United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-87/023 Aug. 1987
SERA Project Summary
Estimating Inaccessible Source
Strengths Using Tracer
Techniques: A Sensitivity Analysis
Joseph A. Catalano
Estimation of the strength of inac-
cessible sources of effluent is a difficult
problem. A method using a line source
of tracer material emitted along the
outer boundary of the area source has
been proposed in the literature. The
present work uses model calculations
to analyze the sensitivity of that method
to several factors. The most important
of these factors are atmospheric sta-
bility, wind direction, and horizontal
separation and height differences
between the tracer and the effluent
sources.
This Project Summary was dew/oped
by EPA'* Atmospheric Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that It fully docu-
mented In a separate report of (he same
title (see Project Report ordering In-
formation at back).
Introduction
A major problem facing agencies
charged with enforcement of emissions
controls for fugitive emissions is the
estimate of the strength of an inaccessible
source. Usually such sources may be
treated as area sources even though they
are in reality a collection of random point
sources. For a variety of reasons, such
sources are often inaccessible. How then,
does one estimate the strength of such a
source? Ludwig, Listen, and Salas
(Atmospheric Environment, Vol. 17, No.
11 PP 2167-2172, 1983) showed how
this might be done using tracer
techniques.
If a line source of known strength were
to be placed immediately downwind of
the area source, and measurements made
of the concentration of both the effluent
from the area source and of the tracer,
then an estimate can be made relatively
easily of the unknown area source
strength. The present work is an attempt
to ascertain the sensitivity of the approach
to several factors. These are:
• Configuration of the Tracer Source
• Size of the Area Source
• Relative Positions of the Tracer and
Area Sources
• Differences in Release Heights for
Tracer and Effluent
• Wind Velocity
• Atmospheric Stability
Rather than do a model exercise ex-
clusively, the method chosen to estimate
the sensitivity involved the use of the
data collected by Ludwig et al. After
screening the data for appropriate cases,
the authors chose a particular data set
and did all of the analysis in that context.
Results
The sensitivity analysis showed that
the method of Ludwig et al. is sensitive to
some of the chosen factors and not to
others. For example, in the actual field
realization of the method, a moving point
source (a vehicle) was used rather than a
true line source. The sensitivity of the
method to this procedure was tested by
using a Gaussian puff model to represent
the moving point source and comparing
the results with those from a calculation
using a true line source. This comparison
yielded little difference in correlation with
the measured concentrations. This test
used 1, 4, 8, and 16 trips per hour to
simulate the moving vehicle with little
discernable difference in the final cor-
relations. Thus, the analysis shows that a
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line source can be approximated by a
moving point source, and the frequency
of the trips made by the moving source
does not seem to be important. Further,
the study also showed that using the EPA
model PAL-DS (Point Area Line Source
model with Deposition and Sedimenta-
tion), very little difference was found
between concentrations emitted from a
line and an area source beyond about 1
km from the source. Beyond about 3 km,
the difference in modeled concentrations
between these source types and a point
source was also very small. This analysis
was done for stability class E, with a 1.8
m/s wind.
Since the size of the area source is not
always known, the effect of this size on
the estimate of the source strength was
examined for a range of sizes from 5000
to 129,600 sq.m. When the, size of the
area source exceeded 25,000 sq. m., the
method was no longer very sensitive to
the size of the area source.
The distance between the tracer line
source and the area source of effluent
was found to have a small effect upon
the estimated strength of the area source.
In general the farther the line source was
from the area source in the direction of
the samplers, the smaller the estimate of
the source strength. The farther the line
source was on the side away from the
samplers, the larger the source strength.
Thus, the effect seemed to be that the
farther the line source was from the
samplers, the larger the source strength
estimate.
As might be expected the effect of
wind direction had a major effect on
estimates of the source strengths. When
the wind was outside the range of plus or
minus 10 degrees, the estimates of the
source strength were too large. Wind
speed showed some effect with the
estimates of strength increasing with
increasing speed.
The effect of atmospheric stability is
very large. When the stability was
changed from category E, the estimates
of source strength increased as the
atmosphere became more unstable. The
method was originally recommended to
be used under stable conditions with
light but continuous winds.
Conclusions
This sensitivity study showed that the
method of Ludwig, Listen, and Salas, is
quite sensitive to atmospheric stability
and should only be used as originally
recommended, that is, during stable con-
ditions. The method is also sensitive to
several other factors. In addition to an
obvious sensitivity to wind direction, the
method is sensitive to wind speed and a
difference in release heights between
the tracer and the effluent. Representation
of the moving tracer source by a fixed
line source is completely justified when
the averaging time for the sampling is
long enough. Since the effluent con-
centrations are likely to be low, long
sampling times will be necessary to
achieve reliable observations of these
concentrations. Thus, using a moving
point source to approximate a line source
is acceptable. Finally, it seems that a field
experiment with a known area source
emission rate would be a very valuable
test of the method.
Joseph A. Catalano is with Aerocomp, Inc., Costa Mesa. CA 92626.
Francis S. Binkowski is the EPA Project Officer (see below).
The complete report entitled "Estimating Inaccessible Source Strengths Using
Tracer Techniques: A Sensitivity Analysis," (Order No. PB 87-199 683/AS;
Cost: $13.95. 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
Agencv
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use S300
EPA/600/S3-87/023
OC00529 PS
U S CltVI* PROtECTIQM
CHICAGO
It- 6060*
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