Seasonal and weekday-dependent diurnal variability of NOx emissions and lifetimes from hourly GEMS NO2 observations

Tropospheric nitrogen dioxide (NO₂) is a key indicator of nitrogen oxide (NO_x) emissions and atmospheric chemical processes.  Tropospheric NO₂ columns retrieved from instruments in low Earth orbit (LEO), such as OMI and TROPOMI, have been extensively used to investigate the spatial and temporal variability of NO_x emissions. However, they typically have only one overpass per day and location, with the availability of data being further reduced due to the presence of clouds. The new geostationary instruments, GEMS, TEMPO, and Sentinel-4, enable the observation of atmospheric trace gases with hourly temporal resolution. The Geostationary Environmental Monitoring Spectrometer (GEMS), launched in February 2020, is the longest-operating and provides hourly daytime observations of NO₂ with a spatial resolution of 3.5 x 8 km² over a large part of Asia.

In this study, four years of GEMS IUP-UB tropospheric NO₂ column data have been analyzed to investigate the seasonal and weekday-dependent diurnal variability of NO_x emissions and lifetime for several emission sources within the GEMS domain. The resulting hourly emission and lifetime estimates are used to assess emission inventories and atmospheric models. For some emission sources, such as Seoul, seasonal emissions differ in magnitude and diurnal pattern, whereas other locations, such as Singapore, show almost no seasonal variation and small diurnal variation. Weekday-to-weekend differences are analyzed on an hourly basis, revealing a clear weekend effect, with small diurnal differences for most analyzed emission sources. Hourly emission profiles used in emission inventories and air quality models are compared with the GEMS-based emission estimates, providing observational constraints on the representation of diurnal NO_x emissions in current atmospheric chemistry models. GEMS-based hourly lifetime estimates are compared to WRF-Chem model results.