Project at a Glance
Title: Collection and analysis of weekend / weekday emissions activity data in the South Coast Air Basin.
Principal Investigator / Author(s): Sullivan, Dana Coe
Contractor: Sonoma Technology Incorporated
Contract Number: 00-305 & 00-313
Topic Areas: Area Sources, Mobile Sources & Fuels, Modeling, Monitoring
This report summarizes the efforts undertaken by Sonoma Technology, Inc. (STI) on behalf of the California Air Resources Board (ARB) to characterize, by day of week, activities associated with emissions of air pollutants. Activity data were collected in the South Coast Air Basin (SoCAB) during summer 2002. Activity data are an underlying component of emission inventories. Emission inventories are needed for evaluating the effectiveness of emission control plans and as input to complex air quality models that characterize the impact of emissions on air quality. Air quality modeling, based on emission inventories, is a critical component of demonstrating that emission control measures in regulatory plans will meet ambient air quality standards established to protect public health and welfare. In recent years, the ozone weekend effect, where ambient ozone concentrations tend to be higher on weekends than on weekdays in major urban areas, has been of particular interest. Although the trend of ozone concentrations in the SoCAB has been downward, the rate of decline has been faster on weekdays than on weekends, so much so that ozone concentrations on weekends now determine the “design value” for emission control plans. This ozone weekend effect is somewhat counter-intuitive because ambient concentrations (and presumably emissions) of ozone precursors decline on weekends compared to weekdays. Because peak ozone concentrations have historically occurred on weekdays, emission inventories were developed from activity factors characterizing average weekdays. If air quality models are to effectively guide emission control plans, it is now necessary to develop emission inventories representative of each day of the week, with particular focus on Saturday and Sunday activity patterns. Some of the factors that can cause increased ground-level ozone concentrations in the SoCAB have been investigated by studying the differences in mobile and stationary source emission activity patterns that occur on weekdays and weekends. A variety of tools were used to characterize day-of-week and time-of-day activity patterns for major types of emissions sources in the SoCAB: (1) telephone and mail surveys; (2) installation of global positioning systems (GPS) in volunteers’ cars; (3) measurements of traffic volumes at fixed locations; (4) acquisition of continuous emissions monitoring systems (CEMS) data for major point sources of NOx; and (5) confirmation of emissions sources and activity patterns by in-neighborhood observers. The results show that activity levels generally decline on weekends (especially Sunday) relative to weekdays for (1) on-road mobile sources (about 10-25%, with larger, 50-75%, decreases for heavy-duty vehicles), (2) commercial off-road mobile sources (about 90-95%), and (3) commercial area sources (about 75-80%). In addition, NOx emissions from point sources declined about 5-10% on weekends. In contrast, activity levels for recreational sources (e.g., barbecues and recreational vehicles [boats, ATVs]) increased about 25-165% on weekends relative to weekdays. As observed in previous analyses, not only does the amount of vehicular activity change but the timing also changes from a bimodal distribution associated with morning and afternoon commutes to work and school on workdays to a broad midday peak on weekends. New insights were gained from the instrumented light-duty-vehicle study: a high proportion, about 35%, of the highway vehicle miles traveled (VMT) occurred at speeds greater than 65 miles per hour and an apparent fewer number of trips per day were made than were counted in a previous study (about 2 less on both weekdays and weekends—4.7 vs. 6.7 on weekdays and 3.8 vs. 5.9 on weekends).
For questions regarding research reports, contact: Heather Choi at (916) 322-3893
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