United States Environmental Protection Agency Atmospneric Sciences Research Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S8-86/040 Mar. 1987 &EPA Project Summary User's Guide for PEM-2: Pollution Episodic Model (Version 2) K. Shankar Rao The Pollution Episodic Model Version 2 (PEM-2) is an urban-scale model designed to predict short term average ground-level concentrations and deposi- tion fluxes of one or two gaseous or paniculate pollutants at multiple recep- tors. The two pollutants may be non- reactive, or chemically-coupled through a first-order chemical transformation. Up to 300 isolated point sources and 60 distributed area sources may be considered in the calculations. Con- centration and deposition flux estimates are made using hourly mean meteoro- logical data. Up to a maximum of 24 hourly scenarios of meteorology may be included in an averaging period. The concentration algorithms and computational techniques used in the PEM-2 program are described and input/output parameters, optional features, capabilities, and limitations of the model are discussed. This Protect Summary was developed by EPA't Atmospheric Sciences Re- search 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 In- formation at back). Introduction The Pollution Episodic Model Version 2 (PEM-2) is an urban-scale air quality model designed to predict short term ground-level concentrations and deposi- tion fluxes of one or two, gaseous or paniculate, reactive or non-reactive pol- lutants in an urban area with multiple point and area sources. PEM-2 uses the concentration algorithms which explicitly account for the effects of dry deposition, gravitational settling, and a first-order chemical transformation. These algo- rithms, derived analytically from a gradient-transfer model, are based on Gaussian plume modeling assumptions. For chemically-coupled pollutants, the surface concentrations and deposition fluxes of both the primary (reactant) and the secondary (reaction product) pollutant are calculated. The complete report contains informa- tion directed to the model user and the programmer. It presents an overview of the PEM-2's capabilities, assumptions, and limitations. Detailed technical dis- cussions on the theoretical basis of the model are presented. Details of the pro- gram's input and a guide to output are included. Input and output for test examples using the various features of the model, flow diagrams, and a complete listing of the computer program are also included. Theoretical Basis The concentration algorithms used in PEM-2 are derived from analytical solu- tions of a steady state gradient-transfer (K-theory) model, which describes the atmospheric transport, diffusion, deposi- tion, and first-order chemical transforma- tion of gaseous or paniculate pollutants from an elevated continuous point source. The eddy diffusivity coefficients in these analytical solutions are expressed in terms of the empirical Gaussian plume dispersion parameters, so that the latter can be conveniently specified as functions of the downwind distance and the atmo- spheric stability class within the frame- ------- work of the standard turbulence-typing schemes. The point-source concentration algorithms for the primary (reactant) and the secondary (product) pollutants are presented for various stability and mixing conditions of the atmosphere. In the limit when deposition and settling velocities and the chemical transformation rate are zero, these algorithms reduce to the well-known Gaussian plume dispersion algorithms. In PEM-2, the ground-level concentra- tions of the primary and secondary pol- lutants resulting from urban area source emissions are computed by numerical integration of the corresponding point- source concentration algorithms. For point sources, PEM-2 provides options for using the standard or new plume rise formulations by Briggs and new schemes for plume penetration of an elevated stable layer. For both point and area sources, PEM-2 uses the Briggs expres- sions for urban dispersion parameters. The capabilities of PEM-2 are as follows: 1. PEM-2 is an urban-scale air quality model applicable to downwind dis- tances of up to 50 km. Up to a maxi- mum of 300 point sources and 50 area sources can be included in the model inputs to estimate concentra- tions at a maximum of 2500 receptors located on a 50 x 50 square receptor grid. 2. PEM-2 calculates short-term (1 to 24 hr) average ground-level concentra- tions and deposition fluxes of one or two gaseous or paniculate pollutants. 3. The two pollutants may be non- reactive, or chemically-coupled through a first-order chemical trans- formation. If only one pollutant is cal- culated, the effects of a first-order chemical decay can be considered. The chemical transformation (or decay) rate may vary from 0.1 to 100 percent per hour. 4. The deposition (and settling) velocities of the two species may be equal or different. Direct emission of the secondary (reaction product) pollutant can be considered for both point and area sources. PEM-2 is based on steady state Gaus- sian plume modeling assumptions. Some of the important assumptions of PEM-2 are as follows: 1. Concentration estimates may be made for each hour using the mean mete- orological conditions for that hour. Average concentrations for a period longer than an hour are determined in the program by averaging the hourly concentrations of that period. 