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S Research Needs
In light of existing uncertainties in knowledge about the
relationships between emissions and the deposition of
acid-forming materials, it is appropriate to consider the
research that may help to clarify understanding. In this
chapter we describe research that we believe is needed to
answer the most central questions in the shortest amount
of time.
A considerable amount of research is currently being
performed with funding from both governmental and private
sources in the United States and Canada as well as in
Europe. We have not attempted to review these research
programs. As a consequence, some of the research
activities that we recommend may be--indeed we know they
are--included in current programs. Others, to the best
of our knowledge, are not. On the basis of our review of
current knowledge as described in this report, we are
convinced that the research that we recommend forms
essential elements in an integrated effort to improve
understanding of the phenomenon of acid deposition.
We do not know whether incorporation into a model of
all the chemical and physical processes currently thought
to be important would account for the concentrations of
sulfate observed in precipitation during both winter and
summer. No quantitative determination of the relative
contributions to the production of sulfate and nitrate by
gas- and liquid-phase processes has been attempted to
date; such attempts are probably several years in the
future.
The details of dispersion of pollutants from sources,
their chemical transformation and transport over long
distances, and their contribution to acids in precipi-
tation systems are sufficiently unknown and complex that
a great deal of research will probably be required before
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they can be described with precision by models. Further-
more, many years of precipitation measurements will be
needed to establish a reliable data set from which tests
of models can be made.
We believe that extensive laboratory,
field, and
modeling studies should be continued, to establish the
physical and chemical mechanisms governing acid depo-
sition. However, it appears to us that useful informa-
tion about the delivery of acids to rural areas by
transport and transformation processes can be determined
fairly quickly by direct empirical observation in the
field. Although the results of such field studies may
not yield complete detailed descriptions of the inter-
actions of all the processes involved, they are likely to
provide basic phenomenological evidence with sufficient
reliability to form the basis for improving the near-term
strategy for dealing with the problem of acid deposition.
The data are essential for enhancing theoretical under-
standing and developing improved deposition models. In
the long term, the ultimate strategy for dealing with
acid deposition will depend on the application of
realistic, validated models.
F TELD STUDIES
Field research capable of testing the possibility of a
nonlinear relationship between emissions and wet
deposition in North America is needed. A limitation of
oxidant in winter is believed to be a principal cause of
such nonlinearity. Because the nature and availability
of oxidants is sensitively dependent on the general
composition of polluted air, field studies should be
conducted in regions particularly involved in the problem
of acid precipitation. Of primary importance are those
experiments directed at discovering the rates of
production of SO4 and Ned in clouds and the
detection of possible limitations of those rates.
Supplemental information, such as the availability of
oxidants under varying conditions, is also important.
We describe below some of the more important issues
currently amenable to study in the field.
Cloud Processes
As described in Chapter 2 and Appendix A, the conversion
of SO2 to H2SO4 in certain cloud systems may be
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relatively complete and rapid, but we lack conclusive
information on either oxidation rate constants or the
completeness of conversion in acidified clouds typical of
the eastern United States and Canada. Most of the mass
of sulfuric acid already existing in aerosols in dry air
is efficiently incorporated into cloud droplets as the
air is cooled and the cloud forms. Models indicate that
nitric acid vapor is also incorporated with high effici-
ency into cloud droplets. If, during the warm months,
when the rate of wet deposition of acid is highest,
complete in-cloud conversion of SO2 and NO2 occurs,
and if the rainout of HNO3 vapor and sulfate aerosol is
highly efficient, then the assumption that the wet
deposition of sulfuric and nitric acid is linearly
related to ambient concentrations of SO2 and NOx
would be justified. This could be readily checked
experimentally with available technology in appropriate
storm systems, such as warm fronts and summertime
convective storms.
Studies of Chemical Mechanisms
A number of key problems relating to the linearity of the
dependence of acid production rates on precursor
concentrations provide the focus for studies of chemical
mechanisms. (1) Tile efficiency and seasonal dependence
of transformation processes are no doubt different for
gasphase and aqueous-phase processes. What are the
relative contributions of these two pathways? (2) What
are the lifetimes of NC~ and SO2 in clouds? Do rates
of acid production differ markedly in various cloud
types? Are the production rates inhibited by increasing
acidity or limited by availability of oxidants? t3) what
are the dominant reaction paths for SO2 oxidation in
clouds? (4) Do competing reactions with other atmospheric
constituents (e.g., formaldehyde), especially in polluted
air, seriously inhibit or lower the effectiveness of
H202 reactions with SC: in clouds? (5) what
processes govern the apparently significant rate of
production of HNO3 in clouds?
(6) What are the
relative rates of production of HNC~ and H2SO4 in
clouds? (7) What is the role of ultraviolet light in the
production of free radicals and H202 in the gas phase
in clouds?
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Dry Deposition
We noted in Chapter 3 that dry deposition of SCt and
NO2 may account for about one half of the total
deposition of sulfur and nitrogen oxides and acids in
eastern North America. Accurate evaluations of the
extent of dry deposition of these pollutants is required
to obtain a quantitative measure of their fates in the
environment. The eddy-correlation technique, in which
simultaneous measurements of the vertical wind speeds and
specific pollutant concentrations are measured with
rapid-response instrumentation, should prove to be a
valuable tool in the study of the deposition of S02,
NO2, HNO3, and other pollutant gases. The develop-
ment, refinement, and application of this method and
possibly other new and accurate methods of measuring dry
deposition, which are suitable for monitoring applicar
Lions, are important research objectives. The challenge
of developing accurate methods is great, adapting as they
must to the great variety of surfaces that cover the
Earth and to the effects of humidity, temperature,
sunlight intensity, and other factors.
