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As all life on earth depends crucially on atmospheric ozone, low earth orbiting (LEO) satellites have been used to monitor atmospheric ozone to reduce its impact on the environment and public health. The continued interest in air pollution and stratospheric ozone variability has motivated the development of a geostationary environmental monitoring satellite (GEMS) for hourly ozone monitoring. This paper provides the atmospheric science community with the world's first assessment of GEMS total column ozone (TCO) retrieval performance and diurnal ozone variation. The algorithm used for GEMS is a more advanced version of its predecessor, the TOMS-V8 algorithm. In addition to calculating total ozone, the algorithm has the advantage of providing ozone profile and retrieval error information. To assess the performance of the GEMS algorithm, the hourly GEMS total ozone was compared with ground-based measurements from four Pandora instruments and other satellite platforms from TROPOMI and OMPS. A high correlation of 0.91 or more with GEMS and Pandora TCO at Seoul, Busan, and Yokosuka but a low correlation of 0.83 at Ulsan, which is significantly smaller than at other sites. Root-mean-squared error (RMSE) showed satisfactory small values, with the lowest RMSE of 2.06 DU. Positive mean biases (MBs) were observed at all sites. This agreement suggests that the GEMS hourly ozone monitoring allows for continuous updates about stratospheric ozone and its related atmospheric changes. The quantitative comparison of GEMS TCO data with TROPOMI and OMPS TCO data shows a high correlation coefficient greater than 0.98 and a low RMSE of less than 1.8 DU over clear sky conditions. GEMS TCO underestimates by - 0.14 % (0.4 DU) with a standard deviation of 2.0 % relative to TROPOMI and overestimates by + 0.1 % (0.3 DU) with a standard deviation of 2.3 % relative to OMPS. It shows that the GEMS TCO agrees very well with the TROPOMI and OMPS TCO. The results are a meaningful scientific advance by providing the first validated, hourly UV ozone retrievals from a satellite in geostationary orbit. This experience can be used to advance research with future geostationary environmental satellite missions, including incoming TEMPO and Sentinel-4.

NASA satellite measurements show that ozone reductions throughout the Northern Hemisphere (NH) free troposphere reported for spring-summer 2020 during the COronaVIrus Disease 2019 pandemic have occurred again in spring-summer 2021. The satellite measurements show that tropospheric column ozone (TCO) (mostly representative of the free troposphere) for 20°N-60°N during spring-summer for both 2020 and 2021 averaged ∼3 Dobson Units (DU) (or ∼7%-8%) below normal. These ozone reductions in 2020 and 2021 were the lowest in the 2005-2021 record. We also include satellite measurements of tropospheric NO