United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory SI
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/013 Apr. 1985
&ER& Project Summary
Ozone Production and Transport
in the Atlanta, Georgia Region
Hal Westberg and Brian Lamb
An air quality monitoring program
was conducted in the Atlanta region
during Jury, 1981. This field study in-
cluded data collection at several
ground stations as well as airborne
measurements using an instrumented
aircraft. Measurements included
ozone, oxides of nitrogen (NOX).
peroxyacetyl nitrate (PAN), individual
hydrocarbons (C, to C10), halocarbons,
condensation nuclei, and visual
distance plus numerous meteorologi-
cal parameters. Isoprene and mono-
terpene emission rates were de-
termined for the most abundant na-
tural vegetation species in the Atlanta
region. Three tracer (SF,) experiments
were performed in order to better
understand dispersion patterns in the
Atlanta area. Specific objectives of
this research were (1) to quantify oxi-
dant transport and production in the
Atlanta plume; (2) to characterize an-
thropogenic and biogenic ambient
hydrocarbon (HC) concentrations; and
(3) to determine the contribution of
natural hydrocarbon emissions to oxi-
dant production in the Atlanta area.
A well-defined pollutant plume was
observed downwind of Atlanta on
most afternoons during the study
period. Ozone levels in the 130 to 160
ppb range were commonly observed
at distances of 35 to 45 miles from the
urban core. Elevated b,cat values were
also observed in the Atlanta plume.
Nonmethane HC concentrations rang-
ing from 113 to 1677 ppbC were
recorded at Atlanta sampling sites
during the 6 to 9 a.m. time period. A
mean concentration of 491 ppbC was
calculated from about 50 samples.
Mean NOX concentrations varied from
44 to 68 ppb at three urban monitoring
sites. A mean HC/NOX ratio of 8.6 was
observed during the 6 to 9 a.m. period
in Atlanta. Biogenic HC including
isoprene, a-pinene, 0-pinene,
A'-carene and myrcene were routinely
detected in air samples collected dur-
ing the 6 to 9 a.m. period. The natural
HC burden in these samples varied
from 2 to 7% at four sampling sites in
the Atlanta area.
Ozone concentrations predicted by
city-specific Empirical Kinetic Model-
ing Approach (EKMA) Ozone Isopleth
Plotting Package, (OZIPP) agreed
reasonably well with observed con-
centrations. This modeling effort con-
sisted of assigning measured precur-
sor and meteorological parameters to
the 8 a.m. Atlanta air parcel,
documenting the air parcel trajectory,
and comparing ozone concentrations
measured in the air parcel later in the
day with model predictions. The
OZIPP ozone concentrations were
generally about 10% below the
observed levels.
The trajectory analysis used to
assign locations to the predicted max-
imum ozone concentrations was
tested against the results of the SF,
tracer experiments. At distances out
to 45 miles, the calculated trajectories
and the paths of the tracer plume
were in good agreement.
An analysis of a recent HC and NOX
anthropogenic emissions inventory for
the Atlanta area indicated that the
HC/NO* ratio calculated from the
emissions data was 1.5 while the
mean ratio observed in ambient
samples was 8.6. Application of a sim-
ple box model using the emissions
data yielded good agreement between
predicted and observed NOX concen-
-------
nations, but the predicted an-
thropogenic HC concentration (99
ppbC) was approximately five times
less than the mean observed concen-
tration (491 ppbC). These results sug-
gest that the anthropogenic HC emis-
sion inventory may be severely
underestimated.
An emission inventory of blogenic
HC developed from a series of vegeta-
tion enclosure samples collected dur-
ing the study yielded area-wide
average fluxes of 2500 ng/m2 h for
isoprene and 90 fig/m2 h for a-pinene.
These levels are similar to those
reported for other areas In the U.S.
Applications of the box model using
these rates provided an upper bound
on the observed biogenic HC concen-
trations. On a 3.2- x 3.2-km (2- x
2-mi) grid average basis, the max-
imum gridded anthropogenic
hydrocarbon was 26,600 ^g/m2 h. The
97- x 97-km (60- x 60-mi) area
average emission flux of an-
thropogenic HC was 1360 jtg/m2 h.
This Project Summary was devel-
oped 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
It has become apparent in recent years
that photochemical smog problems exist
in many sections of the United States.
Ambient ozone concentrations commonly
exceed the National Ambient Air Quality
Standard (NAAQS) around the major ur-
ban centers in California, Texas, and
throughout much of the midwestern and
eastern United States. Oxidant levels in
excess of the standard are recorded in
rural as well as urban regions of the coun-
try. Many studies have been undertaken
to determine the source(s) of the high
ozone concentrations in these urban and
rural areas. The production of oxidants in
urban plumes and subsequent transport
into rural regions is now well
documented. However, considerable con-
troversy still exists regarding
photochemical ozone production from
naturally emitted hydrocarbons. In many
cases, natural hydrocarbon emission
estimates appear to be comparable to an-
thropogenic hydrocarbon emission inven-
tories. However, the direct comparison of
natural and anthropogenic emission inven-
tories is probably not a valid method for
apportioning ozone contributions. Uncer-
tainties in anthropogenic hydrocarbon
emission inventories and in the fate of
naturally emitted organics make it difficult
to predict the impact that each of these
categories will have upon the develop-
ment of oxidants in a particular region.
