United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-86/051 Feb. 1987
v/EPA Project Summary
Ozone Formation in Pollutant
Plumes: A Reactive Plume
Model with Arbitrary
Crosswind Resolution
Noor V. Gillani
A new two-layer reactive plume model
was developed in which arbitrary cross-.
wind resolution of the emission field of
each precursor is preserved and dynamic
plume-plume and plume-background inter-
actions are explicitly accommodated. The
model has a hybrid formulation, having
Lagrangian downwind transport and Euler-
ian crosswind spread. It is applied in a
diagnostic mode to simulate the observed
behavior of plumes of the metropolitan St.
Louis area and the Labadie power plant.
The RAPS emissions inventory gave de-
tailed spatial resolution of the emission
field, numerous stationary and mobile
upper air wind soundings provided the
basis for transport simulation, and aircraft
data provided detailed crosswind profiles
of pollutant concentrations across the
plumes at downwind sections.
Background ozone and sulfate produc-
tion were found to be NOX limited and
poorly correlated with background
NMHC/NOX. Power plant plume ozone
and sulfate production were both pos-
itively correlated with background
NMHC/NOX. Excessive ozone can form in
a power plant plume without any interac-
tion with a neighboring urban plume.
Power plant emissions of NOX, including
those in rural locations, are a major con-
tributor to the regional ozone burden in the
eastern U.S. Their potency, however, is
realized because of the biased precursor
loading of the regional background
(NMHC/NOX around 30 ppbv/ppbv). This
bias is a result of the strong disparity in
the rates of consumption of NMHC and
NOX in urban plumes. Model simulations
of ozone were generally good, even in
crosswind detail, given an appropriate
background characterization. Simulated
values of the rate of SO2 oxidation were
quantitatively not as satisfying.
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 Pro-
ject Report ordering information at back).
Introduction
Damaging effects of ozone on human
health, forests and crops, and materials
occur frequently during summer in rela-
tively polluted regions such as the eastern
United States. While natural emissions of
organic vapors may play a significant role
in the photochemistry leading to ozone
production in the atmosphere, the predom-
inant sources of the precursors NOX and
NMHC are anthropogenic in origin. Specif-
ically, urban-industrial complexes and
power plants (many of which are located
in rural areas) are the principal sources of
ozone precursors. The chemistry of ozone
photosynthesis in the pollutant plumes of
these sources is linked also to the produc-
tion of other irritants (e.g., PAN), acidic
matter (e.g., H2SO4, HN03) which is re-
sponsible for acidic deposition, and fine
aerosols which contribute to visibility
reduction and regional haze.
Ozone, both local and regional, origi-
nates mostly from specific urban and rural
sources. Under typical summer midday
conditions in the eastern U.S., ozone
photosynthesis attains a peak rather
promptly within a mesoscale range of
urban sources as well as rural power
plants. The regional impact follows as this
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peak spreads by atmospheric dilution and
is affected by subsequent nighttime chem-
istry followed by further daytime chemical
reactions in a multi-day regional buildup
process. To properly assess the problem,
plume models must quantify the phenom-
ena of mesoscale ozone peaking and
subsequent dissipation within the time
scale of a day. The results of such models
should be adequate for assessing the local
problem and must also provide the basis
for sub-grid scale parameterizations to be
used in regional models. Regional models
can reciprocate by providing the basis for
estimating the composition of the ambient
environment into which the plumes are
emitted.
Ozone formation in pollutant plumes,
both the typical urban variety and the rural
power plant plumes, is considered here.
The investigative tools include both field
observations and a new reactive plume
model. There was a need to examine and
interpret available in-situ mesoscale data
collected in metropolitan plumes in the
eastern U.S. An effort was initiated to
develop a plume model with spatial resolu-
tion consistent with the available field data
of St. Louis RAPS and MISTT (as well as
PEPE/NEROS and NEROS I) and with the
capability to examine in particular the role
of plume-plume and plume-background in-
teractions. This report describes the cur-
rent state of the new model and its prelim-
inary application to a field observation
scenario. The model awaits some further
development which would make it fully
operational and applicable to arbitrary
metropolitan and rural plume emissions
and their reactive transport.
