United States
                   Environmental Protection
                   Agency
                                                                            •«k:'
 Atmospheric Sciences
 Research Laboratory          x/   ,
 Research Triangle Park NC 27711    '
                   Research and Development
 EPA/600/S3-86/038 Sept. 1986
ŁEPA         Project  Summary
                   Numerical  Simulations  of
                   Photochemical Air  Pollution  in
                   the  Northeastern  United
                   States:  ROM1   Applications
                   Robert G. Lamb
                    The first-generation Regional Qx\-
                  dant Model (ROM1) was used to simu-
                  late pollutant concentrations  during
                  the nine-day period 23-31 July 1980.
                  Two simulations were performed. The
                  first, which is considered to be the base
                  case, used the 1980 NAPAP 4.2 inven-
                  tory for all hydrocarbon and NOX emis-
                  sion rates. The second simulation, or
                  control case, was  identical  in all re-
                  spects except that the county-by-
                  county hydrocarbon and NOX emissions
                  rates were modified in accordance with
                  baseline projections for 1987 contained
                  in State Implementation Plans (SIPs).
                  The one-hour and daily daylight (0900-
                  1600 1ST) averaged ozone concentra-
                  tions produced in each simulation were
                  compared to assess the effectiveness of
                  the proposed emissions changes on air
                  quality.
                    Ozone concentrations in the control
                  case were found to be everywhere
                  lower than those in the base case, but
                  the percentage reduction was not uni-
                  form in space. In areas near the major
                  VOC and NOX sources, the maximum
                  one-hour averaged  ozone levels were
                  reduced by about 25% while in areas
                  farther than 100 km from these sources
                  peak values were only about 10%
                  lower.  Slightly smaller percentage re-
                  ductions were found in the daily day-
                  light average ozone concentrations, ft
                  was also found that the emissions re-
                  ductions lowered peak ozone concen-
                  trations by considerably larger percent-
                  ages than they reduced the median or
                  mean concentration values.
  The analyses of the model results are
prefaced by discussions of a number of
basic issues on regional scale model-
ing, including model initialization, se-
lection of meteorological data, effects
of grid size on model performance, esti-
mating long-term concentration statis-
tics from  short-period  simulations,
probabilistic vs quasi-deterministic
modes of model operation, uncertainty
in emissions estimates, and the charac-
teristics of VOC and NOX sources in the
Northeast, among other topics. Prelimi-
nary results of analyses of the SAROAD
monitoring data, which reveal the char-
acteristics of the ozone problem in the
Northeastern United States, set the
stage for the model simulations.
  This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).

Introduction
  The development of the Environmen-
tal Protection Agency's Regional  Oxi-
dant Model (ROM) began in the late
1970's as a part of the Northeast Corri-
dor Regional Modeling Project
(NECRMP). The NECRMP was initiated
out of the recognition that the adverse
ozone concentrations observed in the
Northeastern United States are due in
large part to the  regional transport of
ozone and its precursor species. The

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principal role envisioned for the ROM in
this project was to assist the states in
developing emissions control plans that
would effect compliance with the Fed-
eral ozone air quality standards in the
most equitable and cost  effective way.
This report describes the second of a
series of applications of the Regional
Oxidant Model in this role. The report
considers projected  1987 emissions,
based on  1982 State Implementation
Plans  (SIPs), and  compares the ozone
concentrations simulated using these
emissions with the corresponding con-
centrations predicted  using 1980 emis-
sions  data. Two questions are of pri-
mary  concern: (1) what impact will
proposed VOC and NOX emissions con-
trols, which were designed to attain the
primary ozone standard in urban areas,
have on ozone  levels in rural and re-
mote  regions?;  (2) what impact will
these emissions changes have on
longer period ozone averages, such as
the 7-hour daily  daylight (0900-1600
LST) average presently  being  consid-
ered as  a possible basis for a new sec-
ondary ozone standard?  The only way
of obtaining answers to questions of
this kind prior to implementing emis-
sions  controls is through models such
as the ROM which simulate the meteo-
rological and chemical  processes that
govern  the evolution of air pollution
episodes.

