Day-of-Week Behavior of Atmospheric Ozone in Three U.S. Cities

Abstract

The weekly cycles of atmospheric ozone (O₃) are of interest because they provide information about the response of O₃ to changes in anthropogenic emissions from week-days to weekends. The weekly behavior of O₃ in Chicago, IL; Philadelphia, PA; and Atlanta, GA, is contrasted. In Chicago and Philadelphia, maximum 1-hr average O₃ increases on weekends. In Atlanta, O₃ builds up from Mondays to Fridays and declines during weekends. In all three areas, volatile organic compound (VOC)/nitrogen oxides (NO_x) ratios are higher during weekends, resulting from greater than proportionate decreases in NO_x relative to VOC emissions. The VOC/NO_x ratios correlate with maximum 1-hr O₃ concentrations in Chicago, a response consistent with a VOC-sensitive airshed. A weak correlation between O₃ concentrations and VOC/NO_x ratios in Philadelphia suggests the impact of transported O₃, which is formed in upwind VOC-sensitive locations that may be hundreds of kilometers away. Ozone concentrations in Atlanta do not correlate with VOC/NO_x ratios but with concentrations of NO_x and total reactive nitrogen (NO_y) carried over from the previous day. When data from 1986–1990 and 1995–1999 are compared, only small differences in the weekly behavior of O₃ are observed in Chicago and Philadelphia. The day-of-week differences in O₃ are amplified in the more recent period in Atlanta, a possible result of urban growth.