Features of the New Model
The principal features of the new model
are outlined below:
• arbitrary crosswind resolution of the
precursor emission field, as well as of
the downwind concentration field;
• full dynamic interaction between typi-
cal urban plumes which are rich in NOX
and NMHC, and power plant plumes
which are initially devoid of NMHC, but
are very rich in NOX;
• full dynamic interaction between a
plume and its ambient environment;
• dynamic simulation of the ambient
environment, both within the boundary
layer and aloft, including the effect of
regional emissions; and
• a chemistry module that is a modified
form of the Demerjian-Schere chemical
scheme and simulates the kinetics of
26 species.
In its present state, the model is not
operational in a generalized mode of-appli-
cation. In particular, the primary emissions
module needs revising and upgrading, and
the chemistry module needs to be general-
ized to accept any scheme. Once these
revisions are implemented, an operational
version of the computer code can be
prepared for more general application by
a wider user community.
Goals of Model Application
Model application involved the follow-
ing specific goals:
• characterization of the chemical
composition of the ambient environ-
ment in terms of precursors, second-
ary products, as well as important
intermediate species (e.g., OH, H02,
H202);
• investigation of the sensitivity of
ozone photosynthesis in the back-
ground to regional emissions of the
precursors;
• investigation of the sensitivity of
ozone formation in a power plant
plume to the composition of the
ambient environment;
• simulation of the transport and chem-
istry of the 1200 release of the Laba-
die power plant plume on July 9,
1976, and comparison of the ob-
served ozone crosswind profile 190
km downwind at 2000;
• simulation of the transport and chem-
istry of the metropolitan St. Louis
plume, emitted over the period of
1200 to 1500 on 9 July 1976, and
comparison of the observed down-
wind ozone crosswind profile at
2000; and
• simulation of the chemistry of sul-
fur and nitrogen oxides in the St.
Louis plumes and in the ambient
environment.
Results of Model Application
The following conclusions based on
field observations and model calculations
are highlighted. They are believed to be of
fairly general qualitative validity for the
eastern U.S.
Ambient Environment
• The ambient environment is dynamic
and chemically quite active both in
the mixing layer and aloft. It is not ap-
propriate to assume the air aloft to
be chemically inactive during the
morning period.
• The boundary layer is even more
reactive, being continuously fed by
regional emissions of precursors and
by entrainment of aged air from aloft.
Both processes can have strong ef-
fects in shaping boundary layer com-
position and chemistry.
• Afternoon values of the concentra-
tion ratio NMHC/NOX (ppbv/ppbv) of
boundary layer air are around 30. In
terms of ozone formation potential,
such an environment is strongly NOX
limited. Regional emission flux of
2 -3x1011 molecules crrr2s-1 of NOX
appears to be appropriate for de-
veloping about 70 ppb of ozone
typically observed in the eastern U.S.
summer background. This flux is con-
sistent with the total eastern U.S. an-
thropogenic emissions of NOX. The
depletion of NOX is quite rapid in the
summer daytime atmosphere.
• The high loading of NMHC in the
boundary layer appears to be a result
of the vast difference in the rates of
NMHC and NOX depletion in urban
plumes. NOX is consumed more rap-
idly, leaving behind the less reactive
hydrocarbons to flow dilute into the
regional background. Regional back-
ground values of NMHC in the Mid-
west and the Ohio River Valley region
are probably around 100 ppbC (about
25 ppbv) on summer days.
• The formation of ozone in the back-
ground is relatively insensitive to in-
creases in the regional emissions of
NMHC. However, observed ozone
formation in the Labadie plume is
sensitive to background NMHC and
requires the ratio of NMHC/NOX in
the regional emission flux to be about
eight by volume. This is considerably
higher than the average eastern U.S.
anthropogenic flux ratio which is less
than two but much less than the cor-
responding ambient concentration
ratio on summer days which is about
30. The significance of natural emis-
sions of NMHC may be greater than
previously thought.
