United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-90/019 Sept. 1990
&EPA Project Summary
1987 Wet Deposition Temporal
and Spatial Patterns in North
America
A. R. Olsen
The purpose of this research
project is to determine the spatial
pattern of wet deposition in North
America during 1987 and to
investigate trends in wet deposition
in North America from 1979 through
1987. The project considers
precipitation-weighted average
concentration and deposition for nine
ion species: hydrogen ion, sulfate,
nitrate, ammonium, calcium, chloride,
sodium, potassium, and magnesium.
To accomplish the research an Acid
Deposition System (ADS) data base
has been established that integrates
wet deposition monitoring data from
several regional and national moni-
toring networks. Networks included
are the National Atmospheric
Deposition Program/National Trends
Network (NADP/NTN), the MAP3S
precipitation chemistry network, the
Utility Acid Precipitation Study
Program (UAPSP) network, the
Canadian Precipitation Monitoring
Network (CAPMoN), and the daily and
4-weekly Acidic Precipitation Moni-
toring in Ontario Study networks
(APIOS-D and APIOS-C). The ADS
data base and statistical summaries
are available to others to conduct
additional research studies.
This Project Summary was
developed by EPA's Atmospheric
Research and Exposure Assessment
Laboratory, Research Triangle Park,
NC, to announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Data Preparation and
Methodology
The spatial and temporal pattern
analyses include data from the six
networks identified above. These
networks operated 69 sites in 1979 and
increased the number of sites to 315 in
1987. During this period, each network
applied specific protocols for selecting
site locations, field operation procedures,
laboratory analysis procedures, and data
quality procedures. In addition, they
participated in the development of unified
data screening and summary procedures
through the Unified Deposition Database
Committee (UDDC) (Olsen et at. 1989).
This project applied the UDDC
procedures in selecting sites and
preparing data summaries for statistical
analysis. The number of sites that meet
the UDDC criteria for annual summaries
increased from 39 in 1979 to 154 in 1987.
In 1987, approximately 50% of the sites
operating met the UDDC criteria. This is
a reflection of the strictness of the
criteria. The 1987 spatial analyses, 1979-
1987 trend analyses, and 1982-1987
trend analyses use specific subsets of
these sites.
The report presents 30 mosaic maps of
1987 annual and seasonal wet deposition
spatial patterns. For each map (ion
species and summary period), the sites
used are selected so that the sites meet
the UDDC criteria for that ion species and
summary period. The number of sites
-------
used range from 154 to 240. For
example, the number of sites for the 1987
annual sulfate maps is 154.
A primary consideration in selecting a
site for temporal pattern analysis is the
continuous operation of the site for the
entire period of interest. Preferably, the
site would also meet the UDDC criteria
for each year during the period. Applying
these criteria severely restricts the
number of sites. A slightly relaxed criteria
was developed that applied to multiple
years of data and still met the objective
of identifying sites with good quality data
throughout the period. The criteria
resulted in 39 sites being selected for the
1979-1987 trend analyses and 148 sites
for the 1982-1987 trend analyses. The
1979-1987 sites have six sites located
west of the Mississippi river, two sites in
Canada, two sites south of North
Carolina, and the rest in eastern United
States. The geographic pattern of sites
for 1982-1987 trend sites is similar but
Ontario Province has 42 sites.
Mosaic maps display the 1987 spatial
patterns. The procedure for developing
the maps is to estimate the spatial
patterns by using a kriging spatial
estimation procedure that includes
measurement error. A separate kriging
analysis was applied to eastern and
western North America, since the regions
have distinctively different spatial
structure. The surface is estimated for
approximately 3700 hexagons with an
area of 2000 square kilometers. The gray
scale map presentation is based on the
percentiles from the frequency
distribution of the estimates for the
eastern and western United States.
The project used two approaches for
trend analysis: graphical displays of
annual summaries and statistical trend
analysis of four-weekly summaries.
