Environmental Engineering Reference
In-Depth Information
emission-inventory ratios averaged approximately 30% lower than the ambient
ratios when converted to comparable units. The observed declines in ambient
NMOC/NO
x
ratios represent shifts toward ratios at which ozone production is less
efficient. The trends do not imply a change in the limiting precursor at the urban
locations where NMOC was monitored. The differences between the morning
(5-8 a.m.) and afternoon (noon-2 p.m. or 4-7 p.m.) ambient NMOC/NO
x
ratios
generally diminished over time. This change suggests that air masses at the
NMOC-monitoring sites were less photochemically aged by mid-day in later years
than in earlier years. One possibility is that NO
x
was removed less rapidly during
later years; another is that some sites may have experienced a relative increase in
fresh emissions during mid-day hours. Further study is needed.
The ozone and ozone-precursor trends indicate that management of ozone in
central California requires a focus on emissions of ozone precursors within and
throughout central California. Ongoing emission reductions will be needed to
further reduce peak ozone concentrations. The analyses do not identify where and
when peak ozone concentrations might be more effectively managed through
reductions of NMOC, NO
x
, or both. Because the CCOS domain is large and
encompasses highly urbanized, rural, undeveloped, and wilderness lands, it is
expected that ozone formation is limited by NMOC in some areas and by NO
x
in
others. Although the analyses are not definitive in this respect, they suggest that
emission reductions that continue to maintain low ratios of NMOC/NO
x
are more
likely to be effective than reductions that cause NMOC/NO
x
ratios to increase.
Further study is needed to identify the specific reductions of ozone precursors of
greatest value within each nonattainment area.
Acknowledgments
The authors greatly appreciate the help provided by many people and especially
thank Dar Mims, Dwight Oda, Martin Johnson, Ajith Kaduwela, Larry Larsen, and Cheryl
Taylor of the California Air Resources Board, Jim Cordova and Saffet Tanrikulu of the Bay Area
Air Quality Management District, and Evan Shipp of the San Joaquin Valley Air Pollution Control
District for their assistance with data, emission inventories, statistical methods, and project
review.
Disclaimer
The statements and conclusions in this report are those of the Contractor and not
necessarily those of the California Air Resources Board, the San Joaquin Valleywide Air Pollution
Study Agency, or its Policy Committee, their employees or their members. The mention of
commercial products, their source, or their use in connection with material reported herein is not
to be construed as actual or implied endorsement of such products.
Questions and Answers
Question:
European data indicate declining peak (episode) ozone and increasing
background ozone. Can you explain the apparent contradiction between these
results and yours?
