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OCR for page 128
5
Precipitation Chemistry
Gary J. Stensland, Douglas M. Whelpdale, and Gary Oehlert
INTRODUCTION
The Role of Precipitation Composition in This Study
The chemical composition of wet deposition has been
central to the question of the effects of acid deposition
and the long-range transport of air pollution since the
issue of "acid rain" rose to prominence in Europe two
decades ago. Early evidence linked acidic substances in
precipitation to harmful effects on surface waters and
fisheries in Scandinavia, and as a consequence interest
has since focused on precipitation acidity and acid
deposition rather than on the more general case of depo-
sition of air pollutants. This focus can be attributed
to several factors: Changes in precipitation composition
could actually be detected, and techniques were available
to make measurements; this was not true for dry deposi-
tion. In addition, precipitation composition was starting
to be used as an indicator of atmospheric quality, so
researchers were aware of the value of such measurements.
Finally, the correlation between adverse effects and wet
deposition could be made more directly than that between
effects and emissions.
The chemistry of precipitation has been studied both
in the expectation that it will provide a measure of the
consequences of emissions in time and space and also as
an indicator of effects on the environment. Unfortu-
nately, problems exist both with the quality of past data
and with the interpretation of the data. These difficul-
ties have stimulated the examination of other historical
data in efforts to improve our understanding of the
phenomenon of acid deposition and to corroborate the
available precipitation-chemistry records.
128
OCR for page 129
129
Wet and Dry Deposition
Atmospheric constituents, and thus pollutant emissions,
reach the surface of the Earth by a variety of processes:
those in which precipitation is involved contribute to
wet deposition; those that (primarily during dry periods)
involve sedimentation, turbulent and molecular diffusion,
impaction, and interception contribute to dry deposition;
and those that include cloud and fog impaction, dew, and
frost are sometimes termed occult deposition. Wet deposi-
tion and bulk deposition (i.e., the sum of wet, dry, and
occult, obtained with the collector always open) have been
measured with varying degrees of reliability for decades;
dry deposition has been measured sporadically in a
research mode but not routinely; and measurements of the
third type of deposition are just beginning. Even his-
torical air-concentration data, from which dry deposition
might have been estimated, are sparse and incomplete.
Out of necessity, therefore, we focus on wet deposi-
tion data. From the point of view of determining his-
torical trends, this is not necessarily a serious
limitation on the regional-to-continental scale, where
the wet deposition is expected to reflect total
deposition to a reasonable extent.
location and at a specific time,
At a specific
however, wet deposition
may not be representative of total deposition.
In addition, omitting dry and occult deposition is
certainly a serious constraint on determining mass balance
and interpreting effects. Over eastern North America the
total wet and total dry deposition are thought to be of
approximately equal magnitude. Dry deposition is, in
general, greater than wet near emission Arms ration
where ambient concentrations of pollutants are higher.
An additional complexity for dry deposition is that the
chemical species behave differently: in general, gaseous
nitric acid, sulfur dioxide, and ammonia are removed more
efficiently than their particulate forms, nitrate,
sulfate, and ammonium.
In high-elevation ecosystems the direct input from
cloud and fog water is likely to exceed that from either
of the other pathways. Interpreting observations of
surface ecosystems must allow for these other inputs,
even though the majority of past deposition data is for
wet or bulk deposition.
~ ~ ~4, _,
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130
Constituents of Interest
We limit this discussion of wet deposition to the
major soluble species in precipitation that account for
most of the measured conductance of the samples. The
species include the following ions: hydrogen (H+),
bicarbonate (HCO3), calcium (Ca ), magnesium (Mg ),
sodium (Na+), potassium (K+), sulfate (SO4 ), nitrate
(NO3), chloride (C1-), and ammonium (Nut). Experience has
shown that a pH value can be calculated from measurements
of the last eight ions in the list, and the calculated
value is usually in good agreement with the measured pH
value. The fact that we can often successfully calculate
the pH of precipitation samples indicates that the small
list of measured ions is probably sufficient for studies
of wet deposition that emphasize the acid precipitation
phenomenon. However, there are exceptions. For example,
samples from remote locations can be strongly affected by
organic acids (Galloway et al. 1982).
