Weekend/Weekday Ozone Observations in the South Coast Air Basin

driving patterns may have been influenced by the holiday. Table 2-1 shows a list of the key daily
parameters of interest. Values of pollutants from 0400-0500 PDT are surrogates for carry-over
from the previous day. Values of pollutants from 0600-0900 PDT represent the morning
commute. The end of the ozone inhibition period, defined to be the time at which [NO] = [O3], is
found by computing the difference [NO] - [O3] from 0700-1400 PDT and finding the hour t at
which this difference transitions from greater than to less than zero. The interpolation involves
the intersection of the two line segments represented by the decreasing NO and the increasing
O 3 . Each variable was computed as defined in Table 2-1. Annual means are compiled by Fujita et
al. (2000). The parameters are listed by site and day-of-the-week with standard errors and
numbers of observations. Monitoring sites include: N. Long Beach, Anaheim, Lynwood, and Los
Angeles–N. Main on the western SoCAB; Reseda, Burbank, Pico Rivera, and La Habra in the
central SoCAB; and Azusa, Pomona, Upland, and Rubidoux in the eastern SoCAB. In the
following discussion, the annual averages are averaged into four time periods covering the years
1981-84, 1985-89, 1990-94, and 1995-98.
Non-methane hydrocarbons (NMHC) were estimated from CO using an empirical
relationship between NMHC and CO in canister samples collected by Desert Research Institute
at three sites in the SoCAB during the summers of 1995 and 1996 (Zielinska et al. 1999). DRI
collected canister and DNPH cartridge samples twice daily (6-9 a.m. and 1-4 p.m. PDT) at
downtown Los Angeles, Burbank and Azusa during six, seven-day periods during each summer
(504 samples). The canister samples were analyzed for methane, CO, CO 2 , and speciated C 2 to
C 12 hydrocarbons. Figure 2-3 shows scatterplots of CO versus NMHC for each of the three
sampling sites and data combined from all three sites. The R-squares of the regression range
from 0.84 to 0.93. These correlations suggest a common source for CO and NMHC. The zero
intercepts for CO at 40 to 90 ppbC NMHC indicate that contributions other than mobile sources
are relatively small near each of the three monitoring sites. We assume that estimates of NMHC
from regression with CO for 1995 and 1996 are reasonably valid for determining day-of-theweek
variations in NMHC concentrations and NMHC/NOx ratios for any year within the 18-year
period of interest. However, they are probably not valid for establishing long-term trends in
NMHC and NMHC/NOx ratios because the slope of the regression between CO and NMHC may
have changed over time with changing emission control technology.
2.1 Trends in Ambient Ozone Concentrations
Ambient ozone trends are governed by changes in the temporal and spatial patterns of
precursor emissions. These changes are driven by emission control measures in the state
implementation plans. The nation’s ozone strategy in the decades of the 70s and 80s favored
VOC control over NOx control, based on the premise that VOC/NOx ratios in most ozone
nonattainment areas were below 10 (i.e., VOC-limited). Despite two decades of increasingly
stringent emission controls, progress toward attainment of the ozone standard during this period
was slow, and the National Research Council (NRC) Committee on Tropospheric Ozone
Formation and Measurement concluded that the 20-year effort to attain the ozone National
Ambient Air Quality Standard (NAAQS) had largely failed (NRC, 1991). The NRC Committee
suggested that past ozone control strategies might have been misdirected due to a significant
underestimation of anthropogenic VOC emissions. Since the beginning of the decade of the
1990s, however, the number of annual exceedances of the federal 1-hour NAAQS for ozone in
2-2