Abstract
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.
Highlights
Introduction The Environmental Trace Gases MonitoringInstrument (EMI)[1] is the first Chinese satellite-borne spectrometer with the aim to measure atmospheric pollutants from space
The results indicate that the Environmental Trace Gases Monitoring Instrument (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
We present a new tropospheric NO2 vertical column density (VCD, i.e., the vertical integral of NO2 concentration from the earth’s surface to the top of the atmosphere) retrieval algorithm dedicated to the EMI
Summary
Introduction The Environmental Trace Gases MonitoringInstrument (EMI)[1] is the first Chinese satellite-borne spectrometer with the aim to measure atmospheric pollutants from space. The Chinese EMI instrument is expected to contribute to the understanding of global air quality and atmospheric chemistry, similar to predecessor European and American satellite missions, e.g., the Ozone Monitoring Instrument (OMI)[2] and TROPOspheric Monitoring Instrument (TROPOMI)[3]. EMI has instrumental characteristics that are similar to OMI and TROPOMI, e.g., the local overpass time at ~13:30, spectral coverage, push-broom imaging technique, and daily global coverage. Both EMI and TROPOMI (launched in 2017) are new-generation satellite-borne air pollutant sensors compared to the OMI that was launched in 2004. Optimization of the NO2 retrieval algorithm was necessary to adapt the unexpected issues of EMI after launch, especially spectral calibration
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