United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S8-83-022/023 Aug. 1983
&EPA Project Summary
Commuter Exposure Model:
Description of Model
Methodology and Code, and
User's Guide
P. B. Simmon and R M. Patterson
A methodology has been designed to
compute commuter exposure statistics
through simulation of the traffic flow,
vehicular emissions, and atmospheric
dispersion of roadway-related air pol-
lutants. The computer modeling pack-
age consists of two programs. The first
calculates the emission factors. The
second computes the emission rates
and derives the exposure statistics.
The commuter exposure model pro-
vides both short-term (single commute)
and annual statistics. The model can
be used to assess the pollutant levels
the commuters are exposed to, identify
high exposure regions, relate the ex-
posure level to the percent of the com-
muting population exposed, supple-
ment health effects data, and assess
the effects of road improvements and
transportation control measures. To
keep the model a manageable size,
certain assumptions were made. The
modeling package consists of two doc-
uments: A User's Guide that describes
the program execution; and a manual
that explains the exposure method-
ology.
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 doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
A topic of increasing concern is the high
air pollutant levels observed on roadways
Commuters are most affected since they
drive during heavy demand periods and
are exposed to the highest pollutant levels.
To investigate this problem, the U.S. En-
vironmental Protection Agency (EPA) com-
missioned a study to develop a commuter
exposure modeling methodology. The
methodology designed computes com-
muter exposure statistics through simula-
tion of the traffic, vehicular emissions, and
atmospheric dispersion of roadway-related
air pollutants
Results
The commuter exposure model can be
used to assess the pollutant levels com-
muters are exposed to in various metro-
politan areas. Since the model treats the
spatial variation of exposure, regions of
the city in which commuters experience
high exposures can be identified from
model output If a single commute pathway
is of interest that pathway can be examined
in detail. The model can relate exposure
level to the percent of the commuting
population exposed The length of ex-
posure time is also readily available for use
in health effects studies. The model not
only determines absolute exposure levels,
but also assesses the effects of road
improvements and transportation control
measures.
The commuter exposure modeling meth-
odolgy simulates the exposure to which
commuters are subject by considering
individually and collectively three distinct
subjects affecting exposure: traffic flow,
emissions, and atmospheric dispersion.
The traffic characteristics that affect ex-
posure are modeled, the resulting traffic
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parameters are translated into emission
rates, and the dispersion of the emissions
by the atmosphere are simulated. The
emission rates are computed from the
emission factors and traffic parameters.
The commuter exposure modeling pack-
age consists of two programs that are to
be run separately on the computer. An
emission factor computation program,
called the emissions preprocessor (PREPRS),
was created to facilitate incorporation of
future emission factor methodology up-
dates. The main model, CEMAP, reads the
output of PREPRS (as well as other data)
and simulates traffic flow, computes emis-
sion rates, and simulates dispersion.
The commuter exposure model is cap-
able of producing two kinds of statistics:
(1) so-called short-term statistics, concern-
ing a single commute; and (2) annual
statistics, which describe average exposure
or the frequency of occurrence of exposure
throughout a year. In both the annual and
short-term modes, the model allows the
user to compute exposures for the "average"
commuter on each of the pathways. (The
"average" commuter is the driver who
commutes mainly during the peak travel
period.) In computing the annual and
short-term exposures, the model takes
into consideration the time period of the
commute. Annual statistics require com-
putations for each of the various meteoro-
logical and traffic condition combination,
while short-term statistics apply to mete-
orological and traffic conditions occurring
during a specific commute.
The commuter exposure modeling pack-
age consists of two documents. The
User's Guide describes program execution
and provides the user with the information
needed to run the program, and the second
document is a detailed description of the
model methodology and code.
Model Limitations and
Recommendations
Certain assumptions had to be made to
produce a practical, useful model of rea-
sonable size and moderate running cost
Care was taken that such assumptions
would have a relatively minor effect on the
model output
First and foremost, the model does not
treat all commuters, especially not those
with short commutes and commutes on
less popular routes. While ideally all com-
muters should be treated, the number of
origin-destination zone pairs in a major
metropolitan area is far greater than can be
reasonably handled. Another potential
model limitation is the semi-objective
method by which commute routes are de-
fined. A background familiarity with the
area to be modeled and experience in traffic
modeling will make better choices of com-
mute routes possible, and the local trans-
portation agency can help select the
commuting routes.
