundary layer.  In the present context of contributing to our
understanding of the mechanisms of dry deposition, it is useful to remember
that these same atmospheric turbulent exchange circulations also maintain
local budgets of pollutant concentration:  contaminants steadily depleted in
the immediate vicinity of receiving surfaces by the deposition process are
continuously resupplied by turbulent mixing from above.  Each pollutant's
special chemical and physical properties determine its particular affinity for
the collecting surface and thus always control, and frequently dominate, the
rate at which dry deposition removes that material from the air.  However, in
the equilibrium flux layer in the free air above the collecting surface, the
aerodynamic pollutant-resupply rate equals the physically, chemically, or
biologically dominated surface removal rate.  This fact makes it possible to
utilize the techniques of modern micrometeorological flux measurement to

-------
evaluate the rate of dry deposition,  even though that removal is being
accomplished by virtually unmeasurable, complex events that take place
essentially in contact with the surface itself.

     Preliminary results of pollutant eddy flux measurements are now being
obtained at a number of laboratories, including ANL.   Unfortunately, since the
experimental equipment used is relatively complex,  these micrometeorological
methods are not suited to routine operation;  therefore, the question before
this workshop as to how dry deposition might best be monitored by a national
network remains unsolved.  Of course, a number of different dry deposition,
pollutant collectors are now available, but their inherent limitations become
clear when one considers the degree to which each of these devices either
interferes with the aerodynamics of the pollutant resupply circulation, or
fails to simulate the collection properties of nearby natural surfaces.  The
problem of selecting an operationally-feasible monitoring methodology is
therefore particularly difficult, and one readily understands why dry
deposition monitoring experiments to date have settled for a variety of
solutions, no one of which is entirely satisfactory.

     Nevertheless, since we do have an appreciable understanding of the
aerodynamic phase of the dry deposition process, some technical guidance for
monitoring methodology does exist.  It remains to select a procedure that does
not excessively violate either the aerodynamic or the collecting surface
criteria (or, at least, one that only violates them in some reproducible way),
and then to calibrate that method1s relative collection efficiency over a
range of physical and chemical pollutant characteristics, terrain and
vegetation types, and meteorological conditions.

     The monitoring problem is indeed difficult, but perhaps not entirely
intractable.  As Peter Medawar said of the useful tendency for new scientific
principles to clear away masses of inconclusive experimental observations,
"Since Newton formulated the law of gravity, we need no longer measure the
fall of every apple."  With more work, and some luck, conceivably something
approaching this ideal might be accomplished for dry deposition.
                                           Paul Frenzen
                                           Associate Director
                                           Radiological and Environmental
                                             Research Division,
                                           Argonne National Laboratory

-------
                                  SECTION 2

                                 INTRODUCTION
     The subject of dry deposition addresses all processes by which airborne
contaminants are removed from the atmosphere at the surface of the earth,
excluding those processes directly aided by precipitation.  Ultimately,
knowledge of the fate of pollutants is desired, particularly if the effects
are harmful..  For surface vegetation, for example, dry deposition can
sometimes be equated to a dose that causes specific harmful effects, but often
the effects of pollutants on biological systems occur after transport away
from the initial surface contacted and after some change in chemical form.
Since the surface flux of atmospheric contaminants affects concentrations
downwind, dry deposition affects the extent of areas in which significant
concentrations of pollutants are found.  Thus, dry deposition exerts a strong,
indirect influence on concentrations of pollutants found in precipitation and
on interactions between pollutants.

     Of concern here is primarily the rate of removal by dry deposition of
pollutants in the atmosphere, to some extent their immediate fate, and less
their effects, except those that aid in determining deposition rate.  Methods
used to assess or predict rates of dry deposition are extremely diverse,
largely because the types of contaminants and surfaces vary greatly.  Since
the final assessment of dry deposition must take place for the actual
environment considered, relevant studies have concentrated on experimental
efforts in the field.  Laboratory and purely theoretical work play a necessary
role in understanding the processes that control dry deposition.  Such
understanding leads to a productive use of data from networks monitoring air
quality, helps extrapolate present deposition results to pollutants not yet
fully investigated, and leads to better parameterizations of surface removal
for use in numerical models of pollutant distribution in the atmosphere.

     The purpose of this report is to evaluate and summarize methods for
determining routinely the dry deposition of gaseous and particulate pollutants
over rather large areas.  Section 5 presents the methods considered during the
workshop and reflects the scientific knowledge and opinions of the
participants.  The needs of each method were kept in mind for measurements
taken at scattered sites of uniform surface characteristics "typical" of the
surrounding area.  In the search for better methods of monitoring, a large
amount of effort at the workshop was devoted to assessing methods normally
used to parameterize the processes that control deposition.  Section 6
describes an application of the parameterizations:  using concentrations
measured at air-quality monitoring stations to calculate the pollutant fluxes
indirectly.  Our current knowledge is, of course, inadequate to achieve this

-------
goal for all important pollutants and surfaces?  some of the research needs
identified are briefly discussed in the second half of Section 6.

     In conformity to the orientation of the workshop, the conclusions and
recommendations in Section 4 are mainly concerned with the general scientific
credibility of methods used to measure dry deposition-  These recommendations,
which are derived from the discussions in Sections 5 and &, are highly
critical of past and present attempts to monitor dry deposition routinely.  No
attempt is made to present a schedule of research that should be accomplished,
since this subject was not explicitly addressed at the workshop.  Indeed it
would have been quite difficult, since competing research interests were
purposefully represented.  Specific details on research and monitoring methods
(such as height of measurements) are not given if such details were not
discussed explicitly.  Finally, no recommendations are made as to which
pollutants should be studied; this decision was not considered one of the
tasks of the workshop, but one more appropriately to be made by those
knowledgeable in health and ecological effects.

-------
                                  SECTION 3

                                   SUMMARY
     This workshop report provides evaluations of various methods for
estimating dry deposition of gaseous and particulate pollutants from the
atmosphere to the surface of the earth»  The intent is to provide the
scientific information needed in order that sound decisions can be made on
research, development, and coordination with regard to dry deposition«
Existing and potential techniques for monitoring and parameterizing dry
deposition are considered systematically and specific recommendations are
given for each method on its present and potential usefulness.  The viewpoints
presented are based on and limited to opinions expressed during discussion
that took place at the workshop.  A priority of research needs was not
established at the workshop and thus is not given here.

     The current capabilities for monitoring dry deposition are felt to be
inadequate because the methods now used, which usually employ surrogate
surfaces or open vessels, do not give the deposition at the actual natural
surface^  Most of the workshop participants felt that "calibration" of such
methods would not be successful or have very limited success c,  However, 20 to
•30% of the scientists present who have been or are making original
contributions to scientific research on dry deposition feel that the use of
surrogate surfaces should be pursued in some fashion.

     Fairly direct measurements of the net dry deposition can be obtained by
measurement of the accumulation of certain chemicals in rather large areas
over long periods of time.  Because of the considerable amounts of effort
necessary in application, this category of method clearly was never intended
to apply to routine monitoring, although the results of such studies are quite
useful in determining the fate of pollutants after reaching the surface.

     Parameterization studies are directed toward understanding the
relationships between vertical fluxes and the properties of pollutants,
surfaces, and the atmosphere.  Estimation of deposition rates needed for
numerical simulations of pollutant behavior in the lower atmosphere usually
are based on parameterizations of the deposition velocity (the ratio of flux
to concentration at a specified height in the atmosphere).  In this report,
evaluations of numerous methods are presented.  Of course, knowledge
accumulated from parameterization studies might allow the use of pollutant
concentrations monitored at a single height to estimate the vertical flux.  To
date, very few species of pollutants have been studied adequately to approach
this goal.  For example, the behavior of many species associated with strong
acids has not been determined, because the chemical sensors presently
available do not meet the specifications required by the techniques for

-------
measuring dry deposition.  In many cases,  there is no "sensor"  but instead a
procedure for chemical analysis best performed in a well-equipped laboratory.
Usually, measurement procedures should be able to detect mean differences of 1
to 10% in successive samples to within an accuracy of 10%,  in order to be
useful to measure deposition rates.  For flux measurement techniques that rely
on detection of turbulent fluctuations in pollutant concentrations near the
ground, the speed of response should be one second or less.

     A simple, but accurate, method for monitoring dry deposition rates
routinely is not available,.  Micrometeorological methods that have potential
in this regard are the techniques of eddy accumulation, modified Bowen ratio,
and variance methods.  It should be remembered, though, that even if these or
other methods prove very useful, the requirements on the selectivity and
performance of chemical and physical sensors are still quite high and, in
many cases, not yet achievable.

     Sites selected for parameterization and monitoring studies ideally should
be located in uniform terrain so that results are not affected by unusual
features of the surface unique to that location.  Monitoring sites, if they
are to be effective, should be located at surfaces that are typical of the
surrounding areas and that should have representative meteorological
conditions and pollutant characteristics.

-------
                                  SECTION 4

                       CONCLUSIONS AND RECOMMENDATIONS
     The rate of dry deposition of atmospheric pollutants depends on many
factors concerning the pollutants, surfaces, and atmospheric conditions.  To
evaluate methods -chat might potentially be used to monitor dry deposition, the
roles of all contributing processes must be considered, at least in broad
categories.  The most important processes that affect deposition rates can
often be identified as those corresponding to the largest resistance to
vertical transport or uptake.

     For small particles that are deposited by diffusion and impaction,
deposition is strongly affected by the resistance of the quasi-laminar
sublayer of air nearest to surface elements and by the microscale roughness
and stickiness of the surface„  For gaseous pollutants, the chemical and
biological nature of a surface, as well as its physical configuration, often
dominates the resistance to air-surface exchange.  Processes of (re)emission
and (re)suspension, which can greatly affect net deposition, are also highly
dependent on surface properties.  Hence, the following recommendation can be
made;

     1,  Since use of artificial collecting surfaces does not simulate the
     net dry deposition of particles and gases to natural surfaces, it
     would "be ill-advised to rely upon continued monitoring "by "open pot"
     or various other types of surrogate surfaces.  As a result,  present
     capabilities to monitor dry deposition in a practical, yet accurate,
     manner are inadequate,

Use of such surfaces has continued for estimating deposition of particles in
some cases, simply because of the lack of better procedures that can be
utilized with small cost and effort.  Notably, knowledge on how to "calibrate"
surrogate surfaces for gaseous or particulate deposition will probably never
be obtained.  At best, future efforts might relate the amount and type of
material collected by a surrogate surface to a corresponding atmospheric
concentration; it would then remain necessary to deduce surface fluxes from
the inferred concentrations, not an easy task in most cases at present.  It
would probably be easier and more accurate over natural surfaces to measure
concentrations directly rather than relying on artificial surfaces intended to
simulate the actual surface.

     In the process of choosing a site for monitoring (which assumes that a
suitable monitoring technique is available), the importance of knowledge of
surface characteristics must be emphasized.  This consideration is reflected
in another recommendation:

-------
     2.  Sites chosen for monitoring should "have a surface "typical" of
     the surrounding area; in the immediate vicinity of measurements,  'the
     surface should be a single uniform type.   The condition of local
     vegetation and the general environmental  conditions that affect the
     surface should be recorded*

Obviously, there might be some difficulty in finding suitable sites in many
locations«  An additional requirement for site selection is:

     3.  Monitoring sites should have pollutant and wind characteristics
     representative of the large surrounding area.  Records should, be
     maintained on the possible presence of pollutants from local
     sources and on local meteorological events such as precipitations
     fog, and dewfall.

The above two requirements for a good site imply that sites currently chosen
for wet and dry deposition monitoring may not be adequate for obtaining
representative estimates of dry deposition<>

     For monitoring, estimates of atmospheric resistance should be made
routinely, since this resistance can control the transfer of pollutants
that can be removed very efficiently by the surface, and since it is important
to identify conditions of atmospheric decoupling at night, when turbulent
vertical transport becomes exceedingly small.   Provided a few essential
measurements of atmospheric conditions are taken, microraeteorological
formulations currently available are adequate for estimating the atmospheric
resistance to vertical transport front a height of several meters to fairly
close to the surface.  We recommend the following?

     4.  At each monitoring site, wind speed,  atmospheric stability,
     and aerodynamic surface roughness should be determined, so that
     atmospheric resistances can be calculated and averaged over time
     intervals preferably not to exceed six hours; one~half to one hour
     is optimum.

     Current techniques intended to monitor dry deposition directly on a
routine basis do not appear to be adequate.  As stated above, surrogate
collecting surfaces do not simulate surface conditions sufficiently well in
most cases.  An alternative method is to attempt to infer surface fluxes from
pollutant concentrations measured at a single height; much work has already
been devoted to develop the technical means to perform the needed
measurements.  Concentration measurements at one height, however, give no
information on transfer processes in the atmosphere and at the air-surface
interface.  The following recommendation must therefore be made:

     5a,  Concentration measurement at a single height should not be  used
     by itself to provide a measure of dry deposition.

     The  availability of measurements of some pollutant concentrations leads
to questions as to what supporting information is required  in order to
interpret these measurements.   Deposition  rates can be estimated as the
product of concentration  and an appropriate deposition velocity.  The latter

                                      8

-------
 quantity can  be  derived  from  accurate parameterizations of the deposition
 processes.  As implied in Recommendation 4 above, micrometeorological aspects
 of parameterizations  can be estimated fairly well above uniform sites.
 However,  interactions of pollutants with the surfaces and the behavior of
 small  particles  in  the interfacial air layers are not well understood.
 Therefore, to interpret  past  and  future measurements of concentrations, at
 least  until means sufficient  for  accurately and routinely monitoring dry
 deposition are developed, further research is needed.  the following statement
 concludes Recommendation 5:

     5b,  In  order  to understand  the relationships between vertical
     fluxes and  concentrations measured at a single pointf research on
     the parameterization of  deposition processes should continue by use
     of a variety of  theoretical  approaches and experimental techniaues
     in the laboratory and the field.

