&ER&
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United States
Environmental Protection
Agency
Environmental Sciences Researc
Laboratory
Research Triangle Park NC 2771
Research and Development
EPA-600/S4-81-070 Oct. 1981
Project Summary
Long-Range Transport and
Transformation of SO2 and
Sulfate: Refinement,
Application, and Verification of
Models
Teizi Henmi and Elmar R. Reiter
A three-layer model of long-range
transport/transformation of SOz and
sulfate, which includes dry and wet
deposition, was refined and applied to
calculate patterns of 24-hour concen-
tration and deposition amounts over
the northeastern United States
bounded by 35-46° N latitude and 75-
95° W longitude for 2 separate days.
Meteorologically, the model is driven
by routine upper-air observations of
the National Weather Service. The
model was modified to reduce computer
requirements and to produce monthly
average concentrations. It was applied
for 2 months over a larger area
extending into Canada, bounded by
35-53° N latitude and 62-95° W
longitude. Sulfur budgets and com-
parisons of observed and calculated
SO2 and sulfate concentrations are
presented and discussed.
This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle 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).
Introduction
There is growing concern by the
public, scientists, government, and
international organizations about the
consequences of SOz pollution and
especially about the increasing acidity
of precipitation. Current opinion is that
this type of pollution threatens human
welfare and the environment. Precip-
itation with increasing acidity has been
observed especially over the north-
eastern United States and Scandinavia.
The annual average pH of the Adirondack
Lakes has dropped from 6.5 in the
1930s to 4.8 today; more than 90 of
these lakes are completely devoid of
fish. It is generally recognized that
precipitation acidification involves long-
range transport and transformation of
sulfur and nitrogen oxides and other
man-made pollutants. Cloud and pre-
cipitation physics also play a role in
lowering the pH of rain. Attempts at
mathematical simulation of the total
process have been hampered by a lack
of detailed knowledge about various
subprocesses and basic input informa-
tion (e.g., rate constants, meteorological
and pollutant emissions data, etc.). Over
the last several years research in this
project has focused on constructing a
practical mathematical model of long-
range transport and transformation of
SOz and sulfate, developing and incor-
porating improved knowledge about
critical subprocesses, and applying the
model to study air quality over the
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eastern United States and nearby
Canada. This report describes refine-
ments to earlier modeling efforts (EPA
reports 600/4-78-003 and 600/4-79-
068) as well as application and verifica-
tion of the models.
The basic model may be described as
a Lagrangian, forward-trajectory type,
which generates average 24-hour
concentrations over a grid of 1-degree
(1/2 degree on a suitable computer)
longitude/latitude spacing. It uses
routine rawinsonde observations of the
National Weather Service to keep track
of pollutants in three diurnally varying
layers:
the daytime, ground based stable
layer,
the daytime (largely convective)
mixing layer, and
the layer at night that extends from
the top of the stable layer to the top
of the most recent daytime mixed
layer.
For this long-range transport model it is
assumed that pollutants in the various
layers are distributed vertically in a
uniform manner; lateral diffusion
depends mainly on wind shear. Constant
transformation rates of SO2 to sulfate
are employed, one each for daytime and
nighttime in accordance with sunlight
effects. Dry and wet removal of SCk and
sulfate are modeled through use of the
deposition velocity concept.
Results
This report describes improvements
and modifications to the earlier model.
Vertical temperature prof iles along each
trajectory were analyzed objectively to
determine the vertical limits of the three
layers. Trajectory computations were
increased to four per day (every 6
hours), were run for 48 hours, and were
composed of 3-hour segments. The
latest information on rates of trans-
formation and on dry and wet deposition
were incorporated into the model. A
major accomplishment (in terms or
required computer capacity) was the
development of a much more efficient
routine to interpolate concentrations
along trajectories to grid points. A
similar scheme was used for precipita-
tion rates measured at weather stations.
The improved model was applied to
calculate geographic distributions of
24-hour average concentrations and
dry and wet deposition amounts of SO2
and sulfate over the region bounded by
35-45° N latitude and 75-95° W
longitude, which encompasses the Ohio
River basin. January 25 and July 11,
1976, were chosen for computations
because observed concentrations were
relatively abundant on those days. Sixty
point sources of S02, each with an
emission rate of more than 10s tons/
year, were estimated to account for 90
percent of total S02 emissions in the
region. Precipitation data were available
for 81 stations. The modeling applica-
tions indicated that removal of S02from
the atmosphere by precipitation
(assumed proportional to precipitation
rate) is relatively inefficient, whereas
the amounts of sulfate were substantial.
