15 January 2009

A new air pollution study found that very large NOx emission reductions that take place on weekends in the United States result in higher or similar ozone concentrations compared to weekdays. These large NOx emission reductions on weekends—attributed mostly to the fewer buses and trucks on the roads—do not result in reductions of ozone pollution, but, on the contrary, lead to higher ambient ozone levels.

The findings from the study suggest that attainment of ozone standards require greater emphasis on reducing VOC emissions, rather than NOx, in major US metropolitan areas. These findings have important policy implications. “EPA’s 2010 NOx emission standards for heavy-duty vehicles will result in no changes, and perhaps, higher levels of ambient ozone levels,” said Doug Lawson of NREL, a co-author of the study.

The study evaluated day-of-week differences in mean concentrations of ozone precursors (NO, NOx, CO, and VOC) at monitoring sites in 23 states over the period 1998-2003. Wednesdays were used to represent weekdays. Statistically significant 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.

Despite the large reductions in ambient NOx and moderate reductions in ambient CO and VOC concentrations on weekends, ozone and particulate matter nitrate did not exhibit large changes from weekdays to weekends. 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.

Increased ozone levels during weekends, at the time when traffic volumes of heavy-duty diesel vehicles are decreased, were discovered in the 1990s in Southern California. The phenomenon, known as the “ozone weekend effect”, seems to be explained by the impact of ambient NOx:VOC concentrations ratios, as opposed to absolute NOx levels, on ozone formation. Several authors believe that under the atmospheric conditions in Southern California, further reductions of ozone would require more emphasis on reductions of VOC emissions rather than NOx.

The results of the new study implicate that the ozone weekend effect is not limited to California, but is occurring at most urban locations in the United States. Thus, further reductions of vehicle NOx emissions, if not accompanied by adequate VOC emission reductions, may be in fact counterproductive in protecting urban air quality. These findings may be also applicable to urban locations outside the United Stated, including metropolitan areas in Europe and Asia.

The paper, titled “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” and authored by C. Blanchard, S. Tanenbaum, and D. Lawson was published in the December 2008 issue of the Journal of the Air & Waste Management Association.

Source: Journal of the AWMA