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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