Ozone Trends and the Ability of Models to Reproduce the 2020 Ozone Concentrations in the South Coast Air Basin in Southern California under the COVID-19 Restrictions

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.

NASA Satellite Measurements Show Global‐Scale Reductions in Free Tropospheric Ozone in 2020 and Again in 2021 During COVID‐19

“…( 2021 ) and Parker et al. ( 2022 ) also show increases up to about +40% for near‐surface ozone concentrations in spring‐summer 2020 over the US in urban environments. Campbell et al.…”

“…These studies suggest that the increases in surface ozone could be due to a weakening of a titration effect on ozone in accordance with 2020 meteorological conditions and relative levels of NO x versus VOC reductions. Campbell et al (2021) and Parker et al (2022) also show increases up to about +40% for near-surface ozone concentrations in spring-summer 2020 over the US in urban environments. Campbell et al (2021) further describe the complex nature of the BL chemistry (NO x limited vs. VOC limited) whereby the widespread emission decreases in the US in 2020 in a general sense led to increases of BL ozone in urban areas, but widespread decreases in rural regions.…”