Environmental Engineering Reference
In-Depth Information
8.4 Effects of VOC and NO
x
Emission Changes
on Ozone and Precursor Concentrations in Central
California from 1990 to 2004
Charles L. Blanchard
1
, Shelley Tanenbaum
1
, Eric Fujita
2
, James Wilkinson
3
,
and David Campbell
2
1
Envair, Albany, CA, USA
2
Desert Research Institute, Reno, NV, USA
3
Alpine Geophysics, Eugene, OR, USA
Abstract
From 1990 to 2004, peak ozone concentrations trended predominantly
downward in central California, but the median rate of decrease in the annual
4
th
-highest peak ozone at 44 trend sites was only 0.3 ppbv per year and trends were
statistically significant at only 6 sites. Ambient monitoring data confirm that emis-
sions of ozone precursors decreased at rates consistent with inventory estimates of
emission trends (~10 to 50 percent reductions). We examine ten possible hypotheses
that might explain why the observed ozone trends did not show larger decreases.
Keywords
Central California, ozone trends, emission trends
From 1990 to 2004, peak ozone concentrations trended predominantly downward
in central California: 36 long-term monitoring sites had downward trends in the
annual fourth-highest daily maximum 8-h ozone and eight sites had upward trends.
Six downward site trends were statistically significant (p > 0.05) for 1990-2004.
The rates of progress were greater post-2000 than in the 1990s, and 11 sites showed
significant (p < 0.05) downward trends for 1990-2007. The median rate of decrease
in the annual fourth-highest peak ozone was modest (0.3 ppbv/year from 1990 to
2004 and 0.7 ppbv/year from 1990 to 2007).
We examine ten possible hypotheses that might explain why the observed
ozone trends did not show larger decreases. The hypotheses fall into three general
categories: (1) emissions-related; (2) exogeneous factors; and (3) the chemistry of
ozone formation.
The hypothesis that emissions did not decline is incorrect. Ambient monitoring
data confirm that emissions of ozone precursors decreased at rates consistent with
inventory estimates of emission trends. The downward trends in mean morning or
mid-day concentrations of oxides of nitrogen (NO
x
), carbon monoxide (CO), and
non-methane organic compounds (NMOC) were statistically significant (p < 0.05)
at 22 of 28 sites, 21 of 25 sites, and 6 of 7 sites, respectively. Although the
