Advancing Chemical Transport Modeling for Air Quality, Satellite Retrievals, and the Clean Energy Transition

Abstract

Tropospheric oxidant chemistry affects air quality by controlling the formation pathways of air pollutants. It also determines the atmospheric lifetime of key greenhouse gases (GHGs) such as carbon dioxide (CO2) and methane (CH4), as well as indirect GHGs like hydrogen (H2). GEOS-Chem is a state-of-the-science atmospheric chemistry model that represents our current understanding of tropospheric oxidant chemistry. The GEOS-Chem chemical transport model (CTM) is used to support the satellite retrievals of air pollutants like nitrogen dioxide (NO2) and to assess the warming potential of GHGs. Recent advances in satellite observations of air pollutants (e.g., NO2) have emerged with the launch of three geostationary satellites: GEMS (2020), TEMPO (2023), and Sentinel-4 (2025). GEMS is the first geostationary satellite that provides hourly NO2 data over East Asia, rather than just one observation per day. As a result, improving our understanding of geostationary satellite retrievals and interpreting hourly data observed from it is an important task. In this work, we first evaluate the ability of GEOS-Chem to accurately simulate the tropospheric oxidant chemistry over East Asia by comparing model output with an extensive suite of aircraft measurements from the KORUS-AQ campaign. Following this validation, we use GEOS-Chem vertical profiles to support geostationary satellite retrievals and investigate how diurnal variation in NO2 profiles affects hourly NO2 satellite retrievals (Chapter 1). Next, we examine how the diurnal variation in column NO2 observed by geostationary satellite differs from that measured in surface NO2 measurements. We leverage GEOS-Chem’s ability to separate the effects of chemistry, transport, and emissions to interpret the observed NO2 variation (Chapter 2). Lastly, during the KORUS-AQ aircraft campaign, we identified a discrepancy between observed and modeled concentrations of methyl hydroperoxide (CH3OOH), which is unexpectedly elevated over the Seoul Metropolitan Area. GEOS-Chem fails to reproduce this behavior. We show that measurement interference from methanediol, a chemical species formed via in-cloud hydration of formaldehyde, may explain part of this discrepancy. We also explore the role of methanediol in oxidant chemistry and formic acid formation (Chapter 3). While air quality is important to human health, transitioning to cleaner energy is also essential to mitigate climate change. The Intergovernmental Panel on Climate Change (IPCC) recommends achieving net-zero anthropogenic CO2 emissions by 2050 to keep global warming to 1.5 ◦C. One proposed solution is switching from fossil fuels to hydrogen. However, hydrogen emissions can affect atmospheric abundances of methane, ozone, and water vapor, making hydrogen an indirect GHG. The global warming potential (GWP) is a commonly used metric to evaluate the climate impact of GHGs. Previous studies using models have shown that soil sink is the largest uncertainty in estimating its GWP. However, current models have known biases in their simulations of OH concentration and reactivity, and how these biases affect the evaluation of hydrogen global warming potential has not been considered. We find that these biases lead to a 20% overestimate in the GWP of hydrogen (Chapter 4).