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OCR for page 93
3
Uncertainties in Trends in
Acid Deposition:
The Role of
Climatic Fluctuations
Raymond S. Bradlley
INTRODUCTION
Acid deposition is the end product of a serf
of
complex processes involving emission of precursors,
chemical transformations in the atmosphere, physical
transport of the pollutants through the atmosphere, and
eventual dry or wet deposition. Day-to-day changes in
weather play an important role in this sequence of
events, since meteorological conditions at the time the
pollutants are emitted determine the direction and the
rate of both horizontal and vertical dispersal. Subse-
quent changes in these conditions determine whether
precipitation will occur and to what extent the
atmosphere will be cleansed of its pollutant load.
Considerable attention has focused on the influence of
meteorological processes on acid deposition, with par-
ticular emphasis on source-receptor relationships to
determine the origin and the route of transport of
deposited materials (National Research Council 1983). In
assessing changes in acid deposition over time scales
longer than seasons, however, short-term meteorological
variability is less significant than longer-term climatic
variability. Climate is a statistical expression of
daily weather events; but, over time periods of years and
decades, changes in climate do occur, and these may be of
considerable importance when evaluating long-term records
of acid deposition. Clearly, temporal and spatial varia-
tions in atmospheric circulation over extended periods
may directly influence patterns of deposition. Climatic
fluctuations may also bring about changes in ecological
conditions that could either accentuate or mask the
effects of acid deposition. Indeed, climatic fluctuations
93
OCR for page 94
94
could induce changes that might be confused with the
effects of acid deposition.
Climatic variability is thus a pervasive factor in all
aspects of the problem of detecting long-term trends
in
acid deposition. Consequently, any long-term records of
acid deposition, whether they be direct or surrogate
measures, must be evaluated in the context of climatic
fluctuations over the same period. To this end, this
chapter summarizes certain aspects of climatic variabil-
ity in the eastern United States over the past 50 to 100
years that may be relevant to evaluating trends in acid
deposition and its effects.
CYCLONE TRACKS
The occurrence of precipitation and
precipitation-
bearing weather systems is of prime interest to those
studying acid deposition, particularly wet deposition.
Much attention has centered on meteorological conditions
preceding acid rain events, generally in an attempt to
understand source-receptor relationships. This chapter
focuses not on this aspect but rather on the general
synoptic weather conditions that produce precipitation in
the eastern United States.
Besides summertime convective storms, which are often
dispersed spatially, the primary sources of precipitation
in the eastern United States are organized frontal system
associated with low-pressure centers (cyclones) traveling
eastward across the region. Airflow into these systems
carries polluted emissions far from the source area. It
is the flow of air from different directions into the
storm systems that carries polluted emissions away from
the source area. Large interannual changes in the primer
tracks that precipitation-producing systems follow, and
hence changes in the flow of air into these systems, will
affect interannual deposition patterns. Similarly,
longer-term changes in the tracks of low-pressure systems
and in the total number of cyclones per year will affect
long-term airflow patterns and acid-deposition trends.
By studying the movement of cyclones over long periods
of time, the primary tracks that they follow can be mappe
(Figure 3.1). These maps, however, provide statistical
summaries only and obscure the marked variability in
storm tracks that characterize any one year. Not only
may the actual tracks followed by individual storms and
the associated airflows vary markedly from month to
OCR for page 95
95
~1~
+ . ~ it': W427~
~-~1~
its ~~
FIGURE 3.1 Primary depression tracks for four months of
the year based on synoptic weather charts for 1951-197^
SOURCE: Reitan (1974).
month, but the total number of storms affecting the
region per year may vary over time (Figure 3.2).
What is the significance of these changes for acid
deposition? The processes by which material passes from
source to receptor are complex and not fully understood.
Every weather situation is unique and must be interpreted
individually. Nevertheless, changes in airflow over a
region that accompany long-term changes in cyclone tracks
and associated frontal systems must play a fundamental
role in producing temporal and spatial variations in acid
deposition. This aspect of climatic variability deserves
further study to quantify its potential significance in
assessing trends in acid deposition.