2. The sources are stationary and the emission rates are constant over the concentration-averaging period. The latter assumption is reasonable since PEM-2 is designed to predict only short-term average concentrations; this assumption is intended solely to limit the amount of input data required by the model. 3. If the hourly emission rates are highly variable over the concentration-aver- aging period, then the average con- centrations may be obtained by averaging externally, with minimal programming, the concentrations cal- culated hourly and stored on tape by the model. This can be done, for example, to calculate the daily mean concentrations with diurnally varying emission rates. 4. Total concentration at a receptor is the sum of the concentrations calculated at the receptor from each source; i.e., concentrations are additive. 5. Pollutants released from a stack are transported downwind at a rate equal to the mean wind speed at the physical stack height. The wind direction is constant for each hour. The horizontal wind field is homogeneous and the effects of directional wind shear are neglected. 6. Diffusion of continuous plumes gives time-averaged Gaussian distributions for concentrations in the crosswind and vertical directions. The diffusion in the downwind direction is negligible compared to advection. 7. The reactant and the product species are coupled through a first-order chemical transformation. The deposi- tion and settling velocities of the species, and the chemical transforma- tion rate are constant over the con- centration-averaging period. The diurnal variation of these parameters can be considered, if necessary, by averaging the hourly concentrations as discussed above. 8. Particulate pollutants consist of par- ticles of a known size (or size distribu- tion) with a representative settling velocity. 9. Pollutant concentration at a receptor due to the distributed area sources depends only on sources located in a narrow upwind sector. Therefore, horizontal diffusion can be ignored for area sources. 10. The crosswind variations of urban area source-strength patterns can be ignored. The contributions of mor remote upwind area sources to concentration at a receptor are quite small. For this reason, it is generally adequate to consider only eight area source grid squares immediately upwind of each receptor grid square. PEM-2 is subject to the same basic limitations of any Gaussian plume-type model. General limitations of the model can be summarized as follows: 1. Receptors farther than 50 km down- wind of a source are ignored. 2. The number of point sources is limited to 300, and the number of area sources is limited to 50. The computer program can be easily modified by the user to increase the maximum number of point and/or area sources, if neces- sary. All sources are stationary. 3. The maximum number of scenarios (sets of hourly meteorological data) in an averaging period is limited to 24. PEM-2 is designed to calculate only short-term (1 to 24 hr) average surface concentrations and deposition fluxes of one or two pollutants. 4. PEM-2 does not make any adjustment for differences in terrain elevation between sources and/or receptors. The model assumes level terrain. No adjustments are made for buildin wake-induced downwash, wake trainment, or other building-related effects on the various effluent plumes. 5. Only a first-order chemical trans- formation/decay is considered. The transformation rate, and the deposition and settling velocities of the species, must be specified by the user, if these options are selected. 6. The model cannot perform calculations for calm conditions; if such conditions occur, the wind speed is arbitrarily set to 1 m/s. 7. PEM-2 does not attempt to deal with wet removal processes; hence, the model does not apply during periods of precipitation. Model input is divided into four main categories. Control parameters and various options must be selected, while deposition and settling velocities, and chemical transformation rate constant, if applicable, must be provided by the user. Hourly mean meteorological data required for input include: atmospheric stability, wind speed and direction, ambient tem- perature, mixing height, and temperature gradient in the elevated stable layer. Emission rates for point and/or area sources must be given as well as their location coordinates on the model grid,{ ------- stack parameter information, and source height. Various output options "are available including printer output or tape storage for the fields of predicted concentrations and deposition fluxes. K. Shankar Rao is with the National Oceanic and Atmospheric Administration, Oak Ridge. TN 37831. James M. Godowitch is the EPA Project Officer (see below). The complete report, entitled "User's Guide for PEM-2: Pollution Episodic Model (Version 2)," (Order No. PB 87-132 098/AS; Cost: $24.95. 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 ------- i U.S. OFFICIAL MA> Environmental Protection Agency Information Cincinnati OH 45268 x^- <\ ••———. /rj- -^X PENALTY ! AFR|3'8/ j °Pi'ATE ••'- — "- - \ /ase s3oo \ /:.,.* r>«.,..=i p..c.n»cc ---~— V^HJU/ VzsbYuV U.S.PCSIAGE ^ 0 .2 2 : __ Penalty for Private Use $300 EPA/600/S8-86/040 0000329 PS U S ENVIR PROTECTION AGENCY REGION 5 LIBRARY 230 S DEARBORN STREET CHICAGO IL 60604 ------- |