Weekend/Weekday Ozone Observations in the South Coast Air Basin

values measured at the regional sites and source-dominated areas. Figure 4.3-3a shows that NOy
and black carbon exhibit very similar diurnal and spatial variations, and weekday/weekend
differences. Both NOy and black carbon are much higher on weekdays, especially during the
ozone inhibition and accumulation periods. During weekdays, their concentrations along the
freeway loops were about an order of magnitude higher than at Dodger Stadium or Industry
Hills. Figure 4.3-3b shows that CO and VOC have similar temporal and spatial variations, which
differ significantly from those exhibited by NOy and black carbon. CO and VOC showed no
significant weekend/weekday differences during the ozone inhibition and accumulation periods.
Overnight carryover appears to be more significant for CO and VOC than for NOy and black
carbon on weekends, especially Sunday morning.
The variations in VOC/NOy ratios(ppbC of VOC to ppb of NOy), shown in Figure 4.3-4,
are straightforward given the variations that are observed for NOy, CO, black carbon, and VOC.
VOC/NOy ratios are lower on freeways because NO concentrations are substantially higher on
freeways than other areas. The high NO concentrations are always accompanied by higher black
carbon and semi-volatile hydrocarbons. Although diesel exhaust is not the sole source of black
carbon and semi-volatile hydrocarbons, it is the predominant source during hot, stabilized
driving conditions on freeways, and diesel trucks are a major source of the large spikes in NO
concentrations on freeways. VOC/NOy ratios on freeways ranged from 0.5 to 2. These ratios are
comparable to the average ratio of 0.9 measured at the truck stop. One notable exception was
early Sunday morning at 2:00 to 5:00 a.m. when VOC/NOy reached about four. Greater
carryover of VOC accounted for the larger VOC/NOy ratio during this time. Larger
concentrations of MTBE during this period indicate greater contributions from gasoline-powered
vehicles. The light-duty gasoline-powered vehicle fleet, as represented by the HF1 samples, has
an average VOC/NOy ratio of about 3.5. This value is similar to the VOC/NOy ratios measured
at Industry Hills and Dodger Stadium.
4.3.2 Pollutant Variations at Ambient Monitoring Sites
Datasets that are comparable to that compiled from the mobile van were acquired at the
SCAQMD Azusa and Pico Rivera monitoring stations. In general, the weekend/weekday patterns
in pollutant concentrations at the monitoring stations and for the on-road measurements are
consistent, as are the associations among pollutants.
Figure 4.3-5a shows time series of hourly average concentrations of NO, NO 2 , black
carbon and CO at the Azusa monitoring station during the study period. Figure 4.3-5b shows the
same data averaged over the carryover, ozone inhibition, and ozone accumulation periods. The
time series for NOx is closely correlated to the time series for black carbon. The day-to-day
variations show a midweek dip, which is coincident with the passage of a trough. Despite
meteorological conditions that were unfavorable for pollutant buildup during the middle of the
week, however, both NOx and black carbon are much higher on weekdays than during the
weekends. CO shows no discernible differences between weekdays and weekends that might be
ascribed to changes in emission patterns. Rather, it reasonably tracks the day-to-day variations in
meteorological conditions. Similar time series are shown for Pico Rivera in Figures 4.3-6a and
4.3-6b. The trends at the Pico Rivera monitoring station are more complex than for Azusa due,
most likely, to its close proximity to a major freeway (I-605) and diurnal variations in wind
speed and direction. During the prevailing sea breeze in the afternoon, the station is upwind of
4-7