United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/070 Dec. 1985
Project Summary
Ozone
Baltimore
Plume
BeHiavior in the Combined
Washington, DC
Hal Westberg
An extensive air quality monitoring
program was carried out in the Wash-
ington, DC-Baltimore region from July
14-August 15, 1980. This field study
included data collection at numerous
ground stations and two aircraft sam-
pling platforms. Measurements includ-
ed ozone, oxides of nitrogen, individual
hydrocarbons (C2-Cio), condensation
nuclei and visual distance, and numer-
ous meteorological parameters. Spe-
cific areas of interest in this study in-
cluded the acquisition of air quality data
for (1) development and testing of a
regional air quality simulation model
and (2) gaining a better understanding
of oxidant production and transport in
plumes emanating from Washington,
DC, and Baltimore. This summary de-
scribes oxidant behavior downwind of
these two cities in cases in which plume
interaction was observed. Seven days
during the 198O study period when
southwesterly winds carried the Wash-
ington, DC, plume into the region
occupied by the Baltimore plume were
selected. A case study analysis was
performed for each of these days. Gen-
eral conclusions are presented regard-
ing oxidant patterns in the regions
where plume interactions occurred.
This Project Summary was developed
by ERA'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
During July and August of 1980, the
U.S. EPA sponsored a large air quality
monitoring program in the northeastern
United States. This program is referred to
as the Northwest Regional Oxidant Study
(NEROS). Federal, state, and private re-
search organizations worked together to
acquire a comprehensive data base that
includes numerous meteorological meas-
urements and gaseous pollutant infor-
mation. Primary objectives of the study
were to establish a data base that could
be used for development and testing of a
regional-scale photochemical air quality
simulation model and to provide states in
the northeastern United States with data
useful for State Implementation Plan
revisions.
Because one of the primary objectives
of the 1980 NEROS study was to obtain a
data base for photochemical modeling
purposes, it was essential that the phys-
ical and chemical characteristics of urban
plumes in the region be well understood.
Consequently, a great deal of effort was
put into characterizing the plumes from
Baltimore, Washington, DC, New York
City, and Boston. Four fixed wing aircraft
and a helicopter were used to monitor the
chemical composition and dispersion
behavior of these plumes. In Baltimore,
Lagrangian plume tracking experiments
were performed with the aid of tetroons.
On one occasion, a tetroon released near
Baltimore allowed tracing of an air parcel
as far as New York City. Many Eulerian-
type studies were conducted in which
aircraft flight paths crisscrossed the
Baltimore and Washington, DC, plume?
at fixed distances upwind and downwind
of the cities.
A great deal of the collected data can be
used to improve understanding of photo-
chemical oxidant production in urban
plumes. The close proximity of the Bal-
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timore and Washington, DC, urban cen-
ters provided a unique opportunity to
examine the effects created when the
plume from one large city impinges on
that of another. This summary describes
plume behavior in instances in which the
Baltimore and Washington, DC, plumes
overlapped. Seven days during the 1980
study period when southwesterly winds
carried the Washington, DC, plume into
the region occupied by the Baltimore
plume were selected. A thorough case
study analysis was performed for each of
these days. General conclusions are pre-
sented regarding oxidant behavior in
regions where plume interactions oc-
curred.
Results
General conclusions relating to oxidant
production and transport in the region
downwind of Baltimore and Washington,
DC, are listed below.
1. Ambient non-methane hydrocar-
bon (NMHC) concentrations meas-
ured during the 0600-0900 period
at ground-level urban sites in Bal-
timore and Washington, DC, were
very similar in magnitude. The
mean NMHC concentrations were
659 and 671 parts per billion carbon
(ppbC) for Baltimore and Washing-
ton, DC, respectively. The NMHC
concentration frequency distribu-
tions were also very comparable;
the majority of readings fell within
the 450 to 750-ppbC range. The
class distribution of individual hy-
drocarbon species was approxi-
mately 60% paraffins, 30% aromat-
ics, and 10% olefins in each city.
