United States
Environmental Protection
Agency
Environmental Sciences Research •-
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-82-063 Sept. 1982
Project Summary
ENAMAP-1A Long-Term
SO2 and Sulfate Air Pollution
Model: Refinements of
Transformation and
Deposition Mechanisms
P. M. Mayerhofer, R. M. Endlich, B. E. Cantrell, R. Brodzinsky, and C. M.
Bhumralkar
The ENAMAP-1 model for long-
range air pollution transport has been
modified in several ways to produce
the newer version, ENAMAP-1 A. The
modeled domain has been increased
to include more of Southeastern
Canada; the meteorological and emis-
sions data for this area have been
added to the United States data base;
the transformation rate for SO2to SOi
and the deposition rates for SO2 and
SOI have been changed to reflect
variations in space and time; and the
transformation rate has been param-
eterized to be a function of latitude
and season. The new transformation
rate is, on the average, several times
larger than the former 1 percent per hr
rate. In ENAMAP-1 A, the dry deposition
rate has been parameterized to be a
function of the underlying terrain and
vegetation, the thermal stability in the
boundary layer, and the time of day.
The wet deposition rate has been
changed to be a function of rainfall
rate and cloud process type (convec-
tive, warm process, or Bergeron
process).
For this project, the ENAMAP-1 A
model was run for each day of January
and August 1977 to produce monthly
averaged values of airborne concen-
trations, dry deposition, and wet
deposition of SO2 and S04. These
values have been compared to values
generated by the previous version of
the model. The boundary exchanges
of SOz and SOI have been computed
for each of 41 states or provinces and
also for 12 smaller areas of special
interest. The course of pollution from
emission to deposition is documented
in the form of maps and tables. For
brevity, only the comparisons are
presented and discussed in this sum-
mary. The remaining results are
discussed in the final report, EPA-
600/3-82-063. In contrast to the
previous computations using ENAMAP-
1, the new computations showed
much larger concentration and de-
position amounts of airborne SOI.
while the amount of SO2 deposition
was decreased. The total sulfur de-
position (SO2 and SO< combined) was
approximately 40 percent of the
previously computed value in winter
and 70 percent of the previous value in
summer. Scatter diagrams of calculated
and observed concentrations showed
reasonably good agreement for SO2;
however, computed SO* concentra-
tions were significantly greater. This
may be interpreted as evidence that
the new model transformation rate
was on the average too large and/or
that SOI deposition was too low.
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This Project Summary was developed
by EPA's Environmental Sciences Re-
search 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
Under contract to the U.S. Environ-
mental Protection Agency (EPA), SRI
International developed and evaluated
an Eastern North American Model of Air
Pollution (ENAMAP-1). The ENAMAP-1
model was adapted from the SRI-
developed European Model of Air
Pollution (EURMAP). The ENAMAP-1
model was designed to study the long-
term transport and deposition of airborne
sulfur pollutants and to calculate
ambient sulfur concentrations for
monthly, seasonal, and annual periods
over the eastern United States and
Canada. The model has been used to
calculate exchanges of airborne sulfur
among various United States and
Canadian regions. ENAMAP-1 was
further tested to study the variability of
the model's calculations of seasonal
sulfur concentrations and depositions
due to year-to-year changes in the wind
and precipitation patterns.
Under another contract with EPA,
work was continued to develop ENAMAP-
1 further. A new version of the model,
ENAMAP-1A, has been developed by
expanding the domain of ENAMAP-1 to
include the area bounded by 29°N and
55°N latitude and 60°W and 104°W
longitude. Other modifications include
1) treating most states and Canadian
provinces as separate receptor
and emitter areas and adding 12
smaller receptor areas; and
2) incorporating deposition and trans-
formation parameterizations ex-
pressed as functions of variables
theorized to be factors governing
the physical processes.
ENAMAP-1 A has been applied to
emissions and meterological data for
January and August, 1977 and the
results have been compared with the
measured concentrations as well as
with the results of ENAMAP-1. Compar-
isons of ENAMAP-1 and ENAMAP-1A
are discussed in this summary. These
comparisons show that the new trans-
formation rate is too high and/or the
S0< deposition rate_is too low.
As the research effort continues, the
model input values will be reassessed.
In addition, the boundary layer is to be
divided into the distinct sublayers with
vertical mixing parameterized as a
function of stability (the previous model
version assumed instantaneous, com-
plete vertical mixing at the source).
Also, the transport wind speed near
mountainous regions will be adjusted to
account for terrain effects and concen-
trations and depositions of nitrogen
compounds will be calculated by the
model.