Qualitative assessment of trend patterns
relies on time trend graphical displays of
percentiles of annual summaries for each
year in the sites associated with either
the 1979-1987 or the 1982-1987 trend
period. Quantitative trend assessment
relies on the modified Kendall seasonal
tau trend statistical test and Sen's
median slope estimation procedure
applied to four-weekly summary data at
each site. The results of the trend
analyses are qualitatively summarized
across sites.
Spatial Patterns for 1987
Thirty mosaic maps summarize and
display the estimated spatial patterns in
1987 for five ion species concentration
and deposition on annual, winter and
summer basis. The five ion species are
hydrogen ion (pH), sulfate, nitrate,
ammonium, and calcium. Simpson and
Olsen (1990) contains the maps and
extensive descriptions of the patterns.
Areas of maximal sulfate and nitrate
deposition vary slightly from year to year,
but have generally remained within the
area bounded by Indiana on the west,
southern Ontario on the north,
Pennsylvania and New York on the east,
and northern Kentucky on the south
(Figures 1 and 2). In 1987, 10% of all
sites used for these spatial analyses had
deposition values exceeding 29 kg/ha for
sulfate and 21 kg/ha for nitrate. Both
maxima occur in western Pennsylvania
and southern Ontario. In 1987, 5% of the
eastern United States is estimated to
have annual sulfate deposition greater
than 30 kg/ha; 25% with annual sulfate
deposition greater than 22 kg/ha. For
annual nitrate deposition, 5% is greater
than 20 kg/ha and 25% is greater than 13
kg/ha in the eastern United States. In the
western United States, annual sulfate
deposition is estimated to be less than 19
kg/ha and annual nitrate deposition less
than 12 kg/ha.
Summertime wet deposition of sulfate
is more than double wintertime wet
sulfate deposition in most areas of the
eastern United States. Summertime
deposition of nitrates is also generally
higher at most of the eastern United
States locations. In the west, seasonal
patterns are generally similar, although
higher uncertainty exists due to terrain
influences on precipitation patterns and
the low density of wet deposition sites
limit the ability to develop patterns for
this type of evaluation. Sulfate
concentrations in summer also are
generally higher than in winter. The area
of summertime maximal sulfate con-
centration extends west along the
southern Great Lakes region as far as
northeastern Illinois. Nitrate
concentrations for the two seasons are
comparable, with the notable exception of
the eastern Great Lakes region, where
wintertime concentrations can be nearly
twice those of summertime.
Similar to earlier years, the area with
highest precipitation acidity (lowest pH) in
1987 is centered on western
Pennsylvania, eastern Ohio, northern
West Virginia, and southwestern New
York, where 5% of the sites used in the
analysis have average annual pH values
less than 4.2 (Figure 3). The area of
lowest pH generally coincides with the
areas of highest sulfate, nitrate, and
hydrogen ion deposition (Figure 4).
Maximal wet deposition of ammonium
and calcium occurs in two regions
(Figures 5 and 6). One region centers
over southern Ontario; the other over
Nebraska and Iowa in the Great Plains
region.
Trends from 1979-1987
Figure 7 shows that annual sulfate
deposition gradually decreased from
1979-1987 for the 39 sites in the analysis.
Although not presented, a similar gradual
decrease is present for sulfate deposition
for the 144 sites during 1982-1987
period. The median sulfate deposition for
the 39 sites decreased from
approximately 26 kg/ha in 1979 to
approximately 21 kg/ha in 1987. All other
ion species also decreased from 1979-
1987, although hydrogen, nitrate, and
ammonium decreases are relatively
smaller. Figure 7 shows calcium
deposition as an example cation species.
Although calcium decreases, the pattern
differs from that for sulfate: the sites
appear more variable within a year and
inter-annual variation appears greater.
The median calcium deposition for the 39
sites decreased from approximately 1.6
kg/ha in 1979 to 1.0 kg/ha in 1987.
An overview of the quantitative
statistical trend results is in Table 1. The
most striking feature of the comparison
across ion species is that trend estimates
are consistent, except for nitrate and
ammonium concentration, with over 50%
of the estimates indicating a decreasing
trend during the 1979-1987 and the 1982-
1987 periods. The percent changes per
year for sulfate and calcium
concentration are greater over 1979-1987
than over 1982-1987; -2.2% and -6.1%
over 1979-1987 for sulfate and calcium,
respectively. In contrast, the percent
changes per year for deposition are more
similar over 1979-1987 and 1982-1987.