Tracers
To develop the most cost-effective strategy for
ameliorating the problem of acid deposition, it is
necessary to know the relative impacts of specific source
regions on specific sensitive receptor regions. In view
of the uncertainties inherent in the calculation of
trajectories, especially during storm conditions, it
seems important to develop tracer techniques that can
yield experimental tracking of air parcels. Such tech-
niques could be applied especially during the meteoro-
logical conditions that are currently believed to provide
the greatest opportunity for long-range transport of
acidic materials. The use of insoluble and chemically
inert gaseous tracers, as is currently planned, provides
a promising approach. Admixtures of insoluble and unique
materials that are subject to reaction with oxidants such
as the HO radical would in addition provide information
on possible limitations of the oxidation of SO2 and
NOx.
In light of the potential advantages of the method of
elemental tracers for understanding the relationships
between source and receptor regions, a considerable
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amount of research and development to establish and test
the method on a regional scale is warranted. The first
priority in this work should be to measure the detailed
patterns of elemental composition of particulate matter
collected at various rural sites in both source and
receptor regions in eastern North America. The particles
should be segregated by size, and a large number of
elements and species should be measured for each size
range. Other data should also be collected concurrently,
including ambient concentrations of pollutant gases, wind
speed and direction, relative humidity, precipitation,
and optical qualities of the atmosphere. High-quality
back trajectories should be calculated for the air masses
sampled for each sampling period to aid in interpretation
of the results. Few of these data now exist.
Research using elemental tracers would attempt to
determine if particles coming from major source regions
have distinctive chemical signatures that can be used to
identify the origins of polluted air masses at long
distances from source regions. For example, are there
elemental tracers that can be used as clear indicators of
coal combustion, in the same way as vanadium apparently
can be used to indicate oil combustion? Can observed
patterns of elemental composition in rural areas be
resolved into linear combinations of known composition
released by certain types of sources? Does the behavior
of fine particles, particularly those bearing soluble
species, provide insight into the behavior of sulfate and
nitrate species that are of primary concern in acid
deposition?
Building on the first stage of research, it may also
be necessary to conduct studies of the elemental
composition of particles in clear air and clouds as well
as on the ground during specific episodes, such as the
dusting of a polluted air mass from the midwestern United
States toward the northeast along a southward-moving warm
front. Changes in elemental composition as a specific
parcel of air moves on a regional scale should help to
identify the types of changes in the characteristics of
the air during transport and as it passes over additional
sources. Much of this work could be conducted in con-
junction with the field studies described in the previous
section.
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Meteorological Studies
The climatology of storm movements in North America is
well developed. The development of a quantitative
relationship between storm type and acid deposition over
eastern North America would help to estimate the long-
range transport of pollutants. Despite the associated
uncertainties, statistical studies of air parcel trajec-
tories associated with the types of storm systems
responsible for depositing acid precipitation would also
be extremely valuable.
An important requirement in the experimental deter-
mination of rates of SO2 and NOx oxidation in clouds
is a realistic evaluation of the airflow in the vicinity
of clouds to establish the quantity of materials nroo-~.~a
by and the time resident in the cloud.
All of these field studies bear directly on the
critical question of the zone of influence of sources on
receptor regions.
Our suggestion that these field measurements be a
first priority is not meant to imply that pertinent
laboratory and modeling studies should not also proceed.
However, in view of the complexities in these systems, we
believe that well-conceived field studies may answer many
of the outstanding questions in a shorter time than that
required for a complete molecular and dynamic description
of the phenomenon.
_ _ _ ,= ~
LABORATORY STUDIES
The direct measurement of elementary rate constants for
the many apparently important reactions related to the
chemistry of acid deposition has been restricted largely
to conditions that are not typical of the lower tropo-
sphere. The direct determination of the rate constants
for reactions of the HO radical with SC2 and NO2
should be made at pressures (near 1 atm of air) and
temperatures characteristic of the troposphere, as should
other measurements.
Many aspects of cloud chemistry can and should be
examined auantitativelv under ~ontr~1 1 ~ 1 Herr
conditions. For example, the mechanisms of the develop
ment Of H2O2 from O3 and other unidentified reactants
should be established. Other important parameters
amenable to laboratory measurement that are required for
the quantitative description of cloud processes are the
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sticking coefficients of the gas-phase reactants such as
HO, HO2, and NO3 on collision with cloud droplets.
In general, laboratory studies are needed to establish
the mechanistic detail that is required in the develop-
ment of the chemically and physically sound models of
acid precipitation.
DEVELOPMENT OF I HEORETICAL MODELS
Models of the development, transport, and deposition of
acids and acid-forming materials based on physical and
chemical principles are under development. These models
will serve as a useful framework with which the latest
data from field and laboratory studies can be combined to
provide a suitable test of theory and improved planning
for further field studies. State-of-the-art theoretical
models that treat quantitatively the complicated gas-
phase chemistry, cloud processes, transport, and
deposition will require many years for development. The
ultimate test of our understanding of the chemistry and
physics of the processes of acid rain is the successful
development and use of such models.
Representative terms from entire chapter:
field studies
