United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/064 Nov. 1985
Project Summary
Green River Air Quality Model
Development: VALMET—A
Valley Air Pollution Model
C. D. Whiteman and K. J. Allwine
Pacific Northwest Laboratory has
developed an air quality model for
application in valleys as part of the U.S.
Environmental Protection Agency (EPA)
.Green River Ambient Model Assess-
ment program. The purpose of the
program is to provide air quality assess-
ment tools applicable in the Green River
Oil Shale Formation region of western
Colorado, eastern Utah, and southern
Wyoming. This region has the potential
for large-scale growth because vast
energy resources, especially oil shale,
are located in the region.
Following a thorough analysis of
meteorological data obtained from deep
valleys of western Colorado, a modular
air pollution model has been developed
to simulate the transport and diffusion
of pollutants released from an elevated
point source in a well-defined mountain
valley during the nighttime and morning
transition periods. This initial version of
the model, named VALMET, operates
on a valley cross section at an arbitrary
distance down-valley from a continuous
point source. The model has been
constructed to include parameteriza-
tions of the major physical processes
that act to disperse pollution during
these time periods.
This Project Summary was developed
by EPA's 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 infor-
mation at back).
Introduction
This report documents an air quality
model that was developed to predict
concentrations of nonreactive pollutants
arising from elevated continuous point
sources that emit pollutants within well-
defined deep mountain valleys. The mod-
el, termed VALMET, is intended to sim-
ulate the effects on pollutant transport
and diffusion of various meteorological
processes that are thought to result in
worst-case pollutant concentrations. The
model is run for situations when pollut-
ants are carried in locally developed
circulations within a valley when these
circulations are "decoupled" from pre-
vailing circulations above the valley. The
primary physical processes included in
the model follow:
Nocturnal Simulation:
• transport by down-valley drainage
flows,
• plume channeling within the valley,
• enhanced horizontal and vertical dif-
fusion due to topography,
• plume reflections off valley floor and
sidewalls,
• pollutant diffusion out the top of the
valley, and
• dilution of the plume due to clean air
inflow from tributaries.
Post-Sunrise Simulation
During Temperature Inversion
Breakup Period:
• convective boundary layer growth,
• plume subsidence in the valley inver-
sion,
• fumigation into growing convective
boundary layers, and
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• transport and diffusion in upslope
flows over the sidewalls.
Overview
The model, while including a variety of
meteorological processes, is highly pa-
rameterized so that it is simple in concept
and easy to run. It is composed of 13
modules, or subroutines, arranged in
such a way that an improved understand-
ing of individual valley meteorological
'phenomena can be easily incorporated in
future versions of the model. The modules
within the model can be replaced by data
if they are available. Thus, the model can
be used in one of two modes. It can be
used in a "screening" mode to calculate
pollutant concentrations within a valley
when little site-specific data are available,
or it can be "calibrated" with site-specific
data so that it can be used as a site-
specific model.
The two-dimensional model was de-
veloped primarily to predict pollutant
concentrations on the valley floor and
sidewalls on a valley cross section an
arbitrary distance down-valley from a
pollutant source during the post-sunrise
temperature inversion breakup period. It
is necessary, however, to know the air
pollution concentration within the valley
cross section at sunrise, as an initial
condition for the post-sunrise simulation.
The model is therefore comprised of two
parts—a nighttime part to predict con-
centrations on the valley cross section at
sunrise, and the daytime part which
predicts concentraions on the valley floor
and sidewalls during the post-sunrise
temperature inversion breakup period.
The temperature inversion breakup period
has been identified by previous inves-
tigators as a period when diurnal fumiga-
tions can produce high pollutant con-
centrations in valleys.
The nighttime simulation, which is
applied during the steady-state period
after valley temperature inversions and
drainage wind systems have become
established, uses a modified valley-fol-
lowing Gaussian plume algorithm to
calculate air pollution concentrations for
points on the valley floor and sidewalls. A
plume rise formulation is used to sim-
ulate the initial rise of a pollutant plume
at the stack due to momentum and
buoyancy of the effluent. Pasquill-Gifford
diffusion coefficients are modified to
accountfor enhanced nocturnal diffusion
caused by rough terrain. The Gaussian
plume is also modified to allow for dilution
of the plume during its down-valley
transport caused by clean airflowing into
the plume from valley tributaries or by
converging downslope drainage flows.
An integral constraint on pollutant mass
is applied to ensure that pollutant mass is
conserved during the plume's transport
down the valley and within any valley
cross section down-valley from the emis-
sion source, except for pollution diffusion
out the top of the valley.
