Central California Ozone Study (CCOS) - Desert Research Institute

CCOS Field Operations Plan Version 2: 5/31/00
Joaquin Valley (SJV) emissions cause frequent exceedances of the 1hr and 8hr standards at
several sites in the interior of the Central Valley.
Ozone season days, May 1 – October 31, were classified for three recent seasons, 1996-
1998, using inspection of 500-mb weather maps to establish gross features (Fujita et al. 1999).
All 552 subject days were classified into eight synoptic categories, three of which are important
for this study (arranged approximately in order of decreasing ozone impact):
1. Western U.S. high pressure – upper-level high centered over the Western U.S.
2. Eastern Pacific high pressure – upper-level high centered off the Western U.S. coast.
3. Monsoonal flow – upper-level high centered in the south-western U.S. or in northern
Mexico such that southerly flow brings moisture north.
The Western U.S. High accounts for proportionately the greatest number of exceedances;
97% of days with highs centered over southern California have 8hr exceedances in the central
San Joaquin Valley (SJV), where the frequency of exceedances on all subject days is 42%.
Similarly, 90% of these days have southern SJV exceedances, where the season frequency is
38%. The Western U.S. High contributes to stagnation conditions throughout central California
by fostering an off-shore gradient which weakens the marine air intrusion and in some cases
even reverses the usual sea breeze. This tends to keep pollutants longer within respective source
regions, although some transport can still occur. The SFBA has 8-hour exceedances on 29% of
days when the Western U.S. High is centered over the Pacific Northwest, while the frequency of
8hr exceedances on all 1996-98 ozone season days is only about 4%. Also, this scenario has the
lowest mixing heights to trap pollutants vertically all over central California. The Eastern Pacific
high fosters greater transport inland, but has greater wind speeds which keep the ozone peak
concentrations somewhat lower than the maximum observed values. Monsoonal flow can have
both mitigating and exacerbating impacts on SJV air quality. Some monsoonal days are
identified where SJV air quality is lower relative to the rest of Central California, but on other
days, the southerly monsoonal flow can weaken the SJV exit flow, contributing to build-up.
During the high pressure ridge scenarios, when synoptic forcing over central California is
weakest but ozone concentrations are greatest, mesoscale features become most important, and
subtle synoptic differences fostering the formation and/or amplification of these features are even
more difficult to classify. The following cluster analysis describes a more objective classification
method to complement the top-down approach. (See Fujita et al. 1999 for more detail.)
Using the target criteria (128 ppb for SFBA, 129 ppb for SV, and 145 ppb for SJV), 43
days from over 85 days identified by local districts were selected for cluster analysis. With the
exclusion of one outlier, three clusters are found for 1-hour exceedances. Three 8-hour clusters
were also found, however the correspondence between 1-hour and 8-hour cluster days is not oneto-one.
Statistical values of meteorological parameters for all 43 days and for each 1hr cluster
were compiled. The 1-hour cluster results are presented here:
Cluster 1 – 22 of 43 days. The San Francisco Bay Area has its highest basin-wide ozone
values, though still less in absolute magnitude than San Joaquin Valley. This cluster is
characterized by the weakest sea breeze (lowest west-to-east component through Carquinez
Chapter 3: FORECAST PROTOCOL 3-2