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 ------- 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 ------- 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 ------- 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 ------- |