United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
^ EPA Project Summary
EPA/600/SR-92/067 September 1992
Sensitivity of Modeled
Ozone Concentrations to
Uncertainties in Biogenic
Emissions
Shawn J. Roselle
This study examines the sensitivity
of regional ozone (O3) modeling to un-
certainties in biogenic emissions esti-
mates. The United States Environmen-
tal Protection Agency's (U.S. EPA) Re-
gional Oxidant Model (ROM) was used
to simulate the photochemistry of the
northeastern United States for the pe-
riod July 2-17, 1988. An operational
model evaluation showed that ROM had
a tendency to underpredict O3 when
observed concentrations were above
70-80 ppb and to overpredict O3 when
observed values were below this level.
On average, the model underpredicted
daily maximum O3 by 14 ppb. Spatial
patterns of O3, however, were repro-
duced favorably by the model.
Several simulations were performed
to analyze the effects of uncertainties
in brogejiic^rnjssipns ^>n predicted O3
arid to study* the effectiveness of 'two
strategies of controlling anthropogenic
emissions for reducing high O3 con-
centrations. Biogenic hydrocarbon
emissions were adjusted by a factor of
3 to account for the existing range of
uncertainty in these emissions. The
impact of biogenic emission uncertain-
ties on O3 predictions depended upon
the availability of NO,. In some ex-
tremely NO,-limited areas, increasing
the amount of biogenic emissions de-
creased O3 concentrations.
Two control strategies were com-
pared in the simulations: (1) reduced
anthropogenic hydrocarbon emissions,
and (2) reduced anthropogenic hydro-
carbon and NO, emissions. The simu-
lations showed that hydrocarbon emis-
sion controls were more beneficial to
the New York City area, but that com-
bined NO, and hydrocarbon controls
were more beneficial to other areas of
the Northeast. Hydrocarbon controls
were more effective as biogenic hydro-
carbon emissions were reduced,
whereas combined NO, and hydrocar-
bon controls were more effective as
biogenic hydrocarbon emissions were
increased.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search 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
--Ozone (O3)~in "the'troposphere is pro-"-
duced by photochemical reactions of ni-
trogen oxides (NO,) in the presence of
hydrocarbons. NO, and hydrocarbons are
emitted into the atmosphere from both
natural (biogenic) and man-made (anthro-
pogenic) sources. In the United States,
anthropogenic sources contribute most of
the NO,, with fossil fuel combustion ac-
counting for 88% of the total. Biogenic
NO, sources, including soils and lightning
flashes, contribute much less to the total
NO, budget. Biogenic sources of hydro-
carbons include plants, shrubs, and trees
and are on the same order of magnitude
as anthropogenic hydrocarbon emissions.
Because of the increase in anthropogenic
emissions due to industrialization, urban
O3 concentrations have reached serious
Printed on Recycled Paper
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levels. Additionally, an increase in rural O3
concentrations was noted from observa-
tional data taken over the past 40 years.
High concentrations of O3 present sev-
eral problems to human health and wel-
fare. Ozone in high concentrations can
cause respiratory illnesses in humans. It
also deteriorates rubber materials such as
automotive tires, wiper blades, etc., short-
ening the lifetime of these materials. Addi-
tionally, O3 affects the growth of vegeta-
tion and can damage plants and trees.
With the many problems of high O3 con-
centrations, a national standard for O3 was
developed to assure that people in the
United States would be protected from
the dangers of this pollutant. A 1-hour
average National Ambient Air Quality Stan-
dard (NAAQS) for O3 was set such that
the concentration would not exceed 120
parts per billion by volume (ppb) on one
or fewer days per year on average.
Because O3 is a secondary pollutant,
not directly emitted into the atmosphere
but rather formed by chemical reactions, it
has been difficult to develop schemes for
reducing its concentration. The precursors
to Oj formation are NO, and hydrocar-
bons. Therefore, precursor emissions must
be reduced if O3 concentrations are to be
lowered. Because of the presence of bio-
genie emissions of hydrocarbons and NO,,
however, development of a procedure for
reducing O3 becomes complicated and the
outcome of any emission controls is not
intuitively obvious. Also, the chemistry of
Oa formation is nonlinear. A further com-
plicating factor is that O3 has a 1-2 day
residence time in the atmosphere, permit-
ting its transport for distances of up to
1000 km. Thus, an O3 problem in one
location may not be caused by local emis-
sions, but from sources far upwind.
