Evaluation of Ozone Profile and NO2 vertical column densities of Geostationary Environment Monitoring Spectrometer (GEMS) in China

Ozone (O3) and nitrogen dioxide (NO2) are critical atmospheric trace gases due to their role in air quality, oxidative chemistry, and climate forcing. Both O3 and NO2 were formerly monitored from space on a daily basis using sun-synchronous polar orbiting satellite instruments. However, the Geostationary Environment Monitoring Spectrometer (GEMS) now provides hourly measurements during the daylight hours over East Asia. Since GEMS is a newly launched satellite instrument, to check the accuracy and uncertainties associated with the first version of its dataset, there is a need for its evaluation through ground-based instruments over the Chinese region. This study presents the systematic evaluation of GEMS hourly measurements of Column Amount NO2 (NO2 VCDs) and Ozone Profile (O3P) by using ground-based measurements. The MAX-DOAS measurements were conducted in Xuzhou in eastern China for one week of each of the four seasons in 2021 to have a better representation of both high and low NO2 concentrations. NO2 variations were well captured in all seasons by both instruments, with higher VCDs in winter and lower during the summer.  A good correlation (R = 0.82) between MAX-DOAS and GEMS was found. However, GEMS underestimated the NO2 VCDs with a consolidated mean deviation (without seasonal discrimination) by -1.87 ± 8.73 x 1015 molec/cm2. The comparison of hourly GEMS NO2 VCDs with in-situ measurements available across China for 2021 has shown a good correlation coefficient of around 0.4-0.6 and higher, particularly in the highly polluted region of the North China Plain. Then, to evaluate the GEMS ozone profile, we used Ozonesonde measurements available for 2021 in Hong Kong, situated in the southern part of China. For the vertical profile of ozone in the troposphere and lower stratosphere, GEMS and Ozonsonde depicted a similar pattern. However, in the troposphere, the Ozonesonde measurements were slightly underestimated, while in the stratosphere, GEMS showed underestimation. It is pertinent to mention that both instruments depicted peak ozone concentrations at an altitude of around 24 to 27 km. The correlation between the two instruments was good, particularly in the lower troposphere (R = 0.74) at 4 km. Further, the lowest layer of the GEMS ozone profile was compared with in-situ measurements for 2021 across the entire Chinese region, and a good correlation coefficient (R: 0.4 - 0.6) was observed. These findings are meaningful scientific advancement enhancing our understanding of the potential of the first geostationary satellite instrument to monitor atmospheric trace gases (O3 and NO2) hourly. More robust validations are recommended in other regions to understand the uncertainties associated with the local conditions and to further improve the GEMS products in future versions of datasets.