16 Mar 2022
16 Mar 2022
Status: this preprint is currently under review for the journal ACP.

Changing Ozone Sensitivity in the South Coast Air Basin during the COVID-19 Period

Jason Schroeder1, Chenxia Cai1, Jin Xu1, David Ridley1, Jin Lu1, Nancy Bui1, Fang Yan1,a, and Jeremy Avise1 Jason Schroeder et al.
  • 1California Air Resources Board, 1001 I Street, Sacramento, CA, USA
  • anow at: ICF Consulting, 980 9th Street, Sacramento, CA, USA

Abstract. The South Coast Air Basin (SoCAB), which includes the city of Los Angeles and is home to more than 15 million people, frequently experiences ozone (O3) levels that exceed ambient air quality standards. While strict regulation of O3 precursors has dramatically improved air quality over the past fifty years, the region has seen limited improvement in O3 over the past decade despite continued reductions in precursor emissions. One contributing factor to the recent lack of improvement is a gradual transition of the underlying photochemical environment from a VOC-limited regime towards a NOx-limited one. The changes in human activity prompted by COVID-related precautions in Spring and Summer of 2020 exacerbated these already-occuring changes in the O3 precursor environment. Analyses of sector-wide changes in activity indicate that emissions of NOx decreased by 15–20 % during Spring (April – May) and 5–10 % during Summer (June – July) relative to expected emissions for 2020, largely due to changes in mobile source activity. Historical trend analysis from two indicators of O3 sensitivity (the satellite HCHO/NO2 ratio and the O3 weekend/weekday ratio) revealed that Spring of 2020 was the first year on record to be on average NOx-limited, while the “transitional” character of recent Summers became NOx-limited due to COVID-related NOx reductions in 2020. Model simulations performed with base-case and COVID-adjusted emissions capture this change to a NOx-limited environment and suggest that COVID-related emissions reductions were responsible for a 0–2 ppb decrease in O3 over the study period. Reaching NOx-limited territory is an important regulatory milestone, and this study suggests that deep reductions in NOx emissions (in excess of those observed in this study) would be an effective pathway for long-term O3 reductions.

Jason Schroeder et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-178', Anonymous Referee #1, 30 Apr 2022
  • RC2: 'Comment on acp-2022-178', Anonymous Referee #2, 18 May 2022

Jason Schroeder et al.

Jason Schroeder et al.


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Short summary
Ozone, a key component of smog, has plagued the Los Angeles region for decades. Ozone is created by complex chemical reactions that can be greatly impacted by anthropogenic emissions. This study makes use of the COVID-19 period to study the sensitivity of ozone chemistry in LA to certain anthropogenic emissions, notably from vehicles. We find that vehicular emissions of key pollutants dropped by up to 25 % during COVID-19, which caused a fundamental shift in ozone chemistry in the region.