For both periods, the percent change per
year for sulfate and calcium deposition
exceeded the percent change in
precipitation.
From 1979 to 1987, 36 of 39 wet
deposition monitoring sites located
mainly in the eastern United States used
for trend analysis show an estimated
decrease in the concentration of sulfate in
precipitation, although most of the
decreases are not statistically significant.
Nevertheless, the median decrease in
sulfate concentration for these 39 stations
during this period is approximately 18%
(using 1987 as base year). Sulfate
deposition shows an estimated decrease
for 34 of 39 monitoring sites and a
-------
max - 18.7
95%
90%
75%
50%
25%
70%
6.9
3.7
2.4
1.8
Based on:
NADP/NTN, UAPSP, MAP3S,
CAPMoN, APIOS-C, APIOS-D
Sulfate Deposition
kg ha'1
mm = 1.1
min =6.4
Figure 1. Annual 1987 spatial distribution of sulfate deposition.
median decrease for all 39 sites of
approximately 27% from 1979 to 1987.
Conclusions
The research project statistical
analyses of wet deposition monitoring
network data enables several conclusions
to be made concerning the 1987 spatial
pattern of major wet deposition ion
species and the trend observed from
1979-1987.
Maximum deposition of individual wet
deposition ion species are located in the
northeastern United States and south-
eastern Canada. These areas have not
changed significantly in eastern North
America in the past decade. The actual
deposition values and areal coverage of
those locations have changed from year
to year, depending on variations in
emissions and meteorological conditions.
Spatial patterns of wet deposition are not
as well characterized in the western as in
the eastern region of North America. This
results from the density of the monitoring
networks being less in the west than in
the east, from the presence of large
terrain features in the west, and from
large separation of emission sources in
the west.
From 1979-1987, more than half of the
39 trend sites have estimated decreasing
concentration and deposition trends for
all ion species, except ammonium
concentration. For sulfate, calcium,
chlorine, sodium, potassium, and
magnesium, more than 75% of the sites
have estimated decreases; and for
calcium, sodium, and magnesium over
90% of the sites have estimated
decreases. In most cases the estimated
decreasing trends are not statistically
significant. Nevertheless, the median
decrease for these 39 sites in sulfate
concentration is approximately 18% and
in sulfate deposition is approximately
27%.
From 1982-1987, less than 10% of the
144 trend sites have a statistically
significant trend in ion species
concentration and deposition. However, a
median decrease of 0% to 4% per year
in concentration and of 0% to 7% per
year in deposition is estimated for the
sites. The median decrease for these 144
sites in sulfate concentration is
approximately 1% per year and in sulfate
deposition 3% per year.
The available monitoring data and
statistical analysis procedures limits
quantitative generalizations of the
estimated magnitudes of decreasing
trends in concentration and deposition of
ion species. The monitoring sites are not
regularly spaced throughout eastern
North America and consequently some
regions have more representation than
others. The statistical analysis procedure
addresses trends at individual sites and
discussion of trends across all sites does
not consider regional trends in a
quantitative way.
References
Olsen, A. R., D. S. Bigelow, W. H. Chan,
T. L. Clark, M. A. Lusis, P. K. Misra, R. J.
Vet, and E. C. Voldner. 1989. "Unified
Wet Deposition Data Summaries for
North America: Data Summary
Procedures and Results for 1984."
Atmospheric Environment, in press.
Simpson, J. C., and A. R. Olsen. 1990.
7987 Wet Deposition Temporal and
Spatial Patterns in North America.
EPA/600/4-90/019, U.S. Environmental
Protection Agency, Research Triangle
Park, North Carolina.