 Cooperative efforts between micrometeorologists, biologists, and aerosol
 scientists, among others, are required? many recent studies have not utilized
 such cooperationo   For example, atmospheric investigations often do not study
 a  minimum set of variables concerning pollutant and surface characteristics in
 order  to  elucidate  the deposition processes.  On the other hand, ecological
 studies  often do not  adequately document atmospheric conditions.

     Based on present knowledge,  it is possible to identify certain
 information that would need to be obtained routinely at monitoring sites in
 order  to  estimate deposition  velocities for evaluating surface fluxes from
 concentrations measured  at a  single height.  Of course, this needed
 information must also be obtained during the parameterization studies that
 form the  basis for this  possible  monitoring technique.  As already stated,
 wind speed at some specified  height above the surface should be measured and
 atmospheric stability estimated,  both for periods not to exceed six hours.
 The type, height, and condition of vegetation present should be known.
 Furthermore,  consideration should be given to a number of other factors that
 appear to be  important in the case of certain pollutant species.  Soil
 moisture  content can  affect the uptake of some gaseous pollutants, especially
 over bare soil.  Some measure of  aridity and atmospheric stability might
 suffice to describe the  gross aspects of physical environment that affect
 plant  physiological functions; relative humidity and solar radiation
 measurements  might suit  this  purpose.  Other variables, particularly those
 dealing with  conditions  of vegetation, will probably be identified as
 important yet suitable for routine evaluation, as a result of future
 parameterization studies.

     Deposition velocities depend on the type of pollutant as well as on
 surface and atmospheric  conditions.  For particles, studies have indicated
 that deposition  is affected by diffusion, impaction, and sedimentation
processes that are continuous functions of particle size in overlapping
 ranges.  Measurements employing a single size fractionation of particles (and
hence providing  samples of "large" and "small" particles)  are not
 satisfactory,  since it is likely that the course and fine fractions each must
be resolved into several size classes.  Gases can be considered in categories
of similar chemical properties only to a limited extent.   Thus, to infer dry

-------
deposition from measurements of concentration at a single height, direct
measurement of each pollutant of interest is required.   The possible
importance of (re)suspension and (re)emission processes further underlies this
needo

     To perform parameterization research such as implied in Recommendation
5b, one would surmise that there are or will be available methods to measure
reliably the deposition rates»  In that case, the measurement techniques might
themselves offer the best means to monitor deposition rates.  The
corresponding recommendation reads as follows:


     6,  Techniques used in field parameterization studies should be
     developed and examined for applicability to routine monitoring,


The methods most recommended are micrometeorological in nature.  Verification
of the techniques requires some direct measurements of accumulation at the
surface; box-budget experiments, though usually too inaccurate, may someday
provide an estimate of errors.  The following statement, at least, can be
made;
     7.  Although micrometeorological methods offer the promise of
     providing a direct and precise measure of dry deposition,
     experimental complexity and sensor inadequacies combine to reduce
     confidence in them.  At this time, no micrometeorological method can
     "be accepted with absolute confidence,  There is need for continued
     intercomparison and comparison with other methods.
Fluxes of particles large enough to be affected significantly by gravitational
settling cannot be measured accurately by standard micrometeorological
techniques.  In fact, if such large particles are not excluded from detection
in some techniques, erroneous results might be obtained.

     Some micrometeorological methods can be identified as possibly worthy for
development for use in monitoring.  They are the variance method, the
eddy-accumulation techniques, and the modified Bowen-ratio technique.  Others
may be indicated as research progresses.  For the variance methods, the main
limitation seems to be in the ability to find or develop chemical sensors that
have fast response (at least 1 Hz) in addition to being linear and having low
noise.  'Ihe other two methods, and others likely to be suggested, rely on
accurate mean measurements of concentration differences.  Therefore, the
following recommendation is made:


     8.  Methods to measure dry deposition that depend on concentration
     differences detected in samples collected in the atmosphere must be
     supported by chemical/physical analysis methods with standard errors
     of 1% or less.
                                       10

-------
Ihis recommendation does not necessarily imply that absolute accuracies must
be > 1% or less°  If the modified Bowen-ratio technique is used, for example,
a single sensor might be rigged to alternately measure at two heights.  "Ihen
the requirement is that the difference, which will usually be 1 to 10% of the
mean^ be measured accurately.  Ihus sensor offsets and slow drifts have almost
no effect, and errors, say of > 10%, in span calibrations would produce errors
of only > 10% in the estimated vertical flux.

     Techniques to measure, let alone monitor, fluxes of species associated
with acid deposition are presently deficient.  Many of the reasons have
already been given in this section.  There is an urgent need to develop
methods with the required accuracy in measuring amounts of strong acids and
species such as ^804, HNC>3, 804°% NO3~, NH4+, NH3, and organic acids and
bases«  Ihis need leads to the following recommendations


     9,  Present capabilities to measure dry deposition for para-
     meterization studies as well as for monitoring must be improved
     in order to support the acid deposition programs of EPA .
                                      11

-------
                                  SECTION 5

                             CRITIQUE OF METHODS
     This section presents brief descriptions and evaluations of various
experimental methods for investigating and estimating dry deposition.  The
methods are representative of most research to date, and include some
especially promising techniques.  Purely theoretical efforts are not
considered separately, but this practice is not meant to imply that such work
is unimportant.  In fact, all of the experimental methods are directed to some
extent towards developing better theories on deposition; the parameterization
studies to be considered are specifically concerned with theoretical and
semi-empirical expressions of processes that affect deposition.

     The main focus is on methods capable of determining dry-deposition rates
of various pollutants either by measurement alone or by a combination of
measurements with numerical simulations.  The methods are sorted into three
categories:  (A) estimates of accumulation, (B) flux monitoring, and (C) flux
parameterization.  These distinctions are somewhat artificial? the categories
only reflect the intent of the methods as they are currently reported in the
scientific literature.  A list of the methods is given in Table 1.

     If the total accumulation of certain pollutants on a given area could be
measured directly, the answers most desired might be found without further
effort.  This approach constitutes the first category, (A) estimates of
accumulation.  Periods of time ranging from months to years are usually
considered; short-term variations cannot be identified.  While areas ranging
in size from the entire surface of the earth to individual roughness elements
can be addressed in principle, a more practical size is that of individual
watersheds characteristically from 1 to 100 km in horizontal dimension,  The
methods of estimating accumulation could potentially be used for routine
monitoring, but the effort required would probably be too large.

     Numerous attempts have been made to monitor dry deposition, i°e°, to
measure fluxes directly on a routine basis.  These techniques usually involve
sampling at individual points; they are considered in the second category,  (B)
flux monitoring.  Single measurements typically last from hours to weeks, and
so shorter-term variations are usually not detected.  Ideally, the point
measurements are taken over a well-defined surface, e.g., one of uniform
vegetation.  Actually, all of the point- and areal-sampling  field methods
listed in this section are potential monitoring techniques, but those listed
in category B are the ones that to date have been used most  often for
monitoring.
                                       12

-------
                 TABLE  1.    SUMMARY OF METHODS GIVEN IN  SECTION 5  TO MEASURE DRY  DEPOSITIO/f
     (Only those methods  for particles that are not affected by gravitational  settling are considered.)
Category Method
A. Estimates of Atmospheric radioactivity
Accumulation Mass-balance studies
B. Flux Monitoring Open pots
Flat filters
Flat plates and shallow pans
Fiber filters
Sticky films
C. Flux Parameterization
1. Field work Box-budget studies
a. Large areas Airborne eddy correlation
b. Small areas Gradient
Modified Bowen ratio
Eddy correlation
Variance
Tracer experiments
Eddy accumulation
Leaf washing
Surface snow sampling
2. Laboratory Chamber studies
Wind-tunnel studies
Pollutant
Partlcles(P)
P
p
p
p
P
P
P

p
p
P
P
P
P
p
P
P
P

P
Gases(G)

G






G
G
G
G
G
G
G
G
G
G
G
G
Monitoring
Potential
low3
low^
low^
low''
low*1
low*
low*1

low*
Iowa
lowh
high
low"
high
low"
high
medium"
medium"
low
low
Research
Potential
medium*1
high"
lowh»'5
lowb-d
j.owb,(J(q
lowb,d
lowb,d

medium"
highf»9
high*
high1 (if*
hlghf
hlghf»l»l
medium1
high1
medium"1
medium"
high"
hlghP
alarge effort  required
kpoor particle sir.e discrimination
^addresses fate of pollutants to some extent, does not discriminate between  particles and gases
aoasy to collect sample but dlffflcult to  interpret, contamination by gaseg  possible
edifflcult to  achieve the absolute accuracies required
'requires fast-response instrumentation
9only pollutant tried so far has been ozone
^instrumentation difficult to maintain
^susceptible to contamination by resuspenslon of particles
3not yet tried for pollutants
^requires small short-term drift of sensors
1data difficult to interpret
mleaf-to-feaf  variability is large
"works only for light winds, subfreezing temperatures
"used mostly to identify important processes
Plevels of turbulence found In the field difficult to achieve realistically
Isee alternative viewpoint by Davidson and Lindberg, Appendix C

-------
     The last, and largest, category of purpose to consider is (C)  flux
parameterizatioru  Many  of the field methods listed may become useful for
routine monitoring as technology develops; at present, however, these methods
in Category C, as opposed to those in Category B, are used primarily for
understanding and parameterizing deposition processes.   With successful
parameterizations, concentrations measured directly or computed in numerical
models can be utilized to calculate the vertical fluxes.  Category C is
further divided into two subcategories according to whether the work is
performed in the field or in the laboratory.  For field work, both large areas
of 1 to 100 km in horizontal extent and small areas of uniform surface
characteristics are considered.  Sampling times vary from 10 minutes to
several days.  During short-term studies over small areas, great effort
usually must be made to obtain sufficient information on the physical,
chemical, and biological characteristics of the surface.  For laboratory work,
sampling periods are highly variable, but usually short.  The type of
laboratory observations considered here is concerned with bulk exchange
processes in rather large enclosures.
                                       14

-------
                        A.  ESTIMATES OF ACCUMULATION
ATMOSPHERIC RADIOACTIVITY AS A TRACER FOR PARTICLES

History,  Radioactive fallout has been used in many studies of the particle
flux to natural surfaces=  Some of these studies have addressed the flux into
closed ecosystems, such as Crater Lake (see references), by using fallout
radioactivity as a tracer "of opportunity".

Usual method of use.  Concentrations in air of a selected radioactive material
are compared with like concentrations in water bodies or in vegetation, e.g.,
in order to evaluate the rate of input of the material in question.  Sometimes
isotope ratios are used to minimize uncertainties about sampling.  The method
is used to give data regarding small-particle uptake in most applications.

Advantages.  This method allows the net uptake rates of well-defined
ecosystems -co be evaluated with confidence, from relatively simple
measurements of vegetation, soil, water, or sediments.  Long-term integrations
are possible.

Disadvantages.  In general, detailed investigation of specific processes is
not possible, i.e., diurnal and often seasonal variations cannot be assessed.
The role of surface characteristics can be addressed only if they remain
constant for long periods of time.  In most applications, it is difficult to
differentiate between dry and wet deposition contributions.  Deposition of
background radioactivity and pollutant particles are likely to be similar only
if they possess similar size distributions and tropospheric distribution
patterns.

Conclusions.  The ability to derive net fluxes of small particles is a feature
that will continue to be attractive.  It seems likely that many other
techniques will be tested by comparison with radioactivity data, and it is
possible that radioactive fallout might eventually provide the only direct
measurement of the small deposition velocities that are expected by some
researchers to be characteristic of transfer to smooth surfaces such as large
water bodies.

Recommendations.  The use of background radioactivity should be more
thoroughly investigated to determine its suitability as a monitoring method
for particle dry deposition.  Experiments using natural radionuclides such as
7Be and 210pb should be considered and encouraged.  During such studies,
meteorological data on winds, temperatures, and humidities should also be
collected.
                                      15

-------
References;

Small, S. H.  (1960)  Wet and dry deposition of fallout materials at Kjeller,
     Tellus, _12, 308-315.

Volchok, H. L., M. Feiner, H. J. Simpson, W. S. Broecker, V. E. Noshkin,
     V. T. Bowen, and E. Willis.  (1970)  Ocean fallout — the Crater Lake
     experiment.  J. Geophys. Res., 75, 1084-1091.
                                       16

-------
MASS BALANCE STUDIES FOR PARTICLES AND GASES

History..  This method was developed largely by ecologists studying the
flowrate of nutrient elements, usually sulfur (S) and nitrogen (N) into and
out of agricultural plots and forests.  In North America, watersheds are being
studied in Tennessee, New York, and New Hampshire.

Usual method of use.  In watersheds, a budget model is formulated to account
for the outflow (e.g., runoff) and sources (e.g., dry deposition, wet
deposition, weathering).  Areas affected by chemical fertilizers are usually
not used,,  Typically, measurements can be made to obtain values for all
important processes except dry deposition, which is inferred by difference.
In controlled fumigation studies, the number of variables to be monitored can
be drastically reduced.

Advantages.  The net deposition, often for a large area, is inferred.
Ecological effects are intensively monitored,

Disadvantages.  To infer dry deposition, all other important inflow and
outflow rates must be evaluated.  Dry deposition is not measured directly but
is inferred, and sufficient accuracy is difficult to achieve because the
result is often a small difference between large numbers.  Some translocation
processes must be understood.  Chemical species (e.g., sulfate, nitrate,
ammonium) may participate in biochemical and geochemical reactions to yield
products not considered but that may cause the budget results to be in error.
It might not be possible to consider gaseous and particulate deposition
separately.  Surface characteristics in some studies can be considered only on
a long-term areal average.

Conclusions.  The approach is not a direct method for measuring dry
deposition, but finds fluxes indirectly.  However, the method is extremely
attractive because it can integrate a large area and the ecological effects
usually are intensively monitored.

Recommendations.  The continued use of this method should be encouraged
especially for watershed and other rather large areas.  Careful work should be
performed to ensure that significant paths for S and N compounds are not
overlooked.  Supporting meterological measurements should include winds,
temperature, and humidities.