PRECIPITATION AND DROUGHT
Since acid deposition results from both wet and dry
processes, variations in the amount of precipitation and
in the frequency and the size of precipitation events
must play an important role in long-term trends in acid
deposition. However, there has been surprisingly little
research on this matter.
Precipitation records for the eastern United States
are characterized by fairly large year-to-year variations
of precipitation superimposed on long-term trends. In
the Adirondack Mountains, for example, precipitation in
some years is approximately 50 percent higher than in
OCR for page 96
96
FIGURE 3.2 Normalized
number of U.S. depression
events, 1905-1977 (after
Hosler and Gamage (1956)
and Zishka and Smith
(1980)).
1.6
1.5
1.4
3 13
>
Z 1. 2
J
~ 1.1
U.
o 1.0
~-
z
c' .8
~ .7
o
6
. _
~ I I I I r I T I I T ~ r T-- I
1
Aim
11
It
.,
If
1
1111!
111~' ~ ,
I iV I
1 /
t
1
1 1
1
1 1
1 1
1 1
1
1
ttl
1
1j
1 1 ~ I I 1 1,, , , I I I
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O — — CtJ N A) ~ ~t `? ~ t~~) ID tD ~~
a, ~ of at at at at ~ at Ad A) ~ at ~ a)
YEAR
others, and precipitation may be above or below the
long-term average for several consecutive years (Figure
3.3). Furthermore, the type of precipitation event may
be significant to total acid deposition. In some areas,
the frequency of days with heavy rain has increased
markedly over the past 20 to 30 years with associated
changes in cloudiness (Changnon 1983). Large precipita-
tion events (greater than 1 in. (25 mm)) tend to have
higher pH than smaller precipitation events but greater
impact on total acid deposition (Likens et al. 1984).
Thus, even if total annual precipitation were identical
in any two years, differences in wet deposition could
arise from differences in the size and the frequency of
precipitation events. Unfortunately, little is known
about the extent to which year-to-year variations in the
number and size of precipitation events affect total acid
deposition, principally because reliable long-term
precipitation chemistry data sets are not available.
Changes in cloud cover (cloud type and extent) have
not yet been studied in any detail. However, the
frequency of convective clouds may be important in
redistributing pollutants from low to high altitudes,
thereby promoting their long-range transport. Further
study of this aspect of climatic variability is warranted.
OCR for page 97
97
140—
t35 -
130 -
425 -
120
o 115
110-
105 -
111
By
100 -
95 -
~ -
,...
2 1
1 ....:]
85- 1 1 1 1 1 1 1 1 1
1910 1920 1930 1940 1950 1960 1970 1980
YEAR
FIGURE 3.3 Annual precipitation in the Adirondack
Mountains, New York. Horizontal line (X) is the
long-term average. Darker line shows low-frequency
trends in the record (based on an 11-point binomial
filter). Shaded areas indicate years of lower than
average precipitation.
The occurrence of individual precipitation events with
respect to the timing of sample collection may be of
considerable importance in comparing historical data
sets, particularly of stream-water chemistry. The acid
shock phenomenon--low-pH water at the start of snowmelt--
is fairly well known (National Research Council of Canada
1981), but even individual precipitation events during
summer months can produce sharp reductions in pH similar
to spring snowmelt changes (Kurtz et al. 1984).
Thus, it is important to take into account the recent
precipitation history of a site (i.e., the period before
sampling) in comparing discrete stream-water samples.
Just as the amount of precipitation is important in
evaluating trends in acid deposition, so is the absence
of precipitation. Drought conditions, defined as periods
of anomalously low precipitation (generally coupled with
OCR for page 98
98
above average temperatures) are relatively common occur-
rences in some areas of the country, such as in the upper
Midwest; in other areas, New England and New York for
example, drought is a relatively uncommon event.
Wet deposition is reduced during droughts, but dry
deposition may increase. In assessing trends in acid
deposition and its effects, the ecological consequences
of drought may be most significant.
