The ozone-climate penalty in the Midwestern U.S

This paper investigates the relationship between ground-level ozone (O-3) and temperature in the Midwestern U.S. during the period 1990-2015. From 1990 to 2015, the overall trend of 95th percentile temperature showed an increase of 0.04 K yr(-1), while summertime 95th percentile O-3 concentrations in the Midwest decreased at an average rate of 0.7 ppb yr(-1) largely because NO2 concentrations decreased by more than 50%. The ozone-climate penalty, defined as the slope of O-3 change with increasing temperature (Delta O-3/Delta T), was by average 0.43 ppb K-1 less in 1999-2007 than in 1990-1998, indicating the early success of NOx emission controls. However, the slope did not continue to decrease in 2008-2015 despite further NOx emission reductions, and it increased more rapidly with increasing temperature (Delta O-2(3)/Delta T-2) by 0.03-0.09 ppb K-2 in most urban areas of the Midwest. This was accompanied by more frequent dry tropical (DT) weather in the Midwest since 2008. We find that O-3 in DT weather was 12 ppb and 17 ppb higher than in non-DT weather in rural and urban areas, respectively. Furthermore, the 2008-2015 period experienced 8% more surface air stagnation days than in 1990-1998. This demonstrates that, in addition to the impact of warmer temperatures, the ozone-climate penalty could be aggravated by altered weather conditions under climate change. It will be challenging for Midwestern cities to attain the National Ambient Air Quality Standard for O-3 if such conditions persist in the future, and future air quality improvements may require even greater efforts to reduce both NOx and VOC emissions in the Midwest. (C) 2017 Elsevier Ltd. All rights reserved.