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OCR for page 1
Summary
During the past 25 years in Europe and the past 10 years
in North America, scientific evidence has accumulated
suggesting that air pollution resulting from emissions of
oxides of sulfur and nitrogen may have significant adverse
effects on ecosystems even when the pollutants or their
reaction products are deposited from the air in locations
remote from the major sources of the pollution. Some
constituents of air pollution are acids or become acidic
when they reach the Earth's surface and interact with
water, soil, or plant life. Several studies have docu-
mented the potentially harmful effects of the deposition
of acids on ecosystems, which are of particular concern
in areas with low geochemical capacities for neutralizing
the acidic inputs (such as parts of the northeastern
quadrant of North America, the Appalachian Mountains, and
some of the mountainous areas of western North America).
Although the pollutants may be deposited in dry form or
in rain, snow, or fog, the deposition phenomenon is often
called acid rain or acid precipitation. In this report
we use the term acid deposition to encompass both wet and
dry processes.
The question of what, if anything, to do about acid
deposition is a complex one, involving generation and
interpretation of scientific evidence, assessment of
risks, costs, and benefits, and political considerations,
both domestic and international. This report deals with
a small, but important, part of the analysis that cur-
rently is being conducted to answer the question--the
scientific evidence concerning the relationships between
emissions of acid-forming precursor gases and deposition
of potentially harmful pollutants. Our purpose is to
assess the current state of scientific information that
can be marshaled to describe those relationships in the
1
OCR for page 2
2
hope that our assessment will be valuable to decision
makers in government and in the private sector. We focus
on conditions in portions of eastern North America, for
which more information is available than for other areas
of the continent.
The central issues of concern in this report are the
adequacy of current scientific understanding about the
relationships between emissions and deposition, the
extent to which the relationships are strongly nonlinear,
and the extent to which distant sources contribute to
deposition in ecologically sensitive, remote areas. We
have reviewed the available scientific evidence that
pertains to the issues of nonlinearity in the relation-
ships between emissions and deposition and long-range
transport. In the report we describe the current state
of understanding about atmospheric processes (Chapter 2
and Appendixes A, B. and C), review the development of
theoretical models (Chapter 3), and analyze the available
observational evidence for source-receptor relationships
(Chapter 4). Much remains to be learned about the
detailed mechanisms involved and their relative impor-
tance for the relationships between emissions and
deposition. As scientists, our training leads us to be
concerned about the current limits of our understanding
of the relevant processes and the uncertainties asso-
ciated with assessing cause and effect. Much of our
report, therefore, has been devoted to exploring the
areas of uncertainty in understanding of the phenomena.
Continuing research on acid deposition is needed to
resolve or reduce the uncertainties and thereby to
provide information useful in making more informed
public-policy decisions regarding acid deposition
(Chapter 5).
Our findings and conclusions are summarized below.
STATUS OF SCIENTIFIC KNOWLEDGE
Current scientific understanding of the relationships
between emissions of precursor gases, such as sulfur
dioxide (SO2) and the oxides of nitrogen (NOk), and
deposition of acids or acid-forming substances, such as
sulfuric acid (H2SO4), nitric acid (HNO3), the anions
sulfate (SOi) and nitrate (NO5), and the cation ammonium
(Nut), is based on theoretical considerations, the
results of modeling exercises, and analysis of observa-
tional data.
OCR for page 3
3
Data
Data are limited that can be used to characterize air
quality, meteorological conditions, and emissions from
which relationships between patterns of emissions and air
quality in rural areas might be discerned in North
America. Most of the historical data on air quality
describe urban conditions. The most reliable information
on rural conditions comes mainly from a single study in
the northeastern United States, the Sulfate Regional
Experiment (SURE), performed in 1977-1978. A long-term
record (18 years) of reasonably reliable data on depo-
sition chemistry is available at only one site in North
America. Reliable data on regional precipitation
chemistry have been collected only over the past 4 or 5
years through monitoring networks set up in the United
States and Canada. There are no regional data from
observations of dry deposition.
