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). 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-toweekend (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. 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. 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) detectors across the SoCAB’s freeway system. In addition, traffic data were collected at ten surface street locations for a period of ten days during Fall 2000. Significant WD-WE differences were observed in total daily traffic volumes and diurnal traffic patterns. The nature of these differences depended upon geography, road class, and vehicle type. Study findings support the following preliminary conclusions. • On weekends in urban zones, daily traffic volumes—which are dominated by light-duty (LD) vehicles—are about 15-30% smaller than those on weekdays and tend to peak around midday rather than during the morning and afternoon rush hours. • On weekends, truck and bus activities decrease to a far greater extent—by factors of 2 to 4—than do passenger vehicle activities. • On weekends in areas just beyond the urban zones, daily traffic volumes increase somewhat on weekends and tend to peak in the late afternoon on Fridays and Sundays. • When segregated by vehicle class, traffic patterns for freeways and surface streets in the central urban zone are similar in many respects. Heavy-duty (HD) truck patterns are a possible exception. On weekdays, a bimodal distribution in HD truck activity was observed on a handful of surface streets, while single-mode patterns were observed on freeways. These results complement those from a major ambient air quality study conducted in Fall 2000 to provide reasons for the causes of elevated weekend ozone levels in the SoCAB. INTRODUCTION Since the mid-1970s and possibly earlier, ozone concentrations in California’s South Coast Air Basin (SoCAB) have been higher on weekends than on weekdays—a tendency that has been more pronounced in the western SoCAB. This so-called “weekend effect” occurs despite the fact that emissions are estimated to be lesser on weekends than on weekdays and poses significant implications for the development of effective ozone control strategies. High ozone concentrations are a result of a complex photochemical process, in which the rate of ozone production is a non-linear function of the mixture of volatile organic compounds (VOC) and nitrogen oxides (NOx) in the atmosphere. The relative ambient concentrations of VOC and NOx and the specific mix of VOCs determine whether the rate of ozone formation is more sensitive to reductions in VOC emissions or reductions in NOx emissions. An understanding of these governing factors is a fundamental prerequisite to the development of less costly and more effective ozone abatement strategies. Given the unlikelihood that weekend-to-weekday (WE-WD) weather differences cause the weekend effect, it has been theorized that day-of-week variations in the temporal and spatial patterns of VOC and NOx emissions, coupled with complex physical and chemical interactions provide better grounds for an explanation . Everyday observations and common sense suggest that aggregate variations in human activities, which produce air pollutant emissions and follow a WE-WD pattern, are the most plausible causes of the weekend effect. However, measurements and data analyses are needed to quantify the nature and extent of these WE-WD differences. On-road mobile sources are major emitters of ozone precursors (VOC and NOx) in the SoCAB. Figure 1 shows the relative contributions of various classes of emissions sources to total emissions in the SoCAB, where mobile emissions were estimated with EMFAC 2000 (version 2.0). (Note that studies since 1987 have shown that mobile source emissions in the SoCAB have been significantly underestimated—an issue that was addressed with the development of EMFAC 2000.) In Figure 1, on-road mobile sources contribute approximately 50% of total reactive organic gases (ROG, which are largely comprised of VOC) and 60% of total NOx emissions in the SoCAB. HD trucks are responsible for about 40% of mobile-source NOx and 10% of mobile-source VOC, while LD passenger vehicles contribute approximately 35% of NOx and 60% of VOC. Therefore, it would seem that WE-WD differences in traffic volumes, timing of traffic peaks, and/or vehicle fleet mix would likely affect the concentrations and relative quantities of ambient VOC and NOx. This paper summarizes and discusses a data collection effort undertaken in the SoCAB to examine and quantify WE-WD differences in traffic patterns. Traffic volume data were acquired from the California Department of Transportation (Caltrans), which operates an extensive network of weighin-motion (WIM) detectors across the SoCAB’s freeway system. In addition, traffic data were collected for ten surface street locations for a period of ten days during Fall 2000. These data were analyzed to characterize WE-WD differences in weekly and daily traffic patterns. DATA ACQUISITION AND ANALYSES FOR FREEWAYS Traffic volume data for freeways were acquired for Caltrans WIM sites. The WIM network consists of sensors embedded in freeways that instantaneously record the number, weight, and speed of passing vehicles. The data are binned by 14 vehicle classes based on vehicle weight and axle spacing. The standard Caltrans protocol for processing WIM data is to summarize, quality assure, and archive two weeks’ worth of data per month for every site. For this study, data from 1997 and 2000 were acquired through a collaborative effort with the California Air Resources Board (ARB) and Caltrans. Figure 2 shows the WIM site locations in the SoCAB for which data were acquired. Traffic volumes were analyzed by day of week, hour of day, and vehicle type. The data were processed for annual and seasonal time periods and for a two-week study period that coincided with data collection on surface streets (September 29–October 11, 2000). When weekday/weekend travel patterns for these three time periods were compared, little difference was observed. Thus, driving behavior appears to be fairly consistent over the course of a year in the SoCAB, and the discussion presented here may be safely limited to annual average results. WIM sites were classified into two groups, interior basin and inflow/outflow, based on characteristics of the observed traffic patterns. The groups were differentiated by day-of-week and diurnal traffic patterns. Two inflow/outflow sites are located along two major freeways (Interstate 5 and Highway 10), which are corridors to and from the SoCAB. Interior basin sites are scattered throughout the central urban zone of the SoCAB. One site, Long Beach, was treated individually because it did not fit well into either group. The Long Beach site’s HD vehicle activity characteristics were unique, probably because the site is located on a stretch of the I-710 freeway that is the main artery to and from the Port of Long Beach. (The Port of Long Beach is the busiest container port in the United States and the sixth busiest in the world.) Traffic volumes and fleet mixes were analyzed for each WIM site. Figure 3 shows the average fleet mixes and traffic volumes for the inflow/outflow sites, the interior basin group, and the Long Beach site. HD vehicles comprise relatively greater fractions of the total traffic volumes at the inflow/outflow and Long Beach sites than at the interior basin sites. HD vehicle volumes decrease on weekends by factors of 2 to 4 throughout the basin, while LD vehicle volumes decrease slightly on weekends in the interior basin but increase slightly at the inflow/outflow sites. Figure 4 shows typical LD vehicle volumes by day of week and hour of day. LD vehicle patterns for Long Beach are similar to those for interior basin sites. At these locations, weekday LD vehicle volumes follow bimodal distributions with peaks during the morning and afternoon rush hours, and weekend LD vehicle volumes peak around midday. At the inflow/outflow sites, weekday LD volumes follow an attenuated bimodal distribution and are relatively high on Friday and Sunday afternoons. The increased volumes on Friday and Sunday afternoons are plausibly a result of vehicles departing for and returning from weekend recreation outside the SoCAB. HD vehicle volumes by day of week and hour of day are shown in Figure 5. HD vehicle volumes are very high at the Long Beach site. Diurnal HD vehicle volumes are similar for the interior basin and Long Beach sites in that the volumes tend to peak at midday. At the inflow/outflow sites, HD vehicle volumes peak in the evenings at Indio (which is in the eastern region of the SoCAB on Highway 10) and in the morning and midday at Castaic (which is in the northwestern region of the SoCAB on Interstate-5). DATA COLLECTION AND ANALYSES FOR SURFACE STREETS Traffic volumes were monitored on surface streets with automated pneumatic devices (loop sensors) that detect ti