References;
Eaton, J. S., and G. E. Likens.  (1978)  The imput of gaseous and particulate
     sulfur to a forest ecosystem.  Tellus, 30, 546-551.

Sprugel, D. S., and J. E. Miller.  (1979)  A field estimate of SC>2 deposition
     velocity to rapidly growing soybeans.  Water, Air, and Soil Pollution,
     12, 233-236.
                                      17

-------
                             B,  FLUX MONITORING

OPEN POTS FOR PARTICLES

History.  Exposed pots were used to measure "total deposition" in early
studies of radioactive fallout and are still in use today.  Recently,
"dry-only" devices that cover themselves automatically when it rains have been
developed.  They were popularized with the development and subsequent frequent
use of the so-called HASL wet-dry collector and a variety of similar devices.
Much early work used pots to provide an indication of the maximum amount of
material that could be deposited; to this end they were variously covered with
sticky material on internal surfaces (see STICKY FILMS) or contained
distilled water to retain deposited particles.  These days, bare stainless
steel or plastic containers are normally employed.

Usual method of use.  Identical containers, usually of about 30 cm diameter,
are normally mounted side-by-side at heights of about one meter, in areas away
from the influence of vegetation, buildings, etc.  A rain-sensitive mechanism
is used to alternately cover the dry collector when it rains, and the wet
collector when it doesn't.  These devices are now available from a commercial
source, at a cost of about $1500 each.  Samples are normally collected over
periods of one week to three months.

Advantages,  These devices have been selected by the National Atmospheric
Deposition Program for use in  their standard collection procedures.
Recommended exposure, collection, and analysis procedures have been specified.
Data obtained at different stations can therefore be compared with some
confidence.

Disadvantages.  While these collectors might collect in a representative
fashion large particles that stick to all  surfaces, it is very unlikely that
they will provide a surrogate  that allows  accurate simulation of the complex
physical interactions between  natural surfaces and fine particles in the
naturally-occuring turbulent flow.  For example,  it has been  suggested that
for fine particles the outside surface of  the pots be used  as the collecting
area rather than the  inside, but in fact the outside, or  any  other  simple
artificial surface, is probably inadequate.  Also, such collectors might be
contaminated by particles resuspended locally.   Determination of whether
compounds arrived in  large or  small particles might be confounded by
attachment of large to small particles.  Reactions of  gaseous pollutants with
particles already stuck to the surface might  contaminate  the  particulate
sample.
                                       18

-------
Conclusions.  Quantification of the surface flux would appear -co be in
considerable doubt, but identification of the chemical species involved seems
possible, even though it will be strongly weighted to the larger particles.
Contamination of particles by trace gases appears possible, much as artifact
sulfa-ce is generated on some filters.  Since the open pot method is a simple
and standard method, its use appears to be defensible.  Care should be taken
to avoid reliance on the "fluxes" estimated from such data, because the
relationship to dry deposition of fine aerosols to natural surfaces is nor
sufficiently known.

Recommendations.  This type of sampling is not recommended to monitor dry
depositions  Use should probably be continued at sites now established, but
future reliance and increased use is not recommended.  At the stations where
such samplers are now operated, continued use may serve to indicate long-term
trends of coarse aerosols.  The procedures and effective surface areas assumed
should be the same for all pots, following the HASL collectors.

References:
Krey, P. W., and L. E. Toonkel.  (1977)  Scavenging ratios.  Proc. Symp. on
     Precipitation Scavenging, Champaign, XL, October 14-18, 1974.  U.S.
     Energy Research and Development Agency Symposium Series, 41, 61-70.

Volchok, H. L., and R. T. Graveson.  (1975)  Wet/dry fallout collection.
     Proc. of the Second Federal Conf. on the Great Lakes.  Interagency
     Commission on Marine Science and Engineering, Argonne National
     Laboratory, Argonne, IL, pp. 259-264.
                                      19

-------
FLAT FILTERS FOR PARTICLES

History,  Several notable studies have been conducted in the United States,
England, and elsewhere by use of a horizontal sheet of filter paper.  Since
certain elements are usually associated with specific size ranges of
particles, the studies often make use of rather thorough chemical analyses to
infer the dependence of deposition velocity on particle size.  Estimates of
deposition velocities for submicron particles are usually in the range 0.1 to
0.5 cm s"1.  This method has been used in both the field and the laboratory.


Usual method of use.  A nearly square piece of Whatman (541) filter paper
about 25 cm on a side is placed horizontally and either covered during rain  or
placed permanently beneath a rain shield.  High-volume samplers are used at
the same time to measure total airborne particulate concentrations.  Sampling
periods are at least several days at each location.

Advantages.  A long-term integrated sample is obtained.  Many different
elements can be considered by analyses in the laboratory.  Operation in the
field is very simple and inexpensive.  The collector might be a slightly more
realistic surface for fine aerosols than open pots.

Disadvantages.  This method has not been fully standardized and appears to be
less popular in the United States than the open-pot HASL collector.
Resuspension from the surface is potentially a serious problem.  As with open
pots (see OPEN POTS), this surface does not accurately simulate the varied
surfaces of the natural surroundings; in most cases, too few particles, large
and small, will be collected.  Attachment of small to large particles may
disrupt the relationships expected between chemical species and effective
particle size range.  Reactions of gases with particles collected may
contaminate the samples.

Conclusions.  The use of filter paper is not significantly superior to the
open-pot method because of the overwhelming failure to simulate natural
surfaces.

Recommendations.  This technique is not recommended as a dry-deposition
moni-coring method.  Attempts at improving this technique will probably not be
highly beneficial.

References;

Muhlbaer, J. L.  (1978)  The chemistry of precipitation near the Chalk Point
     River plant.  Ph.D. Thesis, University of Maryland, College Park, MD.
     322 pp.

Peirson, D. H., P. A. Cawse, and R. S. Cambray.  (1974)  Chemical uniformity
     of airborne particulate material, and a maritime effect.  Nature, 251,
     675-679.
                                      20

-------
FLAT PLATES AND SHALLOW PANS FOR PARTICLES

History.  Various types of horizontal plates, with and without rims, were
used in many early studies of particle deposition.  It was recognized that
these simple devices provided imprecise indications of the rate at which
particles were being delivered to natural surfaces, but nevertheless their use
was wide-spread.  Some tests indicated that particles deposited on the
sampling surface could be more efficiently retained if the surface were
modified by covering it with adhesive, gauze, or water.  In some experiments,
deposition substrates were specially selected to have desirable chemical
characteristics.

Usual method of use.  The devices are usually placed at heights of one meter,
with samples being collected at intervals from several days to months*
Rainfall presents an obvious difficulty.  The material deposited is analyzed
by standard chemical techniques.

Advantages.  Samples are collected easily and the system can be operated by
relatively untrained personnel.  A long-term sample is obtained.  Many
different chemical species can be investigated.

Disadvantages.  Techniques have not been standardized.  As with similar
methods, simulation of actual natural surfaces and their interaction with
atmospheric turbulence is very difficult.  Attachment of small particles to
larger ones may disrupt the relationships expected between chemical species
and effective particle size ranges.  Reactions of gases with particles
collected may contaminate the samples.  If water is placed in pan collectors,
gas transfer can be a dominating factor for some pollutants (e.g., sulfur).

Conclusions.  As with the use of all surrogate-surface systems, the analogy
with the natural case is rather poor and causes considerable doubt about how
results should be interpreted.

Recommendations.  This type of approach is not recommended as a dry-deposition
monitoring method.  Future improvements probably would not be adequate.  (An
alternative viewpoint is presented by Davidson and Lindberg in Appendix C.}

References;

Islitzer, N. F., and R. K. Dumbaulk.  (1963)  Atmospheric diffusion-deposition
     studies over flat terrain.  Inter. J. Air and Water Pollut., "]_,
     999-1022.

Markee, E« H« Jr.  (1967)  Turbulent transfer characteristics of radioactive
     effluents from air to grass.  Proc. U.S. Atomic Energy Commission
     Meteorological Information Meeting, Chalk River Nuclear Laboratories,
     Chalk River, Ontario, Canada, Sept. 11-14, 1967, Atomic Energy of Canada
     Limited Report AECL-2787, 589-601.

Davidson, C= I.  (1977)  The deposition of trace metal containing particles in
     the Los Angles Basin.  J. Powder Technol., 18, 117-126.

                                      21

-------
FIBER FILTERS FOR PARTICLES AND GASES

History.  The use of fiber filter mats was an attempt to avoid the problems of
resuspension for coarse particles and the low deposition rates for fine
aerosols that are present when using flat plates.

Usual method of use.  Fiber filter mats are placed in vegetative canopies in
such a manner that leaves do not shield them, and so that they are not
subjected to avian interferences.  After exposure for one to two weeks, the
filters are removed for mass and chemical analysis.  For gas sampling, the
filters are coated with an active chemical.

Advantages.  Sampling is easy to perform and is inexpensive.  A wide range of
chemical species can be investigated.

Disadvantages.  The surface resistance of the fiber filter mats does not
match that of the vegetation.  Also, the fiber filters (especially when glass)
may be biased due to unrepresentative gas-phase reactions on its surface.

Conclusions.  The surrogate-surface does not represent the behavior of natural
vegetation.  Interpretations of data are difficult.  The method is not
acceptable as a dry deposition monitoring technique.

Recommendations.  The method should not be encouraged and efforts to further
develop it as a method to monitor dry deposition are not warranted.

References;

Israel, A. W.  (1977)  Differences in the accumulation of gaseous and particu-
     late fluorine compounds by forage vegetation and lined filter paper sam-
     plers.  Atmos. Environ., 11, 183-188.

Tomingus, R., and G. Voltmer.  (1978)  Abscheidung von Benz(a)pyren aus der
     Atmosphaere auf Glasfaserfiltern (Deposition of benzo(a)pyrene from the
     atmosphere onto glass fiber filters).  Staub-Reinhalt Luft, 38, 216.
                                      22

-------
STICKS FILMS FOR PARTICLES

History.  Sticky surfaces have been used in a variety cf configurations to
collect different kinds of particles.  For example,  glass slides have been
coated with petroleum jelly or with glycerin to collect spores.
Adhesive-coated acetate films were used in the routine monitoring of
radioactive fallout during the 1950's and 60's.  Workers of the Union of
Soviet Socialist Republics employed flat steel plates and collection vessels
of various geometries that were sometimes coated with glycerin on the
collection surfaces.  In a variation on the more common method, some USSR
workers employed gauze as a collection method instead of a sticky substance.


Usual method of use.  For the radioactive fallout studies, a flat horizontal
surface was usually mounted at heights of one meter, well away from vegetation
and preferably somehow protected from birds.  The exposed surface was coated
with an adhesive substance, which was sometimes similar to flypaper in
consistency but was often some sort of grease.  Surfaces were exposed in all
weather conditions.  In the fallout studies, exposure times varied from one
day to several weeks.  The devices were "calibrated" in terms of the total
(wet plus dry) deposition against the accumulation to soil and results
obtained using open pots.

Advantages.  Obvious advantages include simplicity and lack of expense.  The
technique is easily mastered by relatively untrained personnel, and analysis
can be as complicated as one likes to make its.

Disadvantages.  It seems fairly clear that devices of this kind will provide a
measure of what is in the air, but the relationship of the deposition to the
underlying surface is not obvious, especially when resuspension or surface
emissions are suspected of playing a role in the net transfer.  Early work
(HASL-42a, 1958) indicated that the relationship between gummed film deposits
and collections in open pots was not constant.  Chemical speciation of
deposited material is likely to prove difficult.

Recommendations.  These methods are already largely obsolete.  They should not
receive further encouragement, nor should efforts be made to improve them.

References:

HASL-42a.  (1958)  Environmental Contamination from Weapons Tests.  U.S.
     Atomic Energy Commission, Health and Safety Laboratory, New York, NY.
     E. P. Hardy Jr. and J. H. Harley (eds.).  95 pp.

Aleksandrov, N. N., B. B. Goroshko, V. G° Kovalenko, and F. A. Panfilova.
     (1965)  Influence of the meteorological conditions on the collection
     efficiency of radioactive contaminants.  In:  Radioactive Isotopes in
     the Atmosphere and Their Use in Meteorology, translation of material
     presented at the Conference on Nuclear Technology, Obninsk, U.S.S.R.,
     3-6 February, 1964; Index to Scientific and Technical Proceedings
     Catalog Number 1801, pp. 354-359.

                                      23

-------
Eisenbud, M., and J. H. Harley.  (1955)  Radioactive fallout in the United
     States.  Science, 123, 677-580.
                                      24

-------
                              FLUX PARAMETERIZATION
                              1.   FIELD WORK
                                  a.  LARGE AREAS
BOX-BUDGET STUDIES FOR PARTICLES AND GASES

Historyo  One of the oldest methods of studying atmospheric fluxes is to
evaluate inflow and outflow associated with a particular region during part of
a day and then to estimate the surface deposition from the difference and
details of inputs within the region.  A similar approach is to measure changes
of concentration in a Lagrangian sense, with the mean flow of air over the
area studied.  Such measurements over Great Britain have resulted in a
convincing evaluation of the average deposition velocity for SO2.

Usual method of use.  In most studies, a spatially-uniform area is selected
and concentration profiles are documented at upwind and downwind edges.  These
profiles are then coupled with equally detailed wind profiles to evaluate the
net source or sink term within the region.

Advantages.  The net surface flux over the area in question is evaluated
independently without the need for detailed knowledge of conditions in the .
atmospheric surface layer.

Disadvantages o  Deposition estimates are subject to large errors because very
small differences are found.  Usually, travel distances of the order of 100 km
or more are needed in order for horizontal flux differences to exceed a few
percent even for actively exchanged pollutants.  Wind and concentration
profiles need to be extended above the height at which pollutants mix.  In
practice, during the daytime heights to above 2 km must often be considered,
which usually requires the use of aircraft.  Crosswind uniformity in the flow
field, the surface vegetation, and the topography must be sufficient to ensure
negligible "leakage" through the "sides" of the "box".  Anthropogenic sources
within the region must be well known.  Because of the large areas and rather
long sampling periods required, the capacity to investigate diurnal variations
and the effects of specific surfaces is quite limited.  Chemical reactions
involving the pollutant of interest must either be assumed to be insignificant
or accounted for in a sufficiently accurate manner.

