The Chemistry of Acid Rain. Sources and Atmospheric Processes

333 Pages·1987·5.77 MB·English

by  Russell W. Johnson

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In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
ACS SYMPOSIUM SERIES 349
The Chemistry of Acid Rain
Sources and Atmospheric Processes
Russell W. Johnson, EDITOR
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0 Allied Signal Engineered Materials Research Center
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7-0 Glen E. Gordon, EDITOR
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1 University of Maryland
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10 William Calkins, ASSOCIATE EDITOR
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n Developed from a symposium sponsored by
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ati the Divisions of Petroleum Chemistry, Inc.,
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Environmental Chemistry, and Fuel Chemistry
at the 191st Meeting
of the American Chemical Society,
New York, New York,
April 13-18, 1986
American Chemical Society, Washington, DC 1987
In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Library of Congress Cataloging-in-Publication Data
American Chemical Society. Meeting (191st: 1986:
New York, NY.)
The chemistry of acid rain: sources and atmospheric
processes
Russell W. Johnson, Glen E. Gordon, editors,
p. cm.—(ACS symposium series; 349)
1 "Developed from a symposium sponsored by the
0
0 Division of Petroleum Chemistry...at the 191st
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9.f Meeting of the American Chemical Society, New York,
4 N.Y., April 13-18, 1986."
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0
7- Includes bibliographies and indexes.
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19 ISBN 0-8412-1414-X
k- 1. Acid rain—Congresses. 2. Atmospheric
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1/ chemistry—Congresses. 3. Air—Pollution—
02 Congresses.
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10. I. Johnson, Russell W., 1948- . II. Gordon, Glen,
oi: 1935- . III. American Chemical Society. Division
d of Petroleum Chemistry. IV. Title. V. Series.
87 | TD196.A25A42 1986 87-19404
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In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
ACS Symposium Series
M. Joan Comstock, Series Editor
1987 Advisory Board
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In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Foreword
The ACS SYMPOSIUM SERIES was founded in 1974 to provide a
medium for publishing symposia quickly in book form. The
format of the Series parallels that of the continuing ADVANCES
IN CHEMISTRY SERIES except that, in order to save time, the
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9.f by the authors in camera-ready form. Papers are reviewed under
34 the supervision of the Editors with the assistance of the Series
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7- Advisory Board and are selected to maintain the integrity of the
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In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Preface
DURING THE PAST DECADE, acid deposition, more commonly called "acid
rain" has been the air pollution problem of highest concern in the United
States. It has caused serious political friction between environmentalists and
power companies, between states that burn coal for electric power
production and those upon whom the acid rain falls, and even between the
United States and Canada, where many citizens feel they are victims of acid
1
0
0 exported from the United States. To those who are not experts in
pr
9. atmospheric chemistry, it seems simple enough: What goes up must come
4
3
0 down. If you want less acid rain, reduce emissions of sulfur and nitrogen
7-
8 oxides that produce it. However, the atmosphere is a complex system and
9
1
k- if we do not understand the formation and deposition of acids, there is a
b
1/ definite possibility that we will devise solutions costing tens of billions of
2
0
1 dollars without significantly lessening the severity of problems that have
0.
oi: 1 been attributed to acid rain.
d Although the mechanism for the production and deposition of acid are
87 | not yet fully understood, atmospheric chemists and meteorologists have
9
3, 1 been studying these problems in depth for the past several years and have
er made considerable progress. Methods and instruments for reliable measure­
b
m ments of key species in air and clouds have been developed and exploited
e
ept in field studies. Rates of reactions important in the formation of acids,
S
e: sulfates, and nitrates have been measured. Huge amounts of reliable field
at
D data have been accumulated. Models have been developed and are being
n
atio tested against bodies of field data.
blic The objective of this volume is to describe recent advances in the
Pu understanding of the sources and chemistry of acidic species in the

