United States
Environmental Protection
Agency
Environmental Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-83-059 Jan. 1984
&ER& Project Summary
The ENAMAP-2 Air Pollution
Model for Long-Range Transport
of Sulfur and Nitrogen
Compounds
R. M. Endlich, K. C. Nitz, R. Brodzinsky, and C. M. Bhumralkar
This report describes the Eastern
North American Model for Air Pollution
(ENAMAP-2), which simulates long-
range air pollution transport over
eastern North America. The ENAMAP-
2S version of the model uses SOx
emissions data inventories and
standard daily weather reports to
compute the airborne concentrations of
SO2 and sulfate and their deposition on
the earth's surface. ENAMAP-2S
operates in a Lagrangian manner by
tracking pollution puffs emitted period-
ically over the grid domain. As each puff
travels with the winds, chemical
processes occur, and the puff loses
pollutant mass through dry and wet
deposition processes. For each grid cell,
the deposition amounts and airborne
pollution concentrations are
summarized for periods generally taken
as one month.
The newest form of the model
includes the influences of smoothed
terrain on the winds and divides the
atmospheric boundary layer into three
parts, allowing pollution emissions to
be partitioned among the layers.
Vertical mixing is controlled by
diffusion coefficients computed from
fields of wind shear, stability, and
mixing depth. The transformation and
wet and dry deposition rates are based
on recent information. However, there
are still substantial uncertainties about
them.
This Project Summary was developed
by EPA's Environmental 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
SRI International (SRI) has developed a
numerical model for calculating the
ambient concentrations and depositions
of pollutants. This research was
sponsored by Umweltbundesamt of the
Federal Republic of Germany for
application in Europe and by the U.S.
Environmental Protection Agency for
application in North America. During the
two phases of model development, SRI
has replaced the simplistic parameter-
izations in the North American version
(ENAMAP-1) with more complex param-
eterizations expressed in terms of known
or theoretical physical relationships.
The final version of the North Ameri-
can model (ENAMAP-2) includes
transformation rates dependent on solar
insolation, dry deposition rates
dependent on surface characteristics,
stability, and time of day, and wet
deposition rates dependent on rain type.
In addition, ENAMAP-2 divides the
planetary boundary layer into three
layers. This version of the model accounts
for diurnally changing mixing depths and
the vertical mixing of pollutants based on
vertical mixing coefficients expressed in
terms of atmospheric stability.
The ENAMAP-2S model, which
pertains to sulfur compounds, was
applied to simulate wet and dry
depositions and monthly average
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ambient concentrations of S02 and
sulfate for January and August
1977. These model results were com-
pared to those generated earlier by
the more simplistic one-layer version. In
addition, a model sensitivity study was
conducted. The ENAMAP-2N version was
applied to simulate wet and dry
depositions and average ambient con-
concentrations of NO, NO2, NOi, HN03,
and PAN.
ENAMAP-2S Applications
The sulfur budgets of the emissions
from Ohio, Indiana, and Illinois were
computed from the January and August
1977 ENAMAP-2S and ENAMAP-1
applications to assess the resultant
effects of the model modifications. The
January and August sulfur budgets are
presented in Tables 1 and 2, respectively.
The two models did not differ significantly
for the ambient S02 or sulfate
concentrations. However, the budget
comparisons showed a significant
increase of dry sulfate deposition for
ENAM AP-2S. This was due to the replace-
ment of the low nocturnal dry deposition
rates with the higher rates used for the
daytime. On the other hand, the sulfate
wet deposition showed a decrease for
ENAMAP-2S. Total deposition of sulfate
computed by ENAMAP-2S was approxi-
mately 90% greater than that calculated
by its predecessor for January and only
8% greater for August.
After ENAMAP-2S was applied for the
entire model domain using emissions
from all states and provinces within the
domain for January and August 1977, the
monthly average S02 and sulfate concen-
trations and SO2 wet and dry depositions
did not differ significantly from those
computed from ENAMAP-1. The
significant differences in the sulfate wet
and dry depositions resulted from the
higher nocturnal sulfate dry deposition
rates used in ENAMAP-2S.
The effects of the greater complexities
in ENAMAP-2S seemed to have been
Table 1.
Comparison of ENAMAP-1 and ENAMAP-2S Budgets for Illinois. Indiana, and Ohio
Emissions (kton) for January 1977
Process
ENAMAP-1 ENAMAP-2S
SO2
Total emitted
Wet deposition
Dry deposition
Flux*
Transformation fSO2 -~
SO;
Emitted
Total emitted and transformed
Wet deposition
Dry deposition
Flux*
645.1
-7.2
-213.5
-165.4
-259.0
15.3
403.8
-11.7
-131.7
-260.4
645.1
-7.5
-242.6
-126.7
-268.4
15.3
417.9
-8.6
-262.6
-146.7
"Flux is the amount of SOa or SO< that was transported out of the model domain by the wind.
Table 2.
