United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-90/029 June 1990
Project Summary
The Role of Biogenic Organics
in the Southeast Ozone
Problem: Preliminary
Assessments and Implications
Basil Dimitriades
Literature review and modeling
studies were performed to assess the
role of biogenic VOC emissions in
the photochemical ozone problem of
urban areas. The assessment effort
focused specifically on recent
research results reported by Georgia
Institute of Technology (GIT)
scientists indicating that biogenic
VOCs in Atlanta reduce substantially
the effectiveness of VOC controls in
reducing ambient ozone. The GIT
findings were checked for accuracy
and for consistency with previous
studies and also for applicability in
other urban areas. EPA and SAI
modelers replicated the GIT modeling
study and verified its results and
conclusions. A conflict between the
GIT study and an earlier modeling
study by others was resolved, further
establishing the validity of the GIT
results. Sensitivity studies by EPA
modelers showed, however, that the
GIT findings, while unquestionably
valid for Atlanta, were not necessarily
applicable to all urban non-
attainment areas. Factors affecting
the strength of the biogenic VOC role
in the urban ozone problem were
found to be the abundance and
reactivity of biogenic emissions, the
prevailing wind speeds during ozone
episodes, the maximum afternoon
mixing height, the size of the
urbanized area, and perhaps other
factors also.
This Project Summary was
developed by EPA's Atmospheric
Research and Exposure Assessment
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
Photochemical ozone pollution,
recognized over three decades ago, has
turned out to be one of the most
persistent and intractable air pollution
problems in the US and other parts of the
world. Despite multi-year efforts to
reduce emissions of volatile organic
compounds (VOCs), the ozone air quality
standard continues to be violated in over
100 urban areas in the US, many of
which are in the Southeast part of the
country.
Several reasons have been
hypothesized in the past, explaining slow
progress in alleviating the ozone non-
attainment problem. One possible reason
for the problem was suggested recently
when new modeling studies by Georgia
Institute of Technology (GIT) scientists
for the Atlanta urban area indicated that,
contrary to previous general
understanding, VOC emissions from
vegetation might reduce the effectiveness
of VOC control substantially, at least in
the Southeast. EPA is concerned about
the Southeast ozone problems and about
the role attributed to biogenic VOCs. If
biogenic VOC emissions have indeed
contributed to an underestimation of the
level or type of controls needed in
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Atlanta, similar effects may have
occurred in some other urban areas as
well. Before significant resource
commitments are made, however, to
explore this possibility, the Agency needs
assurances that the evidence pointing to
the need for further studies is credible
and conclusive. The research work
described and discussed here is an
attempt to evaluate the GIT evidence for
the purpose of providing the requisite
assurances. The intent was to identify
and justify future research needs —
clearly, not to suggest changes in
regulatory policy or strategies. Time and
resource constraints dictated that this
effort be limited to reviewing published
information, and doing quick modeling
computations using EKMA and other
comparably simple modeling techniques.
Discussion of Results and
Conclusions
There are three questions regarding
the role of biogenic VOC that need to be
addressed. First is the question of
whether or not all studies agree that the
role of biogenic VOC in the urban ozone
problem is significant. The second
question deals with the magnitude of the
biogenic VOC effect and, specifically,
with the degree to which biogenic VOCs
increase the VOC emission control
requirement for ozone attainment. Finally,
there is the question of whether or not the
GIT results for Atlanta reflect an Atlanta
"uniqueness," or, more generally, the
question of the degree to which the
biogenic VOC effect varies from city to
city for the various US non-attainment
cities.
Studies of the biogenic VOC
involvement in the ozone problem include
smog chamber studies and atmospheric
chemistry and modeling studies. The
unanimous current assessment is that
existing smog chamber data are
consistent with an important role of
biogenic VOCs in the urban ozone
problem comparable to the role of the
anthropogenic VOCs. Smog chamber
experimentation and evidence, however,
while strongly suggestive, are somewhat
unrealistic and for that reason their
validity is questionable. Field
measurements and evidence are
inherently more valid but the biogenic
VOC issue is far too complex to be
resolved through field studies alone.
Ozone models, when properly validated
against theory and smog chamber and
field data, are thought to be both credible
and useful tools for investigating issues
such as the one in hand. One such
modeling study was done in 1983-84.
The investigators modeled the Tampa-St.
Petersburg urban plume using the
ELSTAR photochemical trajectory model,
and interpreted their results to mean that
the biogenii VOC contribution to the
urban ozone problem was not significant.
Because of apoarent conflicts between
that study and other newer modeling
studies, the Tampa-St. Petersburg study
was repeated in the light of the latest
understanding regarding the chemistry
and precursor roles in the ozone-forming
process. The results from the new study
indicated that inclusion of the biogenic
VOCs resulted in a small increment in
peak ozone concentration but the
anthropogenic VOC control requirements
were substantially increased when
biogenic VOCs were included, in
agreement with the GIT results.
There are several factors that can cast
doubts on the modeling estimates of the
magnitude of the biogenic VOC effect.
First, there is the "model uncertainty"
factor which, judging from the differences
among the different models' predictions
reported here and from current judgment,
in general, concerning the validity and
accuracy of current ozone models, will
have to be considered as an important
one. Then there is, also, the biogenic
VOC emissions level, composition and
atmospheric chemistry accuracy factors,
the effects of which were studied to some
extent by the GIT and EPA modelers and
found to be important. Finally, there are
several other factors the importance of
which was established through a
sensitivity analysis done by the EPA
modelers. These latter factors include
atmospheric dilution (i.e., maximum
afternoon mixing height) and size of the
urbanized section of the urban area. The
conclusion from the studies and
theoretical analyses reported here was
that the magnitude of the biogenic VOC
effect, while unquestionably significant,
cannot be quantified with confidence at
this time.
Finally, while there can be little doubt
at this point that biogenic VOCs reduce
significantly the effectiveness of
anthropogenic VOC controls in Atlanta,
this may or may not be the case for each
and every non-attainment urban area. To
explore this possibility, EPA modelers
performed a sensitivity analysis to
determine the degree to which factors
that vary from city to city affect EKMA
computations of the biogenic VOC role.
The conclusion from that study was that
the biogenic VOC factor for some urban
areas may be as strong or stronger than
in Atlanta but for other cities it may be
considerably less important. Each urban
area should be judged separately, based
on its own biogenic VOC emissions
loading factor, the biogenic VOC
composition, the size of its "urbanized
section," the prevailing wind speeds
during ozone episodes, the maximum
afternoon mixing height, and perhaps
other factors also.
In summary, the results from this
assessment effort were:
• Biogenic VOCs in Atlanta are more
important than previously believed.
• "Importance" in other cities varies,
possibly widely.
• Much uncertainty exists in quantitative
estimates of the biogenic VOC role
because of limited data on levels
composition and atmospheric
chemistry of biogenic VOC emissions.
• VOC control requirements in the past
were probably underestimated because
of lack of data on or underestimated
biogenic VOC role.
• NOX emissions may be more important
than previously believed in areas high
in biogenic VOC emissions.
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The EPA author, Basil Dimitriades (also the EPA Project Officer, see below), is
with Atmospheric Research and Exposure Assessment Laboratory, U.S.
Environmental Protection Agency, Research Triangle Park, NC 27711.
The complete report, entitled "The Role of Biogenic Organics in the Southeast
Ozone Problem: Preliminary Assessments and Implications," (Order No. PB90-
188 855/AS; Cost: $39.00, subjttct 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 Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-90/029
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