United States
Environmental Protection
Agency
Research and Development
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
EPA/600/S3-90/059 Aug. 1990
EPA Project Summary
Estimation of Seasonal and
Annual Acidic Deposition through
Aggregation of Three-Day
Episodic Periods
-R,erry,J.,Samsonr JeffreyB, Brook , and Sanfprci Sillman - _ ._ ,,r ^ . _ ,...-
The Regional Acid Deposition Model
(R ADM) simulates the complex physical
and chemical processes involved in the
formation of acidicdeposition. However,
because of the complexity of RADM it
has only been applied to selected epi-
sodes of a few days duration. While the
detail provided by RADM is highly desir-
able to understand the interactions be-
tween emissions and deposition on a
storm-by-storm basis there is also a need
for understand ing the probable long-term
relationship between changing emission
patterns and deposition. The cost of
simulating and interpreting seasonal or
annual deposition patterns using RADM
directly would be high.
A method for aggregating episodic
deposition estimates has been devel-
oped and used to identify meteorologi-
cal situations which merit simulation by
RADM based on their likelihood of pro-
ducing sulfate (SO4=) wet deposition at
multiple locations across eastern North
America, their frequency of occurrence,
and their seasonality. The aggregation
approach is based on four years (1982-
1985) of meteorological and precipita-
tion chemistry data.
The aggregation approach is based
on the stratification of three-day periods
into categories of similar 850 mb wind
flow across eastern North America and
subsequent selection of 30 RADM simu-
lation periods from this stratification
which represent the range of storm pat-
terns present over eastern North
America. The program has also pro-
vided the RADM project with scaling
factors for use in weighing episodic
simulation results to seasonal and an-
nual deposition. The method Is shown
to improve upon the use of a random
selection process in reproducing sea-
sonal and annual wet deposition pat-
terns.
This Project Summary was pre-
pared by ERA'S Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, to announce
keyfindingsof the research project that
is fully documented in a separate report
of the same title (see Project Report
ordering Information at back).
Introduction
The development of policies to ad-
dress problems arising from acid deposition
requires a thorough understanding of pro-
cesses that affect acid deposition. To this
end the United States Environmental Pro-
tection Agency (EPA) has supported in-
vestigations into the physics, chemistry and
meteorology of acid deposition. These ef-
forts have culminated in the development of
the Regional Acid Deposition Model
(RADM), which simulates the processes
involved in the emission, transport, trans-
formation and removal of pollutants contrib-
uting to acid deposition.
The RADM will be used to assess the
impact of different emissions control strate-
gies on the spatial patterns of acid deposi-
tion across eastern North America. The
RADM is an episodic model, simulating
deposition on time scales of about 1 hour to
3 days. Acid deposition policy and assess-
ment issues, however, require deposition
pattern estimates on a seasonal and annual
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bases. Thus, it is necessary to establish a
relationship between individual deposition
events and the long-term climatology of
acid deposition. The relationship between
Individual events and climatology is difficult
because deposition varies greatly from one
eventtothe next in terms of both the amount
of deposition and the relation between
deposition and emission sources. Estab-
lishment of long-term deposition patterns
and source-receptor relationships requires
analysis over an extended time period, pref-
erably Including several years to account
for meteorological variability.
The goal of this study has been to
provide methods for representing aggre-
gate long-term patterns of acid deposition
from a finite number of samples. These
methods involve both the selection of indi-
vidual deposition events which are repre-
sentative of recurring climatic patterns and
the calculation of climatic averages based
onthese events. The aggregation methods
have been applied to the specific problem of
representingtheclimatology of acid deposi-
tion based on in-depth analysis by RADM
for afinite sample of events. An array of 30
three-day events has been identified which
includes the major climatic patterns that
contribute to acid deposition over eastern
North America. Assessment of the impact
of future changes in precursor emissions
will be performed by (1) using RADM to
predict impacts fprthe thirty sample events,
and (2) estimating long-term impacts by
aggregating from the 30 sample events,
based on the techniques described in this
report. While designed specifically in sup-
port of RADM the methods described here
may have broad applicability to other pollut-
ants and to problems that involve the
characterization of long-term climatic im-
pacts of human activities.
Procedure
Theaggregationtechnique is based on
the hypothesis that there are recurring
weatherpatternswhichcanbecharacterized
and described in terms of observable me-
teorological parameters (wind fields, pre-
cipitation, etc.). It is further hypothesized
that distinct weather patterns will be asso-
ciated with characteristic patterns of wet
deposition, both in terms of spatial extent
and deposition amounts. Given those as-
sumptions, long-term wet deposition pro-
cesses may be analyzed based on a finite
numberof individual deposition events, each
of which is "representative" of a major me-
teorological category. The goal of the ag-
gregation project has been to identify the
degree to which weather patterns may be
separated into identifiable categories and to
assess how this information would improve
our ability to extrapolate case study results
to longer-term deposition estimates.
The stratification of events into meteo-
rological categories is used both in the
selection of individual events for in-depth
analysis with RADM and in the calculation
of aggregated estimates for total wet
deposition from 30-event samples. Event
selection has been designed to include
representative episodes from all major me-
teorological categories and to insure that
the selected ensemble includes events with
significant wet deposition at sites through-
out eastern North America. Estimates for
long-term deposition are derived from
known deposition rates (either from obser-
vations or from RADM analyses) forthe 30-
event sample. The aggregation concept
represents long-term deposition as the sum
of deposition from sample events weighted
by the frequency of occurrence of each
meteorological category from which they
were selected. The aggregation technique
has subsequently been modified to make
use of statistical correlations between wet
deposition, category and other meteoro-
logical parameters (precipitation, season,
etc.) in deriving estimates for long-term
deposition.
