United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-86/014 Apr. 1986
Project Summary
Local Source Impact on
Wet Deposition
Aristides A. N. Patrinos
Precipitation chemistry measure-
ments over a network of samplers
upwind and downwind of Philadelphia,
PA, show that a major contribution of
the local sources can be discerned
under certain conditions. For winter
frontal storms with low-level winds
from the southeast, up to as much as a
factor of two increase over upwind
values has been observed for downwind
nitrate deposition. Sulfate deposition
shows an increase of about a factor of
one and one half. The nitrate deposi-
tion increases toward the downwind
direction away from the urban-
industrial sources, indicating that the
maximum is likely to have been beyond
the sampling network for these case
studies. One storm had no increase in
nitrate or sulfate deposition but did
have an increase in total sulfur content
in the precipitation. Reasons for this
difference are being sought.
This Project Summary was developed
by EPA's Atmospheric Sciences
Research 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).
Introduction
Sources in the local and intermediate
range from receptors are expected to
contribute more to deposition than
comparable sources at greater distances
due to the dilution of pollution with
distance. Knowing the deposition-to-
distance relationship is important in
developing effective and efficient control
strategies on nearby source! for
protection of particular sensitive receptor
regions. Furthermore, since wet
deposition monitoring networks are
sparse, with sites located away from
source areas, knowledge of near source
deposition is important in evaluating the
wet deposition sink for budgets of acidic
substances.
Local and intermediate-range
transport was the principal focus in the
history of air quality management.
Recently, attention has been focused on
the long-range contributions from
multiple sources of sulfur in regions
many hundreds of kilometers upwind
from sensitive receptor areas, i.e., the
long-range transport hypothesis.
Furthermore, studies around large point
sources indicated relatively small
contribution to total sulfate and nitrate
deposition. However, studies around
urban pollution sources indicated
substantial impacts on wet deposition.
With the relative importance of local and
intermediate-range sources appearing
unclear. Work Group 2 of the U. S.-
Canada Memorandum of Intent (MOI)
study recommended further studies of
this issue. The National Acid Precipitation
Assessment Program (NAPAP) has
instituted the Mesoscale Acid Deposition
Studies Program to address this problem.
Key Findings
A review of past literature and the
original results of the recent Philadelphia
Mesoscale Acid Deposition Study yield
the following key findings:
• The impact of tall stacks (point
sources with stack heights greater
than 50 m) is small in the local
range (source-to-receptor
distances less than 30 km), with
less than 5% of the emitted SOxand
NOx scavenged by precipitation (for
most regions this corresponds to a
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0.5% annual removal) during
precipitation events. Percentages
for chloride and trace metals are
much higher.
• The impact of area sources such as
large urban and industrial
complexes is significant in the
intermediate range. This
conclusion is based on limited but
reliable field data for summer
precipitation and winter storms.
The summer data were for the
METROMEX study in St. Louis, MO;
the winter data were for the
Philadelphia Mesoscale Field
Study. The nitrate impact is
dominant in the cold season; the
warm season effect is comparable
for nitrate and sulfate.
In both cases, more than 50% of the
emissions may be oxidized and deposited
in acid form within 100 km. The winter
cases demonstrated a gradual increase of
the nitrate impact, with distance from the
source peaking beyond 60 km. The nitrate
effect implicates the significant
transportation sources which are diffuse
and at low heights.
• Preliminary indications, based on
limited but reliable field data,
suggest a minimal impact of
primary sulfates on intermediate-
scale deposition.
• The local scale effect within the
area source is presumably
dominated by below-cloud
scavenging of higher urban sulfur
and nitrate air pollutants, resulting
in higher sulfate and nitrate wet
deposition. However, no clear
effect on pH is evident, due to an
urban excess of neutralizing agents
such as calcium.
The above results are consistent with a
conceptual model in which point source
plumes require time to mix with
oxidant-rich air, thus promoting faster
transformations. Emitted S02 and NO
require time for transformation to more
readily scavenged forms (H2S04 and
HN03); for S02 the process may be
accelerated by reactive scavenging,
which is enhanced under conditions
permitting strong vertical mixing, e.g.,
convective storms. For NO, the gas phase
transformation is an order of magnitude
faster than for SO2 and may continue, via
a different pathway; during the night for
S02 this transformation shuts off at night.
Using emission rates for NOx and SOx
of 200,000 metric tons year-1,
approximate values of 2 and 8 mmol nr2
are estimated for sulfate and nitrate
deposition in the environs of area
sources. In areas of low to moderate
background deposition, the increases
above regional background levels are
relatively large, especially for nitrate
(50% or more). For point source, the
inferred upper bound of 0.5% on wet
removal fraction translates to a rather
significant bound (16 mmol rtr2 of sulfate)
for a large source. This value is not small
in comparison with regional values.
especially in remote regions, i
Experimental difficulties make the '
evaluation of the impact of point sources
on intermediate-range wet deposition
difficult.
To gain a total picture of local and
intermediate-scale deposition impacts,
dry deposition needs to be considered. An
empirical picture comparable to that for
wet deposition is not possible but dry
deposition may be just as important in the
delivery of material to the surface.
Although quantitative estimates are
generally unreliable, on the local scale it
is accepted that dry deposition
contributions exceed those of wet
deposition. Depending on effective stack
height, terrain, and meteorological
conditions, anywhere from less than 1
percent to 50 percent might be removed
within the first 30 km. A complication in
estimating local dry deposition is the
diurnal variation in the dry deposition
process rate, which may create a lag
between the time of maximum
susceptibility and maximum concentra-
tion, depending on the source release
height.
Aristides A. N. Patrinos is with the Brookhaven National Laboratory, Upton, Long
Island, NY 11973.
Francis S. Binkowski is the EPA Project Officer (see below).
The complete report, entitled "Local Source Impact on Wet Deposition," {Order
No. PB86-167 756/A S; Cost: $11.95, 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 Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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