-------
max = 11.9
Based on:
NADP/NTN, UAPSP, MAP3S,
CAPMoN, APIOS-C, APIOS-D
mm = 6.1
Figure 2. Annual 1987 spatial distribution of nitrate deposition
max =4.6
95%
90%
75%
50%
25%
70%
4.8
4.9
5.0
5.2
5.3
5.5
Based on:
NADP/NTN, UAPSP, MAP3S,
CAPMoN, APIOS-C, APIOS-0
min = 6.1
4.6
4.7
4.9
mm =5.2
Figure 3. Annual 1987 spatial distribution of precipitation-weighted pH.
-------
Annual 1987
Hydrogen Deposition
ha'1
50%
0.03
0.02
min =0.01
Based on:
NADP/NTN, UAPSP, MAP3S,
CAPMoN, APIOS-C, APIOS-D
min - 0.04
Figure 4. Annual 1987 spatial distribution of hydrogen ion deposit/on.
Annual 1987
Ammonium Deposition
kg na''
75%
50%
25%
70%
•0.6
•0.5
•0.4
Based on:
NADP'NTN, UAPSP, MAP3S,
CAPMoN, APIOS-C, APIOS-D
min =0.3
min =0.8
Figure 5. Annual 1987 spatial distribution of ammonium deposition.
-------
max = 2.5
95%
90%
75%
50%
25%
10%
1.4
0.7
•0.5
•0.3
Based on:
NADP/NTN, UAPSP, MAP3S,
CAPMoN, APIOS-C, APIOS-D
mm =02
Annual 1987
Calcium Deposition
kg ha'1
min =0.3
Figure 6. Annual 1987 spatial distribution of calcium deposition.
-------
60-
50-
o> 40-
o
1 30-
Q.
0)
Q
03
m 20-
V)
10-
0-
e
e
I
MMMM
I
1
0
O
«
<
DM*
(
<
) <
> e
wwt
1
[ J
>
>
>
>
0
o
n T
1
[ 1
I
O
e
o +
fl ^
•mumm __
T
1 1
o
o
0 0
0
o
A
f
1 1
11
1
o
0
0 0
Io
iL
8
7979 7980 7987 7982 7983 7984 7985 7986 7987
5.0-
TB 4.0-
-C
t
1 3.0-
CO
O
Q.
Q
B 20-
3 *-u
O
O
7.0-
0.0-
<
o J
o
T _
1
i '
9
e
9
i »
0 0
-, f t S
T j f T
1
1 1 -.
i*M*.
1
> . T * 1
8 e
o
0
T 1 °
T
. 1 I-J-, 1
° T T
0,0
7979 7980 7987 7982 7983 7984 7985 7986 7987
Figure 7. Temporal pattern of (a) annual sulfate ion wet deposition and (b) annual calcium ion wet deposition. Boxp/ofs are the 10th, 25th, 50th,
75th and 90th percentiles of sites in the 1979-87 trend subset
Table 1. Median and Percent Change of Trend Estimates of Ion Concentration and Deposition for 7979-7987 and 7982-7987 Trend Sites
Concentration (peq/L yr)
Deposition (meq/m2/yr)
1979-1987
Parameter
Precipitation
H +
SO42'
N03
NH4 +
Ca2 +
Est.
-
-0.05
-0.97
-0.07
0.72
-0.35
%
-
-0.4
-2.2
-0.7
0.7
-6.7
7982-7987
Est
-004
-0 28
042
005
-0.14
%
-0.2
-7.0
1.8
0.3
-2.6
7979-7987
Est.
-0.04
-0.09
-0.88
-0.20
-0.07
-0.29
%
-0.9
-0.6
-3.0
-7.2
-0.6
-5.5
7982-7987
Est.
-1.85
-0.50
-1.08
-0.10
-0.25
-0.32
%
-2.4
-2.9
-3.5
-0.8
-2.0
-6.3
-------
A.R. Olsen is with Pacific Northwest Laboratory, Richland, WA 99352.
David M. Holland is the EPA Project Officer (see below)
The complete report, entitled "1987 Wet Deposition Temporal and Spatial Patterns
in North America," (Order No. PB90 251 836/AS; Cost: $31.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-90/019
------- |