Weekend/Weekday Ozone Observations in the South Coast Air Basin

2. EVOLUTION OF THE OZONE EFFECT IN THE SOUTH COAST
AIR BASIN FROM 1981 TO 1998
This section examines historic trends in the average daily maximum hour ozone in the
SoCAB from 1981 to 1998 and the evolution of the magnitude and spatial extent of the weekend
ozone effect over this period. These trends are associated with day-of-the-week variations in the
diurnal behavior of ozone, nitric oxide (NO), nitrogen dioxide (NO 2 ), carbon monoxide (CO),
and nonmethane hydrocarbons (NMHC). The retrospective data analysis compares the day-ofthe-week
variations in the diurnal behavior of ozone and precursor concentrations in the four
phases of the diurnal cycle: overnight carryover, ozone inhibition, ozone accumulation, and postozone
maximum. The analysis focuses on day-of-the-week differences in the extent of inhibition
of ozone formation during the morning due to titration with NO and the rate of ozone
accumulation from the end of the inhibition period to time of peak ozone. Changes in the rate of
ozone accumulation are correlated to historic trends in CO/NOx ratios, which serve as partial
surrogates for trends in VOC/NOx ratios.
First, we examined the average diurnal variations in ozone and ozone precursors at Azusa
for the summers of 1995 to 1997 to determine the parameters that would best serve as surrogates
for the extent of ozone inhibition and rate of ozone accumulation. Figure 2-1 shows the average
diurnal variations of O 3 , NO, NO 2 , and CO at Azusa for summer 1995. During the carry–over
phase, there is little difference in concentration of NO 2 and O 3 between weekend and weekday.
NO is slightly higher on weekends and CO is about 25% higher on weekends. Although
carryover of ozone precursors is higher on weekends, the differences are relatively small
compared to the higher weekday concentrations of ozone precursors during the morning
commute periods. Fresh NO emissions during this period inhibit radical formation by titrating
ozone with NO. During this inhibition period, formaldehyde (HCHO) and, to a lesser extent,
nitrous acid (HONO) are the main source of HO radicals. We selected the time in the morning
when NO and O 3 crosses over (i.e., t NO=O3 ) as a marker for the end of the inhibition period and
beginning of O 3 production via conversion of NO to NO 2 by peroxy radical. Note in Figure 2-1
that the crossover occurs an hour earlier on weekends. Thus, ozone formation begins earlier on
weekends and provides one reason why ozone might be higher on weekends.
Another reason for higher ozone on weekends is the higher rate of ozone accumulation on
weekends during the ozone accumulation period. Duration of ozone accumulation was estimated
by the difference between time of maximum ozone (t maxO3 ) and t NO=O3 . The rate of ozone
accumulation (ppb/hour) is the increase in ozone from t NO=O3 to t maxO3 divided by the duration of
ozone accumulation. Figure 2-2 shows higher VOC/NOx ratios during weekends, which result in
a higher rate of ozone formation. Ozone is higher on weekends at Azusa because the duration of
ozone accumulation is longer and the rate of ozone accumulation is higher. We examined these
statistics for twelve monitoring sites throughout the basin for the period 1981 to 1999 to develop
a conceptual explanation that accounts for the spatial and temporal variations in the strength of
the weekend effect.
Air quality data for summers (June 1 to September 30) of 1981 to 1998 were obtained
from the latest (February 2000) ARB ambient data CD. The database was validated and screened
for invalid and suspicious data according to the procedures and criteria described by Fujita et al.
(2000a). When forming composite averages by day-of-the-week, holidays were removed if
2-1