Procedure
  The development and initial testing of
the Regional Oxidant Model (ROM)
were described in three earlier reports:
A Regional Scale (1000  km) Model of
Photochemical Air Pollution:  Part  1.
Theoretical Formulation, EPA-600/3-83-
035, May 1983; Part 2. Input Processor
Network Design, EPA-600/3-84-085, Au-
gust 1984; and Part 3. Tests of the Nu-
merical Algorithms, EPA-600/S3-85-037,
June  1985. The  version of  the model
used  in the present study is the first-
generation ROM, referred to as ROM1,
which differs from the ultimate second-
generation model (ROM2) in several key
respects.  First, ROM1 utilizes the 23-
species, 36-step Demerjian-Schere
chemical  mechanism whereas ROM2
will employ Carbon Bond-IV, which
treats 70 reactions among 28 chemical
species. The latter mechanism provides
explicit treatment of the biogenic hydro-
carbon  species isoprene. To make use
of this  provision, ROM2 will use an
emissions inventory in which anthropo-
genic source data are  supplemented
with gridded estimates of the fluxes of
hydrocarbons generated by natural
sources. Another major difference be-
tween the first- and second-generation
models is that the layer thicknesses in
ROM1 are constant in space and time
and the winds  are  horizontally non-
divergent. In ROM2, the layer thick-
nesses will vary spatially and  tempo-
rally to  keep track of meteorological
phenomena, and the winds will be non-
divergent, allowing for large scale verti-
cal motion. Due to these basic differ-
ences between  ROM1 and ROM2, the
principal objectives of the former are to
provide experience in operating large,
complex regional scale air  pollution
models, and to  gain some preliminary
insight into the  effects proposed emis-
sions changes  are  likely to have on
ozone concentrations in the Northeast.
These are the  general topics of the
present study. The primary objective of,
ROM2 will be to provide a credible basis
for formulating regional emissions con-
trol policies for ozone attainment.
  In the course of applying ROM1 con-
siderable knowledge was acquired in
dealing with problems that are unique
to regional scale photochemical mod-
els. One of these is the problem of
model initialization. In  the case of
ROM1, initialization means specifying
the concentrations of each of 23 chemi-
cal species at each of some  7500 grid
cells at the hour the model simulation is
to begin.
  Unfortunately, this requirement
vastly exceeds the information  content
of presently  available air monitoring
data bases. Within the present ROM do-
main, measured hourly ozone concen-
trations are available at about 150 sur-
face sites;  lower  quality  data are
available for NOX at fewer sites, and no
measurements are available for any of
the remaining 23 species. Moreover, no
data are available on the concentrations
of any species above ground level.
Thus, in  practice initialization requires
reconstructing the 3-D spatial structure
of the concentration fields of 23 com-
pounds given the concentration of only
two of the species at a few surface loca-
tions. If the nature of the  pollutant
chemistry were such that the concentra-
tions of all species  were  unique func-
tions of the concentrations of the two
given species, then initialization would
not be a problem. But this  is  not the
case. In fact, it  appears that  the ozone
concentration that evolves from a given
initial mix of species is quite sensitive to
the initial levels assumed for hydrocar-
bon  and NOX, and  as a  consequence
anomalous ozone concentrations may
arise in the course of the simulation that
are merely artifacts of the initialization.
This problem is made all the more acute
in a regional model  by the prolonged
residence time, 4-5 days or more, of the
initially present species within the
model domain. Similar problems arise
in specifying concentrations at  inflow
boundaries. Model  simulations per-
formed in control strategy studies run
long enough that species that enter the
domain through the inflow boundaries
eventually permeate the model area to
nearly the same extent that the initial
concentration field does. After consider-
able study of the problem of recreating
concentration fields  from  sparse data
and of the sensitivity of model predic-
tions to uncertainties in the initial and
boundary concentrations,  it was con-
cluded that  the simplest  and perhaps
most reasonable approach is to assume
"clean" tropospheric values  for both
the initial and boundary concentrations,
at least in model simulations whose
only aim is to compare the effectiveness
of given  emissions controls. This
method is viable only if the model do-
main is large enough  to encompass the
majority of the sources that affect  air
quality in the areas of interest. Because
in this case the influence of  trans-
boundary fluxes on species concentra-
tions at receptors in the interior of the
domain is at most a second-order effect
and well below the  level  of tolerable
error in the predictions.
  Applications of ROM1 also yielded in-
formation on procedures  for selecting
meteorological data, interpreting model
results, and other important aspects of
regional  model applications. This
knowledge will be valuable in the de-
sign and execution  of ROM2 applica-
tions that are planned to support emis-
sions control policy formulation.
Conclusions
  Following are some of the major find-
ings of this study.
(1)  Based  on the 1980 NAPAP version
    4.2 emissions inventory, 70 counties
    in the Northeastern U.S. were iden-
    tified as major sources of VOC and
    NOX. These counties have the
    highest emissions densities (moles
    per area per day) of VOC and NOX of
    all counties in the region, and to-
    gether they produce about one-half
    of all VOC and NOX emissions. The
    areas of highest  measured  ozone
    concentrations are  closely associ-
    ated with the locations of these 70
    counties.