To help understand this complex prob-
lem, computer models have been devel-
oped to simulate the physical and chemi-
cal processes of the atmosphere. A num-
ber of models now exist, ranging from
simple box models to more complex
Eulerian and Langrangian models. These
computer models provide valuable tools
for studying the behavior and structure of
the atmosphere under various conditions
and also provide test-beds for developing
emission control strategies for reducing
O, concentrations.
A commonly heard phrase in the "mod-
eling world" is that computer models are
only as good as their input data. This
statement is true, provided the model is
capable of perfectly simulating physical
processes. Models are not perfect and
inherently contain various degrees of un-
certainty In their formulation (e.g. in the
chemical mechanism, treatment of physi-
cal processes, and numerical algorithms).
In most cases, however, uncertainties in
the input data are more important, espe-
cially when the model is used in control
strategy evaluation. Emissions data, the
most fundamental data required for air
quality modeling, are laden with major un-
certainties. It is extremely important that
emissions uncertainties be minimized in
order to provide reasonable simulations of
the real atmosphere. Development of more
accurate emission inventories will require
many years of research of improving emis-
sion factor estimates, source activity lev-
els, etc. In the interim, a model's sensitiv-
ity to uncertainties in emissions can be
studied to provide insight into the reliabil-
ity of model results. This research project
focuses on uncertainties in the biogenic
emissions database. Uncertainties in the
anthropogenic data are also important, but
are beyond the scope of this study.
The purpose of this study is to further
examine the hypothesis that biogenic emis-
sions have a significant impact on O3 pro-
duction, by analyzing the differences in O3
predictions caused by the range of uncer-
tainty in estimates of biogenic emissions.
Additionally, the results from this research
should give guidance to air quality plan-
ners on the effects of biogenic emissions
uncertainties and the role they may play
in control strategy selection. An opera-
tional model evaluation performed in this
study also provides more insight into the
performance of EPA's ROM beyond the
evaluations that have already been per-
formed.
Procedure
The ROM, which is capable of simulat-
ing most of the physical and chemical
processes responsible for the formation
and transport of O3 on regional scales
(10ekm2), was used to perform simulations
of the period July 2-17, 1988. Most ofthe
northeastern United States was included
in the modeling domain. The region was
defined by a curvilinear coordinate system
with a horizontal grid resolution of (1/4)°
longitude by (1/6)°latitude, or about
18.5km x 18.5km, with a total of 3,328
(64 columns x 52 rows) model grid cells in
each layer. The planetary boundary layer
and capping inversion (or cloud layer) were
simulated using three dynamic layers,
which were free to locally expand and
contract in response to changes in the
physical processes occurring therein.
Anthropogenic emissions used in the
simulations were derived from the 1985
National Acid Precipitation Assessment
Program (NAPAP) inventory. Emissions
from anthropogenic sources of volatile or-
ganic compounds (VOC), nitrogen oxides
(NOJ, and carbon monoxide (CO) were
provided by the NAPAP inventory. Major
point, area, and mobile source emissions
were included in the inventory and varied
daily (Saturday, Sunday, and weekday)
and hourly. Additionally, mobile source
evaporative emissions were adjusted for
daily temperature variations using the em-
pirical model MOBILE3.9. Some station-
ary VOC sources were adjusted for the
average daily temperature and for activity
levels typical of high O3 episodes in the
northeastern United States. The inventory
also accounts for existing emission con-
trols.
Biogenic emissions were obtained from
the Biogenic Emissions Inventory System
(BEIS). BEIS produced estimates of iso-
prene, a-pinene, monoterpenes, and uni-
dentified hydrocarbons from trees and
crops, and also estimated NOX emissions
from grassland soils. For these simula-
tions, no NOX emissions from lightning
flashes were accounted for by BEIS.