Assuming sampling sites were rep-
resentatively placed, these results
imply that hydrocarbon emissions
are nearly the same in Baltimore
and in Washington, DC.
2. The mean hydrocarbon/NOx ratios
were not significantly different for
the two cities. The downtown Bal-
timore site exhibiteda ratioof 7.0±
2.9; the corresponding mean ratio
at the ground-level Washington,
DC, station was 9.6 ± 7.2 The
Washington, DC, ratio appeared to
be more variable than the Baltimore
ratio.
3. Because of the close proximity of
the two cities, meterological factors
that influence oxidant production in
each city are normally the same.
Because oxidant precursor levels
are similar, okone production in the
two plumes would be expected to
be nearly the same. Aircraft meas-
urements verified this assumption.
4. Southwesterly winds in the Wash-
ington, DC-Baltimore region can
lead to direct overlap of the two
urban plumes or to varying degrees
of partial overlap. In the one in-
stance of direct overlap recorded
during the 1980 study period, a
single oxidant plume was observed
downwind of the two cities. Lag-
rangian measurements obtained
with aid of a tetroon traveling at
altitudes between 2300 and 3600 ft
showed no evidence of ozone pro-
duction in the air parcel departing
Baltimore at 11:00 a.m. Ozone
concentrations were about 70 ppb
within the air parcel and to the
north and south of the plume. After
two hours of travel, ozone levels in
the plume were elevated by 40 ppb
compared to ozone concentrations
outside of the plume boundaries. At
5:00 p.m. this same air parcel,
which at that time was located 190
km downwind of Baltimore, exhib-
ited peak ozone concentrations of
170 ppb. Thus, ozone levels in the
plume increased from 70 ppb on the
outskirts of Baltimore to 170 ppb 6
h later at a distance of 190 km
downwind of Baltimore. Because
oxidant precursor concentrations
and atmospheric dispersion condi-
tions on that day were typical of
those encountered during the study
period, it seems reasonable to as-
sume that ozone concentrations
will increase by approximately 100
ppb in an air parcel that passes
directly over both Washington, DC,
and Baltimore during the morning
hours.
In situations in which partial
overlap of the urban plumes oc-
curred, highest ozone levels were
observed in the region of maximum
plume overlap. Because of the
geographic location of the two
cities, the northern portion of the
combined plume contained the re-
gion of greatest overlap. Conse-
quently, a crossplume ozone grad-
ient usually existed; concentrations
were highest in the northern por-
tion of the plume and lower to the
south. When west southwest wind
flow created a side-by-side overlap
of the individual plumes downwind
of Washington, DC, and Baltimore,
a biomodal crossplume ozone pro-
file was observed. Ozone concen-
trations in the interaction region
between the two plume centerlines
were commensurate with an ad-
ditive contribution from each of the
individual plumes.
5. A combined Washington, DC-Bal-
timore plume with enhanced ozone
concentration was documented on
seven days during the 1980 field
study program. On these days.
ozone concentrations in the plume
ranging from 120 to 220 ppb were
measured in the region between
Baltimore and Philadelphia. Back-
ground ozone levels to the north
and south of the plume generally
ranged from 90 to 110 ppb during
the afternoon. Elevated ozone levels
in the Washington, DC-Baltimore
plume were observed as far as 190
km downwind of Baltimore. In the
region 60 to 80 km downwind of
Baltimore, crossplume ozone pro-
files at 1500 ft often differed from
those recorded at higher altitudes.
The low-altitude profiles exhibited
relatively high ozone concentra-
tions over Chesapeake Bay and
along its northern shoreline. The
enhanced ozone concentrations are
probably the result of limited pre-
cursor dilution (shallow mixing
layer) and minimal ozone losses
due to scavenging over the water.
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Hal Westberg is with Washington State University, Pullman, WA 99164.
William A. Lonneman is the EPA Project Officer (see below).
The complete report, entitled "Ozone Behavior in the Combined Baltimore-
Washington. DC Plume," (Order No. PB 86-120664/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
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