Available data on precipitation chemistry and on
annual average ambient concentrations of SC2, NOk,
sulfate, ammonium, and nitric acid indicate elevated
levels of pollutants in the air and acidic substances in
precipitation over much of eastern North America. Ambient
concentrations are much higher than can be accounted for
by emissions from natural sources on a regional scale.
The geographical distributions of SO2 and sulfate
differ somewhat: SO2 concentrations are more localized
in the regions around major concentrations of sources,
and ambient sulfate aerosols appear to be more widely
distributed. The distributions of sulfate in precipita-
tion are similar to those of sulfate in the ambient air.
Currently the molar concentrations of nitrate and sulfate
in precipitation are roughly comparable over much of the
eastern United States.
Ambient concentrations of air pollutants are highly
variable over time, whereas rates of emissions of the
precursor gases SC2 and NOx are less variable. Differ-
ences in temporal behavior are due in large measure to
the variability of meteorological conditions and, for
secondary pollutants such as sulfate and ozone, the
chemical reactivity of the atmosphere and the amount of
solar radiation. Concentrations of sulfate in both the
air and precipitation tend to reach their maximum values
in summer in the northeastern United States; seasonal
variations are less evident in the Midwest and Southeast.
In many areas, ambient SC2 and NOk concentrations are
highest in the winter, although no measurements of these
OCR for page 4
4
parameters in ecologically sensitive areas have been
reported. Nitrate in precipitation shows much less
seasonal variation in the northeastern United States.
Both ambient sulfate concentrations and sulfate in
precipitation have been shown to be statistically related
to aerometric parameters. For example, variations in
ambient sulfate concentrations (measured at ground level)
are related to variations in SO2 and ozone concentra-
tions, relative humidity, winds, and ventilation, whereas
sulfate in precipitation is related to winds, the types
and rates of precipitation, and ambient concentrations of
sulfur oxides. Ambient nitrate data are not amenable to
similar analyses because of uncertainties in the analyti-
cal chemical methods.
Not all sulfates and nitrates in the air or in
precipitation contribute to the acidity of the air or
precipitation.
Acidity in solution is a function of the
concentration of hydrogen ions. Some sulfate and nitrate
in the air and in precipitation may be associated with
cations other than hydrogen, such as ions of calcium or
ammonium. Thus the acidity of deposition is the result
of influences of the variety of cations and anions that
may be present and in general cannot be identified with
one or two anions. However, once deposited, sulfate and
nitrate associated with cations other than hydrogen, such
as ammonium, may still result in acidification of eco-
systems as a result of biological and chemical inter-
actions in soils and water.
Meteorological Processes
One of the greatest difficulties in establishing
relationships between sources of pollution and conditions
in ecologically sensitive areas is that of accounting for
the influences of atmospheric processes on the behavior
of pollutants. These processes include the large-scale
transport of air masses, atmospheric mixing near the
Earth's surface, physical and chemical reactions among
pollutants and naturally present species, deposition of
gases and suspended particles, and cloud processes
leading to precipitation. Transport, mixing, physical
and chemical reactions, and cloud processes are respon-
sible directly or indirectly for the distribution and
rate of deposition of pollutants to the ground. Our
empirical and theoretical understanding of the processes
is strong in some aspects and weak in others.
OCR for page 5
5
Meteorological processes control the transport and
dispersion of pollutants from their sources. Regions of
northeastern North America that are considered to be
sensitive to acid deposition are subjected to highly
variable meteorological conditions, with the result that,
in addition to local sources, a number of geographically
widespread source regions are likely to contribute to the
total deposition of acid-forming chemicals. Nonetheless,
empirical analyses suggest that many of the precipitating
air masses--and therefore most of the pollution-related
ions dissolved in precipitation--reaching several sensi-
tive, remote areas of the northeastern United States and
southeastern Canada have their origins in unwind regions
to the south and southwest. Because of the high variabil-
ity in synoptic-scale meteorological phenomena affecting
sensitive areas, however, all sources in eastern North
America must be considered as contributing in one degree
or another to the phenomenon of acid deposition. Evidence
exists for long-range transport of pollutants leading to
acid deposition, but the relative contributions of
specific source regions to specific receptor sites
currently remain unknown.
