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
------- |