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 NO 2 concentrations decreased by more than 50%. The ozone-climate penalty, defined as the slope of O 3 change with increasing temperature (Δ O 3 /Δ T ), was by average 0.43 ppb K −1 less in 1999–2007 than in 1990–1998, indicating the early success of NO x emission controls. However, the slope did not continue to decrease in 2008–2015 despite further NO x emission reductions, and it increased more rapidly with increasing temperature (Δ 2 O 3 /Δ 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 NO x and VOC emissions in the Midwest. [ABSTRACT FROM AUTHOR]