The data in Table 5.1 demonstrate the relative
importance of these ions at three monitoring sites of the
National Acid Deposition Program (NADP) in the United
TABLE 5.1 Median Ion Concentrations for 1979 for Three NADP Sites
(~eq/L)
GA MN NY
Ion (42 Samples)a (37 Samples)b (49 Samples)C
SO4- 38.9 45.8 44.8
NO3 11.6 24.2 25.0
Cl- 8.2 4.2 4.2
HCO3 (calculated) 0.3 10.3 0.1
Anions 59.0 84.5 74.1
NH4+ 5 5 37 7 8.3
Ca2+ 5.0 28.9 6.5
Mg2+ 2.4 6.1 1.9
K+ 0.7 2.0 0.4
Na+ 17.6 13.7 4.9
H+ 17.8 0.5 45.7
Cations 49.0 88.9 67.7
Median pH 4.75 6.31 4.34
NOTE: See Georgia, Minnesota, and New York in Figure 5.23 for location of these sites.
aThe Georgia Station site in west central Georgia.
bThe Lamberton site in southwestern Minnesota.
C The Huntington Wildlife site in northeastern New York.
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131
States. Concentrations are expressed in microequivalents
per liter (peq/L) to permit a direct evaluation of each
ion's contribution to the anion or cation sum. If all
ions were measured and there were no analytical uncer-
tainty, then the anion sum would equal the cation sum.
The values for hydrogen ion concentration were calculated
from the measured median pH value, and the values for
bicarbonate were calculated by assuming that the sample
was in equilibrium with atmospheric carbon dioxide.
Although the sulfate and nitrate levels shown are similar
at the Minnesota and New York sites, the pH differs
greatly owing to the higher levels of ammonium, calcium,
magnesium, sodium, and potassium ions at the Minnesota
site. These ions are frequently associated with basic
compounds. The data in Table 5.1 suggest, therefore,
that the concentrations of all the major ions must be
considered if we are to understand the time and space
patterns of pH. The data also indicate that sites in
different regions of the United States exhibit large
differences in ionic concentrations.
Currently, sites in states such as Ohio, New York,
Pennsylvania, and West Virginia have the feature shown
for the New York site in Table 5.1, for which hydrogen,
sulfate, and nitrate are the dominant ions. For the New
York site, 98 percent of the acidity coula be accounted
for if all the sulfate were sulfuric acid, whereas
nitrate, as nitric acid, could have accounted for about
55 percent of the acidity. Bower sax and de Pena (1980)
have concluded, by applying multiple linear-regression
analysis for a central Pennsylvania site, that on the
average, sulfuric acid is the principal contributor to
hydrogen-ion concentration in rain, but the acidity in
snow is principally from nitric acid.
Variability in Acid Deposition:
The Role of Meteorology
Any discussion of short- and long-term trends in acid
deposition cannot ignore the role of meteorological and
climatological variability. Even if pollutant emissions
were absolutely constant every hour of the day and every
day of the year, variations in atmospheric conditions
would bring about uneven deposition patterns in both time
and space. For example, both variability throughout the
course of a year and long-term trends in precipitation
records reflect changes in storm tracks and in the fre-
OCR for page 132
132
quency of precipitation-producing circulation patterns
that affect a given region each year. Differences in
precipitation amount from year to year affect total wet
deposition, but even if the precipitation amount were
constant every year, wet deposition of sulfate, nitrate,
and other common ions would not necessarily be, because
of the dependence of the concentrations of these ions on
the size of the precipitation events and the time interval
between the events.
Atmospheric processes vary continuously across a wide
temporal and spatial range; at whatever scale one selects,
atmospheric variability plays a fundamental role in the
acid deposition process. Unless we understand this role
thoroughly, the meaning of trends in acid deposition will
be difficult to resolve unequivocally. (See Chapter 3
for a more detailed discussion.)