Conclusions.  Although this method is outwardly attractive, the experimental
constraints associated with it are extremely demanding.
                                      25

-------
Recommendations.  Ihis experimental method is too expensive and difficult to
be suitable for monitoring directly.  For verification of numerical models and
for testing of parameterizations over complex terrain, however, improvements
of aircraft instruments and remote-sensor capabilities should be encouraged in
order to provide better estimates of dry deposition over scales of
approximately 100 km.

References;

Delmyea, R., and R. L. Petel.  (1979)  Deposition velocity of phosphorus-con-
     taining particles over southern Lake Huron, April-October, 1975.  Atmos.
     Environ., 13, 287-294.

Gillani, N. V.  (1978)   Project MISTTs  Mesoscale plume modelling of
     dispersion, transformation and ground removal of SC>2.  Atmos. Environ.,
     _12, 569-587.

Prahm, L. P., U. Torp, and R. M. Stern.  (1976)  Deposition and transformation
     rates of sulphur oxides during atmospheric transport over the Atlantic.
     Tellus, 28, 355-372.

Smith, F. B., and G. H. Jeffrey.   (1975)  Airborne transport of sulphur
     dioxide from the United Kingdom.  Atmos. Environ,, J3, 643-659.
                                       26

-------
AIRBORNE EDDY CORKEDTION FOR PARTICLES AND GASES

History..  Airborne gust probes have been developed over the last twenty years
to measure the three eddy components of atmospheric motion (y1,  vf ,  w1).
Linking gyroscopes or an inertial navigation system (INS) with a gust probe
provided the means to remove effects of motions of the aircraft platform.
Additions of temperature and water vapor sensors provided the capability to
measure sensible and latent heat fluxes, in addition to the momentum fluxes.
Applications in boundary layer meteorology have been extensive.   Recently, new
pollutant sensorsf potentially suitable for coupling with the gust probe so as
to measure fluxes, have been or are being developed for trace gases (e.g., O3)
and small particles.

Usual method of use.  Rapid-response (~10Hz) vanes and a pitot tube are used
to measure the wind components=  An INS and electronic filtering remove
aircraft motion and noise.  Other rapid-response sensors detect the
concentration of the atmospheric constituents of interest.  Flux measurements
are made in straight, horizontal flight and altitudes and, in patterns that
suit the application.  Instantaneous and mean fluxes, and corresponding
spectral analyses are produced„

Advantages.  Aircraft survey large areas in a very short time; they permit
crosswind and along-wind flux measurements, whereas towers only see the latter
aspect of transport and require much longer time periods to do so.   Deposition
velocities over various types of complex terrain can be studied.  Flight
altitudes can be as low as 10 m for direct measurements of flux to the
surface.  Multiple altitude measurements for budget studies, and higher level
measurements of, e.g., fluxes through cumulus clouds (boundary layer venting),
are feasible.

Disadvantages.  Pollutant sensors sufficiently fast in response are extremely
difficult to construct.  Effects of decoupling and of flux divergence between
the lowest flight level and the surface, as in nocturnal stable air, must be
determined for surface deposition studies.  Over complex terrain,
interpretation of observations could require sophisticated modeling; detailed
knowledge of surface characteristics under the flight paths is needed (but is
obtainable).  "Hie technical complexity restricts uses to research.
Operational costs are high, and although logistically feasible,  aircraft are
normally not operated on station continuously.

Conclusions.  "Hie airborne eddy-correlation method is intended for research
rather than routine monitoring of dry deposition.  The method can provide
accurate measurements of vertical and horizontal fluxes at flight level.
Measurements from the aircraft and towers are complementary in many ways?
primarily, towers provide near-surface reference/calibration measurement for
the aircraft and help to evaluate changes of flux with height, whereas the
aircraft covers large areas not observable with stationary point systems.
                                      27

-------
Recommendations.  TSie use of this method, in conjunction with an extensive
characterization of underlying natural surfaces and complementary
eddy-correlation or other measurements near the ground, may become the best
approach to parameterizing deposition.  For this method to measure fluxes of
acidic gases and aerosols, fast-response pollutant analyzers must be developed
that can be operated in aircraft.  A rapid-response ozone sensor has been used
with the gust probe in recent EPA field studies; improvement of this sensor is
in progress.  Other suitable gas and particle sensors are not yet available,
although some development is underway.

     Further exploration of the airborne eddy-correlation method is
recommended.  Additional studies over complex terrain, with emphasis on
atmospheric deposition, should be attempted.  Since planetary boundary layer
structures can control the amount of pollutant delivered to the surface at
certain times, aircraft measurements throughout the depth of the boundary
layer should be performed occasionally.  In initial studies, comparison with
measurements on towers should be conducted in uniform terrain.  The
measurements of momentum, sensible heat, and water vapor flux should be
obtained as well, since they are required to parameterize surface uptake and
boundary-layer structure.

References;

Bean, B. R., R. Gilmer, R. L. Grossman, and R. McGavin.  (1972)  An analysis
     of airborne measurements of vertical water vapor flux during BOMEX.
     J. Atmos. Sci., 29, 860-869.

Lenshow, D. H., A. C. Delany, B. B. Stankov, and D. H. Stedman.  (1980)
     Airborne measurements of the vertical flux of ozone in the boundary
     layer.  Boundary-Layer Meteorol.   (In press).
                                      28

-------
                              FLUX PARAMETERIZATION
                              1.  FIELD WORK
                                  b.  SMALL AREAS
GRADIENTS FOR GASES AND PARTICLES

History.  Studies of the relationships of fluxes to gradients in the
atmospheric surface layer have led to formulations for eddy diffusivities that
can be applied to differences in pollutant concentration measured between two
or more heights.  This method gained favor among air-pollution meteorologists
following early work to address sulfur dioxide and ozone fluxes.  Initial
uncertainty about the way to estimate an appropriate eddy diffusivity, whether
as heat or momentum, has now been resolved in favor of the former.

Usual method of use.  Concentration differences between two or more levels
within about 10 m above a natural, flat, and spatially-homogeneous surface
are used to evaluate a local gradient that is then taken to be representative
of the area as a whole.  Eddy diffusivities are evaluated as a function of
momentum flux, surface roughness, and atmospheric stability, which are derived
from supporting meteorological data obtained nearby.

Advantages.  Fast-response sensors are not needed.  Interpretation of
gradients is conceptually straightforward and has received a good deal of
attention as a result.  The information needed to determine eddy diffusivity
provides directly several of the parameters needed to infer some of the
processes that control deposition at the surface.

Disadvantages.  The familiar arguments on the required fetch apply with
considerable force, since local surface inhoraogeneities will not only
influence pollutant profiles directly but will also affect the diffusivities
that need to be applied to them.  Artificial sources close upwind cannot be
tolerated.  Concentration differences are likely to be exceedingly small; in
most circumstances vertical differences of about 1% in concentration will need
to be measured over a two-fold height interval.  To measure the small
differences, it is usually necessary to allow each sensor to measure at
several heights so that systematic differences between sensors are eliminated;
sampling over longer time intervals is usually necessary.  With particles,
surface (re)suspension and (re)emission can cause concentrations to increase
near a natural surface, even though a downward flux continues.  The fluxes
measured do not include components associated with gravitational settling.
This method often fails over tall vegetation because of the difficulties of
estimating eddy diffusivities, and because gradients are exceedingly small.
                                      29

-------
Conclusions.  Many gradient studies have failed to recognize either the need
for micrometeorological criteria to be satisfied or the requirement for
exceedingly accurate measurements of concentration differences.  Nevertheless,
the work provides an opportunity to investigate the flux of pollutants for
which the only sensors available are slow in response.

Reconunendations.  This technique is a useful parameterization tool for surfaces
with short or no vegetation.  The need to estimate diffusivity as a function of
height makes this method less attractive than others (e.g., see MODIFIED BOWEN
RATIO) as a monitoring technique.

References;

Dannevik, W. P., S. Frisella, L. Granat, and R. B. Husar.  (1976)  SO2 deposi-
     tion measurements in the St. Louis region.  Proc. Third Symp. on Atmospheric
     Turbulence, Diffusion, and Air Quality, Raleigh, NC, 506-511.

Davidson, C. I., and S. K. Priedlander.  (1978)  A filtration model for aerosol
     dry deposition:  application to trace metal deposition from the atmosphere.
     J. Geophys. Res., 83, 2343-2352.

Droppo, J. G., and J. C. Doran.  (1979)  Measurements of surface layer turbulent
     ozone flux processes.  Proc. Fourth Symp. on Atmospheric Turbulence,
     Diffusion, and Air Quality, Reno, NV, 507-509,

Fowler, D.  (1978)  Dry deposition of SO2 on agricultural crops.  Atmos.
     Environ., 12, 369-373.

Garland, J. A.  (1977)  The dry deposition of sulphur dioxide to land and
     water surfaces.  Proc. R. Soc. London A., 354, 245-268.

Whelpdale, D. M., and R. W. Shaw.  (1974)  Sulphur dioxide removal by turbulent
     transfer over grass, snow, and other surfaces.  Tellus, 26, 196-204,
                                      30

-------
MODIFIED BOWEN RATIO FOR PARTICLES AND GASES

History.  A combination method of Bowen ratio and energy balance is perhaps
one of the oldest and most tested methods for obtaining water vapor fluxes..
An essential assumption, that eddy diffusivities for heat, water vapor, and,
in this case, pollutants are equal, is now accepted with few reservations.
Applications to ozone and carbon dioxide have been reported since 1970.  This
method can be considered a simplified variation of GRADIENTS.

Usual method of use.  The pollutant eddy diffusivities are assumed to be the
same as for sensible heat or water vapor0  The pollutant flux is given by the
product of water vapor flux and pollutant concentration difference, divided by
the difference in specific humidity.  A normalized sensible heat flux and
temperature differences can be used in place of the water-vapor variables.
Differences must be measured over the same height interval and at the same
location.  The fluxes of heat or water vapor would ordinarily be obtained by
energy-balance Bowen-ratio methods or by eddy correlation.

Advantages.  Fluxes are measured without assumptions about the nature of the
surface cover.  Fast-response chemical sensors are not needed.

Disadvantages.  The chemical analyses must be able to detect differences in
successive samples of less than 1% (see Hicks and Wesely, 1978).  This need
for accuracy in differences usually requires periodic interchanging of sensor
sampling points, which results in the need for sampling times of at least one
hour to obtain a' meaningful average.  The energy-balance Bowen-ratio method
has certain difficulties of its own:  (1)  soil heat flux or canopy heat
storage must be estimated, especially near sunrise and sunset; (2) temperature
differences cannot be found accurately in near-neutral conditions; (3) routine
measurement of differences in water vapor content can be difficult.  With
particles, fluxes due to gravitational settling are not detected and
(re)suspension of large particles can invalidate the results.  Measurements
should be well above tall vegetations, which often requires excessive fetches.

Conclusions.  A major limitation is the demand for precision of sample
analysis.  The equipment maintenance required is less than average for
micrometeorological methods, but can still be very demanding.

Recommendation.  This method can be very useful for parameterization studies
at a well-instrumented, well-maintained site for periods as long as one
season.  Utilization for routine monitoring should be investigated.

References;

Allen, L. Ho, Jr., R. J. Hanks, J. K. Aase, and Ho R. Gardner.  (1974)
     Carbon dioxide uptake by wide-row grain sorghum computed by the profile
     Bowen-ratio.   Agronomy J., 66, 35-41.
                                      31

-------
Black, T. A., and K. G. McNaughton.  (1971)  Psychrometric apparatus for
     Bowen-ratio determination over forests.  Boundary-Layer Meteorol., 2^,
     246-254.

Hicks, B. B., and M. L. Wesely.  (1978)  An examination of some
     micrometeorological methods for measuring dry deposition.
     EPA-600/7-78-116, Research Triangle Park, NC, 19 pp.

Leuning, R., M. H. Unsworth, H. N. Newman, and K. M. King.  (1979)  Ozone
     fluxes to tobacco and soil under field conditions.  Atmos. Environ.,
     _13, 1155-1163.

Sinclair, T. R., L. H. Allen Jr., and E. R. Lemon.  (1975)  An analysis of
     errors in the calculation of energy flux densities above vegetation by a
     Bowen-ratio profile method,  Boundary-Layer Meteorol., 8, 129-139.

Verma, S. B., and N. J. Rosenberg.  (1975)  Accuracy of lysimetric, energy
     balance, and stability-corrected aerodynamic methods for estimating
     above-canopy flux of CO2.  Agronomy J., 67, 699-704.
                                      32

-------
EDDY CORRELATION FOR PARTICLES AND GASES

History.  United Kingdom workers first applied the covariance method to
evaluate the transfer of momentum pu'w'  by painstaking hand-evaluation of
chart records of horizontal (u) and vertical (w)  wind data.   Subsequent
efforts extended the method to the cases of sensible heat and water vapor,  and
recently it has been applied to the measurement of vertical  fluxes of a range
of trace gases and small particles.

Usual method of use.  Sensitive and fast-response anemometers are placed at a
height of 1 to 10 m to detect fluctuations in the vertical wind component,  w°
Adjacent to the anemometry, measurements are made of the concentration of the
pollutant of interest, c»  Mean values of both signals are removed, to produce
signals fluctuating about zero, w' and c" .  The product w'c*  is then
calculated and averaged to produce the turbulent flux of the quantity c.  This
flux equals that at the surface if stringent fetch/height criteria are met; in
general, the height of operation of the sensors needs to be  less than 0.5% of
the uniform horizontal upwind fetch.  At the same time, the  sum of exponential
response time and delay time of each sensor used should be less than height
divided by mean wind speed*

Advantages.  Covariance measurements provide absolute evaluations of vertical
fluxes in natural circumstances, without the need for assumptions regarding
appropriate diffusivities and without making any assumption  about the nature
of the surface cover.