Comparison of ENAMAP-1 and ENAMAP-2S Budgets for Illinois. Indiana, and Ohio
Emissions (kton) for August 1977
Process
ENAMAP-1 ENAMAP-2S
S02
Total emitted
Wet deposition
Dry deposition
Flux*
Transformation (SOz — SOI )
Emitted
Total emitted and transformed
Wet deposition
Dry deposition
Flux*
582.9
-164.7
-68.1
-7.5
-342.3
12.4
525.9
-305.7
-170.7
-49.5
582.9
-153.9
-88.7
-1.7
-338.5
12.4
520.3
-238.4
-273.4
-8.5
"Flux is the amount of SOi or SOI that was transported out of the model domain by the wind.
2
smoothed over the month simulation
period. One could have expected the
three-layer model to compute lower
ambient concentrations in Layer 1, close
to the surface, since this version of the
model considers nocturnal mixing
heights. However, due to the exaggerated
nocturnal mixing heights used in these
applications, pollutants from Layer 2
were mixed to the surface throughout the
night.
ENAMAP-2S Sensitivity Tests
In order to minimize the costs of sensi-
tivity testing, ambient concentrations and
depositions were calculated across the
entire model grid using only the
emissions from Ohio (which was near the
center of the grid) for August 1977. The
baseline or nominal case used the model
input parameter values applied in
previous model runs. The parameters
that were adjusted during this sensitivity
analysis included:
1) transformation rate
2) SOz and sulfate wet and dry deposi-
tion rates
3) proportion of nocturnal emissions
injected into Layer 1, and
4) vertical diffusion coefficients.
When compared to air quality
measurements, the ENAMAP-2S
ambient sulfate concentrations appeared
to be significantly higher. On the other
hand, the SO2 concentrations showed no
consistent bias. The results of the
sensitivity analysis indicated that a
combination of 1) reducing the homo-
geneous transformation rate by 25%,
2) increasing the wet and dry depositions
by 25%, and 3) partitioning the nocturnal
pollution injections 25% in Layer 1 and
75% in Layer 2 yielded results which
compared well with measurements.
ENAMAP-2N Description and
Applications
A major portion of this phase of the
project was devoted to the adaptation of
the sulfur version of ENAMAP-2 to a
version designed to simulate, in a linear
fashion, the depositions and complex
non-linear chemical transformations of
nitrogen compounds. In ENAMAP-2N,
the primary gaseous pollutants, NO and
NO2, form an initial puff over each emitter
cell. The concentrations of the two are
calculated and equilibrated with a diur-
nally varying equilibrium constant. The
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NOa can react to form PAN, HN03, and
NOa, with the products formed in a 5:4:1
molar ratio. The reaction rate of NO2
varies dirunally.
The dry and wet deposition rates of the
nitrogen compounds are defined
according to Tables 3 and 4, respectively.
Wet deposition rates for the nitrogen
compounds are expressed as fractions of
those for the sulfur compounds used in
ENAMAP-2S.
Concentration and wet and dry deposi-
tion patterns and source/receptor rela-
tionships for the months of January and
August 1977 for all five nitrogen com-
pounds were determined during this
project. In general, these simulated
patterns seem quite reasonable in terms
of what one would expect for regionally
averaged concentrations. Unfortunately,
no air quality measurements existed for
1977 to evaluate the model.
Table 3. Dry Deposition Velocities
for Atmospheric Nitrogen
Compounds
Compound
NO?, NO
HNO3
PAN
/VOa~ (aerosol)
Va (cm s"V
Day Night
0.2
10
0.25
0.6
0.07
007
0.07
0.6
Table 4. Wet Deposition Hates for
Atmospheric Nitrogen Compounds
Expressed as Fraction of Rates
fa) for SOX
Compound
NO2. NO
HN03
PAN
NO3~ (aerosol)
Relative Rate
0.25 x ctSOz
0.50 x ctHzSO*
0.50 x ctSOz
ocHiSOz
been responsible for the fact that these
differences were insignificant. The dry
deposition of sulfate did vary significantly,
because the nocturnal dry deposition
rates were significantly increased in
ENAMAP-2S applications.
The sensitivity testing of ENAMAP-2
indicated that the resultant effect of
reducing the transformation rate,
increasing the wet and dry deposition
rates, and apportioning some of the
nocturnal emissions into Layer 1 yielded
results that compared well with measure-
ments. Further model evaluations using
these new values and several months of
meteorological data are required before
solid conclusions can be drawn.
R. M. Endlich K. C. Nilz, R. Brodzinsky, and C. M. Bhumralkar are with SRI
International, Menlo Park, CA 94025
Terry L. Clark is the EPA Project Officer (see below).
The complete report, entitled "The ENAMAP-2 Air Pollution Model for Long-
Range Transport of Sulfur and Nitrogen Compounds," (Order No. PB 84-120
930; Cost: $22.50, 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
*US GOVERNMENT PRINTING OFFICE 1984-759-015/7280
Conclusions
The ENAMAP-2S ambient S02
concentrations compared favorably with
measurements, but the calculated
ambient sulfate concentrations were
significantly greater. The ENAMAP-
2S/ENAMAP-1 comparisons showed
that the concentration fields differed
insignificantly, thus the testing of
ENAMAP-2 indicated that the resultant
effects of the more complicated parame-
terizations of ENAMAP-2S were
smoothed over the month simulation
periods. However, the exaggerated
nocturnal mixing heights used in the
ENAMAP-2S applications could have
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