DATA EVALUATION
Given that the object of our evaluation is to deter-
mine trends in concentrations (or deposition) of chemicals
in precipitation, our first task is to identify approxi-
mate data sets and determine their quality. Since
adequate methods to determine dry deposition rates for
most chemical species do not exist, wet-only deposition
data collected with automatic samplers are generally the
most reliable and useful. However, owing to the sparsity
of such data we must also consider data bases in which
less desirable sampling methods were used. Such methods
include bulk sampling and manual wet-only sampling in
which dust leakage into the sampler and water evaporation
and snow sublimation from the collection vessels have
apparently introduced serious bias into the data sets.
The same biases may occur even in an automatic wet-only
sampling device when mechanical failure causes exposure
of the sample to the open atmosphere. In this chapter we
emphasize trends in concentration rather than in depo-
sition to avoid the introduction of spatial variability
caused by variability in the amount of precipitation.
A primary tool in evaluating possible biases in
monitoring techniques is comparison of different data
sets, such as bulk data with concurrent wet-only data.
This approach has not always been used in previous
reports but may be one of the most important evaluation
techniques. However, since local sources can affect bulk
data through high levels of dry deposition, the only
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133
satisfactory comparison must involve concurrence in space
and time. Unfortunately, only in a few cases are the two
sampling methodologies concurrent spatially and
temporally, so that such comparisons are usually far from
ideal.
Several different evaluations are described in this
chapter, with emphasis on evaluating those United States
data sets quoted most frequently in trend studies.
Comparing Three-Month Regional Bulk Data
Concurrently with Wet-Only Data
Most of the published comparisons between bulk and
wet-only ion concentrations are for eastern sites. The
difference between these two sampling methods would be
expected to be greater in locations where local sources
may be influential, for example, where sampling occurs
near tilled fields with an abundance of windblown dust.
Peters amd Bonelli (1982) have reported the results of
a bulk sampling program for the period from about December
1, 1980, to March 1, 1981. They indicated that consider-
able effort in siting was made to avoid the occurrence of
local contamination of regional deposition. States from
Minnesota and Illinois eastward to Maryland and Maine
were included in the study. The NADP network had a large
number of stations operating in the same area for the
same time period. Therefore we are able to present a
simple comparison of the results for the wet-only NADP
concentration data versus the bulk concentration data and
examine geographic variations in the differences between
the two methods.
The wet-only and bulk concentration data were plotted
and examined visually to select five contiguous geo-
graphical areas in which there was some degree of
uniformity in values. The areas selected are shown in
Figure 5.1. The bulk sample at each site was a single
time-integrated sample collected with a continuously open
cylindrically shaped sampler. The concentration ratios
of the, ions (mean of bulk values divided by mean of
wet-only values) are shown in Table 5.2 for each area.
One may observe that in all cases the value of the ratio
is greater than or equal to 1.0 and that there is
considerable variation in the ratio by area and by
chemical parameter. The ratios for calcium, magnesium,
and potassium are greater than 2 (and as high as 30) for
all areas, and bulk sulfate concentrations exceeded
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134
:___
1
15
V
·
1
}41
~ 1~
Jo-''
70
\ 23 ~
. ~ 3:
,
1 -_ _` r~ ~ ~~
my
FIGURE 5.1 Map of five areas (indicated by roman
numerals) used for comparing concentrations of various
ions in wet-only deposition with those in bulk samples.
The arabic numbers indicate the number of bulk sampling
sites within each area. The NADP sites are indicated by
circles. Bulk-measuring sites are distributed uniformly
about the areas but are not co-located with NADP sites.
See also Table 5.2.
wet-only concentrations by factors of 2.0 and 8.1 in
areas III and V, respectively. All ion concentrations
were elevated by more than 90 percent in areas III and V,
with a median ratio value of 3.7 for these two areas.