_ _ _ In those areas where
droughts are relatively common, ecosystems capable of
withstanding periodic water shortages have evolved. In
areas where droughts are relatively rare, the ecological
effects of drought are likely to be more profound and may
result in changes and readjustments within ecosystems
long after droughts per se have ended. Such changes may
complicate attempts to assess the effects of acid depo-
sition on ecosystems. In particular, studies of annual
tree growth increments as a surrogate measure of the
effects of acid deposition on biomass production may not
differentiate unequivocally the long-term effects of
Periodic droughts from the effects of acid deposition.
Also, drought may affect precipitation chemistry
directly. Early monitoring of precipitation chemistry in
the eastern United States (Junge 1956, Jung e and Werby
1958) provides an important data set with which to
compare recent measurements. A direct comparison reveals
significantly lower pH values over most of the East in
the late 1970s compared with those approximately 25 years
earlier.
_ _ , , _ _ _
However, drought in the Midwest during the mid-
1950s and consequent windblown dispersal of aerosols rich
in calcium and magnesium may have resulted in over-
estimating pH values for this interval (Figures 3.4(a)
and 3.4(b)). By adjusting the calculated pH values to
take "excess" calcium and magnesium into account, changes
in pH between the 1950s and late 1970s appear to be less
than reported previously for most of the eastern united
States (compare Figures 3.4(b) and 3.4(c)). Although one
could take issue with the methods employed in making such
adjustments and with the many assumptions necessary
(Butler et al. 1984, Stensland and Semonin 1984), it is
nevertheless clear that climatic variations over years
and decades may have consequences that preclude directly
comparing data for different time periods.
The severity of a drought is commonly expressed in
terms of the Palmer Index (P.I.) (Palmer 1965). The
occurrence of drought for at least two consecutive months
with a P.I. of less than 3 is considered a severe event.
The frequency of such events in the eastern United States
OCR for page 99
99
(a)
(b)
(C)
Ij ,,562 ~
6.4 .
/~ fiO 5fi
~e =~ - ~
3~4.84: Air
1.8~
r>.8 ~$~6.0
44 1:
· ( rm
'
~-~
~ a- ~~
~,44852~4
_ ~ ~~ 4j4
1 ~4
~4.8
~\.4\6.\~ :.8
FIGURE 3.4 1955-56 pH distribution; (a) based on adjusted
Jung e data; (b) after correcting for assumed anomalously
high concentrations of calcium and magnesium; and (c)
September 1980 median pH distribution from National Acid
Deposition Program network. See Chapter 5 for a detailed
discussion of these data (Stensland and Semonin 1982).
OCR for page 100
100
1 11 1
11 1
111 1 111 ~ \1 1 ~ 11
.. ; . . . ..
1890 1900 1910 1920 1930 1940 1950 1960 1970 1980
NEW ENGLAND
M I DO LE AT LANTI C
EAST NORTH CENTRAL
WEST NORTH CENTRAL
SOUTH ATLANTIC
EAST SOUTH CENTRAL
FIGURE 3.5 P.I.s (<3) for regions of the eastern
United States, 1895-1981 (after Diaz 1983). New England
comprises Maine, Vermont, New Hampshire, Massachusetts,
Rhode Island, and Connecticut. The Middle Atlantic
region comprises New York, Pennsylvania, and New Jersey.
The East North Central region comprises Wisconsin,
Michigan, Ohio, Indiana, and Illinois. The West North
Central region comprises Minnesota, North Dakota, South
Dakota, Iowa, Nebraska, Missouri, and Kansas. The South
Atlantic region comprises Delaware, Maryland, West
Virginia, Virginia, North Carolina, South Carolina,
Georgia, and Florida. The East South Central region
comprises Kentucky, Tennessee, Alabama, and Mississippi.
