Unexpected deterioration of O3 pollution in the South Coast Air Basin of California: The role of meteorology and emissions

Abstract

Tropospheric ozone (O3) pollution has long been a prominent environmental threat due to its adverse impacts on vulnerable populations and ecosystems. In recent years, an unexpected increase in O3 levels over the South Coast Air Basin (SoCAB) of California has been observed despite reduced precursor emissions and the driving factors behind this abnormal condition remain unclear. In this work, we combine ambient measurements, satellite data, and air quality modeling to investigate O3 and precursor emission trends and explore the impacts of meteorological variability and emission changes on O3 over the SoCAB from 2012 to 2020. Changes in O3 trends were characterized by declining O3 in 2012–2015, and increasing O3 afterwards with the most extreme O3 exceedances in 2020. Basin-wide increases of MDA8 O3 concentrations over warm season were depicted between 2012 and 2020, with the most significant enhancements (5–10 ppb) observed in San Bernardino County. Persistent heatwaves and weak ventilation on consecutive days were closely correlated with O3 exceedances (r2 above 0.6) over inland SoCAB. While decreasing trends in NOx (−4.1%/yr) and VOC emissions (−1.8%/yr) inferred from emission inventory and satellites during 2012–2020 resulted in a slow transition for O3 sensitivity from VOCs-limited to NOx-limited, model simulations performed with fixed meteorology indicate that unfavorable meteorological conditions could largely offset regulation benefits, with meteorology anomaly-induced monthly O3 changes reaching 20 ppb (May 2020) and the deterioration of O3 pollution in 2016, 2017, and 2020 was largely attributed to unfavorable meteorological conditions. Nevertheless, anthropogenic emission changes may act as the dominant factor in governing O3 variations across the SoCAB when net effects of meteorology are neutral (typically 2018). This work provides a comprehensive assessment of O3 pollution and contributes valuable insights into understanding the long-term changes of O3 and precursors in guiding future regulation efforts in the SoCAB.