Description of the ENAMAP-
1A Model
ENAMAP-1 has been updated in
various ways in the course of developing
ENAMAP-1 A. Algorithms for wet and
dry deposition rates and transformation
rates have been developed to account
for temporal and spatial variability in the
parameters and have been incorporated
in the latest version of the model. These
algorithms have been formulated on the
basis of an extensive literature search;
they represent the state-of-the-art of
the treatment of dry and wet deposition
in long-range transport models. A
description of the model's basic struc-
ture, including grid cell sizes and the
puff advection and diffusion scheme,
can be found in the final report, EPA-
600/4-80-039.
Figure 1 shows the boundaries
ENAMAP-1 An$j«le
covers the
55°N latitu
longitude.
560 km (eight 70-by 70-km grid cells) to
the northern side and 210 km (three 70-
by-70-km grid cells) to the eastern side
of the previous grid. The model has been
modified to calculate interregional
exchanges of sulfur pollution between
41 states and provinces (as opposed to
13 EPA and Canadian regions in
ENAMAP-1).
The transformation rate for SOz to
SC>4 is expressed as the sum of two
components: a homogeneous trans-
formation rate and a heterogeneous
transformation rate. For the homogene-
ous rate, the rate constant is calculated
theoretically as a function of solar
insolation (i.e., latitude and season).
The theoretical rates were based on
tests made in a relatively clean atmos-
phere; therefore, the rates were doubled
in ENAMAP-1 A because of the greater
number of pollutants and reactions
occurring in the actual atmosphere. The
transformation coefficients used in the
homogeneous conversion are presented
in Table 1. An additional term for the
heterogeneous conversion of S02 to
105
100°
35°
65°
90
Figure 1 . The 70 km grid used in the ENAMAP-1 A model.
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SOi is included. There seems to be
some question in the literature as to the
relative importance of the various
heterogeneous conversion mechanisms,
particularly the differentiation between
the strong oxidizing agents (such as
HaOa and O3) and the effect of metallic
catalysts on conversion. Because of the
difficulty in determining the relative
importance of the various heterogeneous
conversion mechanisms, a constant
conversion rate of 0.005 (0.5 percent
h"1) for the heterogeneous conversion is
used in the model. The total transforma-
tion rate varies from approximately 0.01
in winter to 0.04 in summer.
Because of the natural variability of dry
deposition, ENAMAP-1A treats it as a
function of land-use type, stability, and
time of day. Land-use type is defined by
the surface characteristics (land type or
water) and the type of vegetation. Land-
use type was gridded to each 70-by-70-
km receptor cell to incorporate dry
deposition variability at this resolution.
Deposition velocities for each land-use
type for SO2 and SO* for January
(winter) and August (summer) for
stability classes 1 through 6 (very
unstable through very stable) are
applied in ENAMAP-1A. The values vary
from 0.05 cm s~1 for cities to approxi-
mately 1.0 cm s"1 for swamps. To
account for the low absorption by plant
surfaces at night, SOz and SOU deposition
velocities have been reduced to 0.07 cm
s"1 during nighttime hours. The length
of night is adjusted for each season.
Wet deposition is treated as a function
of season and rainfall rate (mm h"1). The
removal rates are based primarily on
considering the washout ratio as a
function of precipitation rate and three
cloud types: cold clouds in which
nucleation of rain is essentially caused
by the Bergeron or ice growth process;
warm or maritime clouds; and convective
clouds. For the model, thefollowing was
assumed: winter precipitation follows
the Bergeron process, fall and spring
precipitation are warm cloud phenom-
ena, and summer precipitation is
confined exclusively to the convective
type of precipitation. The semi-empirical
representation of these removal rates
for use in the model is presented in
Table 1 for both SOz and SO4. The
seasonal variation in the parameters a
and b for SO 2 were adopted to reflect the
variation obtained by Scott for sulfate.
They may be revised at a later date when
data can be obtained to make a more
appropriate distinction between summer
and winter gaseous S02.
The choice of these parameters has
resulted in a significant difference in the
treatment of wet deposition between
ENAMAP-1 and ENAMAP-1A. In
ENAMAP-1, the washout ratio is four
times greater for SOa than for S0« for
both January and August. The washout
ratio for SOa in ENAMAP-1 is approxi-
mately 40 times lower for January and 5
times lower for August than in ENAMAP-
1. The washout ratio for SO< in ENAMAP-
1A is approximately five times lower for
January and two times higher for
August than in ENAMAP-1. Naturally,
these changed rates produce large
differences in the wet deposition
patterns and statistics, as will be shown
below.