Models
Methods are available for estimating the effects of
emissions of SC2 and Nod on regional distributions of
ambient concentrations of these gases. The methods
include statistical analysis of observational data and
theoretical calculations using deterministic models of
the chemical and physical processes involved. The methods
employed to date, however, do not produce reliable esti-
mates of spatial and temporal distributions of acidity,
partly because of in~omnl~t" i nu~nP^r i "= off ban i ~ mom_
~ - ]r ~ ~ _ ~ ~ _ _ %~ ~ ~
~ . . .
pounds in the air that neutralize some of the acidity.
The actual limitations of predictive models for
calculating the effects of sources on sulfate deposition
are not well defined because of deficiencies in knowledge
of atmospheric processes and the lack of coherent
regional data on air quality and deposition. No studies
of the validity or limitations of models for nitrate
concentrations have been performed. No model for wet
deposition of acidity has been developed that takes
account of sources and distributions of all the important
ions in precipitation.
The deterministic models that are available employ by
OCR for page 6
6
necessity approximations to the atmospheric processes
that are known or hypothesized to be important to acid
deposition. All deterministic models are limited in
their usefulness by the sparseness of meteorological data
compared with that needed as input to project reliably
the movement and mixing of air masses.
ISSUES
Applicability of Models to Decisions on Control Strategy
Results from current air-quality models applied to
regional-scale processes have provided guidance on the
significance of dynamic processes influencing sulfur
deposition. The results of the models are qualitatively
consistent with observations, thus demonstrating impor-
tant temporal and spatial scales of the source-receptor
relationships. Qualitatively, the models have pointed to
the importance of certain geographical groupings of
(SO2) sources and the potential influence of the
sources on certain receptor areas. However, current
models have not provided results that enable us to have
confidence in their ability to translate SO2 emissions
from specific sources or localized groupings of sources
to influences on specific sensitive receptors. Little
has been done in modeling to translate NOx emissions
into nitrate deposition or to link sulfate and nitrate to
acid (H+) deposition.
A predictive capability that
includes accounting for important cations is considered
an essential requirement where long-range transport
processes are involved.
Because of the simplifying assumptions made in order
to develop practical, economical regional-scale air-
quality models and because data are not available to
validate or verify the models, workers in the field
generally have only limited confidence in current results.
The models and their results are useful research tools;
but given the state of knowledge of the physics and
chemistry of the atmosphere in the context of long-range
transport in air pollution, we advise caution in using
deterministic models to project chances in patterns of
deposition on the basis of changes in patterns of
emissions of precursor gases.
For practical purposes, deterministic models have been
and will continue to be used for research on atmospheric
OCR for page 7
7
transport and transformation chemistry, and this use may
lead to improvements in our understanding of deposition
of acidifying substances in eastern North America. Before
models can be relied on for development of refined
strategies for dealing with acid deposition that take
into account the specific locations of sources and
receptors, however, we will need a greatly improved base
of meteorological data and more precise treatment in the
models of both chemical and meteorological aspects such
as vertical distributions of meteorological variables,
including clouds. In the near term, we believe that
collection and analysis of field data are likely to lead
to improved understanding more quickly than refinement of
deterministic models. In fact, such data are needed to
improve the models themselves. Confidence in control
strategies will be strengthened to the extent that they
are founded on scientifically sound, verified models.
Laboratory evidence suggests that an alternative model
of the chemical processes involved in acid deposition may
be postulated that represents the gas-phase reactions
leading to the oxidation of SO2 more correctly than the
model of Rodhe, Crutzen, and Vanderpol, which has been
widely used for this purpose. In keeping with results of
laboratory experiments, the alternative model employs a
series of reactions that results in oxidation of SO2
without net consumption of a major oxidant, the hydroxy
radical. When these gas-phase reactions are incorporated
into the model, the previously reported nonlinearity in
the relationship between changes in ambient concentra-
tions of SO2 and changes in ambient concentrations of
sulfate aerosol is greatly reduced (see Chapter 3).