Nine model simulations were performed
with the ROM using different levels of
emissions. The different levels of anthro-
pogenic emissions included (1) base case
emissions, (2) emissions reduced by hy-
drocarbon controls, and (3) emissions re-
duced by hydrocarbon and NOX controls.
Three different levels of biogenic emis-
sions were used with each of these an-
thropogenic emissions data sets: (1) the
reference amount of biogenic emissions,
(2) biogenic emissions reduced by a fac-
tor of 3, and (3) biogenic emissions in-
creased by a factor of 3. With the different
combinations of emissions data sets, a
matrix of nine model simulations was
formed.
Results and Discussion
The ROM was operationally evaluated
using observed hourly O3 concentrations
. taken during the period of simulation^The
evaluation showed that the model tended
to underpredict O3 if observed concentra-
tions were above 70-80 ppb and to
overpredict O3 if concentrations were be-
low this level. On average, the model
underpredicted daily maximum O3 by 14
ppb (15%). Spatial patterns of maximum
O3 were predicted well by the model, even
though magnitudes were underpredicted.
Analysis of several time series, taken from
different locations within the modeling do-
main, showed that the model predicted
diurnal O3 variations better in urban areas
than in rural areas. Also, the time-series
analysis showed that the model
overpredicted concentrations during the
nighttime hours.
The effect of uncertainties in biogenic
emissions on O3 production was studied.
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Decreasing biogenic hydrocarbon emis-
sions by a factor of 3 substantially low-
ered the predicted concentration of reac-
tive organic gases (ROG) by more than
30%. in response to the change in bio-
genic emissions, predicted NO, concen-
trations increased slightly. Ozone predic-
tions were lower in most locations with the
decrease in biogenic hydrocarbon emis-
sions; episodic maximum O3 decreased
by 5% to 15% over much of the modeling
domain. However, analysis of the rural
interior portion of New England, where
the predicted chemical system was ex-
tremely NO,-limited, showed ah increase
in O3 as the level of biogenic hydrocarbon
emissions was decreased. Analysis of daily
and hourly data showed that in most ar-
. .e.as_of ,tne,Nortbe,as.t,jie.creasing .bjogenic,..__
hydrocarbons caused the smallest pre-
dicted O3 values to increase but the larg-
est predicted values decreased.
Simulations using biogenic hydrocarbon
emissions increased by a factor of 3 were
also examined. In response to the increase
in these emissions, modeled ROG con-
centrations increased by well over 100%
in most of the model domain but NO,
decreased slightly. Predicted episodic
maximum O3 increased by 15% over many
areas of the Northeast. Increases were
equally apparent in both the daily and
hourly data. Some rural locations, which
were NO,-limited in the model, were pre-
dicted to have lower O3 concentrations
with increased emissions of biogenic hy-
drocarbons. With increased amounts of
biogenic hydrocarbons, the smallest O3
predictions were reduced but the largest
predictions were increased.
The effectiveness of two simulated con-
trol strategies for reducing O3 was also
examined; one strategy reduced only an-
thropogenic hydrocarbon emissions and
the other reduced both anthropogenic hy-
•'drocarbon and NO~emissionsr Both -strat -«
egies resulted in a reduction in modeled
O3 concentrations. An exception to this
was in extremely NO,-limited areas of the
Northeast where application of hydrocar-
bon controls alone caused a slight in-
crease in O3. ROG concentrations were
similar for the two control strategy simula-
tions, but NO., concentrations were lower
for the strategy that reduced both hydro-
carbons and NO, emissions. Modeled O3
was reduced in NOx-limited areas with the
application of the combination of both hy-
drocarbon and NO, controls. Most of the
Northeast was predicted to be NO,-limited
except in the vicinity of major NO, sources,
such as are found in the New York City
area. For these NO,-rich (hydrocarbon-
limited) areas, the hydrocarbon-only con-
trol strategy performed better in the simu-
lations. The usefulness or effectiveness of
the combined strategy (hydrocarbon and
NO,) for reducing O3 predictions increased
over NO,-limited areas of the Northeast
as the level of biogenic emissions in-
creased. The strategy of only reducing
hydrocarbon emissions, however, was pre-
dicted to become more effective when bio-
genic emissions were decreased in the
simulations. For the most part, however,
the combined strategy of hydrocarbon and
..NO, .controls, was pxedjctecJLto J3e_mpre_
effective than the control of hydrocarbons
alone, except in the vicinity of New York
City. This may result in part from the fact
that the controls are not spatially or tem-
porally homogeneous.