Peters and Bonelli (1982) reported both sample volume and
an estimated sample volume derived from the data from the
nearest National Weather Service precipitation gauge
site. Using these data, we calculated the medians of the
ratios of the measured divided by estimated volumes to be
0.88, 0.79, 0.81, 0.69, and 0.45 for areas I, II, III,
IV, and V, respectively.
These ratios suggest that bulk
samples may be concentrated up to a factor of 2 by
evaporation. Thus, the many ratios greater than 2.0 in
Table 5.2 suggest that differences between concentrations
of ions in bulk samples and in wet-only samples cannot by
fully explained by evaporation.
Even when an average bulk to wet-only ion concentration
ratio for an area in Table 5.2 has a value close to 1, an
individual bulk site in that area may still differ widely
in concentration from the wet-only data. Each site needs
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135
TABLE 5.2 Ion-Concentration Ratios (Mean of Bulk Values Divided by Mean
of Wet-Only Values) for All Sites in Areas I to V for the Period December 1980-
February 1981
Ion
Area Ca2+ Mg2+ K+ Na+ Cl- NO3 NH4 SO2- NBa NWb
I 2.5 5.6 2.7 1.2 1.0 1.1 1.1 1.0 23 3
II 3.7 4.8 5.9 2.9 2.0 1.5 2.0 1.3 70 11
III 7.2 3.5 11.2 2.7 2.1 1.9 3.5 2.0 45 4
IV 2.7 2.3 3.1 1.9 1.6 1.1 1.3 1.3 26 3
vc 30.5 19.2 6.0 2.8 6.6 3.9 2.1 8.1 15 -
NOTE: Areas are depicted in Figure 5.1.
a Number of bulk sampling sites used to determine the means.
b Number of wet-only sampling sites used to determine the means.
C Since there were no NADP sites in area V, computer-generated contour maps (for
weighted-average concentration) for all data available for the period 1978 - mid-1983
were used to estimate average NADP ion concentrations for this area.
SOURCES: NADP data were provided by Illinois State Water Survey. Bulk data were
obtained from Peters and Bonelli (1982~.
to be evaluated separately for each chemical parameter to
determine if bulk concentrations are likely to be similar
to wet-only concentrations.
Comparing Bulk and Wet-Only Data
at Hubbard Brook, New Hampshire
A comparison of bulk and wet-only data from the
Hubbard Brook site shows good agreement for most major
ions, in contrast to other bulk versus wet-only com-
parisons (cf. Tables 5.3 and 5.4 (below)). Weighted-
average concentrations of the colocated Hubbard Brook
bulk samples and NADP wet-only samples were determined
for the period from June 1979 to May 1982. Table 5.3
presents the results as 3-year averages except for pH,
for which the averages of three 12-month periods are
given.
Bulk concentrations of sulfate plus nitrate were 4.8
peq/L (7 percent) higher than for the wet-only values.
The sum of calcium plus magnesium, an indicator of dust
input, was actually lower for the bulk than for the
wet-only data. Apparently, dry deposition of sulfate and
nitrate to the continuously exposed bulk collector,
either through gaseous precursors or in association with
dust particles, was not an important factor. Another
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136
TABLE 5.3 Comparison of Bulk and Wet-Only Weighted-Average Concentrations
(~eq/L) at Hubbard Brook, New Hampshire
Hubbard Brook NADP
Ion (bulk)a (wet only)b
SO2- 49.9 48.5
NOT 27.3 23.9
C1- 6.6 62
NH4 11.0 9.4
Ca2+ 5.6 5.8
Mg2+ 2.2 3.0
K+ 1.0 0.4
Na+ 5.3 8.4
H+ 60.3 47.9
pH 4.24, 4.19, 4.22 4.39, 4.27, 4.32
Weighted-average concentrations for the period June 1979-May 1982. Data provided by
John Eaton, Cornell University, personal communication 1985.
