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The Science of
Ozone Formation in the San Joaquin Valley
When the state adopted the air quality improvement
plan for the San Joaquin Valley in 1994, the fundamental
premise for ozone reduction was straightforward: reduction
of "ozone precursors" would result in a corresponding
reduction in ozone. Ozone precursors are the individual
chemical emissions that combine through a chemical reaction
to create ozone, namely volatile organic compounds (VOC)
and oxides of nitrogen (NOx). Eight years later, despite
significant reductions in precursor emissions and a
reduction in the number of days the federal standard
has been exceeded, the Valley's air quality has improved
but not as much as would have been expected from the
reduction in precursor emissions. And the air quality
still does not meet national health standards.
Since 1986, a partnership of the oil industry, agriculture,
business and government called the San Joaquin Valley
Study Agency has closely examined the air quality in
the Valley through a continuing set of studies, the
most recent being the Central California Ozone Study
(CCOS). That study has found that simply reducing the
main constituents of ozone equally, everywhere and on
everyone has not produced the ozone reductions that
were expected. Unfortunately, everything we have learned
in the past 16 years about ozone formation in the Valley
points to a process far more complicated than anyone
imagined.
Scientists now believe:
- As the concentration of precursors in the air is
reduced, the chemical reaction that forms ozone becomes
more efficient. This means that as the tons of VOC
and NOx are reduced, the corresponding rate of reduction
in ozone is less and less. This means that more and
more tons of emissions need to be removed from the
air to achieve the comparable ozone reductions. For
example, if a 20% reduction of precursors resulted
in a 5% reduction of ozone, it would now take a 30%
reduction in precursors to achieve the next 5% ozone
reduction, etc.
- In many regions, if VOC and NOx are reduced in equivalent
amounts, ozone reduction is less efficient than if
one or the other of the precursors is reduced alone.
Unfortunately, which precursor to reduce is not always
clear, and often depends on when and where the precursors
are present. That is, not all the solutions for all
of the subregions in the district would be the same.
- Meteorological conditions always control the production
of ozone. Hot, still days most often produce levels
of ozone that exceed the standards. Therefore, current
ozone reduction strategies are based on controlling
precursor emissions assuming the "worst case"
weather conditions. Unfortunately, controlling emissions
to meet worst-case situations does not avoid all violations
of the standards. Results of the CCOS may provide
more information to understand how the variables that
control ozone production combine to create ozone levels
that exceed standards even when worst case conditions
do not occur.
What we do know about the chemistry of the Valley's
air quality is due in large part to the unprecedented
$18 million partnership of the federal and state governments,
industry and agriculture, with the studies conducted
under the leadership of the Study Agency. The CCOS ranges
north-south from Redding to the Mojave Desert, and from
the Pacific ocean to the Eastern Sierra Nevada Mountains.
It consists of a comprehensive field program, a master
database of field measurements, data analysis and modeling
and emissions inventories. Much of what is known to
date about air quality in the Valley is the direct result
of this effort. The inventories and modeling from this
study will be essential to develop control strategies
and measures to achieve attainment in the future.
The complicated interactions that produce ozone appear
to suggest that, at least in the San Joaquin Valley,
localized geographic and temporal emission controls
may be more effective than region-wide responses. Unfortunately,
despite our improved understanding of the chemical and
regional behavior of ozone forming emissions, more information
is necessary to be able to tailor effective control
strategies for the region and smaller areas within the
region where exceedences may be localized.
The long-term and broad-based commitment to the development
of good science must continue to be an integral component
of our attainment strategy.
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