I/.
&EFK
United States
Environmental Protection
Agency
Environmental Sciences Research ^*
Laboratory '.
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-82-078 Oct. 1982
Project Summary
Estimating Concentrations
Downwind from an
Instantaneous Puff Release
William B. Petersen
This document provides an ap-
proach to estimating concentrations
downwind of an instantaneous puff
release. Dispersion of the puff is
described by the Gaussian puff equa-
tion, using the dispersion parameters
presented in Meteorology and Atomic
Energy. The primary purpose of this
work is to provide estimates of the
instantaneous peak concentration or
average concentration through the
use of simple equations and nomo-
grams. Example problems are pro-
vided to demonstrate the use of the
equations and nomograms. A com-
puterized Gaussian puff model is also
presented.
This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory. Research Tri-
angle Park. NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
' ordering information at back).
Introduction
This document is provided m re-
sponse to the need for making disper-
sion estimates of accidental releases of
hazardous chemicals. Every year there
are news stories of industnal-or trans-
portation-related spills requiring the
evacuation of nearby residents. The
main objective of this work is to provide
the air pollution specialist a meth-
odology for estimating peak concen-
trations downwind of an instantaneous
release.
The equations, nomograms, and
methodology described in this docu-
ment were not designed for the fireman
at the site of an accidental spill. He will
not have the time or the technical back-
ground to use the equations and
nomograms properly. The factors af-
fecting his decision to order an evacua-
tion will be based primarily on the size of
the spill, and the nature and toxicity of
the substance. However, material provided
in this document would be valuable to
the air pollution specialist who is
assisting the fire department in estab-
lishing procedures to determine toxic
corridors. The dispersion algorithms
described would provide the basis for a
model catering to the specific needs of
the user A computer algorithm de-
scribed in this document could also be
adapted for on-site use at a chemical
plant using real time meteorology to
predict downwind concentrations.
Approach
The document is composed of three
major sections. The first part of Section
2 describes a technique for estimating
peak concentrations of instantaneous
puff releases as a function of atmos-
pheric stability and downwind distance.
This discussion was designed so that
within minutes after a spill, estimates of
peak concentration could be provided to
the agencies directly concerned with
the immediate health and welfare of the
public. These initial estimates could be
provided without the aid of a computer.
The remainder of Section 2 is concerned
* US GOVERNMENT PRINTING OFFICE 1982-559-017/0856
-------
with estimating average concentrations
and exposure. Section 3 provides a
discussion of a generalized Gaussian
puff equation with the same assump-
tions and limitations of the approach
described in Section 2. The equations in
Section 3 are the basis for a computer
model described in Section 4.
The modeling effort is significantly
simplified if only the magnitudes of the
concentrations must be predicted,
without regard to exactly where the
concentrations will occur. If concentra-
tion estimates are to be made at specific
points, it is important to account for puff
trajectory; and, because methods for
determining the trajectory of the puff
contain deficiencies, estimating con-
centrations at point locations is very
difficult. Releases are seldom point
sources, but are more typically small
area sources. Small area source re-
leases can be modeled to some extent
by using initial dispersion parameters.
The initial horizontal dispersion is
typically calculated by dividing the initial
horizontal dimension of the area source
by 4.3. This method will give reasonable
concentration estimates at downwind
distances greater than about five times
the horizontal dimension of the source.
As with the lateral dispersion, the initial
vertical dispersion parameter is typi-
cally evaluated by dividing the initial
height of the source by 2.15.
Many sources are not truly instan-
taneous. However, if the travel time is
long compared to the release time, then
the puff model can be used to estimate
concentrations. If the travel time to a
receptor is on the same order or smaller
than the release time, the Gaussian
plume model can be used. Where the
release time is finite but smaller than
the travel time, the instantaneous puff
model will provide a worst-case esti-
mate of peak concentrations for the
same total release. It is difficult to
evaluate the effect on average concen-
trations over a given sampling time for a
finite release using simple models,
since the puff is distorted in the
downwind direction.
Recommendations
The model estimates are appropriate
for travel times where the atmosphere
can be assumed to be steady state. A
number of factors affect the length of
time the atmosphere will remain
approximately steady: season of the
year, latitude, synoptic conditions, and
local meteorology such as sea breeze
and terrain flows. During periods of
transition (such as sunrise or sunset),
the stability, mixing height, wind profile,
wind speed, and wind direction are
likely to change; therefore, particular
care should be exercised in character-
izing the atmosphere during these time
periods. For situations where the travel
time of the puff is greater than an hour,
the factors which affect the dispersion
and trajectory of the puff should be
incorporated in the concentratioh
estimates.
The Gaussian instantaneous puff
model is applicable for neutrally buoy-
ant nonreactive releases. The dis-
persion of a small negatively buoyant
release can be modeled using the puff
equation, provided the material mixes
rapidly with the ambient air and
disperses like a neutrally buoyant
source.
The EPA author William B. Peterson is with the Environmental Sciences
Research Laboratory, Research Triangle Park, NC 27711.
The complete report, entitled "Estimating Concentrations Downwind from an
Instantaneous Puff Release," (Order No. PB 82-261 959; Cost: $9.00,
subject to change) will be available only frorh:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer cay? be contacted at:
Environmental fdfences 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
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS 0000329
------- |