December 2008
Differences between Weekday and Weekend Air Pollutant Levels in Atlanta; Baltimore; Chicago; Dallas–Fort Worth; Denver; Houston; New York; Phoenix; Washington, DC; and Surrounding Areas
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Charles L. Blanchard and Shelley Tanenbaum,
Envair, Albany, CA
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Douglas R. Lawson,
National Renewable Energy Laboratory, Golden, CO
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AbstractWe evaluated day-of-week differences in mean concentrations of ozone (O3) precursors (nitric oxide [NO], nitrogen oxides [NOx], carbon monoxide [CO], and volatile organic compounds [VOCs]) at monitoring sites in 23 states comprising seven geographic focus areas over the period 1998– 2003. Data for VOC measurements were available for six metropolitan areas in five regions. We used Wednesdays to represent weekdays and Sundays to represent weekends; we also analyzed Saturdays. At many sites, NO, NOx, and CO mean concentrations decreased at all individual hours from 6:00 a.m. to 3:00 p.m. on Sundays compared with corresponding Wednesday means. Statistically significant (p 0.01) weekend decreases in ambient concentrations were observed for 92% of NOx sites, 89% of CO sites, and 23% of VOC sites. Nine-hour (6:00 a.m. to 3:00 p.m.) mean concentrations of NO, NOx, CO, and VOCs declined by 65, 49, 28, and 19%, respectively, from Wednesdays to Sundays (median site responses). Despite the large reductions in ambient NOx and moderate reductions in ambient CO and VOC concentrations on weekends, ozone and particulate matter (PM) nitrate did not exhibit large changes from weekdays to weekends. The median differences between Wednesday and Sunday mean ozone concentrations at all monitoring sites ranged from 3% higher on Sundays for peak 8-hr concentrations determined from all monitoring days to 3.8% lower on Sundays for peak 1-hr concentrations on extreme-ozone days. Eighty-three percent of the sites did not show statistically significant differences between Wednesday and weekend mean concentrations of peak ozone. Statistically significant weekend ozone decreases occurred at 6% of the sites and significant increases occurred at 11% of the sites. Average PM nitrate concentrations were 2.6% lower on Sundays than on Wednesdays. Statistically significant Sunday PM nitrate decreases occurred at one site and significant increases occurred at seven sites.
ImplicationsWeekends provide an opportunity to examine how urban O3 and PM nitrate respond to large reductions in precursor emissions, although future emission reductions may differ in spatial and temporal distribution from those that presently occur on weekends. Weekend reductions of urban NOx emissions generate little change in peak O3 and mean PM nitrate concentrations at most urban U.S. locations. The findings from this study suggest that timely attainment of O3 standards will require greater emphasis on reducing VOC emissions in major metropolitan areas.
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