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(2) The characteristic spatial scale of
   the major VOC and NOX sources in
   the Northeast is estimated to be a
   few tens of kilometers. This means
   that if a model with a grid size much
   larger than this is used  to simulate
   photochemical air pollution in this
   region, significant systematic errors
   can occur not only in the predicted
   peak concentrations  of secondary
   species, such as ozone, but also in
   the predicted response  of the con-
   centrations of these species  to
   changes in VOC and NOX emissions.
   The latter point is of critical impor-
   tance in the use of models in regula-
   tory studies and requires further de-
   tailed analysis.

(3) A comparison of the 1980 NAPAP
   version  4.2 emissions inventory
   with the earlier 1979 NECRMP  in-
   ventory revealed differences of 300
   percent  and more in the gridded
   (-18 x 18 km) VOC and NOX emis-
   sion rates. This represents a level of
   uncertainty  in the base emissions
   that is roughly ten times the magni-
   tude of the changes in emissions
   that are contemplated in present
   control strategies.
(4) Air quality  models that treat re-
   gional scale and larger areas, i.e.,
   domains > 1000 km in  extent, can
   be operated in either of two modes,
   which are called the probabilistic
   mode and the quasi-deterministic
   mode. In the  former,  the model
   predicts the probabilities, expec-
   tations and  other statistical prop-
   erties of concentrations at specific
   sites at specific times. In the quasi-
   deterministic mode, the model pro-
   vides statistics of concentrations at
   given times or integrated over given
   periods within given receptor
   classes rather than at specific sites.
   In the present study, the  model is
   run only in the quasi-deterministic
   mode, yielding  information on the
   concentrations of 23 different chem-
   ical species in four receptor classes
   —Urban, Suburban, Rural, and
   Wilderness.
(5) Two criteria were tentatively pro-
   posed for selecting  historical me-
   teorological data for  use in re-
   gional scale modeling studies  of
   photochemical  oxidant:  (1) the
   meteorological scenario should
   begin on a day when the  median
   value  of the  maximum  hourly
   ozone concentrations observed at
   all measuring  sites in  the model
   domain is near the seasonal mini-
   mum value; (2)the scenario
   should  be long  enough that the
   frequency distribution of mea-
   sured hourly ozone values during
   the scenario period approximates
   the corresponding seasonal distri-
   bution closely enough to give the
   results  of the model  simulations
   broad applicability. Moreover, the
   scenario must be more than about
   5 days  long, to  minimize the  ef-
   fects of the initialization procedure
   on predicted concentrations in re-
   mote areas. A 9-day scenario is ap-
   parently not long enough to model
   the processes that control concen-
   trations above the 90-th  percentile
   level at any site.
(6) Two 9-day simulations  were per-
   formed with ROM1 using meteoro-
   logical data from 23-31 July, 1980.
   One simulation, the base  case, used
   the 1980 NAPAP version 4.2 emis-
   sions data as input. The other simu-
   lation, the control case, use pro-
   jected 1987 baseline emissions.
   Comparisons of the predicted ozone
   concentrations in the base case with
   corresponding values in the control
   case revealed the following data:

      (a) In general, at any given loca-
         tion and hour the  concentra-
         tion in the  control case is less
         than or  equal to that  in  the
         base case;
      (b) Within each receptor group,
         peak concentrations are re-
         duced by larger percentages
         than the median values  are
         reduced. Reductions of 0 to
         50% occur in the  peak com-
         pared to 0 to 25%  reductions
         in the median. This is true of
         all concentration  averaging
         times.
      (c) Ozone is  reduced more  at
         sites near the major VOC and
         NOX sources than at locations
         far away.  For example, me-
         dian ozone levels at suburban
         locations  (defined to  be
         within 50 km of major source
         centers)  were reduced up to
         25% whereas in Wilderness
         areas (greater than  100  km
         from major sources) reduc-
         tions were less than 15%.
     (d)  The maximum ozone concen-
         trations  in rural and wilder-
         ness areas occurred on differ-
         ent  days in the  control case
         than in the base case, even
   though meteorological condi-
   tions were identical in  both
   simulations.  This suggests
   that the source-receptor rela-
   tionship between VOC/NOX
   sources and remote sites is
   strongly nonlinear.
(e) Overall, the 1987 emissions
   reductions appear to have
   two basic effects on  ozone:
   they cause a  delay in ozone
   formation, and they  reduce
   the total quantity produced.

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     The EPA author, Robert G. Lamb (also the EPA Project Officer, see below) is with
       Atmospheric Sciences Research  Laboratory, Research  Triangle Park,  NC
       27711.
     The complete report,  entitled "Numerical Simulations of  Photochemical Air
       Pollution in the Northeastern United States: ROM 1 Applications," (Order No. PB
       86-219 201/AS; Cost: $16.95. 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:
             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
Official Business
Penalty for Private Use $300

EPA/600/S3-86/038
                                                      ps

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