Interpreting Geostationary Environment Monitoring Spectrometer (GEMS) geostationary satellite observations of the diurnal variation in nitrogen dioxide (NO₂) over East Asia

Nitrogen oxide radicals (NO_x≡ NO+NO₂) emitted by fuel combustion are important precursors of ozone and particulate matter pollution, and NO₂ itself is harmful to public health. The Geostationary Environment Monitoring Spectrometer (GEMS), launched in space in 2020, now provides hourly daytime observations of NO₂ columns over East Asia. This diurnal variation offers unique information on the emission and chemistry of NO₂, but it needs to be carefully interpreted. Here we investigate the drivers of the diurnal variation in NO2 observed by GEMS during winter and summer over Beijing and Seoul. We place the GEMS observations in the context of ground-based column observations (Pandora instruments) and GEOS-Chem chemical transport model simulations. We find good agreement between the diurnal variations in NO₂ columns in GEMS, Pandora, and GEOS-Chem, and we use GEOS-Chem to interpret these variations. NO₂ emissions are 4 times higher in the daytime than at night, driving an accumulation of NO₂ over the course of the day, offset by losses from chemistry and transport (horizontal flux divergence). For the urban core, where the Pandora instruments are located, we find that NO₂ in winter increases throughout the day due to high daytime emissions and increasing NO₂/NO_X ratio from entrainment of ozone, partly balanced by loss from transport and with a negligible role of chemistry. In summer, by contrast, chemical loss combined with transport drives a minimum in the NO₂ column at 13:00-14:00 local time (LT). Segregation of the GEMS data by wind speed further demonstrates the effect of transport, with NO₂ in winter accumulating throughout the day at low winds but flat at high winds. The effect of transport can be minimized in summer by spatially averaging observations over the broader metropolitan scale, under which conditions the diurnal variation in NO2 reflects a dynamic balance between emission and chemical loss.