• Production of OH and hence the rate
of gas-phase oxidation of SO2 and
N02 in the background are strongly
sensitive to ambient NOX. Gas-phase
formation of acidic products in the
ambient air appears to be inversely
related to the concentration of ratio
NMHC/NOX.
Power Plant Plumes
• Power plant plumes are a major
source of ozone in the eastern U.S.
The NOx-starved and NMHC-loaded
background air is very responsive to
NOX loading from the power plant
emissions. Following an initial deple-
tion of ozone within the plume due
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to reaction with primary NO in the
fresh plume, there is rapid ozone
recovery and ultimately a substantial
ozone bulge in the plume, as NMHC
and free radical species are entrain-
ed from the background.
• The ozone-forming potential of
power plant emissions is, however,
strongly dependent on ambient
NMHC which are not emitted by the
power plants. Presumably, while the
polluted atmosphere under convec-
tive conditions is conducive to rapid
ozone generation in power plant
plumes, the clean atmosphere is not.
An increase in ambient NOX raises
ambient ozone and reduces the po-
tential for excess ozone formation in
the plume. Ozone formation in the
power plant plume is thus positively
correlated with ambient NMHC/
NOX.
• There is also more direct evidence of
the influence of background entrain-
ment on the evolution of plume ozone
dynamics. The developing crosswind
profiles clearly show that plume
ozone recovery is most rapid in plume
edges where the NMHC/NOX ratio is
highest.
• It appears that the ozone recovery in
the plume is complete when the
cross-sectional average value of
NMHC/NOX in the plume exceeds
about 1 ppbv/ppbv.
• The rates of oxidation of SO2 and
N02 in the plume appear to be pos-
itively correlated with ambient
NMHC/NOX. The correlation for SO2
appears to be much better than the
correlation of the S02 oxidation rate
with ambient ozone. Hence, use of
ambient ozone as a surrogate for am-
bient NMHC/NOX in parameteriza-
tions of the SO2 oxidation rate is not
well founded. However, given the
dearth of reliable ambient data for
NMHC and NOX and the approx-
imate level of accuracy of estimates
of the SO2 oxidation rate based on
field measurements, the continued
temporary use of ozone may be justi-
fiable in empirical parameterizations.
Metropolitan Plumes
• Urban emissions are rich in both
precursors of ozone, and hence,
urban plumes are less reliant on
background entrainment for ozone
photosynthesis than are power plant
plumes.
• Metropolitan emissions are typically
characterized by a diverse spatial
distribution of the precursor mix and
of source configurations. There is a
corresponding wide range of spatial-
temporal variability of ozone distribu-
tion. There is good evidence for the
"source intensification" effect where
plumes intersect. Improper treatment
of the primary emissions distribution
does lead to distortion of the ozone
distribution. Urban emissions are
capable of generating substantial
amounts of ozone rapidly. The pro-
duction of ozone then slows down as
the NOX becomes depleted long be-
fore the NMHC, thus passing on an
excessive amount of NMHC to the
regional background. This relative
overloading of the background re-
mains potent in terms of potential
ozone production. This potential is
realized when NOX emissions, as
from a power plant, are released into
the background.
Research needs
• Proper chemical characterization of
the primary emission field and of the
ambient environment remain high
priority needs for reliable prediction
of ozone formation in the polluted
atmosphere.
• Sub-grid scale effects are very impor-
tant near precursor sources in the
context of regional models. Meso-
scale field observations and model
'applications must be directed at the
study and quantification of such
effects.
Noor V. Gillani is with the Mechanical Engineering Department, Washington
University, St. Louis, MO 63130.
John F. Clarke, Francis Pooler, Jr.. and William E. Wilson are the EPA Project
Officers (see below).
The complete report, entitled "Ozone Formation in Pollutant Plumes: A Reactive
Plume Model with Arbitary Crosswind Resolution," (Order No. PB 86-236
973/AS; Cost: $11.95, 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 Officers can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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Environmental Protection
Agency
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