Nonlinearity
There is, admittedly, much to be learned about the
relationships between emissions and deposition. However,
on the basis of analysis of currently available data in
eastern North America and within the limits of uncer-
tainty associated with errors in the data and in
estimating emissions, we conclude that there is no
evidence for a strong nonlinearity in the relationships
between long-term average emissions and deposition. This
conclusion is based on analysis of available data on
historical trends (mainly at the Hubbard Brook Experi-
mental Forest in New Hampshire), the ratios of pollutants
OCR for page 8
8
in emissions and deposition, and comparison of the
percentages of emissions of SO2 and NOk that are
deposited as sulfate and nitrate in precipitation (see
Chapter 4); it is also supported by theoretical
calculations taking account of the latest results of
realistic laboratory studies (see Chapter 3).
The only available direct evidence of strong non-
linearity between average emissions and average depo-
sition of sulfur compounds is found in the extensive data
taken in Europe over the past 25 years. Analysis of the
European data has focused on historical trends in bulk
sulfate deposition and on the spatial distribution of the
ratio of sulfate to nitrate in monthly bulk samples. The
analysis has also employed the original Rodhe-Crutzen-
Vanderpol model. The observed trends in Europe are
somewhat uncertain because of changes in sampling and
analytical techniques and analytical laboratories
throughout the period (see Chapter 4).
Reasonably reliable historical data indicating trends
in North America are available only from the Hubbard
Brook site (see Chapter 4). These data, from 13 years of
weekly bulk samples, show no evidence of strong
nonlinearity.
Analysis of the spatial distributions of the molar
ratio of SO2 to NOx in emissions and the molar ratio
of sulfate to nitrate in precipitation provides addi-
tional though indirect evidence that there is no strong
nonlinearity in the relationships between long-term
average emissions and deposition in eastern North America
(see Chapter 4). The molar ratio of sulfate to nitrate
in precipitation does not vary substantially over a large
region in eastern North America; and for annual average
data, it is similar to the average molar ratio of SO2
to NOx in emissions. Analysis of the data from the
Midwestern and northeastern United States also ndicates
that the percentage of emitted SO2 that is deposited as
sulfate in precipitation is approximately equal to the
percentage of emitted NOk deposited as wet nitrate in
that region. Since the conversion of NC2 to HNog and
its subsequent incorporation into cloud water are
believed to be relatively rapid and efficient processes,
the data suggest that the combined gaseous and aqueous
conversions of SC2 to sulfate are similarly efficient.
It is therefore improbable that the oxidation of SO2 is
sufficiently hindered by a lack of oxidant to cause a
disproportionately small reduction in sulfate concentra
OCR for page 9
9
Lions in precipitation as a result of a given reduction
in S0: emissions.
The North American data therefore suggest that (a)
whatever atmospheric processes are taking place,
pollutants are being thoroughly mixed over a region with
linear dimensions up to 1,000 km, and (b) the formation
of sulfate is neither enhanced nor retarded relative to
the formation of nitrate. The conclusion is clouded by
three types of uncertainty: the limited amount and
uncertain quality of the data, the natural variability of
atmospheric processes, and the lack of firm understanding
of the physical and chemical processes involved.
If improved measurements indicate that the
relationships between emissions and deposition in Europe
and eastern North America are different, the differences
between the two regions in meteorology, or latitude, or
other factors, such as the spatial distribution of
sources, may be responsible.
Influences of Local and Distant Sources
Theoretical and observational evidence exists for the
long-range transport of air pollutants leading to acid
deposition (see Chapters 3 and 4). However, the relative
importance for deposition at specific sites of long-range
transport from distant sources as compared with more
direct influences of local sources cannot be determined
from currently available data (see Chapter 4) or reliably
estimated using currently available models (see Chapter
3).