These climatological regions do not correspond exactly to
the regions that we have defined in this report to
characterize trends in acid deposition (see Chapter 1,
Figure 1.2). However, the regions denoted here are
somewhat analogous; i.e., New England plus the Middle
Atlantic States are nearly equivalent to our Region B.
the South Atlantic plus the East South Central States are
nearly equivalent to our Region C, and the East North
Central States are somewhat similar to our Region D.
is shown in Figure 3.5. Over the past 90 years droughts
have occurred in all areas of the eastern United States
at some time. Most significant was the drought of the
1930s, which began to affect the area east of the
Mississippi River, except New England, around 1930. The
drought later became centered over the North Central
States, lasting until the end of the decade in the West
North Central region (see Figure 3.5). In the 1950s, a
major but less severe drought affected an area from the
South Atlantic seaboard to the upper Midwest; once again
New England for the most part escaped its effects, as did
the two other regions of the North (the Middle Atlantic
and East North Central States). In the mid-1960s, how-
OCR for page 101
101
ever, an exceptionally severe drought affected New England
and the Middle Atlantic States. Dendroclimatic studies
ity in the Hudson River
va..ey since the seventeenth century (Figure 3.6), and
from this work it appears that the 1960s drought was one
of the most severe in the past 300 years (Cook and Jacoby
1979). Interestingly, this was a cool drought accompanied
by below-average temperatures and anomalous northwesterly
airflow. The most recent drought (1976-1977) also
affected New England and the Middle Atlantic States but
was not so extreme as the 1960s drought.
have reconstructed drought sever
. . ~ ~ . . .
AIR STAGNATION EPISODES
It is well known that high air pollution levels are
often associated with certain weather conditions, the
most noteworthy of which are slow-moving anticyclones
(high-pressure systems). Anticyclones are generally
associated with subsiding air motion, clear skies, and
strong nighttime radiative cooling, giving rise to
pronounced temperature inversions that limit vertical
dispersal of pollutants. Since pollutants are not
readily dispersed during air stagnation episodes,
relatively high levels of major contaminants, such as
ozone, may occur for extended periods during these
events. Thus, in evaluating the relationship between
ecological indicators and acid deposition one should
.
account tor long-term changes in the frequency of air
stagnation events in each region under study.
In addition, changes in air stagnation trends may play
a role in changes in regional visibility over time.
Visibility is also affected by variations in temperature
and relative humidity, since hydroscopic sulfate aerosols,
a primary ingredient in reducing visibility, absorb water
from the air. Comparisons of long-term visibility data
should thus account for variations in these parameters
(Sloane 1983, 1984). This is discussed further in
Chapter 4.
The frequency of air stagnation episodes in the
eastern United States has been studied by Korshover
(1976), who found the maximum frequency of such cases to
be centered over Georgia and South Carolina (Figure
3.7). Seasonally, the fall (August-October) has the
maximum number of air stagnation cases. An analysis of
the number of stagnation cases each year in the eastern
United States over the past 50 years reveals an increase
OCR for page 102
102
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OCR for page 103
103
If
100 /
/ 175 _
~~4950/J ioo
25 ~ ~
\
'25
10Q
if,
in,
fly
-
,,,25
FIGURE 3.7 Geographical distribution of number of air
stagnation days in the eastern United States over the
period from 1936 to 1970 (after Korshover 1976).
from a low of 30 days/yr in 1936 to 1940 to a high of 57
days/yr in 1979 to 1983 (Figure 3.8). This does not
imply that all parts of the country experienced an
increase in stagnation days over the 50 years, but it
does mean that the total number of cases within the
entire eastern United States has increased. Neverthe-
less, it is clear that air stagnation conditions are
variable in time and space and thus play a role in
evaluating spatial and temporal trends in acid deposition
and its effects.
TEMPERATURE
Changes in temperature over time are not of direct
significance for trends in acid deposition. Neverthe-
less, they may affect a biological indicator, such as
tree growth, and thereby confound efforts to isolate
unequivocally any acid deposition signal in the tree ring
records. (See Chapter 6.)
OCR for page 104
104
An analysis of long-term temperature data for the
United States shows that the record since 1%95 divides
into three periods (Diaz and Quayle 1980) (Figure 3.9)
The first period (1895 to 1920) was characterized by
relatively low temperatures, the second period (1921 to
1954) by a warming trend, and the third period (1955 to
1979 or later) by a return to cooler conditions. When
the changes in temperature between these periods are
examined by region, it becomes clear that over the past
60 years the most pronounced changes in temperature have
occurred in the eastern half of the United States (Figure
3.10). In particular, since the early 1920s summer
(June-August) temperatures have decreased 1.5°F (0.8°C)
or more in a zone centered on Tennessee and Kentucky.
