Office of FreedomCAR and Vehicle Technologies Health Impacts Program (Poster)

We evaluated day-of-week differences in mean concentrations of ozone precursors [nitric oxide (NO), nitrogen oxides (NOx), volatile organic compounds (VOC) and carbon monoxide (CO)] at monitoring sites in 23 states comprising seven geographic focus areas for the years 1998-2003. We used Wednesdays to represent weekdays and Sundays to represent weekends; we also analyzed Saturdays. Statistical t-tests applied to every monitor showed substantial and statistically significant differences between Wednesday and weekend mean concentrations of ozone precursors in all study areas. At half the sites, nine-hour (6 am through 3 pm) mean concentrations of NO, NOx, and CO declined by at least 65, 49, and 28 percent, respectively, from Wednesdays to Sundays. Large reductions in mean concentrations of ozone precursors on weekends did not reduce mean concentrations of peak ozone significantly, and in many areas, mean peak ozone increased on weekends. Ozone accumulation began earlier on Sundays than on Wednesdays, on average. We also evaluated day-of-week differences in mean concentrations of fine particle and nitrate at available monitoring sites for the same time period and same geographic focus areas. Based on the monitors differences in Wednesday and Sunday means, PM nitrate showed almost negligible weekend decline, despite large NO and NOx declines