###### Abstract  
The current U.S. emission control requirements for on-road motor vehicles are driven by the ozone problem in the South Coast Air Basin (SoCAB) in southern California. Based on ozone modeling performed for Air Quality Management Plans (AQMPs), the SoCAB ozone attainment plan requires large (>80%) amounts of emission reductions in oxides of nitrogen (NOx) from current levels with more modest (~40%) controls on Volatile Organic Compounds (VOC). The shelter in place orders in response to the 2020 COVID-19 pandemic resulted in an immediate reduction in emissions, but instead of ozone being reduced, in 2020 the SoCAB saw some of the highest observed ozone levels in decades. We used the abrupt emissions reductions from 2019 to 2020 caused by COVID-19 to conduct a dynamic model evaluation of the Community Multiscale Air Quality (CMAQ) model to evaluate whether the models used to develop ozone control plans can correctly simulate the ozone response to the emissions reductions. Ozone modeling was conducted for three scenarios: 2019 Base, 2020 business-as-usual (i.e., without COVID reductions), and 2020 COVID. We found that modeled ozone changes between 2019 and 2020 were generally consistent with the observed ozone changes. We determined that meteorology played the major role in the increases in ozone between 2019 and 2020; however, the reduction in NOx emissions also caused ozone increases in Los Angeles County and into western San Bernardino County, with more widespread ozone decreases further to the east. 20221352820220220223528202  
and nitrogen dioxides (NOx) were lowered, but the reduction was modest and transient. Their findings are important on regional scales but, because they spatially aggregated data, important local scale variations (e.g., within the South Coast Air Basin (SoCAB) of California) may be obscured in their analysis. [PERSON] et al. [3] considered the Air Quality Index (AQI) in 20 major cities worldwide and found substantially decreased NO\({}_{2}\) concentrations and smaller, mixed impacts on ozone concentrations from COVID-19 lockdowns. For Los Angeles, they found lower NO\({}_{2}\) AQI but no noticeable effect on ozone AQI during their study period of 19 March-7 May 2020. [PERSON] et al. [4] utilized TROPOMI satellite data to disentangle the impact of the COVID-19 lockdowns on urban NO\({}_{2}\) from natural variability and found that, after accounting for meteorology, NO\({}_{2}\) reductions were between 9.2% and 43.4% among 20 cities in North America, and approximately 33% in Los Angeles from 15 March to 30 April 2020. [PERSON] et al. [5] utilized TROPOMI satellite data and ground-based observations and concluded that lockdown events have reduced the population-weighted concentration of nitrogen dioxide and particulate matter levels by about 60% and 31% in 34 countries, with mixed effects on ozone during lockdown dates up to 15 May 2020. [PERSON] et al. [6] evaluated the impacts of the COVID-19 lockdowns on emissions and ozone air quality by using observational data and the National Air Quality Forecasting Capability (NAQFC) air quality modeling system, which utilizes CMAQ at 12 km resolution. Their study included the entire U.S. and evaluated the period March-September 2020. They found variable impacts on maximum daily 8-hr average (MDA8 ozone concentrations across the U.S., including widespread decreases in rural regions but also localized increases near some of the highly populated urban areas  
The impacts of COVID-19 on air quality have also been investigated with a modeling study by [PERSON] et al. [7], who used the global Community Earth System Model (CESM v2.2, Boulder, CO, USA) to investigate the response of secondary pollutants (ozone, secondary organic aerosols) in different parts of the world in response to modified emissions of primary pollutants during the COVID-19 pandemic. They found ozone decreases for most of the U.S. but increases in ozone within the SoCAB as well as a few other localized regions. [PERSON] et al.