During the same Period winter h~mn~r~t',r-~ H=~1 inch ; n
this region by 3°F (1.7°C) or more. The decade of the
1960s in particular was notable for a succession of cold
winters, which may have contributed to the onset of
.
forest decline in the eastern United States. This point
is discussed in more detail in Chapter 6. Such changes
are quite significant and may be of particular ecological
importance near the normal geographical range limit of a
species or at higher elevations. It is also in these
areas, where vegetation is under environmental stress,
that increases in acid deposition might have the greatest
impact. Thus, separating the climatic and acid
deposition signals in vegetation can be complex and may
produce equivocal results.
The changes in mean monthly or seasonal temperature
that have been observed are only one isolated measure of
climate. It is probable that these temperature changes
reflect adjustments in large-scale circulation patterns
of the eastern United States involving a multitude of
other, more subtle changes in climate, such as length of
the growing season, frequency of frosts, growing degree
days, type and amount of cloudiness, vapor pressure, net
radiation, and wind direction. Such changes may be
significant ecologically, yet they are rural v Id
Such chances
to careful long-term analysis because adequate data sets
are lacking. Even with good long-term data and reliable
models of the important interactions, it would be
extremely difficult to isolate the multiplicity of
effects that such changes could have on ecosystems from
the additional influence of acid deposition. AS it is,
both data and models are generally inadequate as a means
of resolving the unique effects of any particular factor.
OCR for page 105
105
cry 1 00
it
~ 90
lo:
80
70
A:
it
o
i
it
o
try 20 _
m
~ 10 _
At
<: O 1 1 1 1 1 1 1 1 1
~ 1 936-40 41 -45
_
46 50 51-55 56-60
61-65 66-70 71-75 76-80 81-83
PERIOD
FIGURE 3.8 Air stagnation days in the eastern U.S.,
1936-1983. Horizontal line (X) is the long-term
average. Dashed line signifies the 3-year mean from 1980
to 1983 (data courtesy of J. Korshover, Air Resources
Laboratory, NOAA, Silver Spring, Md.).
\20~
90t
60
30
pF} o
—30
—60
_90
-,20
1921
195S
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ' ~ ~ ~ ~ ~ ~ I ~ l ~ I ~~ ~ ~ ~ ~ ~ ~ ~ · ~ · - ~ ~ - - ~ · - - ~ · - - ~ · - - ~ · - - ~ - - - ~ · - - ~
95 99 1 7 ~ ~ ,5 t9 23 27 3' 35 ~ 43 47 51 55 ~ 63 67 7t 75 ~
YEA.
t189S.1979}
FIGURE 3.9 Cumulative departures of long-term mean
monthly temperature in the contiguous United States;
monthly means compiled from January 1895 to March 1979.
SOURCE: Diaz and Quayle (1980).
OCR for page 106
106
Mean Winter Temperature Difference ( F)
(1954-55 to 1977 78) Minus (1920-21 to 1953-54)
1 _ ~ is -lS J ~
Mean Summer Temperature Difference ( F)
(1955-1977) Minus (1921 1954)
Mean Annual Temperature Difference ( F)
(lq~iR-lq77) MinL,c (1921-1954)
!\/lean Spring Temperature Difference ( F)
(1955-1977) Minus (1921 1954)
~>SrK
Mean Autumn Temperature Difference ( F)
(1955-1977) Minus (1921 1954)
FIGURE 3.10 Mean seasonal and annual temperature
differences (OF) for (1955-1977) minus (1921-1954).
Shading indicates significance at the 99% (dark) and 95%
(light) levels, respectively (after Diaz and Quayle 1980).
