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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