Weekend/Weekday Ozone Observations in the South Coast Air Basin

1.1 Introduction
Since the mid 1970’s it has been documented that ozone levels in California’s South
Coast Air Basin (SoCAB) are higher on weekends than on weekdays, in spite of the fact that
emissions of ozone precursors, oxides of nitrogen (NOx) and to a lesser extent volatile organic
compounds (VOC), are lower on weekends than on weekdays (Elkus and Wilson, 1977; Horie et
al., 1979; Levitt and Chock, 1976; Zeldin et al., 1989; Blier et al., 1996; Blier et al., 1999; and
Austin and Tran, 1999). Similar weekend ozone effects have been observed in San Francisco
(Altshuler et al. 1995), the northeastern cities of Washington D.C., Philadelphia, New York
(SAIC, 1997) and Chicago, with a lesser effect in Atlanta (Pun et al. 2001). The weekend effect
is an observable demonstration that the rate of ozone production is a non-linear function of the
mixture of VOC and NOx in the atmosphere. Depending upon the relative concentrations 2 of
VOC and NOx and the specific mix of VOC present, the rate of O 3 formation can be most
sensitive to changes in VOC alone or to changes in NOx alone or to simultaneous changes in
both VOC and NOx. Understanding the response of ozone concentrations to specific changes in
VOC or NOx emissions is fundamental to understanding the weekend ozone effect.
Previous studies have shown that while peak ozone levels in the SoCAB have dropped
from ~ 0.45 ppm in the early 1980s to ~ 0.18 in the late 1990’s, the occurrences of higher ozone
levels on weekends have become more frequent and widespread over the past two decades. From
the mid-1980s to mid-1990s, decreases in peak ozone levels were greatest in the western and
middle portions of the SoCAB and on weekdays than on weekends; hence the magnitude of the
weekend ozone effect increased in the 1990s over the 1980s (Blier and Winer, 1996). Along with
a strengthening of the weekend ozone effect, the occurrences of highest ozone levels shifted
toward the eastern basin. During 1992-94, daily maximum ozone in the western and central basin
typically increased by about 30 percent from Friday to Saturday, decreased slightly from
Saturday to Sunday, and decreased back to Friday levels on Monday (Austin and Tran, 1999). By
1996-98, ozone levels were consistently higher on Sundays (Austin and Tran, 1999). During the
period from 1965 to 1972, peak ozone levels occurred on Sunday in the western portion (Long
Beach, Lennox, and West Los Angeles) of the SoCAB and on Saturday at downtown Los
Angeles. Ozone levels were generally higher on weekdays in the central and northern portions of
the basin (Azusa, Burbank, Reseda) (Elkus and Wilson, 1977). These trends in weekday ozone
patterns parallel a trend to lower VOC/NOx ratios.
Mobile sources are the single largest source category for ozone precursor pollutants,
accounting in 2000 for about 62%, 88%, and 95% of the average daily (summer) reactive organic
gases (ROG), NOx, and CO emissions, respectively, in the SoCAB according to official
government inventories (based on EMFAC2000 estimates) (CARB, 2001) 3 . According to
emissions backcasts from the 2000 base year, the ratios of current emissions to emissions in 1980
are 0.34 and 0.75 for ROG and NOx, respectively. During the past 20 years, NOx emissions
from gasoline vehicles decreased by 37 percent, while NOx emissions from diesel increased by
2 Gas-phase species are usually measured as molar ratios (e.g., ppbv or ppmv), which are technically mixing ratios.
The term concentration denotes units of mass per unit volume (e.g, µg/m 3 ). In common usage, these terms are used
interchangeably.
3 Vehicle emission models have historically underestimated emissions (Ingalls et al., 1989; Pierson et al. 1990; and
Fujita et al., 1992). Although many improvements have been incorporated into vehicle emissions models, the
uncertainties of the emission estimates are not known.
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