Disadvantages,  Fast-response chemical sensors often need to be developed»   In
practice, the fetch/height constraints and the increase in the frequency of
turbulent transfer mechanisms near the _ surface combine to require the sum of
exponential response time and delay time to be less than about one second.
This makes the task extremely demanding, and only a few pollutant fluxes have
been measured by this method.  Particle flux due to gravitational settling is
not detected and significant resuspension of large particles can invalidate
results..

Conclusions.  Provided the standard micrometeorological criteria are met, this
method is capable of providing accurate measurements of surface fluxes.

Recommendations.  In parameterization studies, the measurements of momentum,
sensible heat, and latent heat flux should be obtained as well as the
pollutant flux, since these are required to obtain surface-related information
and are relarively easy to obtain.  Monitoring capabilities  should be
assessed.

References;

Desjardins, R. L., and E. R. Lemon.  (1974)  Limitations of  an eddy-correla-
     tion technique for the determination of the carbon dioxide and sensible
     heat fluxes.  Boundary-Layer Meteorol., 5f 475-488.
                                      33

-------
Galbally, I. E., J. A. Garland, and M. J. G. Wilson.  (1979)  Sulphur uptakes
     from the atmosphere by forest and farmland.  Nature, 280, 49-50.

Jones, E. P., and S. D. Smith.  (1977)  A first measurement of sea-air CO2
     flux by eddy correlation.  J. Geophys. Res., 82, 5990-5992.

Wesely, M« L., B. B. Hicks, W. P. Dannevik, S. Frisella, and R. B. Husar.
     (1977)  An eddy-correlation measurement of particulate deposition from
     the atmosphere.  Atmos. Environ. 11, 562-563.

Wesely, M. L., J. A. Eastman, D. R. Cook, and B. B. Hicks.  (1978)  Daytime
     variations of ozone eddy fluxes to maize.  Boundary-Layer Meteorol., 15,
     361-373.
                                       34

-------
 VARIANCE FOR PARTICLES AND GASES

 History.  Experimental studies of turbulence in the atmospheric surface layer
 in  the  1960's and 1970's have shown that turbulent variations of temperature
 and humidity are highly correlated and can be parameteriEed in terms of
 surface layer variables.  The accompanying theory of the similarity of heat
 and water vapor behavior can be extended to trace gases and fine particles,
 except  perhaps in some cases where fluxes are very small.  While the variance
 method  has not yet been tried directly, very recent field tests have indicated
 that the method is feasible.

 Usual method of use.  The variances of pollutant concentration and of
 temperature or humidity fluctuations are measured simultaneously at the same
 height  and location.  Sensible heat flux (or evaporation) is measured also, so
 that the pollutant flux can be estimated as the product of the temperature
 (or humidity) flux times the standard deviation of pollutant concentration,
 divided by the standard deviation of temperature (or humidity).  Rather the
 standard deviations from total variance, a selected bandpass for variance
 averaged over periods of 10 min to 1 hr can be used.  See MODIFIED BOWEN RATIO
 for a discussion of means likely to obtain sensible heat flux or evaporation
 rate.

 Advantages.  Fluxes can be measured without assumptions about the nature of
 the surface.  Measurements of vertical wind-speed fluctuations might be
 avoided.  As compared to eddy correlation, lag times can be much larger, so
 that long sample lines can be used for the chemical sensor, and the
 time-response requirements can be slightly relaxed if a low-frequency
 band-pass variance is computed.

 Disadvantages.  Suitable fast-response chemical sensors must be developed.
 Sensor  random noise is a more serious problem here as compared to eddy
 correlation.  If temperature is used, the method will usually fail during
 near-neutral conditions; if humidity is used, suitable humidity sensors are
 expensive and difficult to maintain over long periods of time, and the method
 will fail in dry conditions and often in cold conditions.  The direction (up
 versus  down) of the pollutant flux cannot be determined.  With particles,
 fluxes  due to gravitational settling are not detected and (re)suspension of
 large particles can invalidate the results.  Measurements should be well above
 tall vegetative canopies, which results in the usual need for large fetches.

 Conclusions.  If the pollutant sensor has very low noise levels, this method
might be superior to direct eddy correlation.  Even if heat or water vapor
 flux is measured simultaneously by eddy correlation, a variance method might
be more feasible than eddy correlation for the pollutant flux because
low-frequency band-pass filtering can reduce the requirement for small
response and delay times.
                                      35

-------
Recommendations.  This method might be especially attractive for pollutants
for which sensors are available that have long delay times and response times
only slightly too large for eddy correlation.  Of the micrometeorological
methods, the variance technique might hold the most promise for monitoring of
some pollutants.

References:
McBean, G. A.  (1973)  Comparison of the turbulent transfer processes near the
     surface.  Boundary-Layer Meteorol., 4_, 265-274.

Swinbank, W. C., and A. J. Dyer.  (1967)  An experimental study in micrometeo-
     rology.  Quart. J. Roy. Meteorol. Soc., 93, 494-500.

Wesely, M. L., and B. B. Hicks.  (1978)  High-frequency temperature and
     humidity correlation above a warm wet surface.  J. Appl. Meteorol.,
     17, 123-128.
                                       36

-------
'TRACER EXPERIMENTS FOR PARTICLES AND GASES

Historyo  Studies on the possible effects of inadvertent releases of
radioactivity into the atmosphere led to a series of sophisticated field
studies involving the release of tagged material such as 13 lj an  Mr concentration measurements are also
made.  Tracer materials have ranged from radioactive gases to fluorescent
particles.  Alternative methods include the use of two tracers, one of which
is inert, to infer deposition by assessment of depletion of the active tracer
over a relatively large area.

Advantages.  Studies of the deposition of tagged material from plumes
traversing various surfaces address the question of inadvertant pollution
release in a direct and easily understandable manner.  If the tracer material
is carefully chosen, sampling of deposited material can be quite accurate.

Disadvantages.  The tracer particle and pollutant particle size distributions
may not be the same, which may lead to different removal rates.  For both
gases and particles at short distances from release points, mixing is
insufficient for effective simulation of deposition of pollutants in the
planetary boundary layer well downwind from sources, i.e., the turbulent
structure of eddies that carry the tracers to the surface may be artificially
weighted towards small eddies? vertical gradients are distorted so that
concentrations in the air above the surface can be misleading.  The dependence
of deposition rates on wind speed may be hard to determine because of dilution
varying with wind speed and turbulence intensity.  Also, surface sampling can
be tedious, and the logistical problems of following the plume over long
distances can be severe.

Conclusions.  Releases of special tracers can be used to develop
parameterizations suitable for wider application if care is taken to document
local horizontal and vertical concentration gradients.  Results from tracers
in early studies were quite useful, when alternative methods were few.

Recommendations.  Tracer studies can be recommended for the study of
deposition processes only if some unique advantage is found.  The distribution
of tagged pollutants within vegetated canopies could be the subject of a
tracer study.  Another study worthy of consideration deals with resuspension
of radioactive soil particles.  When tracers are used in studies of
atmospheric dispersion, dry deposition often must be considered anyway; a
little extra effort might yield valuable information on deposition processes.
                                      37

-------
References;

Garland, J. A., W. S. Clough, and D. Fowler.  (1973)  Deposition of sulphur
     dioxide on grass=  Nature, 242, 256-257.

Markee, E. H., Jr.  (1967)  Turbulent transfer characteristics of radioactive
     effluents from air to grass.  Proc. U.S. Atomic Energy Commission
     Meteorological Information Meeting Chalk River Nuclear Laboratories,
     Chalk River, Ontario, Canada, Sept. 11-14, 1967, Atomic Energy of Canada
     Limited Report AECL-2787, 589-601.

Owers, M. J., and A» W. Powell.  (1974)  Deposition velocity of sulphur
     dioxide on land and water surfaces using a 35S method.  Atmos. Environ.,
     B, 63-67.

Slade, D. H.  (1968)  Meteorology and Atomic Energy.  U.S. Atomic Energy
     Commission, TID 25190, Washington, DC, pp. 206-208.
                                       38

-------
EDDY ACCUMULATION FC,-: PARTICLES AND GASES

Historyo  During the middle 1960's, the eddy-accumulation method was first
considered seriously as a possible means of avoiding the need for
rapid-response sensors for scalar quantities in eddy correlation«  Most often,
a  simplified instrument for measuring the flux of water vapor was sought.  In
later considerations, emphasis has been on trace gases, especially pollutants
and carbon dioxide.  A successful application of this method has not yet been
reported,,

Usual method of use.  See the discussion under EDDY CORRELATION0
Fast-response vertical velocity signals are manipulated to control the
sampling rates of two separate mean-measurement systems„ one associated with
updrafts and the other with downdrafts.  As with conventional eddy correlation,
the anemometer must be well aligned in the vertical, preferably with high-pass
filtering applied to the velocity signal as well.  The eddy flux is evaluated
from the differences in concentrations of the material of interest in air
accumulated by the two sampling systems.  The usual micrometeorological siting
and sampling constraints apply.

Advantages.  The technique offers the benefits of a genuine eddy-correlation
calculation while sidestepping the need for fast-response pollutant sensing.
For some pollutants, analysis could be performed on samples taken to a
chemistry laboratory,,

Disadvantages»  Differences in concentration between the two samples are
likely to be small, a few percent or less.  Aerosol sampling poses special
problems for this technique because inlet sampling efficiency is likely to be
a  function of particle size, horizontal wind velocity, and inlet velocity.
Particle fluxes resulting from gravitational settling should not be detected,
but significant local resuspension of large particles can invalidate results.

Conclusions.  This method is a suitable technique for investigation of the
dependence of dry flux upon micrometeorological and surface variables.  The
lack of success in past attempts is evidence of the difficulties involved.
Nevertheless, active interest persists because of the potential benefits.
While the eddy-accumulation approach offers high potential as a dry-deposition
monitoring technique, it suffers in at least three aspects:  (1) lack of a
suitable electromechanical sampling system, (2) variations of sampler inlet
efficiency for aerosols, and (3) the need for chemical analysis precision
errors to be less than 1%.

Recommendations.  Work to develop dry-deposition measurement systems based on
the principle of eddy accumulation should be initiated.  Monitoring
capabilities should be assessed.

References;

Desjardins, R.  L.   (1977)   Energy budget by an eddy correlation method.  J.
     Appl.  Meteorol., 16,  248-250.
                                      39

-------
Desjardins, R. L.  (1977)  Description and evaluation of a sensible heat flux
     detector.  Boundary-Layer Meteorol.,  11, 147-154.

Hales, J. M., and T.  W. Horst.  (1974)  A flux meter of direct field
     measurement of deposition and resuspension rates.   In:  Pacific Northwest
     Laboratory Annual Report for 1973 to the USAEC Division of Biomedical and
     Environmental Research, Part 3.,  Pacific Northwest Laboratory, Richland,
     WA,  pp. 176-178.
                                      40

-------
LEAF WASHING FOR PARTICLES AND GASES

History»  The enrichment of pollutants in rainfall as it penetrates foliage is
recognized to be a major route by which several elements are delivered to the
floor of a forest„  Dry deposition between rains can be a major source of the
contaminants washed to the ground.  Recent studies at Oak Ridge National
Laboratory and elsewhere have attempted to quantify this source by measuring
the amount of sulfate and certain trace metals that can be rinsed from leaves
taken from the canopy, from artificial surfaces placed in the canopy, and from
the canopy itself during precipitation.

Usual method of use.  Investigations require detailed rain chemistry studies,
with collection and analysis of rainfall incident upon a canopy and of
simultaneous throughfall arriving at the floor.  An obvious innovation is to
undertake careful washing of individual leaves, so as to yield measurements of
fluxes of particles and, to a lesser extent, gases during prescribed periods.
Samples are taken at the beginning as well as at the end of each sample period
in order to account for contaminants already present and to estimate the
contributions from leaf leachate.  Internal foliage sources may also be
estimated by comparison of "wash" concentrations from leaves with those from
inert surfaces exposed in the canopy during the same sampling period.

Advantages.  This method results in direct measurements of deposition.
Results obtained for selected foliage samples can be extended to entire
canopies by relating deposition representative of height intervals to leaf
areas sampled in each interval.

Disadvantages.  Because of the great spatial variability in the amount
deposited to leaves, a very large number of samples must be taken and
analyzed.  Leaves can be a highly variable source of some chemical species,
which might be mistaken for the consequences of atmospheric deposition.  A
redistribution of pollutants within the canopy can occur by various means,
which can confuse interpretation.  Due to the great spatial variability of
contaminants in the "throughfall" rain samples (under a canopy), relating
deposited material to material washed down naturally might be doubly
difficult.  This method will not respond reliably to species that may be
unstable, e.g., H2S04 and HNC>3.  Nitrates may evaporate and strong acids may
be neutralized between rains.  Peak doses may be difficult to identify because
of the time elapsed between samples for collection of sufficient materials for
analysis.

Conclusions.  The full benefits of this method have yet to be explored, but
for suitable canopies, it offers a means to monitor dry deposition, especially
for stable species such as sulfate.  Measurements of wind speeds and
atmospheric stability should be taken during experiments„

Recommendations.  The use of the method should be encouraged, especially for
mass-balance experiments in watersheds (see MASS BALANCE STUDIES}.
Simultaneous experiments on comparing concentrations in rainfall with
throughfall must be conducted in order to check results.  (This rainfall and
                                      41

-------
throughfall method itself will be useful in dry deposition studies if
comparisons are successful.)

References;

Lindberg, S. E., R. C. Harriss, R. R. Turner, D. S. Shriner, and D. D. Huff.
     (1979)  Mechanisms and rates of atmospheric deposition of selected trace
     elements and sulfate to a deciduous forest watershed.  Oak Ridge National
     Laboratory, TM-6674, Oak Ridge, Tennessee.  514 pp.

