Weekday Versus Weekend On-Road Mobile Source Emissions Activity Patterns in California’s South Coast Air Basin

Since the mid-1970s, episodes of high ozone concentrations tend to occur more often on weekends rather than weekdays in California’s South Coast Air Basin (SoCAB). These weekend episodes seem counterintuitive because air pollutant emissions estimates are reduced for weekends (relative to weekdays).1,2,3,4,5 If weekend emissions are reduced, then why are ozone episodes more likely to occur on weekends? And, what does the so-called “weekend effect ” imply for development of effective ozone control strategies in the SoCAB? It is extremely unlikely that systematic weekday-to-weekend (WD-WE) differences in weather tendencies could explain the weekend effect. Rather, differences in the relative proportions of ambient air pollutants correlate with weekend ozone peaks.6 Therefore, researchers postulate that an aggregate shift in human activities on weekends, which causes certain pollutants to be emitted in disproportionately lesser amounts, is the most plausible explanation for the weekend effect.7,8 This paper examines variability in one of the SoCAB’s most important generators of air pollutants—vehicular traffic. Traffic volume data were acquired from the California Department of Transportation (Caltrans), which operates an extensive network of weigh-in-motion (WIM) detector