Trends in the historical data at the Hubbard Brook
Experimental Forest appear to reflect general trends in
emissions (see Chapter 4). Available meteorological
analyses of trajectories of precipitating systems at
three locations in the Northeast (Whiteface Mountain and
Ithaca in New York and south central Ontario) indicate
that much of the acidity in precipitation--as well as
much of the precipitation--comes from air masses arriving
from the South and Southwest.
Based on the analysis of spatial distributions of the
annual average molar ratios of pollutants in emissions
and deposition, it appears that the atmospheric processes
in eastern North America lead to a thorough mixing of
pollutants, making it difficult to distinguish between
effects of distant and local sources (see Chapter 4).
OCR for page 10
10
IMPLICATIONS FOR EMISSION-CONTROL STRATEGIE S
The implications of our findings and conclusions for
choosing among possible emission-control strategies,
should they be deemed necessary, are limited. We do not
believe it is practical at this time to rely upon cur-
rently available models to distinguish among alternative
strategies. In the absence of other methods, analysis of
observational data provides guidance for assessing the
consequences of changing SO2 emissions for wet
deposition of sulfate.
If we assume that all other factors, including
meteorology, remain unchanged, the annual average
concentration of sulfate in precipitation at a given site
should be reduced in proportion to a reduction in SO2
and sulfate transported to that site from a source or
region of sources. If ambient concentrations of NOk,
nonmethane hydrocarbons, and basic substances (such as
ammonia and calcium carbonate) remain unchanged, a
reduction in sulfate deposition will result in at least
as great a reduction in the deposition of hydrogen ion.
It can be stated as a rule of thumb that the farther a
source is from a given receptor site, the smaller its
influence on that site will be per unit mass emitted.
Analysis of air-mass trajectories and modeling may
provide insight into the relative contributions of sub-
regional groupings of sources to sulfate deposition in
ecologically sensitive areas. Interpretation of this
information, however, is subjective, and it will entail
considerable judgment in assigning zones of influence of
sources, even for long-term averages. This subjectivity
has been a source of major differences in expert opinion,
and it will continue to be until scientific knowledge
improves considerably.
On the basis of analysis of the spatial distributions
of the molar ratios of pollutants in emissions and
deposition and assuming that all other emissions and
conditions remain unchanged, we would expect that if the
molar ratio in emissions in eastern North America were
changed by changing SO2 emissions, a similar change
would occur in the ratio of sulfate to nitrate in wet
deposition. If, as described in Chapter 4, dry
deposition is linearly proportional to emissions, then
the average annual ratio in total deposition in the
region should also respond to changes in the emission
ratio. Because the analysis is based on spatial
distributions, its applicability is limited to
OCR for page 11
11
circumstances in which the spatial distribution of
emissions is not changed. Because we cannot rely on
current models or analyses of air-mass trajectories, we
cannot objectively predict the consequences for
deposition in ecologically sensitive areas of changing
the spatial pattern of emissions in eastern North
America, such as by reducing emissions in one area by a
larger percentage than in other areas.
RESEARCH NEEDS
We believe that extensive laboratory, field, and modeling
studies should be continued if we are to establish the
physical and chemical mechanisms governing acid deposition
(see Chapter 5). It appears to us, however, that useful
information about the delivery of acids to ecologically
sensitive areas by transport and transformation processes
can be determined more quickly by direct empirical
observation in the field than by other means. Although
the results of such field studies may not yield complete
detailed descriptions of the interactions of all the
processes involved, the studies are likely to provide
basic phenomenological evidence with sufficient reliabil-
ity to form a basis for improving the near-term strategy
for dealing with the problem of acid deposition in
eastern North America. Indeed, the data are essential to
enhance theoretical understanding and to develop improved
deposition models. In the long term, however, the
ultimate strategy for dealing with acid deposition will
depend on the application of realistic, validated models.
Representative terms from entire chapter:
eastern north