Parker, G. G», S. E. Lindberg, and J. M. Kelly.  (1980)  Atmosphere-canopy
     interactions of sulfur in the southeastern United States.  In:
     Atmospheric Sulfur Deposition.  D. Shriner, C. Richmond, and S.E.
     Lindberg (eds.).  Ann Arbor Publishers, Ann Arbor, MI.

Richter, A., and L. Granat.  (1978)  Pine forest canopy throughfall
     measurements.  Department of Meteorology, University of Stockholm, and
     the International Meteorological Institute, Stockholm, Sweden, Report
     1978-03-01, 29 pp.

Wedding, J, B., R. W. Carlson, J. J. Stukel, and F. A. Bazzaz.  (1977)
     Aerosol deposition on plant leaves.  Water, Air, Soil Pollut., 7,
     545-550.                                                       ~
                                      42

-------
 SURFACE SNOW SAMPLING FOR PARTICLES AND GASES

 History.   Only recently have  experimental methods been developed to permit
 snow sampling and  analysis  for trace contaminants at low concentrations.  The
 methods currently  in use have been of primary interest in the Arctic and
 Antarctic  regions, although some work in urban areas has been performed.

 Usual method of use-  Fresh surface snow is collected immediately following a
 storm and  analyzed for the  species of interest, providing baseline values that
 correspond to the  wet deposition rates.  After one or more days, the surface
 snow is again analyzed.  Ihe  increases in contaminant levels provide a measure
 of  dry deposition  onto the  snow.

 Advantages.   When  fluxes are  small or when winds are very light, this method
 is  more likely to  be successful than micrometeorological methods.  The
 sampling of  snow provides a simple and direct flux measurement.

 Disadvantages.   Data interpretation may be difficult.  Snow sublimes, causing
 the pollutant to become more  concentrated.  Strong acids have freezing points
 lower than water,  and they  are known to migrate downward in snow.  Since
 blowing and  drifting snow redistributes contaminants and thus makes
 representative sampling very  difficult, this method should probably be used
 only for periods of light winds.

 Conclusions.   The method will probably provide useful information under
 limited ambient conditions  and should be applied in certain parameterization
 studies.   Wind speeds and atmospheric stability should be measured
 concurrently.

 Recommendations»  Obviously, this method is useful only for snow surfaces.
 Low winds  and temperatures  several degrees below freezing are necessary.  The
 method  may be helpful for use in certain northern areas, but will require
 additional research before being implemented.

 References;

 Barrie, L. A.,  and J. L.  Walmsley.  (1978)  Study of sulphur dioxide
 deposition velocities to snow in northern Canada.  Atmos. Environ.,  12,
 2321-2332.                                                          ~

 Dovland, H., and A. Eliassen,,   (1976)   Dry deposition on a snow surface.
     Atmos. Environ., 10,  783-785.

Farland, E. J., and Y.  T.  Gjessing.   (1975)   Snow contamination from
     washout/rainout and dry deposition.   Atmos.  Environ.,  9,  339-352.
                                      43

-------
                          C.  FLUX PARAMETERIZATION
                              2.  LABORATORY
CHAMBER STUDIES FOR GASES

History,.  In studies of ecological effects of air pollutants such as SO2 and
03, the use of chambers has become an accepted method, although direct
transfer of laboratory results to natural circumstances is quite difficult.
Measurement of the flux of pollutants into plants or even into individual
leaves is possible, under fairly controlled circumstances.  Emphasis often has
been on the processes that control deposition, rather than precise
quantitative evaluation.  The increased use of chambers in the field has
improved the quantitative aspects of the experiments.

Usual method of use.  Controlled amounts of pollutants are administered in
stirred chambers to plants, leaves, or other surfaces.  Uptake is determined
by measurements of differences in concentrations in outflow versus inflow, by
the rate of change of concentration with time in the chamber, or by
measurement of uptake of radioactive substances.  This method can be applied
to vegetation or other types of surfaces, such as soils.

Advantages.  The roles of individual biological or chemical factors can be
examined in considerable detail.  Relative evaluation of deposition rates for
different types of surfaces can be obtained quickly.  The method can make use
of chamber equipment that is often developed in conjunction with related
investigations of ecological impacts.

Disadvantages.  Realistic conditions are difficult to achieve.  Vegetation in
the laboratory, for example, is quite different from vegetation grown in the
field, because of decreased light levels.  Since turbulence conditions in the
field cannot be truly simulated in chambers, inaccurate results are obtained
if aerodynamic resistances are important relative to surface resistances.
Relatively limited information is obtained on procedures to add up transfer
to individual surface elements in order to obtain the total deposition from
the atmosphere.

Conclusions.  There are many different types of chamber techniques, some of
which contain aspects of small wind tunnels.  Chamber methods provide an
important way to investigate the relative effects of biological or chemical
factors on the surface resistance to gaseous pollutant fluxes.

Recommendations.  There is no obvious manner in which chambers can be used to
monitor fluxes routinely.  It is essential that parameterization studies
involving chambers be continued.
                                      44

-------
References;

Garland, J. A-, and J. R. Branson.  (1977)  The deposition of sulphur dioxide
     to pine forests assessed by a radioactive tracer method,  Tellus, 2^9,
     445-454.

Hill, A. C.  (1971)  Vegetation:  a sink for atmospheric pollutants.  J. Air
     Pollution Control Assoc,, 21, 341-346*

Milne, J. W., D. B. Roberts, and D. J,, Williams.  (1979)   The dry deposition
     of sulphur dioxide—field measurements with a stirred chamber.  Atmos.
     Environ,,, 13, 373-379.

Payrissat, M., and S. Beilke.  (1975)   Laboratory measurements of the uptake
     of sulphur dioxide by different European soils.  A-cmos. Environ.,
     9, 211-217.

Regener, V. H., and L. Aldaz.  (1969)  Turbulent transport near the ground as
     determined from measurements of ozone flux and the ozone gradient.  J.
     Geophys. Res., 74, 79-80.

Rich, S., P. E* Waggoner, and H. Tomlison.  (1970)  Ozone uptake by bean
     leaves.  Science, 169, 79-80,,
                                      45

-------
WIND-TUNNEL STUDIES FOR PARTICLES AND GASES

History.  Many formative studies of pollutant interactions with surfaces have
been conducted in wind tunnels, primarily in Europe and later in the United
States.  These studies tended to approach the problem in a reductionist
manner, by investigation of selected gaseous species and types of particles
deposited to artificial surfaces of various kinds.  The work has been
expanded to address the case of live vegetation and water surfaces.  Much of
our present understanding of the manner in which surface properties influence
dry deposition has been attained by use of wind tunnels.

Usual method of use.  Selected pollutants are "tagged" in some way (often by
use of radioactivity or some fluorescent material) and injected into
wind-tunnel flow above a surface of particular interest.  Surface materials
are then sampled to determine uptake rates.  Concentrations are usually
measured very close to the surface, within the lowest few centimeters, in
order to maximize effective fetch/height ratios.

Advantages.  Detailed studies are possible of deposition velocities of
particles and gases to surfaces.  The effects of varying wind speed, chemical
characteristics of gases, particle size, surface roughness element
configuration, etc. can be investigated.

Disadvantages.  Particles used are sometimes not similar to those normally
found in nature, not only because of the size distribution but also because of
chemical characteristics.  Only small vegetation can be considered since
tunnels are limited in size.  The fetches available are clearly constrained.
Samples of vegetation grown indoors can differ appreciably from those grown in
the field.  The extreme gustiness characteristic of turbulence in-the
atmospheric surface layer during unstable conditions cannot be reproduced.
Water surface effects may not be adequately reproduced in wind tunnels, giving
rise to doubts about the accuracy of particle deposition measurements to
bodies of water.

Conclusions.  Deposition processes can be closely scrutinized by wind tunnel
experiments, but great care needs to be exercised that the experimental set-up
satisfies the theoretical requirements, especially regarding the nature of the
surface used, characteristics of turbulence, and the available fetch/height
ratio.

Recommendations.  Wind tunnel studies offer a unique opportunity to test the
relative importance of different surface mechanisms that potentially
contribute to the process of dry deposition.  Studies of the roles of factors
such as wetness, microscale roughness, stickiness, and electrostatic
characterics should be increased.

References;

Chamberlain, A* C.  (1966)   Transport of gases to and from grass and
     grass-like surfaces.  Proc. Roy. Soc. London A, 290, 236-265.


                                      46

-------
Moller, Uo, and G. Schumanno  (1970)  Mechanisms of transport from the atmo-
     sphere to the earth's surface.  J. Geophys. Res., 75, 3013-3019.

Sehmel, G. A.  (1971)  Particle diffusivities and deposition velocities over
     a horizontal smooth surface.  J» Colloid Interface Sci., 37, 891-906.

Slinn, So A., and W. Go No Slinn.  (1980)  Modeling of atmospheric particulate
     deposition to natural waters<,  Ins  Armospheric Inputs of Pollutants to
     Natural Waters, S, S, Eisenreich (ed.K  Ann Arbor Publishers, Ann Arbor,
     Michigano  (In press) <.
                                      47

-------
                                  SECTION 6

                  INDIRECT CALCULATION OF DEPOSITION RATES
     Another method for determining deposition is always implied to some
extent when parameterization is discussed.  This more indirect method is the
application of parameterizations to measurements of pollutant concentrations
and a few selected companion variables in order to compute the dry deposition
rates indirectly.  The use of concentrations monitored at a network of
stations is emphasized here/ but concentrations computed by numerical models
is inherently considered also.  The following description summarizes this
method„

CONCENTRATION MONITORING AND INTERPRETATION FOR PARTICLES AND GASES

History.  For airborne particulate and gaseous pollutants, there is no
historical data base on dry deposition except for some exceptional cases such
as architectural surfaces.  However, there are data bases on air quality and
there are networks for measuring air quality in operation at the present; time.
Thus, an approach -co monitoring dry deposition is to use the measurements of
air-pollutant concentrations at a single height and employ parameterizations
of dry deposition to yield the deposition rates.

Method of use.  At one height above a well-specified, uniform surface typical
of the area, pollutant concentrations are measured and typically averaged over
one-hour intervals.  From micrometeorological observations and knowledge of
surface behavior, deposition velocities are inferred.  Pollutant flux is
obtained as the product of deposition velocity and concentration.  Companion
variables that form the basis for the meteorological calculations are wind
speed, atmospheric stability, and surface roughness; depending on pollutant
and surface characteristics, a range of other properties must be monitored.

Advantages.  Monitoring stations are already in place for some pollutants.
Other types of observations are minimized.

Disadvantages.   Single-height concentration monitoring is fundamentally
deficient.  Meteorological information alone will not yield a measurement of
deposition velocity because it cannot account for all surface variables, such
as the physiology of vegetation.  Thus, careful observations of surface
characteristics must be made.  While it is probably not essential that
the concentrations be measured at extremely uniform sites, micrometeorological
measurements  should be.   The alternative, less exact but more practical method
is to  infer atmospheric variables from standard meteorological measurements
and assume diurnal cycles of stability.
                                       48

-------
Conclusions.  Dry deposition is not actually monitored.   The develooment of
accurate paramecerizations is crucial.  In many cases,  empirical correlations
between the gross aspects of the physical environment and surface conditions
must be estimated in order to supply the parameters needed.   For example, the
resistance to gas uptake through stomaca of plant leaves can be approximated
according to atmospheric stability categories.

Recommendations.  Until reliable methods suitable for directly monitoring dry
deposition are developed, use of monitored concentrations may be the only
alternative that is practical yet sufficiently accurate.  Parameterizations
of SC>2 and 03 fluxes to many surfaces are already available, but are not well
founded for submicron particles and many gases.  For surfaces, additional work
above tall plant canopies such as forests is needed.

References;

Sheih, C. M., M. L. Wesely, and B. B. Hicks.  (1979)  Estimating dry
     deposition velocities of sulfur over the eastern United States and
     surrounding regions.  Atmos. Environ, 9, 1361-1358.
                                      49

-------
                             SOME RESEARCH NEEDS
     An attempt is made here to describe some of the processes controlling dry
deposition that need to be better understood, and to suggest methods of
research that might be employed.  These considerations complement the
discussions on parameterization procedures presented earlier in Section 5.
The major problem in parameterizing the flux of specified pollution species to
a given surface can often be identified by noting where the resistance to
vertical transport is greatest.  For example, uptakes of certain gases by
vegetation is mainly controlled by the sizes of the leaf stomatal openings.
For submicron particles, deposition to smooth inert surfaces is limited by the
slow diffusion through the quasi-laminar layer of air closest to the surface;
any means of effectively short-circuiting that resistance can result in large
changes in deposition rates.

     Practical considerations limit the number of chemical species and types
of surfaces that can be investigated.  The surfaces chosen for study usually
should be those most prevalent or characteristic of the area or region
considered.  At a minimum, the pollutants to be examined should be the
criteria pollutants and particles in many small size intervals, with emphasis
on submicron diameters.  Of course, studies are aided by grouping together
pollutants with similar chemical and physical properties.  Categories might
depend on solubility in water or reactivity with various substrates, for
example»  For particles, the processes of diffusion, impaction, and
sedimentation can sometimes be studied separately, but care must be taken
because these ara smooth functions of particle size in overlapping size
ranges.