In order to improve our knowledge of
the factors governing oxidant production
and transport in different regions of the
United States, a comprehensive air quality
monitoring program was conducted in the
Southeast during the summer of 1981.
The Atlanta, GA area was selected for
study because it provided a large, isolated
city surrounded by natural vegetation.
Specific goals of the research program in-
cluded: (1) measurement of an-
thropogenic ozone precursor concentra-
tions in Atlanta, (2) development of a
natural hydrocarbon emissions inventory
for a 100-by-100-km area around Atlanta,
(3) documentation of ozone production in
and around Atlanta, and (4) acquisition of
a meteorological data base sufficient for
air quality modeling. All of these objec-
tives were achieved, although the data
base is somewhat limited in that the field
study spanned only three weeks. Air
quality data were collected between July
13 and August 3, 1981. The purpose of
this report is to provide a tabulation and
discussion of the data collected during the
study period.
Procedure
The data collection phase of this pro-
gram consisted of a joint effort involving
research teams from Washington State
University and the Environmental Protec-
tion Agency (Atmospheric Sciences
Research Laboratory, Research Triangle
Park). Two special field study laboratories
and an instrumented aircraft were used to
obtain air quality information in the Atlan-
ta region. Parameters that were measured
include ozone, oxides of nitrogen (NOJ,
halocarbons, Peroxyacetyl Nitrate (PAN),
individual HC in the C2 to C,0 molecular
weight range, and numerous meteorologi-
cal parameters. Additional air quality in-
formation was obtained from existing
monitoring stations operated by federal,
state, and local agencies. Figure 1 shows
the location of surface stations and lists
the pollutants that were measured at each
site.
Aircraft sampling missions were con-
ducted on 14 days during the study
period. Two flights were flown on most
of the 14 days. An early morning flight
was designed to establish boundary con-
ditions on the upwind side of Atlanta.
The morning mixing height as well as
ozone and oxidant precursor transport
aloft were monitored during the morning
flight. Afternoon flights were conducted
in the region downwind of Atlanta to
establish dimensions of the urban plume
and to locate the region of maximum
ozone buildup.
Results and Discussion
Specific components of the intensive
monitoring program conducted in Atlanta
during July, 1981, included measurement
of ozone precursor concentrations in
Atlanta, chemical characterization of the
upwind air flow, and monitoring of oxi-
dant production and transport downwind
of Atlanta. The results of this sampling ef-
fort were used for evaluation of the city-
specific Empirical Kinetic Modeling Ap-
proach (EKMA) model Ozone Isopleth
Plotting Package (OZIPP). Trajectories
calculated from wind observations during
the study were used to locate predicted
maximum concentrations, and several SFe
urban tracer releases were performed to
aid in the analysis of pollutant transport
originating in Atlanta. This work also in-
cluded the documentation of available an-
thropogenic HC and NOX emission inven-
tories and the estimation of a similar
biogenic HC emission inventory based
upon series of vegetation enclosure
samples collected during the field study.
The various tasks directly related to the
investigation of ozone formation were ac-
complished through the operation of an
instrumented aircraft during 14 days along
with the use of two special field monitor-
ing laboratories and several existing air
quality monitoring sites during a total of
22 sampling days.
Oxidant Precursors
Early morning (6 to 9 a.m.) total
nonmethane hydrocarbon (NMHC) con-
centrations were obtained from gas
chromatographic analyses of individual
species. The average concentrations
varied between 113 and 1677 ppbC with a
mean equal to 491 ppbC at four ur-
ban/suburban sites. These values are
somewhat low compared to similar
measurements from a number of other
U.S. cities. The mean distribution of HC
was approximately 55% paraffin, 15%
olefin, and 30% aromatic, but this
distribution varied widely from day to day.
Biogenic HC were routinely detected in
the early morning samples at the DeKalb
CC site and on most days at Georgia
Tech and DeKalb-Peachtree. These in-
cluded isoprene, a-pinene, 0-pinene,
-------
Dekalb-Peachtree
HC, PAN, NO* Halocarbon, 03
Georgia Tech HC, PAN, NOX, 03
Fulton Co. Health HC, NO,
• South Dekalb C. C.
Chattahoochee River
Figure 1. Location of ground level sampling sites in Atlanta.
A3-carene, and myrcene. The total
biogenic contribution to ambient NMHC
varied from 2% to 7%. Isoprene typically
constituted 1% of the total NMHC.