atmosphere.
We thank the authors for their contributions to this Volume.
RUSSELL W. JOHNSON
Allied Signal Engineered Materials Research Center
Des Plaines, IL 60017-5016
GLEN Ε. GORDON
University of Maryland
College Park, MD 20742
July 20, 1987
xi
In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Chapter 1
A Decade of Acid Rain Research
Glen E. Gordon
Department of Chemistry and Biochemistry, University of Maryland,
College Park, MD 20742
1 Much progress has been made in our understanding of the sources,
0
h0 formation and deposition of acid and sulfate. Large field studies can be
c
9. conducted with good quality control of analyses and data. In the gas
4
3 phase, ∙OH radicals are known to be capable of converting SO to sul­
7-0 fate fast enough to be important. Rates for ΗO, O and O in 2cloud
8 2 2 3 2
9 droplets are fast under certain conditions. However, serious gaps in
1
k- our knowledge still exist, especially methods for measuring and predict­
b
1/ ing dry deposition and estimates ofthe supply of reactants to active
2
0 cloud systems. A focal point for development of the U. S. control
1
0. strategy is the Regional Acid Deposition Model (RADM). Uncertainties
1
oi: in some features of the model are likely to be so large that it may not
d provide credible predictions in a time soon enough to be useful to legis­
7 | lators or regulators. Hybrid receptor models may be able to provide
8
19 some answers for sulfur species more quickly, although RADM should
3, ultimately yield more detailed predictions for more species. Many prob­
ber lems attributed to acid rain, especially damage to trees at high altitudes,
m
may be largely due to some other species, e.g., HO or O.
pte 2 2 3
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S
e: In the late 1940s and the 1950s, concerns about air pollution increased enormously
at because of episodes such as the London Fog of 1952, the Donora, PA episode of
D
n 1948, and other similar incidents. The air was physically being cleaned up by use of
catio eofl eSct0ro wstaetriec bperiencgi prietlaetaosresd t oin r emmeotrvoep moloitsatn v aisreibalse, eamndi sist iwonass ;f heoltw theavte trh, el aSrgOe^ acmomou­nts
bli bined 2with particles and droplets in the air was responsible for the abnormally high
u
P death and illness rates observed during the episodes. London attacked its problems by
a variety of clean-up methods, the most important being the banning of coal-burning
in individual living units, resulting in a considerable improvement in both air quality
and local climate!
The main U. S. response was restrictions on the use of high sulfur fuels within
metropolitan areas, which had the effect of forcing in-town sources to switch to low
sulfur oil and gas and for new plants to have tall stacks and be built outside of cities.
As documented by Altshuller CI), these measures reduced urban levels of S0 to
2
nearly rural levels.
Nearly simultaneously with the U. S. success in reducing urban S0 levels, the
2
Community Health and Environmental Surveillance System (CHESS) reported that
adverse health effects result not from S0 itself, but from the secondary sulfates and
2
0097-6156/87/0349-0002$06.00/0
© 1987 American Chemical Society
In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1. GORDON A Decade of Acid Rain Research 3
Η^0 formed by atmospheric chemical reactions after release of primary S0 (2).
4 2
Furthermore, the moves taken to reduce S0 concentrations in cities simply moved
2
the sources to rural areas and increased the altitudes of release, but did not reduce the
total amount released. Since S0 is converted slowly to sulfate over long distances,
2
concentrations of particulate sulfates were nearly as great over large rural areas of the
eastern U. S. as in cities (1). Not only were these ubiquitous sulfates of concern
because of health effects, but it was soon recognized that particulate sulfates were
largely responsible for the haze that blankets huge areas of the East during Summer
(3), even the "smoke" of the Great Smoky Mountains, which had previously been
attributed to particles formed from terpenes emitted by trees. This set of problems
related to the release of SCX, mainly by coal-fired power plants, was the impetus for
large field studies, especially the Sulfur Regional Experiment (SURE), supported by
the Electric Power Research Institute (EPRI) (see G. M. Hidy, This Volume).
At about this point in the mid-1970s, results of CHESS were discounted, largely
1 because of the measurement methods used (2). This does not necessarily mean that
0
0 sulfates are not harmful to humans, but that we have no proof that they are. This
h
9.c would have removed much of the impetus for studies of atmospheric S02 and sul­
4 fates; however, at about that time, Likens and others (4) published contour plots of
3
7-0 the pH of rainfall in the Eastern U. S. for the mid-1950s and the mid-1970s which
98 purported to show that the acidity of precipitation had increased greatly over this
k-1 period. Earlier, Swedish scientists had observed increasing acidity of lakes in Scan­
1/b dinavia, causing many species of fish to disappear from affected lakes. The decrease
02 of fish life in many lakes of New England and Upper New York State was thought
0.1 by many to be the result of acid rain. Later, damage to trees, especially high on
doi: 1 mdeopuonstiatiionn s. loSpiensc,e b aobtohu itn tEwuor-othpier dasn do fi nth teh ea cniodr othf eraasitn fUa.l lS i.s, Fw^aSs Ca^t trainbdu toende t-oth aircdid,
87 | HN03, the focus of atmospheric research in the eastern U. S. continued to be on
9 sulfur species and, secondarily, nitrogen species.
1
3, Just when acid deposition was becoming the atmospheric research priority in the
er East, people in the West were becoming increasingly concerned about visibility de­
b
m gradation, again a problem largely caused by sulfates. Ironically visibility degradation
e
pt is of much greater concern in the West, where visibility is much better than it is in the
Se East! The reason appears to be that mountains of the West can be seen for distances
e: of 100 km or more when haze levels are low, whereas the topography of the East and
at
D buildings in areas where most people live prevent one from seeing more than about
on 20 km even in clear air.
ati Thus, atmospheric research in the eastern U. S. has been dominated by the need
c
bli for a better understanding of sulfur species, first because of presumed human health
u
P effects of SO9, then because of human health effects of sulfates, and now because of