     Table 2 summarizes identified research needs, grouped according to scales
of the phenomena of interest and correlated with experimental methods of
investigation given in Table 1.  The smallest; scale sizes, those associated
with re in Table 2, are often best studied in the laboratory, where, for
example, reactivity with various substrates can be examined.  Further work on
particle deposition is needed to determine the effects of surface stickiness
and microscale roughness.  Continued work on diffusiophoresis, thermophoresis,
and electrophoresis is needed, but with greater emphasis on the environmental
conditions that exist in the field.  For example, electrical charges on
particles vary with the "age" of the aerosol, resulting in varying effects of
electrophoresis on deposition.  Another problem is that pollutants are often
(re)suspended or (re)emitted, due to a variety of causes that are often not
appreciated until what appears as anomolous results are found in laboratory
and field experiments.  Small-scale surface phenomena that need to be studied
in this respect include decomposition of organic material and chemical
reactions at surfaces that "recycle" pollutants.
                                      50

-------
                     TABLE  2.    SUMMARY OF RESEARCH PRESENTLY  NEEDED ON DRY DEPOSITION PROCESSES
Contributing Process
    (resistance)
Length, Seal*
  or Height
     (m)
     Topics of Concern
      Recommended Methods
    of Experimental Research
re, resistance of individual
  surface elements
    «1
 I, resistance of
  quasi-laminar layer
  enveloping surface elements
rs, bulk surface resistance,
  composed mainly of re's and
     -0.005
    <30
Particles:  surface wet-ness,
  stickiness, microEcale
  roughness, electrostatic
  attraction
Gases<  numerous chemical  and
  biological factors as they
  interact with pollutants
  having various properties3

Particle deposition as
  affected by impactlon and
  diffusion3
                            Relationship of mean vertical
                              aloft to transport to  individual
                              surface elements
                                                                 All with medium or high research
                                                                   potential identified in
                                                                   Table 1
Wind-tunnel studies plus all
  small-area field flux-
  parameterization techniques
  (C.t.b. in Table 95

Mlcrometeorological techniques''
  applied in conjunction with
  leaf washing, tracer studies,
  mass-balance studies, and
  similar methods
ra, aerodynamic resistance
  In the atmospheric surface
  layer
ra, and rj as affected by
  medium-scale surface
  discontinuities
    ~50, day
    ~ 5, night
                                           OO
Similarity theory applied to
  vertical gas transport0;
  Transport in very stable
  conditions0

Increased aerodynamic roughness.
  Increased aeration of plant
All mic=roroeteorologtcal
  techniques"
                                                                 Complex geometry^i  box-budget
                                                                   studies, airborne eddy-
                                                                   correlatJon, atmospheric
                                                                   radioactivity, mass balance
                                                                   studies
                                                                 Simple geometry^:  wind-tunnel
                                                                   studies, tracer studies
rD, resistance in the
  planetary boundary layer
  '1500, day
  - 100, night
Vertical flux divergences due to
  physical and chemical processes
Box-budget studies, atiborne
  eddy correlation
aprobably where initial research emphasis shoultJ be placed
''the gradient, modified Bowen-Ratlo, eddy~correl«tl"n, variance, and edrjy-accumulatlon methods
cnot felt to be particularly important  in research presently needed
''not specifically considered at the workshop

-------
     Next to the actual surface is the quasi-laminar boundary layer with a
resistance r£ often considered separately.  A considerable amount of
wind-tunnel work has been performed on phenomena of this scale size. If
possible, both laboratory and theoretical studies should consider the high
levels of atmospheric turbulence and its effects on the interfacial air layer.
For example, the intermittent penetration of strong gusts into deep plant
canopies during unstable conditions strongly modifies both the concentration
of certain pollutants and the thickness of the boundary layers of air around
surface elements.

     A difficult theoretical and experimental problem concerns the process by
which transfer to surface elements is added up to yield the total deposition
from the atmosphere.  Micrometeorological techniques usually only yield an
estimate of bulk surface resistance (rs in Table 2) that may not be adequate
by itself to identify the controlling process.  For present purposes, the
goal sought should be simple parameterizations of bulk surface and interfacial
sublayer resistances or filtration efficiency.  Both numerical and analytical
models have been attempted; they usually rely to some extent on empirical
relationships found in laboratory and field work.  A by-product of such models
is the calculation of the accumulation of harmful pollutants to parts of the
surface.

     On the largest scale sizes, those concerning ra and r^ in Table 2,
application of the parameterizations gives deposition velocites for uniform
surfaces; the deposition rates for large diverse areas are computed as an
average of contributions from each surface weighted in proportion to the total
area covered by each surface.  Several sources of error can affect the
estimates.  For example, the fluxes themselves alter the concentrations, so
that use of a single concentration for many types of surfaces is inaccurate.
Also, the effects of surface discontinuities may result in altered deposition
rates near boundaries of an area with a single type of surface.  Obstacles
such as trees and isolated buildings have an effect, not to mention the
unknown extent to which complex terrain can alter the parameterizations.
Obviously, experimental and theoretical methods used to assess these effects
should be developed further.  Perhaps box-budget experiments can be used to
determine errors; such procedures may in the future be sensitive enough to
detect the large errors that are of greatest concern.
                                       52

-------
                                  APPENDIX A

                             LIST OF PARTICIPANTS
Dr. Len A. Barrie
Atmospheric Environment Service
4905 Dufferin
Downsview, Ontario M3H 5T4
CANADA
Telephone:  (416) 667-4796

Mr. Pranz Burmann
U.S. Environmental Protection Agency
Mail Drop 75
Research Triangle Park, North Carolina  27711
Telephones  (919) 541-2106
            (FTS) 629-2106

Dr. Cliff I. Davidson
Department of Civil Engineering
Carnegie - Mellon University
Pittsburgh, Pennsylvania  15213
Telephone:  (412) 578-2951

Dr. James G. Droppo
Atmospheric Sciences Section
Pacific Northwest Laboratories
Post Office Box 999
Richland, Washington  99352
Telephone:  (509) 444-4706
            (FTS) 444-4706

Dr. Jack L. Durham
U.S. Environmental Protection Agency
Mail Drop 57
Research Triangle Park, North Carolina  27711
Telephone:  (919) 541-2183
            (FTS) 629-2183
                                      53

-------
Dro Charles T. Elly
U«S. Environmental Protection Agency
Region V
536 South Clark Street
Chicago, Illinois  60605
Telephone:  (312) 353-9087
            (FTS) 8-353-9087

Dr. Paul Frenzen
Argonne National Laboratory, D-203
9700 South Cass Avenue
Argonne, Illinois  60439
Telephones  (312) 972-4143
            (FTS) 972-4143

Dr. John A* Garland
Environmental and Medical Sciences Division
AERE Harwell, Oxfordshire
0X11 ORD
ENGLAND
Telephone:  (0235) 24141

Dr. Gary Glass
U.S. Environmental Protection Agency
6201 Congdon Boulevard
Duluth, Minnesota  55804
Telephone:  (218) 727-6692 x526
            (FTS) 783-9526

Dr. Dan Golomb
U.S. Environmental Protection Agency
Energy Effects Division
Mail Code RD-682
401 M Street  SW
Washington, District of Columbia  20460
Telephone:  (202) 426-0264
            (FTS) 426-0264

Dr. Robert G. Henderson
Mitre Corporation
1820 Dolly Madison Boulevard
McLean, Virginia  22101
Telephone:  (703) 827-6661

Mr. Bruce B.  Hicks
Argonne National Laboratory, D-181
9700 South Cass Avenue
Argonne,  Illinois  60439
Telephone:   (312) 972-5792
             (FTS) 972-5792
                                       54

-------
Dr. Mohammed Ibrahim
Atmospheric Environment Service
4905 Dufferin Street
Downsview, Ontario M3H 5T4
CANADA
•Telephone:  (416) 662-4594

Dr. Edward Klappenbach
U.S. Environmental Protection Agency
Region V
536 South Clark Street
Chicago, Illinois  60605
Telephone:  (312) 886-6225
            (FTS) 8-886-6225

Dr= Wo Steve Lewellen
Aeronautical Research Associates
  of Princeton, Inc.
50 Washington Road
Post Office Box 2229
Princeton, New Jersey  08540
Telephones  (609) 452-2950
            (FTS) 483-2000

Dr. Steven E. Lindberg
Building 1505
Oak Ridge National Laboratory
Post Office Box X
Oak Ridge, Tennessee 37830
Telephone:  (615) 574-7857
            (FTS) 624-7857

Dr. Maris Lusis
Ministry of the Environment
880 Bay Street
Toronto, Ontario  M5S 178
CANADA
Telephones  (416) 437-4411
Dr. Brand Niemann
Meteorology Group
Teknekron, Incorporated
2118 Milvia Street
Berkeley, California  94704
Telephones  (415) 548-4100
                                      55

-------
Dr. David F. Parkhurst
School of Public and Environmental Affairs
Indiana University
Bloomington, Indiana  47401
Telephone:  (812) 237-0193

Dr. Roger Reinking
Environmental Research Laboratory
National Oceanic and Atmospheric Administration
325 Broadway
Boulder, Colorado  80302
Telephone:  (303) 497-6167
            (FTS) 320-6167

Dr. George A. Sehmel
Atmospheric Sciences Section
Pacific Northwest Laboratories
Post Office Box 999
Richland, Washington  99352
Telephone:  (509) 444-4127
            (FTS) 444-4127

Dr. Jack Shreffler
U.S. Environmental Protection Agency
Mail Drop 80
Research Triangle Park, North Carolina  27711
Telephone:  (919) 541-4524
            (FTS) 649-4524

Dr. W. George N. Siinn
Joint Center for Graduate Study
100 Sprout Road
Richland, Washington  99352
Telephone:  (509) 375-3176

Dr. Donald H. Stedman
Department of Atmospheric and Oceanic Science
University of Michigan
2450 Hayward
Ann Arbor, Michigan  48109
Telephone:  (313) 763-4381

Dr. Dennis A. Tirpak
U.S. Environmental Protection Agency
RD 676
Washington, District of Columbia  20460
Telephone:  (202) 755-0455
            (FTS) 755-0455
                                      56

-------
Dr. Marvin L» Wesely
Argonne National Laboratory, D-181
9700 South Cass Avenue
Argonne, Illinois  60439
Telephone:   (312) 972-5827
             (FTS) 972-5827

Dr. Joe Wisniewski
Mitre Corporation
1820 Dolly Madison Boulevard
McLean, Virginia  22102
Telephone:   (703) 827-6661
        THE FOLLOWING INDIVIDUALS WERE INVITED BUT COULD NOT ATTEND:
Dr. A. P. Altshuller
U.So Environmental Protection Agency
Mail Drop 59
Research Triangle Park, North Carolina  27711
Telephone:  (919) 5412191
            (FTS) 629-2191

Dr. Bradford R. Bean,
Environmental Research Laboratory
National Oceanic and Atmospheric Administration
Boulder, Colorado  80302
Telephone:  (302) 499-1000

Dr. Jeremy Hales
Pacific Northwest Laboratories
Batelle Boulevard
Post Office Box 999
Richland, Washington  99352
Telephone:  (509) 942-6694

Dr. Herbert L. Volchok
Environmental Measurements Laboratory
U.S. Department of Energy
376 Hudson Street
New York, New York  10014
Telephone:  (FTS)  660-3619
                                      57

-------
                                 APPENDIX B

                              LIST OF OBSERVERS
Richard Coulter
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois  60439

Patricia Irving
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois  60439

John Parker
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois  60439

Douglas Sisterson
Argonne National Laboratory
9700 South' Cass Avenue
Argonne, Illinois  60439

Jimmy Sheih
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois  60439

Steve Spigarelle
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois  60439

Clarence Stevens
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois  60439

Tom Tisue
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois  60439
                                      58

-------
Richard Williams
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois  60439
                                      59

-------
   APPENDIX C





DISSENTING VIEWS
        60

-------
DISSENTING VIEW #1t  Mr. Franz  Burmann and Dr. Richard  J.  TS ore-son


            UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
  September 15,  I960

  Comments on the Report of the "U.S. EPA Workshop on Dry Deposition Methodology"
  Dr. Franz Burmann
  Dr. Richard J. Thompson

  Dr» Jack L. Durham
       Dry deposition  measurement methodology is a combination of two disciplines:
  (I) the micrometeorological  measurement or inference of air transport parameters,
  and (2) quantitative chemical analysis.  The workshop has critiqued various methods
  mainly from the viewpoint of physical mass  transport parameters, which  is expected
  since most  of  the  workshop participants were micrometeorologists.   The degree of
  accuracy of chemical analysis is stated for  several of the  techniques; however, the
  unreasonable difficulty of such demands on the chemical analyst is not discussed.

       While  a legitimate need may exist to monitor the "dry deposition of pollutants,
  especially the  strong  mineral acids" (see the Preface), known atmospheric chemical
  behavior is ignored  in regard to the preservation of sample integrity. For example, the
  presence of strong  mineral acids in fine aerosols has been demonstrated; however, the
  presence  of  such acids in a dry deposition  sample (from a bucket,  plate, or filter-
  collector) is highly improbable because eventual neutralization by  ambient ammonia
  will occur over the collection period (several days).  Of at least equal  importance is
  the simultaneous collection of large mineral particles that are bases, which neutralize
  the acids during extraction.  Our previous experience has been  that dust-fall bucket
  samples invariably  yield a basic aqueous extract.   Such  a result is contrary to the
  suspected acidification of surface waters in the northeast.

       Not demonstrated  is the requirement  of non-surrogate surface  dry deposition
  measurement procedures to be able (a) to detect mean concentration differences of  I
  to 10% in successive samples to within an accuracy of 10%, or (b) to monitor pollutant
  air concentrations  with a  response time of  one second  or  less.   However,  if such
  requirements are valid,  the successful  achievement of  chemical/physical  measure-
  ments in a routine monitoring network is an unreasonable  expectation.   In general the
  error in flowrate measurement for aspirated samples is not better than  ± 10%.  If this
  fluctuation error could be reduced to ± 1% (which is unlikely), the chemical measure-
  ment random errors are expected to be too large to be acceptable.  For example, the
  uncertainties of the NBS analysis  of urban particulate Standard Reference Material
  1648 are (see attachment):

           Nitrogen (N0~)                  2a = 1.07% ± 0.06

           Nitrogen (NhJ)                   2a = 2.01% ± 0.08

           Sulfate                          20= 15.42% ± 14

                                        61

-------
Thus, the precision of the chemical  analyses  for  nitrate and ammonium may  be
sufficient (disregarding  biases  due to  sample  volatility), but that for  sulfate is
unacceptable.  It  is not reasonable to expect that  the analysis of  routine network
samples can be performed at significantly lower  levels of random error than obtained
by NBS for urban aerosols.