OCR for page 107
107
SUMMARY
Acid deposition is a consequence of atmospheric
processes acting on air pollutants. As such, the
phenomenon is subject to the variability of the
atmospheric system on both short (meteorological) and
long (climatological) time scales. In assessing trends
in acid deposition, climatic trends must be an implicit
part of the assessment. Furthermore, in trying to
understand the effect that acid deposition may have had
on various ecosystems it is equally important to have a
clear understanding of the effects of climatic variations
on the ecosystems. Because this type of analysis has
rarely been carried out, research should focus on the
following topics:
1. The effects that climatic variability may have on
those systems thought to be affected by acid deposition,
such as aquatic and forest ecosystems.
2. The implications of long-term climatic changes for
trends in acid deposition both in the past and in the
future.
REFERENCES
Butler, T. J., C. V. Cogbill, and G. E. Likens. 1984.
Effect of climatology on precipitation acidity. Bull.
Am. Meteorol. Soc. 65:639-640.
Changnon, S. 1983. Trends in floods and related climatic
conditions in Illinois. Climatic Change 5:341-363.
Cook, E. R., and G. C. Jacoby, Jr. 1979. Evidence for
quasi-periodic July drought in the Hudson Valley, New
York. Nature 282:390-392.
Diaz, H. F. 1983. Some aspects of major wet and dry
periods in the contiguous United States, 1895-1981. J
Climatol. Appl. Meteorol. 22:3-16.
Diaz, H. F., and R. G. Quayle. 1980. The climate of the
United States since 1895: spatial and temporal
changes. Mon. Weather Rev. 108:249-266.
Hosler, C. L., and L. A. Gamage. 1956. Cyclone frequencies
of the United States for the period l9O5-1954. Mon.
Weather Rev. 84:388-390.
Junge, C. E. 1958. The distribution of ammonia and
nitrate in rainwater over the United States. Trans.
Am. Geophys. Union 39:241-248.
.
OCR for page 108
108
Junge, C. E., and R. T. Werby. 1958. The distribution of
chloride, sodium, potassium, calcium and sulfate in
rainwater over the United States. J. Meteorol.
15:417-425.
Korshover, J. 1976. Climatology of stagnating anticyclones
east of the Rocky Mountains, 1936-1975. NOAA Tech.
Mem. ERL/ARL-55. National Oceanic and Atmospheric
Administration. 26 pp.
Kurtz, J., A. J. S. Tang, R. W. Kirk, and W. H. Chan.
1984. Analysis of an acidic deposition episode at
Dorset, Ontario. Atmos. Environ. 18: 387-394.
Likens, G. E., F. H. Bormann, R. S. Pierce, J. S. Easton,
and R. E. Munn. 1984. Long-term trends in
precipitation chemistry at Hubbard Brook, New
Hampshire. Atmos. Environ. 18: 2641-2647.
National Research Council. 1983. Acid Deposition:
Atmospheric Processes in Eastern North America.
Washington, D.C.: National Academy Press. 375 pp .
National Research Council of Canada. 1981. Acidification
in the Canadian Aquatic Environment: Scientific
Criteria for Assessing the Effects of Acid Deposition
on Aquatic Ecosystems. National Research Council
Publication No. 18475 196.
Palmer, W. C. 1965. Meteorological drought. U.S. Weather
Bureau Res. Paper No. 45. U.S. Department of Commerce.
58 pp.
Reitan, C. H. 1974. Frequencies of cyclones and
anticyclones for North America 1951-1970. Mon. Weather
Rev. 102: 861-686.
Sloane, C. S. 1983. Summertime visibility declines:
meteorological influences. Atmos. Environ. 17: 763-774.
Sloane, C. S. 1984. Meteorologically adjusted air quality
trends: visibility. Atmos. Environ. 18: 1217-1229.
Stensland, G. J., and R. G. Semonin. 1982. Another
interpretation of the pH trend in the United States.
Bull. Am. Meteorol. Soc. 63: 1277-1284.
Stensland, G. J., and R. G. Semonin. 1984. Response to:
"Comments on effect of climatology on precipitation
acidity." Bull. Am. Meteorol. Soc. 65: 640-643.
Zishka, K. M., and P. J. Smith. 1980. The climatology of
cyclones and anticyclones over North America and
surrounding ocean environs for January and July
1950-1977. Mon. Weather Rev. 108:387-401.
Representative terms from entire chapter:
eastern united