To keep the calculation of annual aver-
age exposures within reasonable computa-
tional bounds, no peaks (other than diumal)
are allowed in the traffic distribution. If the
volume input to the model is annual average
daily traffic (AADT), the lack of peaking
characteristics is not a problem. However,
if average daily traffic (ADT) is input the
lack of weekly and seasonal peaks could
affect the annual average exposures. This
effect can be alleviated by adjusting ADT
to AADT using seasonal factors when
computing annual average exposures.
Furthermore, the acceleration and de-
celeration rates were assumed to have a
constant value, although effect assumption
is expected to be minor. Single, constant
rates may be chosen that reflect the average
emissions from a distribution of rates.
Three assumptions are necessary in
computing the emission rates to keep the
number of model computations and the
computer storage required at realistic levels
First an annual average ambient air tem-
perature is used when the model is in the
annual mode. Next in the morning all
vehicles on the commute routes are as-
sumed to be in a "warmed-up" mode of
operation. (The vehicles reached this state
while traveling to the beginning of the
commute route.) Finally, when computing
pollutant concentrations resulting from
nonpathway sources, fixed cold-start and
hot-start percentages are assumed for the
morning and evening commutes for each
locale type
Four potential model limitations concern
the simulation of atmospheric dispersion.
The Gaussian dispersion formulations pre-
sented preclude treatment of fumigation
or stagnation conditions. The Hanna-
Gifford dispersion treatment used to com-
pute concentrations resulting from non-
pathway sources assumes that the emis-
sion rate in grid squares adjacent to the
receptor square is the same as the emission
rate in the receptor grid square The effect
of precipitation is ignored although some
scavenging or rainout is sure to occur.
Finally, on-roadway and in-vehicle concen-
trations are assumed to be equal, even
though use of the windows, air conditioner,
heater, etc., in the vehicle will affect this
relationship.
P. B. Simmon and R. M. Patterson are with Atmospheric Science Center, SRI
International, Menlo Park, CA 94025.
William B. Peterson is the EPA Project Officer (see below).
The complete report consists of two parts entitled, "Commuter Exposure Model:"
"Descript/oq of Model Methodology and Code," (Order No. PB 83-215 566;
Cost: $13.50, subject to change).
"User's Guide," (Order No. PB 83-214 494; Cost: $14.50, subject to change).
The above reports 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
irU.S. GOVERNMENT PRINTING OFFICE: 1383-659-017/7146
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Step
1A
Step
IB
Prepare
Test
Sample
Step
1C
Prepare S9
Mix
Step 1
Maintain
Cell Cultures
Step 2
Step
2
Treat Test
Cultures
Prepare Test
Cultures
Step 3
Prepare Test
Sample
Step
3
Incubate
Plates
Step 4
Treat Test
Cultures
Step
4
Figure 1. Ames assay steps.
ronmental Assessment Biological
Tests. EPA-600/8-81-024, NTIS
PB 82-228966, Litton Bionetics,
Inc., Kensington, MD, October
-1981, 150pp.- -----
Collect and
Analyze Data
(Plate Counts)
StepS
Collect
Cytotoxicity Data
Step 6
Evaluate Data
Figure 2. CHO clonal toxicity assay steps.
D. Brusick, R. Young. B. Myhr. and D. Jagannath are with Litton Bionetics, Inc.,
Kensington, MD 20895.
Raymond G. Merrill is the EPA Project Officer (see below).
The complete report, entitled "Quality Control and Quality Assurance Procedures
for Level 1 Health Effects Bioassays,"(Order No. PB 84-111228; Cost: $13.00,
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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
-.•U.S. GOVERNMENT PRINTING OFFICE: 1983-659-017/7234
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Step 1
Step 2
Step 3
Collect
Macrophage
Step 1
Prepare RAM Cell
Suspension
StepS
Step 6
Figure 3.
Step 2
Step 3
Collect and Evaluate
Data
Rabbit alveolar macrophage
(RAM) cytotoxicity assay steps.
Step 4
StepS
Figure 4.
Care for Animals
Prepare and Administer
Compound
Evaluate Quanta/
Toxicity and
Animal Observations
Analyze Data and
Designate Toxicity
In vivo rodent toxicity asSayste"ps.
Evaluate Quantitative
Toxicity and
Animal Observations
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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