Investigating the Effects of NOx Emission, Chemistry, and Transport on the Diurnal Variation of NO2 over East Asia Using Ground-Based and Geostationary Satellite Observations

Nitrogen oxides (NO_x ≡ NO + NO₂) are detrimental to health and are precursors of tropospheric ozone and secondary aerosol. The Geostationary Environment Monitoring Spectrometer (GEMS), launched in 2020, provides an unprecedented opportunity to observe the diurnal variation of NO₂ column over East Asia. This diurnal variation offers information on the emission, chemistry, and air quality impacts of NO_x, but it needs to be carefully interpreted. Here, we investigate the key drivers of the diurnal variation of NO₂ using the GEMS observations during the winter and the summer of 2021-2022 over Eastern China and South Korea. We place the GEMS observations in the context of ground-based column observations from Pandora and surface NO₂ observations from air quality networks. We use the GEOS-Chem chemical transport model to understand the drivers of the NO₂ diurnal variation and separate the effects of emissions and chemistry in different seasons. Our results indicate that diurnal variations in transport can complicate the interpretation of the NO₂ observations, and we show how to address this complication.