Weighted-average concentration for wet-only samples from the NADP collector for the
period June 1979-May 1982.
typical problem with bulk data, concentration effects
owing to sample evaporation, was probably not a major
factor given the close agreement of bulk ion concen-
trations with the wet-only concentrations. This finding
suggests that the funnel and bottle design of the Hubbard
Brook bulk sampler effectively prevented this problem for
long-term averages at this site. The three (12-month)
weighted averages of pH for the bulk samples are lower
than the corresponding values for the wet-only samples.
The 3-year weighted-average concentration of hydrogen
ions in bulk samples is 26 percent larger than in wet-
only samples. Such a difference is likely to be
significant for these data.
In addition to different sampling devices and dif-
ferent laboratories for sample analyses of the Hubbard
Brook data, different screening and sampling procedures
were used for bulk data and the wet-only data. More bulk
data than wet-only data were deleted from the data base
for the 3-year period. Since the two sampling devices
were generally serviced on different days of the week, it
was not possible to calculate the data as exactly matched
pairs. It would be useful to undertake a more detailed
study involving a week-by-week comparison--both to learn
more about the long-term record at this important site
and to help to relate trends in bulk measurements to
those in wet-only measurements.
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137
Comparing U.S. Geological Survey Bulk Data
with National Acid Deposition Program Wet-Only Data
Nine U.S. Geological Survey (USGS) bulk collection
sites have operated in New York state and Pennsylvania
since 1965 (Barnes et al. 1982), and several investigators
have summarized and analyzed these data. (See later
discussion.) Here, we compare these data with NADP
wet-only data (Table 5.4).
The first column of the table gives the mean and the
standard deviation of all New York NADP data available
through the end of 1982; the seven NADP sites began
operation at different times between November 1978 and
TABLE 5.4 Ion Concentrations (~eq/L) and pH for All New York NADP Sites and for
Two New York USGS Bulk Sites
Time-Adjusted
Weighted Average Weighted Average
Mean and Standard
Deviation for Mays Mays
Ion Seven NADP Sitesa Hinckleyb Points Hinckley Point
Ne 23 24 23 24
Ca2+ 8.4 + 2.0 30.2 74.2 26.3 71.6
Mg2+ 3.5 + 0.8 7.7 17.1 6.7 16.5
K+ 0.8 + 0.6 2.1 2.9 1.8 2.8
Na+ 6.2 + 3.5 5.4 10.4 4.7 10.0
NH4+ 16.5 + 5.5 31.2 22.7 37.3 27.5
Sumf 35.4 76.6 127.3 76.8 128.4
NO3 30.2 + 5.4 41.7 46.5 62.1 54.7
SO2- 64.8 + 13.4 72.5 82.2 65.9 76.4
C1- 5.8 + 3.3 8.9 15.2 8.9 15.2
pH 4.22 (4.15-4.33)g 3.99 3.99 3.90 3.86
H+ 60.4 + 10.3 102.3h 102.3h 126.4h 138.3h
aVolume-weighted concentrations for 1978-1982.
bHinckley USGS bulk data for 8/31/75 to 9/15/77.
CMays Point USGS bulk data for 8/31/75 to 8/31177.
Time adjustments made using time-trend results for 1965-1978 period as determined by
Kendall's test (Barnes et al. 1982). An adjustment factor for a 4.5-year time shift was
multiplied by the USGS bulk concentrations: Ca2+, Mg2+, K+, and Na+ concentrations
were multiplied by 0.87 and 0.965 for Hinckley and Mays Point, respectively; SO2-
concentration by 0.909 and 0.93; NO3 concentration by 1.489 and 1.176; NH4+
concentration by 1.194 and 1.211; and H+ concentration by 1.236 and 1.352.
eN is the number of samples at USGS sites.
f"Sum" gives the total microequivalents per liter potentially available for neutralizing
acids in precipitation.
g Range.
h USGS weighted-average H+ concentrations are likely influenced by a few outliers.
Median concentrations are ~50% less.
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138
June 1980. The sample volume weighted-average concentra-
tions were calculated for each NADP site, and the means
of these values are shown in Table 5.4.