But data for verification are not avail-
able. Comparisons of observed and
calculated S02 and sulfate concentra-
tions indicated that calculated S02
concentrations generally were too low
and sulfate too high, suggesting that the
rate used in the model for transformation
of S02 to sulfate was too high. Table 1
shows correlation coefficients between
observed and calculated concentrations;
all are significant at the 99.9 percent
confidence level. Although calculated
and observed concentrations were
related statistically, the correlations
were not particularly high; for SO2 they
were disappointingly low. This was
attributed in part to the too-high
transformation rate and to the use of
observed concentrations in urban areas
where the impact of nearby (unac-
counted for) small sources could be
large. Table 2 shows the sulfur budget
for the region that was modeled. More
than 50 percent of the emitted sulfur
was transported out of the region on
both dates. Less than 10 percent was
removed from the atmosphere by
precipitation, compared to 30 percent in
our earlier model, a consequence of
revising the formula for wet removal.
The model described in the foregoing
paragraphs was applied in a climate-
logical mode after some modifications,
largely to reduce the computing time.
Trajectories from each S02 source were
begun every 6 hours, and were based on
3-hour time segments; they were
pursued for up to 3 days, and were
based on average winds for only one
layer, the seasonal average afternoon
mixing layer. The vertical distribution of
pollutants was assumed uniform in the
mixing layer. The lateral distribution
was assumed to depend on the devia-
tion of each trajectory from an average
trajectory from each source (meander)
and on the vertical wind shear. Puffs of
S02 were formulated with a Gaussian
distribution along and across each
trajectory segment. Based upon data for
January, 1977, for 72 major SO2
sources in the eastern United States
and southeastern Canada, the four
average dispersion components (mean-
der and shear, along and across tra-
jectories) displayed some interesting ,
characteristics. The greatest dispersion '
component out to 72 hours was meander
along trajectories, followed by meander
across trajectories (which had many of
the smallest standard deviations), shear
along trajectories, and finally shear
across trajectories. For all components
the rate of increase of dispersion tended
to become small by 72 hours.
Table 1.
Correlation Coefficients (r) Between Observed and Calculated
24-Hour Average Concentrations
January 25, 1976
Juty 11. 1976
S02
Sulfate
= 0.68 (N=162)
r = 0.77 (N
r = 0.44 (N = 216)
r = 0.85 (N=86)
Table 2. Calculated Sulfur Budget (Tons) for Region Modeled (Sulfur Emission -
24,492 Tons)
Jan 25, 1976 July 11, 1976
Removal by Wet Deposition
in SOZ
in Sulfate
Removal by Dry Deposition
in S02
in Sulfate
Total Deposition
Amount Exported
808
90
718
10.167
9,240
927
10.975
13,517
2,015
129
1.886
7.495
6,406
1,089
9,510
14.982
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Calculations of concentration, trans-
formation, dry, and wet deposition all
were handled in a straightforward
manner as in the basic model. The
climatological model was run for the
regions bounded by 35-53° N latitude
and 62-95° W longitude for the months
of January, 1977, and March, 1979. In
order to compute sulfur budgets the
region was broken into four subregions:
United States, Canada, Great Lakes,
and Atlantic Ocean. Monthly precipita-
tion amounts from approximately 700
stations were used to calculate wet
deposition amounts of S02 and sulfate.
Emissions of SOz for 72 point sources,
each with an annual emission rate of at
least 105 tons, were used as inputs to
the model.
Figure 1 shows the distribution of
average SOz concentrations for January
1977. The highest concentrations
tended to occur in the general vicinity of
greatest emissions, i.e., along the Ohio
River basin and northward into Canada
(north of Lakes Erie and Huron). The
general pattern of SOz concentrations
in Figure 1 is similar to that for sulfate
and for dry and wet deposition amounts
of SOz and sulfateand to the general
patterns for March, 1979. For January,
1977 observed average concentrations
of S02 and sulfate were available for
more than 90 and 60 locations, re-
spectively, in the United States. The
respective correlations with observed
concentrations were 0.538 and 0.477,
significant at the 99 percent level. As for
the basic model, the correlation co-
efficients were not impressive. Observed
SOz and sulfate concentrations for
March, 1979 were not available.
