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Changes in ozone chemical sensitivity in the U.S. from 2007 to 2016
Citation:
Koplitz, S., H. Simon, B. Henderson, J. Liljegren, G. Tonnesen, A. Whitehill, AND B. Wells. Changes in ozone chemical sensitivity in the U.S. from 2007 to 2016. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. American Chemical Society, Washington, DC, 2(3):206-222, (2021). https://doi.org/10.1021/acsenvironau.1c00029
Impact/Purpose:
This problem address ozone concentrations across the United States and how they have changed between 2007 and 2016. In particular, it re-analyzes modeling and empirical data to understand how ozone concentrations have changed and, importantly, how the ozone formation regimes (NOx limited versus VOC limited) have changed. It does this using both a day-of-week analysis, as well as Higher-ordered Decoupled Direct Method (HDDM) to understand changes in ozone formation chemistry between 2007 and 2016. This work is significant to understanding what continues to drive ozone NAAQS exceedances, which is important to targeted effective emissions reduction strategies. This research is of interest to federal partners, state and local agencies, and academic researchers.
Description:
Anthropogenic nitrogen oxide (NOx) and volatile organic compound (VOC) emissions in the U.S. have declined substantially over the last decade, altering the NOx-VOC chemistry and ozone (O3) production characteristics of many areas. In this work we use multiple air quality analysis tools to assess how these large reductions in NOx and VOC have affected O3 production 18 regimes across the U.S. between 2007 and 2016. We first compare observed and modeled evolution of NOx-limited and NOx-saturated O3 formation regimes using a day-of-week (DOW) analysis. This comparison builds confidence in the model’s ability to qualitatively capture O3 changes due to chemistry and meteorology both within years and across periods of large emissions decreases. DOW analysis, however, cannot definitively differentiate between emissions and meteorology impacts. We therefore supplement this analysis with sensitivity calculations from CAMx-HDDM to characterize modeled shifts in O3 formation chemistry between 2007 and 2016 in different regions of the U.S. We also conduct a more detailed investigation of the O3 chemical behavior observed in Chicago and Detroit, two complex urban areas in the Midwest. Both the ambient and modeling data show that more locations across the U.S. have shifted towards NOx-limited regimes between 2007 and 2016. The model-based HDDM sensitivity analysis shows only a few locations remaining NOx-saturated on high-O3 days in 2016 including portions of New York City, Chicago, Minneapolis, San Francisco and Los Angeles. This work offers insights into the current state of O3 production chemistry in large population centers across the U.S., as well as how O3 chemistry in these areas may evolve in the future.