The table also presents weighted-average concentrations
of data from Hinckley and May s Point, two of the USGS
bulk sampling sites in New York; later in this chapter
these two sites are listed with three others for con-
sideration of regional time-trend analysis. The Hinckley
and May s Point sites were chosen for inclusion in Table
5.4 because these sites tend to represent the extremes in
concentrations found at the USGS stations, with the
Hinckley site tending to have the lower concentrations
and the May s Point site the higher concentrations (Barnes
et al. 1982).
Because the two data sets do not overlap in time
(1978-1982 for NADP and 1975-1977 for USGS) the USGS data
are presented in two ways.
The two middle columns in
Table 5.4 present the data from 1975-1977. The two right
hand columns present the USGS data adjusted to account
for the difference in time periods. Temporal trends of
the concentrations of the ions listed in the table were
determined by Barnes et al. (1982) for the period from
1965 to 1978. We estimated the data for the 1978-1982
period by multiplying the USGS data (1975-1977) by a
factor derived from the calculated trend from 1965-1978
(see footnote d in Table 5.4 for details) .
Regardless of whether the actual or adjusted USGS
values are used, large differences exist between the NADP
and USGS data sets. Calcium concentrations are three to
nine times higher in the bulk data. Magnesium, potassium,
ammonium, and nitrate concentrations are also higher in
the bulk data (factors of 1.4 to 4.9). Sodium concentra-
tions are about the same, and the hydrogen-ion concentra-
tion is much higher in the bulk data, although one might
anticipate the reverse. The bulk sulfate values are
somewhat higher than the wet-only values at May s Point
but are about the same as Hinckley values.
The overall conclusion is that the USGS bulk concen-
tration data are significantly different from NADP
wet-only data for many ions. The sum of the basic
cations is larger at the bulk sites than at the wet-only
sites by more than twofold.
-
OCR for page 189
189
120
100
-
cn
0 80
LLl 60
by
o
C'
Z 40
_:
\ so2- +1
\NO3 +
Cl
-/\
\ so2
_^
_ \\ H
~ 1
~ _~_
~ NO3
20 —NH4
1 1 1 1
o
NY 20
Sow +
on
~ ~ SHO 2-
_N,
{~' NO3
~
_ / NH4
80 82
NC 41
~so2~+
_ NO3 +
Cl -
~ iso2~
_
_ ,~
Hit 1
/ ~ NO3
— N H 4
1 1 1 1
80 82 80 82
CALENDAR YEAR
~ Ca2 +
| M92+ +
1 Na +
K +
NH4
FIGURE 5.24 Calendar year median ion concentrations for
three NADP sites for 1979-1982.
NADP Data
Because the data have only recently become available,
no studies have been published that examine the time
pattern for NADP stations (in particular for the 1979-1982
time period). Here an initial examination is presented.
When the NADP began in 1978 most of the stations were
located in the northeastern quadrant of the United States,
although there were fewer stations in other areas. These
stations have compiled relatively complete records for
the 1979-1982 period. Samples were collected for each
week that had precipitation and problems of contamination
or equipment malfunction were infrequent. Results are
presented for 12 sites whose locations are shown in
Figure 5.23.
The median annual concentrations for major ions and
combinations of ions are shown in Figure 5.24 for three
of the sites (ILll, NY20, and NC41), and volume-weighted
OCR for page 190
190
~ N ~ I ~ ~ ~
I ~ ~4 '' ~ ~ ~ ~
~ N; I ~ I
I ° ° —
1 1 1 1 1
+ ~ N ~
I ~ ~
_ 1 1 1 1 1 1 1 1
O O O O
C-l O 00 ~
N ~ ,
O ~ O I
v, I _ z Z
~1 1 1 1 1 ~ ¢ 141 - o
l
, ~ ~ ~
I _ Oz I C~
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g ~ _
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(l/bad) SNOIlVUlN3~:)NOO G31H!313M-3WnlOA
c:
z
LL
c,
z
LL
J
~:
C~
o
N N , + ~ ~ ' ~
Y Z ~ ~ Z ~ Y
~ ~ ~r U) (D
N
O + O I
— Z Z t~ N tD
1 1 1 1 14 1 1~ °
N ~ O I
i/ ~
~:
LL
t~d (D
t(~0 Z
J
::
1 -
N ~ ~ ~ ~ ~
_, O — _ Z Z ~, ,,, C`'
O O O O O O O
C~l O 00 C.C) ~ ~
r~ _
(l/bad) SNOIl~lUlN3ONOD 031HE)13M-3Wnl OA
OCR for page 191
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Representative terms from entire chapter:
wet deposition
191
+ +
-
192
average concentrations (to be referred to as weighted
concentrations) are shown for all twelve sites in Figure
5.25.
