Response of the Summertime Ground-level Ozone Trend in the Chicago Area to Emission Controls and Temperature Changes 2005–2013

Abstract:

Despite strenuous efforts to reduce the emissions of ozone precursors such as nitrogen oxides (NO_x), concentrations of ground-level ozone (O₃) still often exceed the National Ambient Air Quality Standard in U.S. cities in summertime, including Chicago. Furthermore, studies have projected a future increase in O₃ formation due to global climate change. This study examines the response of summertime O₃ to emission controls and temperature change in the Chicago area from 2005 to 2013 by employing observations of O₃, O₃ precursors, and meteorological variables. We find that meteorology explains about 53% of the O₃ variance in Chicago. O₃ mixing ratios over Chicago are found to show no clear decline over the 2005–2013 period. The summertime ground-level O₃ trend consists of a decrease of 0.08 ppb/year between 2005 and 2009 and an increase of 1.49 ppb/year between 2009 and 2013. Emissions of NO_x and concentrations of NO₂ have been decreasing steadily from 2005 to 2013 in the Chicago area. Concentrations of volatile organic compounds (VOCs) in Chicago, however, have more than doubled since 2009, even though emission inventories suggest that VOC emissions have decreased. We believe that O₃ production in Chicago became more sensitive to VOCs starting in 2008/2009 and may have switched from being NO_x-limited to VOC-limited. The warmer climate since 2008 has also contributed to the increasing ozone trend in the Chicago area. Increased attention should be paid to improving the quantification of VOC sources, enhancing the monitoring of reactive VOC concentrations, and designing VOC mitigation measures.