The climatological model was used
also to estimate monthly mass budgets
of sulfur for subregions of the area that
were studied. The results for January,
1977 and March, 1979 indicate that:
The budgets for the 2 months were
different, due to differing meteoro-
logical conditions.
During January, 1977, inflow of
sulfur to Canada from the United
States was only 23 percent of the
total sulfur over southeastern
Canada, but during March, 1979, it
was 40 percent.
Less than 3 percent of the total
sulfur over the northeastern United
States was imported from Canada
in both months.
Major portions of sulfur were
removed from the atmosphere by
wet and dry deposition.
Of the total sulfur emitted by the
northeastern United States and by
southeastern Canada, less than 10
and 3 percent, respectively, were
transported to the Atlantic Ocean,
much less than in other estimates.
Finally, the climatological model was
used to calculate the acidity (pH) of
precipitation. Since it has been shown
50
45
85 ^80 7570
Figure 1. Distribution of SOz concentrations (jjg/m3) for January 1977.
that there is a significant correlation
between the pH and the concentration
of sulfate in precipitation, it was
assumed that the pH of precipitation
could be expressed as the wet deposition
amount of sulfate (as generated by the
model) divided by the volume of precip-
itation per unit area (based on observed
precipitation amounts). For March,
1979, the average pH of precipitation
samples measured at each of 17
stations in the United States was
compared to corresponding calculated
pH values; the correlation coefficient
was 0.825, significant at the 99 percent
confidence level.
Recognizing that the acidity of precip-
itation is dependent on ions other than
sulfate, available data on nitrates and
ammonium were utilized to evaluate
previously proposed formulations, some
also involving additional ions in precip-
itation in ratios found in sea water.
Clearly, the matter of precipitation
acidity is extremely complicated, but the
results of evaluations indicate that
inclusion of nitrogen oxides in the long-
range transport model will improve pH
predictability.
Conclusions
The correlations between calculated
and observed concentrations of SOz and
sulfate for the basic model (24-hour
average concentrations) and for the
climatological model (monthly average
concentrations) are statistically signif-
icant, but in both cases there is much
more scatter than desirable. Some
possible causes of discrepancies are as
follows:
SOz annual emission data were
used rather than more specific,
shorter term values.
SO2 emissions less than 10s
tons/year were neglected. Such
small sources that are relatively
close to a receptor may have a large
impact on concentrations there.
Measured values of SOa and
sulfate used in this study may be
unreliable.
Model parameters such as dry and
wet deposition velocities and the
transformation rate of SOz to
sulfate likely can be improved.
Under certain conditions (e.g., at
fronts and in the vicinity of cols)
trajectory calculations may be
inaccurate.
Small-scale, close-to-the-source
dispersion is neglected entirely.
In spite of these difficulties the
modeled patterns of concentrations and
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deposition amounts are not unreason-
able, and were used to calculate sulfur
budgets for the northeastern United
States and southeastern Canada. The
results show that major portions of the
emitted sulfur were removed by wet and
dry deposition. For the sulfur over
Canada the contribution transported
from the United States was substantial,
whereas that from Canada to the United
States was small. Only a small fraction
of the total emitted sulfur was trans-
ported to the Atlantic Ocean.
The basic objective of this project, to
develop reasonable working models
that include the essential features of
long-range transport/transformation/
dispersion/deposition, has been
achieved. Future incorporation into the
models of new knowledge about the
basic phenomena involved (especially.
nitrogen oxides transformation to
nitrates) and the use of more appropriate
input information will permit improve-
ments in the modeling results.
Teizi Henmi and ElmarR. Reiter are with Colorado State University, Fort Coll ins.
CO 80523.
George Holzworth is the EPA Project Officer (see below).
The complete report, entitled "Long-Range Transport and Transformation of SO 2
and Sulfate: Refinement, Application, and Verification of Models," (Order No.
PB 82-101 759; Cost: $9.50. subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
it U.S GOVERNMENT PRINTING OFFICE, 1981 559-017/7415
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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