Using Orbital and Suborbital Observations to Constrain US Air Quality Trends

Abstract

Ozone (O3) and particulate matter (PM) pollution in surface air are harmful to human health and the environment. Tropospheric ozone is produced by photochemical oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxide radicals (NOx = NO + NO2) and PM is directly emitted or formed in the atmosphere from oxidation and subsequent condensation of precursor gases (SO2, NH3, NOx, VOCs) into the aqueous aerosol phase. In effort to improve US air quality, the Environmental Protection Agency (EPA) has been regulating these precursor emissions since the 1970s under the Clean Air Act. As emissions have drastically decreased and the dominant pollution sources have changed, regulatory control strategies must adapt in order to continue to limit human and environmental exposure to ozone and PM. This thesis aims to address the emissions and formation processes that control ambient ozone and aerosol concentrations in surface air and the troposphere in the US over the past decade. The objectives of this work are to investigate the (1) processes controlling aerosol formation and composition in response to changes in emissions, (2) gas phase chemical cycling in the upper troposphere, and (3) consistency between long-term trends in US NOx emissions and satellite observations. In order to address these questions, we rely on a wide range of atmospheric observations from surface networks, aircraft campaigns, and satellite observations, with a specific focus on the Southeast US in summer where observations are particularly comprehensive. Additionally, we use the 3D chemical transport model, GEOS-Chem, as a tool to interpret these atmospheric observations and identify gaps in our understanding of the chemical and physical processes controlling atmospheric pollutants. In this thesis, we identify a fundamental discrepancy between thermodynamic equilibrium theory and observations of inorganic aerosol composition in the eastern US in summer that show low ammonium-sulfate aerosol ratios. In addition, we show that from 2003 to 2013, while SO2 emissions have declined due to US emission controls, aerosols have become more acidic in the Southeastern US. To explain these observations, we suggest that the large and increasing source of organic aerosol may be affecting thermodynamic equilibrium. Next, we investigate the chemistry controlling NO and NO2 in the upper troposphere by identifying large inconsistencies between observed and modeled NO/NO2 ratios over the Southeast US during August–September 2013. We suggest that either unrecognized chemistry or errors in modeled cycling between NO, NO2, and O3 could explain this discrepancy. We discuss how either explanation will have important implications for global tropospheric chemistry and for the interpretation of satellite observations of NO2. The EPA reports a steady decrease in NOx emissions from fuel combustion over the 2005–2017 period, while satellite observations show a leveling off after 2009 suggesting emission reductions and related air quality gains have halted. In the final section of this thesis, we show the sustained decrease in NOx emissions is in fact consistent with observed trends in surface NO2 and ozone concentrations, and that the flattening of the satellite trend reflects a growing influence from the non-anthropogenic background.