POMINO-GEMS: A Research Product for Tropospheric NO₂ Columns from Geostationary Environment Monitoring Spectrometer

Abstract

Abstract. Nitrogen dioxide (NO2) is a major air pollutant. Tropospheric NO2 vertical column densities (VCDs) retrieved from sun-synchronous satellite instruments have provided abundant NO2 data for environmental studies, but such data are limited by insufficient temporal sampling (e.g., once a day). The Geostationary Environment Monitoring Spectrometer (GEMS) launched in February 2020 monitors NO2 at an unprecedented high temporal resolution. Here we present a research product for tropospheric NO2 VCDs, referred to as POMINO-GEMS. We develop a hybrid retrieval method combining GEMS and TROPOMI observations as well as GEOS-Chem simulations to generate hourly tropospheric NO2 slant column densities (SCDs). We then derive tropospheric NO2 air mass factors (AMFs) with explicit corrections for the anisotropy of surface reflectance and aerosol optical effects, through pixel-by-pixel radiative transfer calculations. Prerequisite cloud parameters are retrieved with the O2-O2 algorithm by using ancillary parameters consistent with those used in NO2 AMF calculations. Initial retrieval of POMINO-GEMS tropospheric NO2 VCDs for June–August 2021 reveals strong hotspot signals over megacities and distinctive diurnal variations over polluted and clean areas. POMINO-GEMS NO2 VCDs agree well with our POMINO-TROPOMI v1.2.2 product (R = 0.97, and NMB = 3.6 %) over Asia. Comparison with ground-based MAX-DOAS VCD data at nine sites shows a small bias of POMINO-GEMS (NMB = –15.7 %); however, the correlation for diurnal variation varies from -0.66 to 0.90, suggesting location-dependent performance. Surface NO2 concentrations estimated from POMINO-GEMS VCDs are consistent with measurements from the Ministry of Ecology and Environment of China at 855 sites (NMB = –24.1 %, and R = 0.95 for diurnal correlation averaged over all sites). POMINO-GEMS data will be made freely available for users to study the spatiotemporal variations, sources and impacts of NO2.

Retrieval of Tropospheric NO2 Vertical Column Densities from Ground-Based MAX-DOAS Measurements in Lhasa, a City on the Tibetan Plateau

“…The AD and RD depend on the tropospheric NO 2 abundance, and their monthly variations show a reverse phase of that of the tropospheric NO 2 VCDs (Figure 2). Similarly, previous studies for urban, suburban, rural and remote stations found apparent station dependence, i.e., usually negative deviations in the unban condition and positive deviations in the remote condition for satellite tropospheric NO 2 products [14]. The possible reasons for our finding are:…”

“…The AD and RD depend on the tropospheric NO2 abundance, and their monthly variations show a reverse phase of that of the tropospheric NO2 VCDs (Figure 2). Similarly, previous studies for urban, suburban, rural and remote stations found apparent station dependence, i.e., usually negative deviations in the unban condition and positive deviations in the remote condition for satellite tropospheric NO2 products [14]. The possible reasons for our finding are: (1) the ground-based MAX-DOAS is more sensitive to the NO2 in the lower troposphere, while the satellite s sensitivity is higher in the upper troposphere; (2) the layer of high NO2 concentration was close to the surface in the normal urban condition, but the relative NO2 profile had its maximum at higher altitudes during the lockdown period in Lhasa.…”

“…The comparison results for the tropospheric NO 2 VCDs between ground-based MAX-DOAS and satellite (TROPOMI and GEMS) show that it is a challenge to precisely monitor NO 2 in the lower troposphere using satellite over the TP, even in the plateau city of Lhasa. The small correlation coefficients, large deviations, and different diurnal patterns of tropospheric NO 2 between ground-based MAX-DOAS and satellite are influenced by multiple complex factors, such as the difference in the sensitive altitudes of the two methods [14], low level and small variation range of tropospheric NO 2 , low signal-to-noise ratio (reduced due to measurement errors over mountain terrain [24]), and horizontal NO 2 inhomogeneity [16,28]. The satellite retrieval of tropospheric NO 2 should be further improved and validated over the TP in future studies.…”

“…Tropospheric NO 2 vertical column density (VCD) is an important indicator reflecting the air quality. Tropospheric NO 2 VCDs are usually measured by satellite or groundbased remote sensing [14][15][16][17]. As a kind of ground-based remote-sensing technique, multi-axis differential optical absorption spectroscopy (MAX-DOAS) is widely used to retrieve tropospheric NO 2 VCDs, which are also applied in the validation of a satellite product [18][19][20].…”

“…To the best of our knowledge, relatively long-term variations in tropospheric NO 2 VCDs, especially those during the COVID-19 outbreak in 2022, have not been reported yet. Currently, the validations of satellite products, especially for the new-generation geostationary satellites monitoring the atmospheric composition over the TP, are the focus of the remotesensing community [14].…”