Testing the aggregation technique
poses a special problem because the most
important issue, response of the method to
changes in precursor emission rates, can-
not be tested empirically. A variety of
techniques have been used to explore ag-
gregation accuracy. These include the es-
tablishment of statistical correlations be-
tween observed wet deposition, wind flow
pattern and other meteorological variables
and application of the aggregationtechnique
to arrays of randomly selected events.
Discrepancies between aggregate esti-
mates and observed deposition are com-
pared to the natural uncertainty arisingfrom
year-to-year variation in wet deposition.
Lastly, an abbreviated model for source-
receptor relationships is used to identify
similarities and differences between ran-
domly selected 30-day ensembles and long-
term averages. These combined tests es-
tablish both the level of accuracy and limi-
tations of the aggregation technique.
Identification of representative deposi-
tion events and methods of aggregation are
based on meteorological and statistical
analyses of deposition events that occurred
over a 4-year period (1982-1985). Events
during this period were divided into cat-
egories with similar three-day patterns of
850 mb wind flow across eastern North
America. The three-day time period and
850 mb wind fields are broadly representa-
tive of the evolution of distinctive storm
patterns that are likely to affect atmospheric
transport and deposition rates. Events have
been selected for analysis by RADM from
among the meteorological strata based on
their likelihood of producing sulfate (SO4=)
wet deposition at multiple locations across
eastern North America, their frequency of
occurrence, and their seasonality.
Results and Discussion
The following specific tasks have been
completed as part of this project:
1. An extensive data base with daily
precipitation and wet deposition of
sulfate, nitrate and total H+ was de-
veloped. The data base covered the
years 1982-1985 and included 23 sites
in the northeastern, southern and
midwestern U.S. and southern
--Canada. Meteorological information
used in subsequent analysis was also
collected.
2. Techniques of cluster analysis were
used to stratify 3-day events over the
4-year period into categories based
on the meteorological data described
above. Four independent stratifica-
tion schemes were investigated:
stratification by 850 mb wind field,
stratification by 850 mb geopotential
and mean surface pressure, stratifi-
cation by 850mb temperature advec-
tion and 500 mb vorticity, and stratifi-
cation by 500 mb vorticity advection
and 850 mb relative humidity. These
four stratification schemes were
evaluated to find the combination of
meteorological parameters which best
explained variations in sulfur wet
deposition.
3. A set of 19 meteorological categories
was compiled based on similarity in
wind flow at 850 mb. These 19 cat-
egories were each subdivided into
"wet" and "dry" cases, making 38 cat-
egories. These 38 categories were
used as the basis for selection of
individual days and derivation of ag-
gregated estimates for annual depo-
sition.
4. Accuracy and validity of the 38-cat-
egory stratification was evaluated
based on the ability to predict annual
sulfate deposition at eastern and
midwestern sites from afinite sample
of 3-day events. We compared esti-
mates for annual deposition derived
from a random sample of 3-day events
with estimates from a cluster-based
sample of events. The standard de-
viation between estimated and actual
deposition was lower by 25% with
cluster-based selection.
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5. Selection of 30 individual 3-day events
for in-depth study with RADM was
made. Selection of events was de-
signed to satisfy the following criteria:
a. representation of the meteorologi-
cal categories that account for the
bulk of dry deposition in the eastern
U.S.
b. Inclusion of days representing awide
variety of circulation patterns, in-
cluding relatively rare winter circula-
tion patterns.
c. Inclusion of a number of days with
very little wet deposition, to allow for
simulation of dry deposition pro-
cesses.
d. Selection of individual events within
each-category- by-a-*random-'"proi'-
cess, ratherthan prescribing specific
events.
e. Accurate representation of total an-
nual deposition at all northeastern
and midwestern sites from the en-
semble of individual events.
The selected ensemble allows for ac-
curate estimation of summer, winter, and
total annual deposition at all sites in the
northeast and midwest.
Conclusions
Analysis of observed wet deposition
during the 1982-1985 time period shows
that deposition rates are statistically corre-
lated with meteorological category with
greater than 95% confidence at most sites
in eastern North America. The aggregation
technique has been used to obtain estimates
for annual deposition at 21 sites in eastern
North America based solely on observed
deposition during the selected 30-day en-
semble. The resulting estimates differ from
the observed 4-year average wet deposi-
^^oaby^arnp^inLs-Csmp^arableJp.npOTalyear-
to-year variations. Aggregated estimates
based on meteorological strata are more
accurate than estimates from randomly se-
lected events.
The aggregation project has served
two purposes:
1. to provide the RADM project with a list
of simulation periods which represent
the range of storm patterns present
over eastern North America. The
selection of storm types was based on
their likelihood of producing SO4= wet
deposition at multiple locations across
eastern North America, theirfrequency
of occurrence, and their seasonably;
and
2. to provide the RADM project with scal-
ing factors for use in weighing epi-
sodic simulation results to seasonal
and annual deposition.
The aggregation methodology has po-
tential application to other airpollution issues
including regional oxidants and regional
aerosols. Research is continuing to refine
The approach and to quantifyTts uncertainty
and sensitivity, and to apply and test its
performance for ambient concentrations as
they impact dry deposition and visibility.
. S. GOVERNMENT PRINTING OFFICE-. 1990/748-012/20087
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Perry J. Samson, Jeffrey R. Brook and Sanford Sillman are with the Department of
Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Ml
48109-2143.
John F. Clarke is the EPA Project Officer (see below).
The complete report, entitled "Estimation of Seasonal and Annual Acidic Deposition
through Aggregation of Three-Day Episodic Periods," (Order no. PB90-252 628/AS;
Cost: $31.00 cost 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 Research and Exposure Assessment 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/S3-90/059
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