The daytime simulation uses numerical
techniques that simulate the fumigation
of the nocturnal plume onto the valley
floor and sidewalls as a convective bound-
ary layer grows upwards from the heated
valley surfaqes and as subsiding motions
occur over the valley center after sunrise.
The rate of growth of convective boundary
layers and subsidence within the valley
temperature inversion are simulated us-
ing the bulk thermodynamic model of
Whiteman and McKee. This model is
driven by sensible heat flux, estimated as
a fraction of the solar radiation using a
highly parameterized surface energy
budget. The effects of such factors as
snow cover, soil moisture, cloud cover, or
surface albedo are not explicitly included
in the model but can be incorporated into
the model in the future through an
expanded energy budget module. The
shape of the topographic cross section of
the valley is explicitly included in the
model through the valley floor width and
sidewall inclination angles at the valley
cross section of interest. The retarding
effect on temperature inversion breakup
and pollution dispersion due to warm air
advection above the inversion is also
included in the model. Fumigated pollut-
ants are transported from the valley cross
section in upslope flows that develop
within the convective boundary layers
over the slopes. Pollutants are diffused
through model grid elements during this
transport up the slopes in the growing
convective boundary layer. Pollutant con-
centrations decay exponentially within
individual grid elements high on the
sidewall as they are dropped from the
simulation as the inversion top subsides
below them.
The output from the nighttime simula-
tion includes the steady-state pollutant
concentration at valley floor and sidewall
grid elements on the valley cross section
of interest. The fraction of plume mass
that has diffused out the top of the valley
during the plume's travel is also an output
of the model. Since an analytical formula
describes the concentrations within a
valley cross section, cross valley and
vertical profiles of pollutant concentration
can be calculated and plotted. The plume
centerline concentration is an output of
the model.
The primary output of the daytime
simulation is the maximum 1- and 3-h
average pollutant concentrations in each
of the model grid elements on the valley
floor and sidewalls. The time-varying 5-
min average concentrations for each of
the grid elements between sunrise and
the time of inversion destruction is also
an output of the model. In addition to
these primary outputs, intermediate mod-
el outputs come from individual modules
in the program. The local standard time of
sunrise, the duration of the daylight
period, and the solar flux on a horizontal
surface at solar noon come from the solar
module. The convective boundary layer
height and inversion top height as a
function of time come from the temper-
ature inversion breakup module.
Twenty-seven input parameters are
necessary to drive the model. These input
parameters include the date, site location,
topographic characteristics of the valley
cross section, temperature inversion char-
acteristics at sunrise, emission and stack
characteristics, down-valley wind
speed(s), atmospheric stability, grid ele-
ment length, and sensible heat flux
parameters. If known, the rate of warm
air advection above the valley can be
input. The necessary model inputs can be
obtained from topographic maps, engi-
neering information on the pollutant
source, and one or more seasonal meteor-
ological data collection campaigns in the
valley of interest using tethered balloon
data collection systems and/or doppler
acoustic sounders.
Conclusions and
Recommendations
The model shows promise for use as a
planning tool and eventually as a regu-
latory tool. Further development, testing,
and tracer evaluation of the model will be
necessary before sufficient confidence
can be gained to justify the model's use in
a regulatory setting. The priorities for
further development and testing are
provided in the report. Testing of the
model's sensitivity to input parameters
and an initial evaluation of the model with
tracer experiment data are high priority
tasks. These tests will, no doubt, result in
future modifications to the initial version
of the model.
The authors stress that the model's
ultimate utility in addressing and provid-
ing solutions to potential air pollution
problems in mountain valleys will depend
on the further evaluation of the model. To
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have confidence in model predictions, it is
necessaryto test the model against actual
air pollution data. Several parameters in
the model (A0, k, cry, and CTZ) are, at
present, poorly understood for mountain
valleys due to a dearth of experimental
data, and theoretical research should be
focused on the need for information on
both turbulent diffusion and valley energy
budget studies. The use of full physics
models may help in providing some of the
a nswers necessary to i mprove the present
model.
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C. David Whiteman and K. Jerry Allwine are with Battelle. Pacific Northwest
Laboratory, Rich/and. WA 99532.
Alan H. Hubor is the EPA Project Officer (see below).
The complete report, entitled "Green River Air Quality Model Development
VALMET—A Valley Air Pollution Model," (Order No. PB 86-104 106/AS; Cost:
$16.95, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-85/064
0000329 PS
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