Average concentrations of NOX
measured during the 6 to 9 a.m. period
ranged from 10 to 200 ppb, and the mean
NOX concentrations measured at four sites
were between 44 and 68 ppb. Mean
HC/NOX ratios at these sites were similar
with a mean of 8.6, which is quite com-
parable to ratios observed in other urban
areas.
Oxidant Production and
Transport
The NAAQS for ozone of 120 ppb was
exceeded on 3 different days at a site 24
miles downwind of urban Atlanta, and the
standard was exceeded at urban sites dur-
ing one of those days. The maximum
hourly average ozone concentration was
172 ppb observed in the urban center.
Daily maximum hourly average ozone
concentrations were less than 60 ppb on
seven days, in the range 80 to 110 ppb on
fourteen days, and greater than 150 ppb
on one day. Maximum hourly average
concentrations of PAN closely tracked the
ozone levels and varied from less than 1
ppb to 4 ppb.
During most of the flight days,
moderate winds from the northwest pro-
duced a well-defined, broad ozone plume
downwind of Atlanta by midafternoon.
Ozone levels in the 130 to 160 ppb range
were commonly observed within the
mixed layer 35 to 45 mi from the urban
core. These concentrations were generally
50 to 70 ppb above background concen-
trations.
Stagnant wind conditions produced by
a stationary front over Atlanta created the
worst-case ozone formation of all the
study days. Winds were light and
variable, and the vertical mixing height
was limited to approximately 925 m during
the day. Unlike the days with northwest-
erly winds, maximum ozone concentra-
tions were observed in the city at the sur-
face and aloft. The maximum ozone con-
centration recorded with the aircraft was
approximately 190 ppb on July 24. The
hourly average maximum at the surface
on that day was 172 ppb. Outside of ur-
ban Atlanta, the maximum surface con-
centration was 120 ppb.
EKMA Evaluation
Ozone production during the two types
of wind conditions was modeled using
city-specific EKMA with the OZIPP com-
puter code. Model input data were taken
from surface and aircraft measurements
during each modeled day. Maximum
ozone concentrations predicted with
OZIPP were approximately 10% less than
those observed. In addition, a simple air
trajectory analysis based upon available
surface and upper-air data indicated that
the predicted ozone maxima occurred
very near the locations and times where
ozone maxima were actually observed.
Results from three SFe tracer tests also in-
dicated that predicted trajectories correct-
ly simulated gas transport paths
originating in Atlanta and extending as far
as 45 mi downwind.
Precursor Emission Inventories
Anthropogenic HC and NOX emissions
in Atlanta, recently inventoried by the
State of Georgia, are predominantly from
mobile sources. Annual average emission
rates from all sources for a 97- x 97-km
(60- x 60-mi) area centered over Atlanta
are 133,000 tons HC/day and 275,000
tons NOx/day. These values yield an
HC/NOX ratio equal to 1.5/1, assuming
average molecular weights of HC and NOX
to be 14.5 and 46 g/m respectively. This
ratio is significantly less than observed in
the ambient concentrations. Application
of a simple box model of the urban area
predicted an NOX concentration essentially
equal to the mean observed NOX concen-
tration. However, predicted HC concen-
trations for the same conditions were 5.0
times less than the mean observed con-
centration. The results suggest that the
discrepancy in the HC/NOX ratio is the
result of severe underestimation of an-
thropogenic HC emission rates.
Biogenic HC emission rates from the
predominate tree species in Atlanta were
measured using a branch enclosure
technique. Results from 20 samples were
similar in magnitude and temperature
response to measurements conducted
elsewhere in the U.S. At 27°C, the
isoprene emission rate from several
deciduous species was 11 /ig/g/h, and
the a-pinene emission rate from loblolly
pine was 0.7 /xg/g/h. These rates were
combined with biomass density factors to
yield emission flux estimates for
deciduous and coniferous forests. In turn,
land-use distributions for Atlanta obtained
from an EPA analysis of Landsat data
were used with the estimated fluxes to
estimate the daily average emission rate: •
330 tons/day for isoprene and 24 tons/
day for a-pinene. On a unit area basis,
these rates become 2500 jtg/m2 h and 90
/tg/m2 h, respectively. For comparison,
the average (3.2- x 3.2-km grid) flux of
anthropogenic HC for the 97- x 97-km
area was 1350 /ig/m2 h and the highest
gridded flux of anthropogenic HC was
26,600 /*g/m2 h. Unlike the results for the
anthropogenic emissions, biogenic HC
-------
concentrations predicted with the box
model represented an upper • bound on
typical measured levels.
Hal Westberg and Brian Lamb are with Washington State University, Pullman,
WA 99164-2730.
William Lonneman is the EPA Project Officer (see below).
The complete report, entitled "Ozone Production and Transport in the Atlanta,
Georgia Region," (Order No. PB 85-173 839/AS; Cost: $20.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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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