United States
Environmental Protection
Agency
Atmospheric Sciences *> ,f
Research Laboratory * • i
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-86/069 Mar. 1987
&ER& Project Summary
Comprehensive Field Study
Plan to Relate Pollutant
Sources to Acidic Deposition:
A Preliminary Study of
Uncertainties
D. A. Stewart, J. E. Langstaff, G. E. Moore, S. M. Greenfield, M. K. Liu,
D. J. McNaughton, N. E. Bowne, R. Kaleel, and M. K. Anderson
An experimental program was de-
signed to empirically relate acidic
deposition to precursor emissions.
Several technical issues requiring fur-
ther study prior to field experiments
were raised. Preliminary estimates of
uncertainty were made in order to
assess confidence in the experimental
design. The five general areas studied
included uncertainties in measurements,
local scale data analyses, regional scale
data analyses, model simulations and
data analyses for regional experiments.
Measurement uncertainties are large
compared to deposition losses for gases
on the local scale. On a regional scale,
the existing ambient sulfate measure-
ment network has a resolution of order
500 km which is adequate, but char-
acteristic spacing of SO2 patterns re-
quires resolution of less than 100 km.
Model simulations indicated the fre-
quency of tracer detectability at a
receptor from a specific source was
small and limited by meteorology. Also,
the frequency of detectability is depen-
dent on source strength. Local source
modulations were modeled and attain-
able modulation signals were found to
be of insufficient magnitude to be
detected over background concentra-
tions when measurement uncertainties
were considered. RjMUta from these
analyses of the effect* of uncertainty
were considered hi th*!ftn*l experi-
mental design.
This Project summary was developed
by EPA's Atmospheric Sciences
Research Laboratory, Research
Triangle 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
The Atmospheric Sciences Research
Laboratory of U.S. Environmental Protec-
tion Agency funded a program to design
an experimentally based study to provide
empirical relationships relating acidic
deposition in ecologically sensitive areas
to sources of precursor emissions. In
addition, the experiments are to provide a
data base for evaluation of regional acidic
deposition models. The program has been
named the comprehensive experiment or
COMPEX and the design plan is presented
in a companion report entitled, "Com-
prehensive Experimental Design Plan to
Relate Pollutant Sources to Acidic Deposi-
tion." In the course of designing the
program, several questions arose on
technical issues requiring further study
prior to conducting the experiments. This
report describes preliminary studies per-
formed to clarify these issues and increase
the confidence in success of the COMPEX
plan. Studies are divided into five general
areas: 1) summary of measurement un-
certainties, 2) local scale data analyses,
3) regional scale data analyses, 4) model
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simulations, and 5) data analyses from
the regional experiments.
Measurement Uncertainties
A primary consideration in COMPEX is
the ability to design a program which
would provide empirical source/receptor
relationships within reasonable uncer-
tainty levels. The first component of the
uncertainty studies is a review of uncer-
tainties associated with measurement
techniques required in the design.
COMPEX requires new experimental
techniques or new applications of pre-
viously used techniques. The report sum-
marizes information on uncertainties
associated with systems to be used in the
study with the exception of PMCP per-
fluorocarbon tracer measurements pro-
posed for use. Feasibility experiments
proposed in the COMPEX design include
studies of this tracer. .
Local Scale Analysis
Uncertainties
The second element of the uncertainty
studies involved examination of local scale
data to better understand the temporal
and spatial characteristics of concentra-
tions and the relationships among pol-
lutants and tracers. The study analyzed
data from the Electric Power Research
Institute's (EPRI) Plume Model Validation
and Development (PMV+D) experiments
within 20 km of the Kincaid power plant.
Results can be summarized as follows:
• Within local scale distances « km),
ambient concentrations of sulfur di-
oxide, oxides of nitrogen, and inert
tracers are strongly related when there
is no interference from background.
Ambient concentrations respond to
variations in emission rates.
• Close agreement of concentration data
among pollutants and tracers indicates
that depositional losses within 20 km
of sources are negligible and within
the measurement uncertainty.
• Uncertainty in experimental measure-
ments is large.
Primarily, the results indicate the dif-
ficulty in detecting deposition effects over
short distances and the need, when
simulating sources, to match the tracer
release rates to the actual source emis-
sions rates.
