Remote sensing of trace gases with Chinese satellite instruments

Remote sensing from hyperspectral satellite instruments, such as OMI, TROPOMI and OMPS, can simultaneously obtain the spatio-temporal distribution of several species of trace gases, which has been widely used to study the emissions, regional transport and physical and chemical evolution of trace gases. Nevertheless, there were very few relevant studies using Chinese satellite instruments, because the poor spectral quality makes it extremely difficult to retrieve data from the spectra of the Environmental Trace Gases Monitoring Instrument (EMI), the first Chinese satellite-based ultraviolet–visible spectrometer monitoring air pollutants. In this study, we performed on-orbit wavelength calibration to calculate daily instrumental spectral response functions (ISRFs) and wavelength shifts to diminish the fitting residuals. For the retrieval under the low signal-to-noise ratio (SNR) of EMI, an adaptive iterative retrieval algorithm is set up to select the retrieval setting best with minimum uncertainty. Besides, we used simulated irradiance instead of measured irradiance to obtain the requisite daily solar spectrum for the following retrieval algorithm, because EMI only provides the solar spectrum once every six months. Through these algorithm updates, several trace gases, such as Ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂) and formaldehyde (HCHO), were retrieved from EMI with comparable accuracy of OMI and TROPOMI. The retrieval results from EMI were used to locate emission sources, evaluate regional transport and trace the change of air quality due to important events, such as COVID-19 pandemic, China International Import Expo and Beijing Winter Olympic Games.