Weekend/Weekday Ozone Observations in the South Coast Air Basin

(HPLC/UV). PAMS carbonyl data normally include only formaldehyde, acetaldehyde, and
acetone. Total non-methane organic compounds (NMOC) are the sum of quantifiable peaks by
EPA method TO-14A, including unidentified but excluding halocarbons, or by continuous
instruments with flame ionization detection. Measured NMOC will be lower for laboratories
employing water management. NMOC also refers to the sum of NMHC plus carbonyl
compounds by TO-11.
3.1 Weekday Differences in Peak Levels and Diurnal Variations of Ozone
Historic trends in the average daily maximum hour ozone in the SoCAB show that both
the magnitude and spatial extent of the weekend ozone effect have increased over the period
from 1981 to 1998, especially during the 1990’s. Reductions in peak ozone levels in the central
portion of the basin were also greater during the 1990s. Data for the summers of 1999 and 2000
show a continuation of these trends through the end of the decade. Figures 3.1-1a through 3.1-1d
show the mean diurnal variations in ozone mixing ratios on Saturdays, Sundays, and weekdays
for the years 1980-85, 1986-1990, 1991-94, 1995-98, and 1999-2000 for Los Angeles – North
Main, Azusa, Upland, and Rubidoux (listed in order of location from the western to eastern
portion of the basin), respectively.
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). 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 1990’s. The diurnal
variations of ozone at Los Angeles – North Main show a shift of about 2 hours in the timing of
peak ozone during the past two decades. While ozone reached its peak earlier in the western
basin in the 1980s and early 90s, there is now little difference in the timing of maximum ozone
between the western and central basin. With decreasing rates of ozone accumulation and
therefore lower ozone concentrations, we speculate that the onset of ventilation has become a
more significant factor in the diurnal variation of ozone in the western basin. Diurnal variations
in patterns of transport and solar radiation, factors that have not changed with time, are probably
more important in the timing of maximum ozone in downwind areas.
The plot of the mean day-of-the-week variations in daily maximum ozone mixing ratios
in Figure 3.1-2 shows that the weekday differences are similar throughout the western and
central basin. Table 3.1-1a lists the mean daily maximum ozone at Los Angeles – North Main,
Azusa, Pico Rivera, and Upland by day of the week for summer 1999 and 2000. Ratios of the
mean daily maximum ozone to the midweek values (mean of Tuesday to Thursday) are shown in
Table 3.1-1b. The peak ozone levels on Sundays and Saturdays are about 40 and 25 percent
higher than midweek values, respectively. The majority of ozone exceedances in the SoCAB
now occur on Sundays.
3-2