Toward diurnal observations of tropospheric NO2 from Chinese polar-orbiting satellite series

Nitrogen oxides (NO_x=NO₂+NO) play an important role in atmospheric chemistry and human health. Satellite measurements of atmospheric NO₂ have been made available by satellite ultraviolet‒visible spectrometers such as GOME, SCIAMACHY, OMI, and TROPOMI. To enhance the stereoscopic monitoring of air quality, China has launched the series of Environmental trace gases Monitoring Instruments (EMI) onboard multiple satellites including GaoFen-5 (2018 May–2020 April), GaoFen-5A (since 2022 December), GaoFen-5B (since 2021 September), and DQ-1 (since 2022 April), as well as more satellites to be planned. The EMI-series instruments can measure NO₂ at two local overpass times, i.e., 10:30 in the morning (DQ-1) and 13:30 in the afternoon (others), with a nadir spatial resolution of 12×13 km². In this study, we optimized a consistent retrieval algorithm for tropospheric NO₂ from the EMI series by fitting spectra with an earthshine reference over the remote ocean, masking cloudy pixels by the cloud fraction, and calculating the air mass factor by using high-resolution a priori profiles from a global chemical model and surface Lambertian-equivalent reflectivity. We have compared the EMI NO₂ retrievals with ground-based remote sensing measurements from NDACC, TCCON and other MAX-DOAS networks, as well as other satellites such as OMI, TROPOMI and GEMS. In general, both ground and satellite validations show good correlations and high precision of EMI NO₂ datasets. Finally, we explored the potential of detecting the diurnal variability of atmospheric NO₂ in different global cities by using EMI NO₂ observations. Such diurnal patterns can be helpful for the understanding of atmospheric chemistry and the design of geostationary satellite instruments in different global regions.