Weekend/Weekday Ozone Observations in the South Coast Air Basin

Although there is a large uncertainty in J NO2 , calculated by this method the analysis
showed that J NO2 increased over the period from 1980 to 2000, Figure 3.6-1. This long-term
trend in J NO2 is consistent with the trend to lower soot emissions. However, the maximum J NO2
values were slightly higher on weekdays than on Sunday for the entire period from 1980 to 2000.
These weekend/weekday variations in J NO2 are not consistent with the hypothesis that increased
soot concentrations reduce J NO2 on weekdays.
The ratio of ozone to potential ozone, [O 3 ]/([O 3 ] + [NO x ], is a measure of the fraction of
the possible ozone mixing ratio for a given amount of NO x . A value of zero indicates that there is
no ozone while a value of unity indicates that all of the NO x has been converted to ozone. The
long-term trend of the ratio from 1980 to 2000 is not constant (or consistent??)with the soot
hypothesis, Figure 3.6-2. If all other conditions were constant, a higher J NO2 should yield a more
complete conversion of NO x to ozone. The figure shows that this has not happened. The ratio of
ozone to potential ozone has dropped markedly on weekdays over this time interval. The figure
shows that there is a more complete conversion of NO x to ozone on weekends, but as the
previous EKMA analysis has shown this is caused by the more efficient production of ozone due
to lower NO x mixing ratios rather than an increase in J NO2 .
3.7 Summary of Findings
Weekday Differences in Peak Levels and Diurnal Variations of Ozone
• The peak ozone levels on Sundays and Saturdays are currently about 40 and 25 percent
higher than midweek values, respectively. The majority of ozone exceedances in the SoCAB
now occur on Sundays.
• While the reductions in peak ozone for weekdays were roughly linear over the past two
decades, most of the reductions on weekends occurred during mid to late 1990s. Weekday
differences are similar throughout the western and central basin.
• In the early 1980s, the highest ozone levels occurred on weekdays at Upland and Rubidoux,
and on Saturdays and weekdays at Azusa. Occurrences of highest peak ozone levels on
Sundays were observed at Los Angeles – North Main. By the late 1990s, the highest ozone
levels occur on Sundays at all sites with the exception of Rubidoux where the highest ozone
occurred on Saturday.
• The largest reductions in peak ozone levels over the past two decades are in the central basin.
In the western basin, represented by Los Angeles – North Main, the mean peak current
(1999-2000) ozone levels are about 60 ppb lower than in 1980-85 for all days of the week. In
contrast, decreases in peak ozone in the central basin (Azusa and Upland) have been greater
on weekdays (~ 100 to 110 ppb) than on Saturdays (~ 70 to 90 ppb) or Sundays (60 to 70
ppb).
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