      Also,  the determination  of sample  acidity should be expected to have  errors
much larger than those for sulfate, principally because the environmental sample will
contain a complex mixture of strong and weak acids and bases.

      It is notable that  the report  does  not  endorse  a reliance or  expanded use of
surrogate surfaces, including the dry dust bucket. Also, no functional definition of dry
deposition is made which would permit effective monitoring, except  for sophisticated
research  methods  dependent  upon  the chemical analyses of samples  at  a relative
standard error of  ± 1%, or the  analysis of ambient concentration fluctuations at  I Hz.
In general,  the chemical analysis methods cannot provide the required precision, and
the chemical monitors do not possess sufficiently rapid response.

      CONCLUSION:  The report does  not  identify for routine monitoring suitable
methodology  that  is presently in existence.   Recommendations  are given for the
development of dry deposition monitors based  on fundamental principles  of micro-
meteorology  and  analytical  chemistry.     The   extreme   requirements   for
precision/response time of the chemical analysis/monitor make it unlikely that such
methods  will be developed rapidly, as evidenced by  the total  absence of even one
routine dry  deposition monitor based on eddy accumulation, modified Bowen ratio, or
variance techniques.

Attachment
                                         62

-------
U.S. Department of Commerce
    Juanita.M. Kreps
       Secretary
        •*.  *
  National Buram of Standsrai
   Emm Afnbvr. Director
                                                           of
                                               is   of
                      Standard  Reference Material  1648

                                  Urban  Particulate  Matter
        This Standard Reference Material is intended for use in the calibration of methods used in the chemical analysis
        of atmospheric paniculate matter and materials with similar matrices. The material is atmospheric paniculate
        matter collected in an urban location.

        The certified values are based on measurements of 6 to 30 samples by each of the analytical techniques indicated.
        The estimated uncertainties include those due to sample variation, possible methodology differences, and errors
        of measurement (see Preparation and Analysis).  The certified values are based on a sample size of at least
        100 mg of the dried material The material should be dried at 105 °C for 8 hours before use.
Element
Arsenic* c
Cadmium' bcd
Chromium" c
f, a be
Copper
Nickel* bd
— • abed
Zinc
Uranium
Mg/g
115 ±10
75 ± 7
403 ± 12
609 ± 27
82 ± 3
4760 ± 140
5.5 ± 0.1
Element
Iron*""
Lead'""





Weight %
3.91 ±0.10
0.655 ± .008





           Atomic Absorption Spectrophotometry
           Isotope Dilution Mass Spcctroroetry
           Neutron Activation Analysis
           Polarography
           Spectrophotometry
       The overall direction and coordination of the technical measurements leading to certification were performed
       under the chairmanship of J. K. Taylor.

       The technical and support aspects involved in preparation, certification, and issuance of this Standard Refer-
       ence Material were coordinated through the Office of Standard Reference Materials by W. P. Reed.
       Washington. D.C.  20234
       November 16, 1978
        J. Paul Cali, Chief
Office of Standard Reference Materials
                                                    63

-------
Preparation and Analysis
This SRM was prepared from urban paniculate matter collected in the St. Louis, Missouri, area in & baghouse
especially designed for this purpose. The material was collected over a period in excess of 12 months aed, there-
fore, is a time-integrated sample. While not represented to be typical of the area in which it was collected, it is
believed to typify the analytical problems of atmospheric samples obtained from industrialized urban areas.
The material was removed from the filter bags by a specially designed vacuum cleaner and combined into a
single lot.  This product was screened through a fine-mesh sieve to remove most of the fibers and other
extraneous material from the bags. The sieved material was then thoroughly mixed in a V-blender, bottled, aad
sequentially numbered.
Randomly selected bottles were used for the analytical measurements. Each analyst examined at least 6 bottles,
some of them measuring replicate samples  from each bottle.  No correlation was found between measured
values and the bottling sequence. Also, the results of measurements of samples from different bottles were not
significantly different than the measurements  of  replicate samples from single bottles.  Accordingly, it is
believed that all bottles of this SRM have the same composition.
The analytical methods employed were those in regular use at NBS for certification of Standard Reference
Materials, except as noted in the following paragraphs. Measurements and calibrations were made to reduce
random and systematic errors to no more than one percent, relative. The uncertainties of the certified values
listed in the table include those associated both with measurement and material variability.  They represent the
95 percent tolerance limits for an individual sub-sample, Le., 95 percent of the sub-samples from & single uisit of
this SRM would be expected to have a composition within the indicated range of values 95 percent of the time.
The following values have not been  certified because either they were not based on results of a reference
method, or were not determined by two or more independent methods. They are included for isIorm&tiGn only.
All values are in units of Mg/g of sample, unless otherwise indicated.
          Aluminum
          Antimony
          Barium
          Bromine
          Cerium
          Cesium
          Chlorine
          Cobalt
          Europium
          Hafnium
          Indium
          Iodine
          Vanadium
  (3.3 wt. 9
 (45)
(737)
(500)
 (55)
  (3)
  (0.45 wt.
  (18)
  (0.8)
  (4.4)
  (1.0)
  (200"
(130)
Lanthanum
Magnesium
Manganese
Potassium
Samarium
Scandium
Selenium
Silver
Sodium
Thorium
Titanium
Tungsten
 (42)
  (0.8  wt.%)
(860)
  (1.0  wt.%)
  (4.4)
  (7)
 (24)
  (6)
  (0.40 wt.%)
  (7.4)
  (0.40 wt.%)
  (4.8)
                                                 64

-------
The values listed below are based on measurements made in a single laboratory, and are given for information
only. While no reason exists to suspect systematic bias in these numbers, no attempt was made to evaluate such
bias attributable to either the method or the laboratory. The method used for each set of measurements is also
list-d. The uncertainties indicated are two times the standard deviation of the mean.
          Nitrogen (NO3)            (1.07% ± 0.06)
          Nitrogen (NH4)            (2.01% ±  .08)
          Sulfate                  (15.42% ±  .14)
          SiOj                    (26.8%  ±  .38)
          Freon Soluble             (1.19%±  .47)
          The above values were determined by the methods indicated below:
          Nitrate      - Extraction with water and measurement by ASTM Method D992.
          Ammonia    — NaOH addition followed by steam distillation and titration.
          Sulfate      - Extraction with water and measurement by ASTM D516.
          SiO:        - Solution and measurement by ASTM Method E350.
          Freon Soluble  - Extraction with Freon  113,  using the method described  in "Standard
                          Methods in Examination of Water and Waste Water," 14th Edition, p. 518,
                          American Public Health Association, Washington, D.C.
J. W. Matwey supervised the collection of the material as well as sieving and bottling.  The following members
of the staff of the NBS Center for Analytical Chemistry performed the certification measurements: R. W. Burke;
E. R. Deardorff; B. I. Diamondstone; L. P. Dunstan; M. S. Epstein; M. Gallorini; E. L. Garner; J. W. Gramlich;
R. R. Greenberg; L. A. Machlan; E. J. Maienthal; and T. J. Murphy.
                                               65

-------
DISSENTING VIEW #2:  Dr. Cliff  I. Davidson and Sreven E.  Lindberg

              ALTERNATIVE VIEWPOINTS ON SURROGATE SURFACES

       Surrogate surface deposition measurements of various types  have
  been  conducted for years,  and  will probably continue  to  be used  on a widespread
  basis.   Simplicity in obtaining the data,  control over contamination,
  ability  to  collect sufficient  material for trace level analysis, and low
  expenses are the  primary reasons.  It is  acknowledged that certain
  applications of such measurements in particular may  not  provide  as much
  accuracy as more  sophisticated techniques, however sufficient data are
  not available to  establish conclusively that  all types of surrogate
  surfaces should not be used.
       Several surrogate surface studies reported in the literature
  have  shown  consistencies in  the data which suggest that  semi-quantitative
  information can be obtained.   For example, Peirson et. al. (1973) showed
  dry deposition velocities  in the range of 0.2-0.Son/sec  for 15 out of
  23 trace elements studied.   Horizontal sheets of filter  paper were used
  for these measurements,  dough (1973) has determined the relationship
  between  deposition velocities  measured using  similar filter samplers
  in a  wind tunnel  with theoretical deposition  velocities  for smooth surfaces
  and a grass sward as  influenced by particle size and wind speed.  Similar
  experiments were  reported  by Klepper and  Craig (1975) using 0.8  ym
  radioactively labelled aerosols to compare deposition velocities to bean
  leaves and  inert  surfaces  (tape).  Deposition velocities for leaves
  were  similar to those measured for upward facing inert surfaces  under
  the conditions tested.  A  comparison of deposition to vegetation with
                                        66

-------
 that to surrogate surfaces  has also been reported  by Servant  (1976) who
 used inverted plastic containers and leaves from four species of trees.
     Huntzicker et.  al.  (1975) used teflon  plates  to measure  lead  dry
 deposition  in the  Los Angeles  Basin.   It was  found that  the dry deposition
 rate was  similar within  the Basin, and decreased at several sites  mono-
 tonically with  distance  away from the  Basin when plates  were  set up  in
 surrounding mountain, deserts  and coastal areas.   Lead deposition  also
 decreased monotonically  between  1m and 150m from a freeway  near central
 Los Angeles.
     In another Los  Angeles area study, Davidson (1977)  measured
 simultaneously  airborne  size distributions and  teflon plate deposition
 of lead,  zinc and cadmium.   It was shown that the  size data could  be
 used with equations  for  sedimentation  to predict deposition on  the plates.
 This suggests that teflon plate data can provide a reliable lower  limit
 to dry deposition on natural surfaces  under certain conditions, such  as
 when sedimentation is the dominant transport mechanism onto the plates.
     Lindberg et. al. (1979) compared  in-canopy petri dish  dry  deposition
 data for  several atmospheric constituents to that  obtained  by leaf
 washing experiments  in a deciduous forest (Walker  Branch Watershed,
 Tennessee).  Agreement to within a factor of two was obtained for
 cadmium,  zinc, manganese, and sulfate, although poorer agreement was
 observed  for lead (factor of 10).  Leaf surface absorption, rather than
 nonrepresentative petri  dish data, could be a possible reason for  the
 discrepancies in the Pb values.  This study also showed consistently
higher dry deposition velocities, based on petri dish data, for those
elements found in larger particle sizes.  The deposition velocity  for
particulate sulfate to flat plates when extrapolated to the full
                                       67

-------
forest canopy at Walker Branch is in rough agreement with values for the
total sulfur dry deposition velocity to a pine forest canopy determined
by eddy correlation techniques (Hicks and Wesely,  1978).
     Comparisons between airborne particle size data and teflon plate
deposition have been conducted by Elias and Davidson (1980) in a remote
High Sierra canyon.  As in the Tennessee study, consistently greater dry
deposition velocities were found for those elements associated with
larger particle sizes.  Values of dry deposition velocity for each
element were also consistent from one experiment to the next.
     The data from these studies do not provide detailed information on
the  relations between surrogate surface and natural surface deposition.
The  consistencies in the data, however, suggest that it may be possible
to establish such relations, i.e., to calibrate surrogate surfaces in
some fashion.  We do not suggest that these surfaces be  used to predict
airborne concentrations, however.  Uncertainties associated with these
relationships cannot be predicted at this time.
     We suggest that the comparison of surrogate surface, micro-
meteorological, leaf washing, and other methods to  estimate dry deposition
be performed in wind tunnels and in the field over  natural surfaces.
This would be especially useful  for those dry collectors that  have
already been applied in a  research mode  (e.g., teflon discs, polyethylene
plates, or filter  paper).
     Recommended research  includes  determination of the  relationships
among material  suspended in the  atmosphere, collected by surrogate
surfaces, and deposited on natural  surface elements (as  determined  by
washing and microscopic examination, or  other techniques).   Such
comparison studies might lead to a  better design and application  of
                                        68

-------
certain artificial surfaces than those routinely used for monitoring,

such as the use of flat plates situated in the foliar canopy as opposed

to the use of buckets at ground level.
                                        Cliff I./ Davidson
                                        Assistant Professor of
                                          Civil Engineering and
                                          Engineering and Public Policy
                                        Carnegie-Mellon University
                                        Steven E. Lindberg
                                        Geochemist
                                        Environmental Sciences
                                        Oak Ridge National Laboratory
References

Clough, W. S.  (1973) Transport of particles to surfaces.  Aerosol
     Science 4. 227-234.

Davidson, C. I.  (1977)  The deposition of trace metal containing particles
     in the Los Angeles Basin.  J. Powder Tech. 18, 117-126.

Elias, R. W. and C. I. Davidson.  (1980) Mechanisms of trace element
     deposition from the free atmosphere to surfaces in a remote High
     Sierra canyon.  Atmos. Environ, (in press).

Hicks, B. and M. Wesely.  (1978) Recent results for particle deposition
     obtained by eddy correlation methods.  ERC-78-12.  Argonne
     National Laboratory, Argonne, Illinois.

Klepper, B. and D.  K. Craig.  (1975)  Deposition of airborne parti-
     culates onto plant leaves.  J. Environ. Qua!. 4. 495-499.
                                       69

-------
References  (Continued)


Lindberg, S. E., R. C. Harriss,  R.  R.  Turner,  D.  S.  Shriner and
     D. D. Huff  (1979) Mechanisms  and rates of atmospheric deposition of
     selected trace elements and sulfate  to a  deciduous forest watershed.
     Oak Ridge National Laboratory, TM-6674, Oak Ridge, Tennessee, 514 pp.

 Servant, J.  (1976) Deposition  of atmospheric  lead  particles  to  natural
      surfaces in field experiments.   In, Atmosphere-Surface Exchange  of
      Particulate and Gaseous Pollutants, ERDA  Symposium No. 38,
      CONF 740921, NTIS.
             U.S. Environmental Protect 'V
             Region 5, Library (5PL ]-;
             230 u. DearVi:i .'7,>..yt, P.-J -
             Chicago, IL   60C04
                                         70

-------