Understanding the factors controlling ozone pollution in East Asia

Abstract

East Asia is one of the most severely polluted regions in the world. A key component of this pollution is tropospheric ozone, which poses a threat to both human and ecosystem health. Despite targeted efforts to reduce emissions, tropospheric ozone has continued to rise steadily in the region over the past twenty years. This dissertation investigates the factors controlling ozone pollution in East Asia using integrated data analysis from satellites, aircraft campaigns, and in-situ measurements, as well as chemical transport modeling. Specific topics addressed in my dissertation include the following: Infer the spatial distribution of surface ozone concentrations in Asia using multispectral satellite ozone retrievals (Chapter 1): Over the past two decades, satellite instruments have provided unprecedented information on global air quality, but direct inference of surface ozone from space remains challenging. Here, we develop a novel approach that combines multispectral ozone retrievals from the thermal infrared Tropospheric Emission Spectrometer (TES) and the ultraviolet-visible Ozone Monitoring Instrument (OMI) with a chemical reanalysis. Our results show the potential of combining satellite measurements and chemical reanalyses to augment air quality assessments in regions lacking robust surface monitoring networks. Diagnose the causes of persistently high surface ozone in South Korea (Chapter 2): Despite substantial efforts to reduce emissions, South Korea continues to experience widespread exceedances of their ozone standard. I examine trends in ozone and $NO_2$ from 2015–2019 across South Korea, identifying volatile organic compounds (VOCs) as a dominant driver of ozone formation under current conditions. Simulations with anthropogenic emissions zeroed out reveal a significant external background contribution, implying that the air quality standard in South Korea is not practically achievable unless this background external to East Asia can be decreased. Determine the origin of increasingly high background ozone in East Asia (Chapter 3): From Chapter 2, we find that severe surface ozone pollution in East Asia is due in part to an elevated background subsiding from the free troposphere. We find that increasing background ozone is driven by enhanced stratospheric downwelling in recent years. This growing stratospheric contribution poses a substantial obstacle to achieving air quality standards, suggesting that mitigation strategies must account for both anthropogenic emissions and climate-driven changes in stratosphere–troposphere exchange.