Conclusions and
Recommendations
The sensitivity of regional ozone (O3)
modeling to uncertainties in biogenic emis-
sions was studied using EPA's ROM. The
ROM was used to simulate the photo-
chemistry of the northeastern United States
for a 15-day period in July 1988. A matrix
of nine simulations was performed using
different levels of biogenic and anthropo-
genic emissions in each simulation. Bio-
genic emissions were increased and de-
creased by a factor of 3 to account for the
existing range of uncertainty in these emis-
sions. Anthropogenic emission estimates
were reduced by two different control strat-
egies—one using hydrocarbon controls
and another using both hydrocarbon and
NO, controls.
The model results suggest that biogenic
-hydrocarbon-emissions-contribute signifi-
cantly to the formation of O3 in the north-
eastern United States. Uncertainties in bio-
genic emissions used in the model may
lead to significant errors in O3 predictions.
Because of horizontal transport, the influ-
ence of biogenic emissions (even when
biogenic emissions were reduced by a
factor of 3) on the New York City area can
be very important.
There appears to be a broad spectrum
of predicted chemical regimes within the
Northeast, ranging from the extremes of
NO,-limited to hydrocarbon-limited envi-
ronments. Based upon modeled concen-
trations, most of the Northeast appears to
be NO,-limited except in the vicinity of
major NO, sources, such as are found in
the New York City area. Reduction of NO,
emissions appears more beneficial to most
of the Northeast, whereas hydrocarbon
emission reductions appear better in the
vicinity of New York City (and other hydro-
carbon-limited areas). Application of one
strategy type for the whole Northeast may
not be the most effective means for re-
ducing high O3 concentrations. Instead,
the design of control strategies for the
Northeast should consider the geographic
distribution of emissions and the chemical
regime of the area, whether it is NO,-
^ _______
Because of the model's tendency "to
underpredict higher O3 concentrations,
analysis of control strategies could not
fully assess the regional effectiveness of
the strategies for reducing high O3 con-
centrations. Also, the simulations have
shown that uncertainties in biogenic emis-
sions may lead to errors in predicting O3
concentrations and thus could possibly
lead one to choose a less effective control
strategy.
The following recommendations are sug-
gested from results of this study: (1) Un-
certainties in biogenic emission estimates
need to be reduced to improve predic-
tions of ozone concentrations. (2) Field
validation of rural hydrocarbons and NO,
is needed because of the sensitivity of
predicted O3 to biogenic emissions and
background NO, concentrations. Monitor-
ing programs planned for the Southeast
Oxidant Study and for the Lake Michigan
Ozone Study should help immensely be-
cause of the existing dearth of NO, and
rural hydrocarbon measurements. (3) New
York City and other high concentration
.-areas should be- treated as special -cases-
and examined in further detail with urban
scale models. (4) Additional modeling stud-
ies should be conducted to determine the
relative effectiveness of hydrocarbon ver-
sus NO, controls. A project recently initi-
ated by the EPA, which uses ROM to
simulate the effects of across-the-board
hydrocarbon and NO, emissions reduc-
tions, will provide a systematic approach
to determining the effectiveness of emis-
sion control strategies.
•U.S. Government Printing Office: 1992 — 648-080/60059
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The EPA author, Shawn J. Roselle, (also the EPA Project Officer, see below) is with
the Atmospheric Research and Exposure Assessment Laboratory, Research
Triangle Park, NO 27711.
The complete report, entitled "Sensitivity of Modeled Ozone Concentrations to
Uncertainties In Biogenic Emissions," (Order No. PB92-192202/AS; Cost:
$35.00; 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:
Atmoshparic Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangel Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
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EPA/600/SR-92/067
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