Weekend/Weekday Ozone Observations in the South Coast Air Basin

Saturday and are almost entirely absent on Sunday. The percentage of CO-associated NOx
for the three freeway loops during this time of day, CO2, CV2, and PO1, are 47, 35, and 41
percent, respectively, on Monday, 38, 40, and 30 percent on Wednesday, 43, 59, and 57
percent on Saturday, and 63, 68, and 59 percent on Sunday.
• At the Azusa and Pico Rivera monitoring stations, the percentage of NOx associated with CO
is consistently larger on weekends and the largest fractions of NOx associated with black
carbon occur midday during weekdays. These patterns are consistent with the diurnal and
day-of-the-week variations in relative traffic volume of gasoline and diesel powered vehicles.
• At Industry Hills, the most regionally representative site where the mobile sampling was
conducted, the percentages of CO-associated NOx in the 10:30 to 11:15 a.m. samples are 44,
48, 62, and 72 percent for Monday, October 2, Wednesday, October 4, Saturday, October 7,
and Sunday, October 8, respectively. Based upon STI’s analysis of the study period
meteorology, Monday, October 2 is meteorologically similar to Sunday, October 8, and
Wednesday, October 4 is similar to Saturday, October 7. The Saturday/Wednesday ratios of
the CO and BC associated NOx are 1.01 and 0.58, respectively. The Sunday/Monday ratios
of the CO and BC associated NOx are 0.84 and 0.25.
• The weekend/weekday differences in the relative contribution of NOx associated with CO
and black carbon at Industry Hills are consistent with those found at the Azusa and Pico
Rivera monitoring stations. These results show that the contribution of gasoline-powered
vehicles to ambient NOx on a typical Saturday during the ozone accumulation period is
comparable to their contribution on weekdays. This conclusion is consistent with traffic
counts, which show comparable light-duty gasoline vehicle traffic volumes on weekday and
Saturday during this time of day. The contribution of gasoline-powered vehicles to ambient
NOx is about 25 percent lower on Sunday compared to weekdays. In contrast, the
contributions of diesel vehicles during the ozone accumulation period to ambient NOx on
Saturday and Sunday are about one-half and one-third of its weekday contribution,
respectively.
VOC and NOx Source Apportionment by Chemical Mass Balance
• Gasoline exhaust is the predominant source in all samples ranging from 60-80% for on-road
samples and samples taken at more regionally representative locations.
• The diurnal and weekday/weekend variations in the relative contributions of NMHC are
more variable for diesel exhaust than gasoline exhaust.
• The relative contributions of diesel exhaust are greatest during the weekday on freeway loops
with the greatest fraction of diesel traffic. Diesel exhaust contributed 20 percent of NMHC
on the Pomona Loop on Monday, October 2, compared to only 3 percent on Sunday, October
8. The same percentages are 18 and 6 percent on Wednesday, October 4 and Saturday,
October 7.
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