Regional Scale Analysis
Uncertainties
Ambient concentration data are avail-
able on a regional scale from the EPRI
Sulfate Regional Experiment (SURE) and
the data provides a data base for studying
concentration relationships on a regional
scale. The primary product of the regional
data analysis is an evaluation of the scale
of the spatial concentration patterns and
the required resolution for sampling in a
program such as COMPEX. The spatial
resolution of the ambient sulfate concen-
trations is of the order of 500 km which
indicates the adequacy of both the SURE
network and the proposed COMPEX
monitoring grid. SURE data were not of
adequate resolution to determine the
characteristic spacing of the SO2 patterns.
Data indicate that the scale of patterns is
less than 100 km. The SURE data analysis
also allowed an estimate of the uncer-
tainties in representing spatial concen-
trations with mean values from point
measurements. The analysis provided a
means of studying the errors involved
with spatial averaging but also indicated
difficulties in detecting concentrations
changes resulting from local source
modulation experiments.
Model Simulations
Numerous uncertainty questions were
studied using regional model simulations.
First, relative to the long range tracer
experiments, the simulations indicated
that point source releases of tracers in
transport studies did not adequately de-
scribe the resultant tracer or emissions
distributions from large emission areas.
In addition, the simulations suggested
tracer release rates which are adequate
to assure detection at large source/
receptor separation distances. The fre-
quency of detectability was analyzed as a
function of these rates and multiples of
concentration over background levels. The
frequency of detectability or the frequency
of source receptor interactions is in gen-
eral small and limited by meteorology.
The frequency is reduced when emission
levels for the tracers are reduced. The
rate of reduction is larger for tracer re-
leases which are intermittent rather than
continuous.
Small scale simulations were performed
to evaluate the feasibility of local source
modulation and deposition experiments.
Results indicated that planned emissions
modulation may not be of sufficient
magnitude to be detected over background
concentrations. Supplementary results
indicate that the time series analyses of
the modulation patterns may likewise be
insufficient to provide a detectable
modulation signal over temporal cycles
in the concentration data. Model simula-
tions relative to source depletion and
mass balance techniques for estimation
of dry deposition rates may also be
hampered by problems with the detec-
tability of deposition losses over local to
mesoscale distances.
Data Analysis from
Regional Experiments
Analyses of data from other more
specialized experimental programs were
performed to evaluate aspects of the
COMPEX design. Limited data available
for analysis from the CAPTEX program
was used to evaluate the feasibility of
using ground level tracer concentration
data to estimate trajectories for the trans-
mittance approach described in the
COMPEX plan. The CAPTEX tracer data
indicated that the tracer data could be
used to provide trajectory information
using a sampler network with the resolu-
tion of that proposed for the COMPEX
experiments.
Data from the SURE program and the
MAP3S precipitation chemistry program
were analyzed to examine the repre-
sentativeness of a one year experimental
program in generating empirical source/
receptor relationships and potential
categories for use in statistical analyses.
The data suggest that the use of a single
year period for an empirical analysis may
not be satisfactory. Meteorological cate-
gorization schemes require additional
study and need to consider broad classes
of data to provide adequate sample sizes.
Data collection activities in the COMPEX
program require both modifications to
increase the statistical data base and to
relate the program to previous data col-
lection efforts.
The last element of the uncertainty
analysis is an analysis of data from the
ACURATE experimental program to deter-
mine the frequency of source/receptor
interactions. ACURATE examined the
long-range transport of krypton-85 re-
leases over a one and one half year
period. The data show a surprisingly small
frequency of interaction between a point
release and single receptors. The rela-
tionship decreases with distance which
emphasizes the need for program modifi-
cations to increase the sample size of the
COMPEX data base.
Conclusions
The uncertainty studies were performed
in parallel with modifications of the
COMPEX plan. Numerous suggestions
from the studies were incorporated in the
plan, particularly in the areas of dis-
tributed tracer releases, release rates,
release configuration, and sampling
resolution.
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Some of the studies undertaken in
support of the preliminary plan my in-
corporate assumptions which differ
slightly from those of the final experi-
mental plan but the general results still
provide a valuable contribution in deter-
mining feasibility and expected results of
the experiments.
D. A. Stewart. J. E. Langstaff. G. f. Moore, S. M. Greenfield, and M. K. Liu
are with Systems Applications, Inc., San Rafael, CA 94903; D. J. McNaughton,
N. E. Bowne, R. Kaleel, and M. K. Anderson are with TRC Environmental
Consultants, Inc.. East Hartford, CT06108.
Francis Pooler. Jr., is the EPA Project Officer (see below).
The complete report, entitled "Comprehensive Field Study Plan to Relate
Pollutant Sources to Acidic Deposition: A Preliminary Study of Uncertainties,"
(Order No. PB 87-140 943/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
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