Results of Model Application
for January and August 1977
To determine the effects of using the
new sulfur deposition and transforma-
tion algorithms, the more complete
Canadian emissions data, and the
increased model domain, the model was
run for January and August 1977.
Separate runs were made for the states
and provinces for which emissions data
were available. For each month, fields of
SOa and SO^ concentrations, dry
deposition, and wet deposition resulting
from sulfur emissions in each of the
individual areas were then combined
into maps showing the monthly area
totals. Interregional exchange tables were
also generated, but are not presented in
this summary. For comparison purposes,
plots of S02 and S04 deposition and
concentration patterns generated from
applications of the previous version of
the model (ENAMAP-1) are included in
this section.
SOa and SO^ Concentrations
Calculated and observed SOa January
1977 concentrations were in good
agreement, except that calculated
concentrations were low (<10//g/m3
versus >32 fjg/m3) in Minnesota and
Tablet. Comparison of ENAMAP-1 and ENAMAP-1 A Transformation and Deposition Parameters
ENAMAP-1
ENAMAP-1 A
Transformation
Dry Deposition
SO2
so;
Wet Deposition
so;
1%/h
1.18 cm/s (winter)
1.34 cm/s (spring/fall)
1.49 cm/s (summer)
0.22 cm/s (winter)
0.25 cm/s (spring/fall)
0.28 cm/s (summer)
28. OR %/h,
where R is the
precipitation rate
(mm/h).
7.OR %/h
where R is the
precipitation rate
(mm/h)
(2[a+b In (latitude)J+0.5f/o/h,
where in winter, a=2.5 and b=-0.61;
in spring/fall, a=4.4 and b=-1.O;
and in summer, a=6.3 and b=- 1.4.
0.05 - 0.95 cm/s.
depending on stability
and land use.
0.15 -0.95 cm/s.
depending on stability
and land use.
100 (aRh) %/h.
where in winter, 3--0.009 andb=O. 70;
in spring/fall, a--0.036 and b=0.53;
and in summer, a=-0.140 and b=O. 12.
100 faR") %/h.
where in winter, a=-0.021 andb=O. 70;
in spring/fall. a=-0.091 and 6=0.27;
and in summer, a=-0.390 and b=O.06.
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Wisconsin. Concentrations calculated
by ENAMAP-1A were higher at the
northern and southern boundaries of
the model domain than those calculated
by ENAMAP-1. The concentrations from
the new model run were closer to the
measured values in this respect; pre-
viously the calculated concentrations
were too low in Alabama and Georgia.
The slightly higher S02 concentrations
calculated.by ENAMAP-1A were due to
its much lower wet and dry removal
rates, which, to some degree, counteract
the higher transformation rate.
The calculated SOz concentrations for
August 1977 were similar to the
measured concentrations in pattern and
in magnitude. The new model results
from the northern and southern states
were higher than the previous model
results, which made them closer to the
measured values in these areas. The
higher SOz concentrations calculated
by ENAMAP-1A were due to the lower
wet and dry removal rates, even though
the transformation rate was higher.
The ENAMAP-1A January 1977 503
concentrations were approximately
twice as large as both the previously
calculated values and the measured
values, but the pattern of the isopleths
was very similar to the earlier run. The
reason for the higher SOJ concentrations
calculated by ENAMAP-1A was the
higher transformation rate and lower
wet removal rate, which overshadow
the higher dry removal rate.
As in the January 1977 SO* model
results, the August SO^ concentrations
from ENAMAP-1A increased by a factor
of two over the previously calculated
concentrations. This made them much
higher than the measured values. The
new calculated concentrations were
higher than the previous values because
of the higher transformation rate,
although the wet and dry removal rates
were also several times higher than
before.
Table 2 compares the modeling
results using the former and revised wet
and dry removal and transformation
rates and the SOz snd SOJ emissions
from Illinois, Indiana, and Ohio. For
January 1977, the revision of wet
removal rates led to a significant
reduction, nearly 20-fold for S02 and 4-
fold for SOi. Dry deposition of SOz
decreased by a factor of nearly three,
while dry deposition of SO* increased by
a factor of three. Transformation of S02
to S04 increased by a factor of 2.6.
As was the case for January 1977,
the wet SOz deposition calculated by
Table 2.
Comparison of ENAMAP- 1A and ENAMAP-1 Results (KTON) for Illinois,
Indiana, and Ohio Emissions for January and August 1977.