The results shown in Figures 5.24 and 5.25(a) can be
compared to evaluate differences in the two data presen-
tations. For site ILll the anions and hydrogen ion have
similar median and weighted concentration patterns,
although hydrogen ion is lowest in 1979 for the weighted
concentration and in 1982 for the median concentration.
For ammonium and the base cation sum (the sum of calcium,
magnesium, potassium, sodium, and ammonium) the year of
maximum concentration differs for the median and weighted
concentrations. There are also differences in median and
weighted concentrations at sites NY20 and NC41. For site
NY20 the concentration patterns of ammonium ion and the
base-cation sum differ between the two figures, while for
site NC41 the sulfate and hydrogen ion concentration
patterns differ in the two presentations.
To compare the 12 sites, volume-weighted average
concentrations are shown in Figure 5.25. From Figure
5.25 (b) for the three sites in Ohio, we see that almost
every curve has some site-to-site differences even though
the sites are spatially close together. In particular,
sulfate has a different pattern for each site. From
Figure 5.25(c) we see that the various curves of the
first two sites, GA41 and NC25, are quite similar in
shape but different from NC03. However, the patterns for
site NC03 are quite similar to those for site NC41 in
Figure 5.25(a). From Figure 5.23 we see that GA41 and
NC25 are adjacent, as are NC03 and NC41, suggesting a
spatial consistency for the annual concentration patterns.
In summary, the most persistent pattern for all ions
and all sites is higher concentrations in 1980 and 1981
and lower concentrations in 1979 and 1982. However, not
all sites or all ions follow the general pattern. Even
multiple sites within a single state have different time
patterns (for example, in North Carolina and Ohio). It
is not uncommon to have most of the major conservative
ions (sulfate, nitrate, ammonium, and calcium) following
the same time pattern (for example, see ILll). This
suggests that for the 1979-1982 period meteorology, as
opposed to the temporal changes in source emissions, was
important in determining time trends of ion
concentrations.
193
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
· The eastern half of the United States and south-
eastern Canada south of James Bay experience concentra-
tions of sulfate and nitrate in precipitation that are,
in general, greater by at least a factor of 5 than those
in remote areas of the world, indicating that levels have
increased by this amount in northeastern North America
since sometime before the 1950s.
· The northeastern quadrant of the United States,
consisting of Region B. the northernmost states of Region
C, and most of Region D, is the area most heavily affected
by acidic species (the ions of hydrogen, sulfate, and
nitrate) in precipitation.
· Spatial distributions of annual (precipitation-
weighted-mean) concentrations and wet deposition for the
major ionic species in precipitation are adequately known
on the regional to continental scale in North America to
afford comparison with spatial patterns of emissions and
other possible indicators of acid deposition, such as
visibility and the quality of surface waters. The 1980
concentration and deposition fields are acceptably
representative of the broad-scale patterns present in the
7-year period 1977-1983.
· Data on the chemistry of precipitation before
1955 should not be used for trend analysis.
.
Precipitation is currently more acidic in parts
of the eastern United States than it was in the mid-19SOs
or mid-1960s; however, the amount of change and its mech-
anism are in dispute. Changes in natural dust sources,
anthropogenic acid sources, and sampling methods are all
major factors that need to be considered in interpreting
the changes in precipitation.
