First observation of tropospheric nitrogen dioxide from the Environmental Trace Gases Monitoring Instrument onboard the GaoFen-5 satellite

The Environmental Trace Gases Monitoring Instrument (EMI) is the first Chinese satellite-borne UV–Vis spectrometer aiming to measure the distribution of atmospheric trace gases on a global scale. The EMI instrument onboard the GaoFen-5 satellite was launched on 9 May 2018. In this paper, we present the tropospheric nitrogen dioxide (NO2) vertical column density (VCD) retrieval algorithm dedicated to EMI measurement. We report the first successful retrieval of tropospheric NO2 VCD from the EMI instrument. Our retrieval improved the original EMI NO2 prototype algorithm by modifying the settings of the spectral fit and air mass factor calculations to account for the on-orbit instrumental performance changes. The retrieved EMI NO2 VCDs generally show good spatiotemporal agreement with the satellite-borne Ozone Monitoring Instrument and TROPOspheric Monitoring Instrument (correlation coefficient R of ~0.9, bias < 50%). A comparison with ground-based MAX-DOAS (Multi-Axis Differential Optical Absorption Spectroscopy) observations also shows good correlation with an R of 0.82. The results indicate that the EMI NO2 retrieval algorithm derives reliable and precise results, and this algorithm can feasibly produce stable operational products that can contribute to global air pollution monitoring. Improvements to a high-resolution spectrometer onboard China’s recently-launched GaoFen-5 (GF-5) satellite can give researchers a new tool to assess atmospheric trace gases. Cheng Liu from the University of Science and Technology of China and his colleagues reported the first successful retrieval of nitrogen dioxide from the GF-5 orbiter thanks to crucial adjustments to calibration algorithms. The satellite’s payload Environmental Trace Gases Monitoring Instrument (EMI) uses visible spectroscopic measurements to detect nitrogen dioxide in the atmosphere, which can be used to identify the spatial distribution of pollution emissions on the Earth. To overcome on-orbit issues including detector saturation, the team has optimized the spectral retrieval settings by spectral precalibration, fitting wavelength adjustment, reference spectrum selection, etc. Comparisons to other satellite and ground measurements revealed the optimization effort yielded reliable results for monitoring pollution levels.