effects of sulfate and acid upon plant and animal life (and, to a lesser extent, on buil­
ding materials, statues, etc.) in the East, and because of visibility degradation in the
West.
The huge increase of population and automobile traffic in the Los Angeles Basin
during World War Π gave rise to a new air pollution phenomenon called "smog",
which was found to result from atmospheric reactions of hydrocarbons, CO and
nitrogen oxides (ΝΟ) during frequent strong inversions in the Basin under influence
χ
of abundant sunshine in southern California. Originally the "photochemical smog"
phenomena of southern California was seen as quite a different problem from the
SO^-and-fog problem of London and the eastern U. S., in part because episodes of
the latter tended to occur in Fall and Winter, whereas smog is usually associated with
warm weather and sunshine. Indeed, in the earlier years, when concentrations of
sulfur oxides and particles were much higher in cities, the phenomena may have been
different However, under today's conditions, the two sets of problems clearly are
closely related. Huge veils of particulate haze that blanket entire regions of the East
In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
4 THE CHEMISTRY OF ACID RAIN
now occur mainly between May and October. Improved knowledge of gas-phase
kinetics resulting from studies of photochemical smog and of stratospheric problems
of supersonic transports and chlorofluorocarbon compounds during the early 1970s
has revealed many connections between sulfur chemistry and photochemistry because
of the involvement of highly reactive transient species such as hydroxyl radicals
(ΌΗ), as well as of more stable oxidants produced by photochemistry, e.g., 0,
3
F^O^ In recent years, most air pollution alerts in eastern cities have occurred be­
cause of high levels of oxidants (mainly 0) during Summer, when sulfate levels are
3
also very high.
Present Knowledge about Acid Rain
As demonstrated by papers presented at this Symposium, the increase of our know­
ledge about acid deposition in recent years has been enormous. The SURE project
(Hidy, This Volume) demonstrated that huge field projects can be conducted with
1
00 good quality control of samples, analyses and data. Kok, Tanner (This Volume) and
h
c others have developed highly sophisticated systems for measuring concentrations of
9.
4 many species, including the very important Η2θ, in clouds and clear air with air­
3 2
0 craft. In the area of mechanisms, we know that oxidation by -OH radicals is the
7-
8 dominant gas-phase reaction in the conversion of S0 to HS0 and sulfate (5). Fur­
9 2 2 4
1 thermore, we know that oxidation in solution by F^O^ is rapid and that by 0 and O^
k- 3
b (the latter catalyzed by metal ions or carbon soot) can oe important under some condi­
21/ tions (6; Schwartz, This Volume).
0
0.1 Despite these advances, large gaps in our knowledge still exist. As demonstrated
oi: 1 bsyit imona narye pwapelelr se sitna bthliissh seydm, bpuots uiunmde, rmsteatnhdoidnsg foofr dcroyl ldeectpioonsi tainodn raenmalayisniss poof owre. tT dheep o­
d
7 | Environmental Protection Agency (EPA) does not plan to do routine direct measure­
98 ments of dry deposition for the time being. At most network stations, airborne con­
3, 1 centrations will be measured and dry deposition rates will be calculated from them
er (Hicks et al.. This Volume). Some stations will be equipped to do fast response
mb eddy-correlation and airborne concentration measurements for further research on the
pte method and comparison with results from nearby stations that will measure only air­
Se borne concentrations over longer averaging times (7, £). Although the importance of
e: ΌΗ radicals is clear and millions of dollars have been spent, no reliable, portable in­
Dat strument for real-time measurement of their concentrations at low altitudes has been
n developed. We know that the reaction of H0 with S0 in cloud droplets is fast, but
atio little is known about the supply of F^Oj to c2l2oud dropl2ets, which may limit the
blic amount of sulfate formed.
Pu Some of our largest areas of ignorance involve in-cloud processes. Most clouds

evaporate, releasing any sulfate formed as sulfate aerosol. However, there is not
usually enough airborne sulfate present to account for the sulfate in rain simply by
washout of sulfate aerosol beneath the clouds (2). Thus, it appears that much of the
sulfate brought down by rain must be formed in the clouds that are causing the rain.
There is a lot of "action" in large storm clouds, e.g., strong updrafts and mixing,
which may provide a good environment for extensive chemical reactions. However,
the clouds must be supplied with reactants if sulfate is to be formed, and it's not clear
if this happens. Unfortunately, most in-cloud studies have been conducted in gentle
clouds. We may never be able to study large storm clouds, but investigators of the
PRECP project (PRocessing of Emissions by Clouds and Precipitation) have devel­
oped methods for studying air flowing into and out of such systems (10, H). A
recent study by Dickerson etal. (11) demonstrated that air pollutants are rapidly
transported to the upper troposphere by thunderstorms. After being transported to
such high altitudes, they have much longer residence times and can be transported
much greater distances than can pollutants confined to low altitudes. Thus, one of the
most serious deficiencies in our knowledge is the almost complete lack of vertical
In The Chemistry of Acid Rain; Johnson, R., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.