.
Precipitation sulfate concentrations and possibly
acidity leave increased in the southeastern United States
(Region C) since the mid-1950s.
· For Hubbard Brook in New England, general agree-
ment exists among several reports on the magnitude and
direction of trends since 1964 in some species: hydrogen
ion shows no overall significant trend; sulfate has
decreased at approximately 2 percent/yr; sodium, chloride,
calcium, magnesium, and potassium have shown strong
decreases with time; nitrate appears to have increased
until about 1970-1971 and subsequently leveled off; the
analyses for the ammonium record are contradictory.
194
· On the basis of past analyses of the USGS bulk
data from New York and Pennsylvania since the mid-1960s a
consistent network-wide trend does not emerge: results
differ from station to station (with a few exceptions).
Thus the USGS data base should be screened carefully when
general conclusions about regional-scale trends are
drawn. The USGS stations at Mays Point and Hinckley, New
York, show sulfate trends based on time-series analysis
to be consistent with those at Hubbard Brook, making it
more likely that a regional trend exists in this
parameter.
.
On the basis of preliminary analyses of CANSAP-
APN and NADP data for the eastern half of the continent,
changes in anthropogenic emissions during the period
1979-1982 are not readily apparent; concentration changes
from year to year occur concurrently in both anthropogenic
and natural constituents and are thought to be more
strongly affected by meteorological rather than emissions
changes during this period.
.
Because data from the WMO BAPMoN network have not
yet been rigorously quality assured, trend analyses have
not yet been carried out.
.
The uncertainties associated with available time-
trend estimates of precipitation chemistry make it unwise
to draw quantitative conclusions on the basis of direct
comparisons between changes in concentrations or deposi-
tion and changes in emissions. In cases in which
artifacts may be introduced in a precipitation-chemistry
data set, for example, by changes in analytical methods
over time, even qualitative conclusions related to
emission changes may not be possible.
.
In general, individual sites or groups of a few
neighboring sites cannot be assumed a priori to provide
regionally representative information; regional repre-
sentativeness must be demonstrated on a site-by-site
basis.
· General conclusions drawn from merging bulk
precipitation data sets with wet-only precipitation data
sets are not valid unless detailed site-by-site analyses
justify the approach. Similar evaluations should be made
when different wet-only precipitation data sets are
merged.
· Conclusions regarding time trends depend to some
extent on the statistical analysis technique chosen (see
Table 5.12). They also depend on the period of the
record used for analysis, and this point is frequently
ignored.
195
· Trends for particular ions in precipitation
(including hydrogen) should be interpreted only in
relation to trends of all the major ions present.
Recommendations
· Expand the scope of deposition monitoring to
include dry and occult deposition.
· Strive for improved understanding of the char-
acteristics of all forms of deposition, particularly as a
function of altitude and species.
.
Continue long-term, wet-only national monitoring,
preferably under the auspices of a single jurisdiction.
· Use measures other than pH or hydrogen-ion
concentration as a measure or an indicator of the acid
deposition problem.
.
Investigate the role of organic acids in
precipitation acidity.
.
sites.
Discourage bulk sampling except in specific
circumstances in which it can be demonstrated that bulk
and wet-only samples are providing comparable data.
· Use other major ionic species (in addition to
hydrogen and sulfate) in future trend analyses; also use
ancillary information such as meteorological/
climatological data.
· Investigate the role of meteorological and
climatological variability in temporal and spatial
variations of acid deposition.
· Monitor air concentrations in closely located
· Take great care in all aspects of selecting and
maintaining a station, particularly its siting and
operation, if it is to be used in the future for trend
analysis.
.
Devote more effort to the analysis of available
data.
· Emphasize network design to ensure that networks
are capable of detecting changes, particularly those
changes that may result from strategic emissions
reductions.
· Establish a mechanism and a procedure to compile
data, to perform quality control and assurance, to
analyze routinely, to synthesize results, and to publish
multinetwork data.
196
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