Weekend/Weekday Ozone Observations in the South Coast Air Basin

3.2 Weekday Differences in Diurnal Variations of VOC, NOx, and VOC/NOx Ratios
The diurnal variations in the mixing ratios of VOC and NOx depend on the spatial and
temporal variations of their emissions, and the diurnal variations in the chemical and physical
processes that affect their transformations and dispersion. Tables 3.2-1 through 3.2-5 show the
mean concentrations of NO, NO 2 , NOx, and NMHC and NMHC/NOx ratios by 3-hour periods
for each day of the week for Azusa, Los Angeles – N. Main, Pico Rivera, and Upland for the
summers of 1999 and 2000. The averages for NO, NO 2 and NOx include only periods that
correspond to the PAMS NMHC data (i.e., every third day for type 1, 3, and 4 sites). The mean
NMHC/NOx ratios are means of the individual 3-hour periods. The ratios of the concentrations
of NO on Saturday and Sunday to midweek (Tuesday to Thursday) are about one-half and onethird,
respectively, during the 12-hour period from 0300 to 1500 (PDT). These ratios are constant
throughout this period with mean Sunday/midweek ratios of 0.86, 0.35, 0.34, and 0.33 during the
carryover (0000-0300), ozone inhibition (0600-0900), ozone accumulation (0900-1200), and
peak ozone (1200-1500) periods, respectively. The corresponding Saturday/midweek ratios are
1.15, 0.60, 0.59, and 0.54, respectively. The four-site mean weekend to midweek ratios for NO 2 ,
NOx and NMHC during 0600-0900 are 0.73, 0.48, and 0.61, respectively, on Sunday and 0.89,
0.69, and 0.77, respectively, on Saturday. The day-of-the-week variations for NO and NO 2 are
shown as column plots in Figure 3.2-1a for times corresponding to the carryover, ozone
inhibition, ozone accumulation, and peak ozone periods. The corresponding day-of-the-week
variations are shown for NMHC and NMHC/NOx in Figure 3.2-1b. The ranges in the columns
are the standard deviations of the mean mixing ratios at the four sites. The decrease in mixing
ratios on weekends relative to weekdays is proportionately greater for NO than NMHC for all
times of the day resulting in lower weekend NMHC/NOx ratios.
The four-site mean NMHC/NOx ratios during 0600-0900 ranged from 31 to 59 percent
higher on Sunday (mean of 46 percent) and 20 to 39 percent higher on Saturday (mean of 29
percent). The mean 6-9 a.m. NMHC/NOx ratio (ppbC/ppbv) is 4.9 and 5.5 on Saturday and
Sunday, respectively and increases to a high of 6.7 and 7.5 during the period of peak ozone
(1200-1500). The mean NMHC/NOx ratio is 4.6 on Monday and ranges from 3.7 to 3.9 for other
weekdays. The mean midday NMHC/NOx ratios do not exceed seven. NMHC/NOx ratios have
steadily decreased over time because of the greater proportional reduction of VOC emissions.
Current NMHC/NOx ratios in the SoCAB are about half relative to the ratios observed during
the 1987 Southern California Air Quality Study (SCAQS). SCAQS (Lawson, 1990) included
eleven days of intensive measurements during the summer of 1987. One of the eleven days
occurred on a Saturday. All other sampling days were on weekdays. Mean 7-8 a.m. NMHC/NOx
ratios of 8.8 (standard error of 1.0) and 7.5 (standard error of 0.4) were observed during SCAQS
at Los Angeles – North Main and Azusa, respectively (Fujita et al., 1992). Current mean 6-9 a.m.
weekday NMHC/NOx ratios range from 3.2 to 4.6 at Los Angeles – North Main and from 3.5 to
5.6 at Azusa. The mean 7-8 a.m. NMHC and NOx mixing ratios during SCAQS were 1087 ppbC
and 139 ppb, respectively, at Los Angeles – North Main and 812 ppbC and 117 ppb, respectively
at Azusa. Current mean 6-9 a.m. weekday NMHC and NOx mixing ratios range from 322 to 544
ppbC and 119 to 172 ppb, respectively, for Los Angeles – North Main and 348 to 598 ppbC and
96 to 122 ppb, respectively, for Azusa. These data indicate that the ~50 percent decrease in
NMHC/NOx ratios in the SoCAB from 1987 to 1999-2000 can be attributed primarily to
reductions in hydrocarbon emissions.
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