United States
                     Environmental Protection
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).

                      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

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
  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
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
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
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
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

                                                                       U.S. OFFICIAL MA>
Environmental Protection

Cincinnati OH 45268

x^- 	 <\
••———. /rj- -^X PENALTY
! AFR|3'8/ j °Pi'ATE
••'- — "- - \ /ase s3oo
\ /:.,.*
r>«.,..=i p..c.n»cc ---~— V^HJU/ VzsbYuV

^ 0 .2 2 :
Penalty for Private Use $300

         0000329  PS


         230 S DEARBORN STREET
         CHICAGO            IL  60604