气候变化领域集成服务门户

Despite strenuous efforts to reduce the emissions of ozone precursors such as nitrogen oxides (NOx), concentrations of ground-level ozone (O-3) 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 O-3 formation due to global climate change. This study examines the response of summertime O-3 to emission controls and temperature change in the Chicago area from 2005 to 2013 by employing observations of O-3, O-3 precursors, and meteorological variables. We find that meteorology explains about 53% of the O-3 variance in Chicago. O-3 mixing ratios over Chicago are found to show no clear decline over the 2005-2013 period. The summertime ground-level O-3 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 NOx 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 O-3 production in Chicago became more sensitive to VOCs starting in 2008/2009 and may have switched from being NOx-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. (C) 2014 Elsevier Ltd. All rights reserved.