January 1977
August 1977
Emission
S02
Total SOz emitted
Wet deposition
Dry deposition
Flux*
Transformation (SO 2 -> SOI)
ENAMAP-1
638.7
145.6
372.7
19.7
100.7
ENAMAP-1A
645.1
72
213.5
165.4
259.0
ENAMAP- 1
576.8
210.6
286.4
2.4
77.4
ENAMAP- 1 A
882.9
164.7
68.1
7.5
342.3
so;
Total SOI emitted and trans- 165.9 403.8 127.6 525.9
formed
Wet deposition 44.6 117 65.5 305.7
Dry deposition 44.3 131.7 36.6 170.7
Flux'' 77.0 260.4 25.5 49.5
*Flux is the amount of SOi or SO< that was transported out of the model domain by the wind
ENAMAP-1A for August 1977 decreased,
but not nearly as much. However, unlike
the January results, the wet SOI
deposition for August increased (by a
factor of nearly five). Dry SOz deposition
decreased as they did in the January
case, but this time by a factor of nearly
four. Dry SO* depositions increased by a
factor of nearly five. Transformation
increased by a factor of nearly 4.5.
Dry and Wet Depositions
Dry depositions calculated by ENAMAP-
1A were much lower than the ENAMAP-
1 results, which displayed a closed
isopleth of 1024 mg/m2 of S02 deposi-
tion over eastern Pennsylvania (absent
in ENAMAP-1A results). The January
SOz wet deposition results indicate that
the wet deposition was drastically
reduced by using the new coefficients in
ENAMAP-1A. This was the largest
change of any of the SO2 or SO* wet or
dry deposition results.
The new dry deposition of SO^
increased by a factor of approximately
two, similar to the change in SOJ
concentration. The wet SO< deposition
results were reduced in ENAMAP-1A
because of the.lower wet removal rate.
The increases or decreases in deposition
from ENAMAP-1A compared to ENAMAP-
1 are shown in Table 2.
Summary and Concluding
Remarks
This project summary describes a
new version of the ENAMAP-1 model,
ENAMAP-1 A, of long-range airborne
pollution transport and removal. The
new version covers a larger geographical
area and includes emission data and
weather observations from southeastern
Canada as well as from the eastern
United States.
The transformation and deposition
parameterizations in ENAMAP-1 A have
been modified to be functions of those
variables theorized to be factors in
governing the relevant physical pro-
cesses. The new transformation rate
for SO2 to SO4 in ENAMAP-1 A varies
with solar insolation (i.e., it is dependent
on latitude and season). It is several
times larger than the rate used pre-
viously. This factor, combined with
greater SOJ deposition rates, signifi-
cantly increased the SOI deposition
amounts, while the deposition amounts
for SOz correspondingly decreased. In
ENAMAP-1A the total monthly sulfur
depositions for August were much less
than those for ENAMAP-1. Unfortu-
nately, measured deposition data to
compare with the simulations were not
available. However, the computed SOa
and SOJ concentrations can be compared
to air quality data. The SOz concentra-
tions yielded by ENAMAP-1 Afor January
were closer to measured values than
previous computations, particularly in
the northern and southern parts of the
domain. The pattern of August SO2
concentrations were very similar to
previous computations. The ENAMAP-
1A S04 concentrations were too large
for both January and August, however.
Because of the lack of SOz and SOJ
deposition measurements, it was not
possible to assess the overall accuracy
of the ENAMAP-1 A results. As such
data become available, total model
evaluation will become feasjble. The
present overestimation of SO4 concen-
trations can be taken as evidence
suggesting that the new transformation
rate was too high and/or the new SOl
deposition rates were too low.
Research under the existing contract
is continuing. ENAMAP-1 A is undergo-
4
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ing further refinement to include the
effects of terrai n on the wind flow and to
divide the boundary layer into three
sublayers with vertical mixing among
them. This modification will enable
emissions from near-ground sources to
be injected into sublayer one, the lowest
sublayer, and tall stack emissions to be
injected into sublayer two. The effects of
these changes on the computations can
then be determined.
P. M. Mayerhofer, R. M. Endlich, B. £. Cantrell, R. Brodzinsky, and C. M.
Bhumralkar are with SRI International. Menlo Park, CA 94O25.
Terry Clark is the EPA Project Officer (see below).
The complete report, entitled "ENAMAP-1A Long-Term SO2 and Sulfate Air
Pollution Model: Refinements of Transformation and Deposition Mechanisms,"
(Order No. PB 82-237017; Cost: $10.50, 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
OUSGPO: 1982 — 559-092/0522
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