Global reanalysis products cannot reproduce seasonal and diurnal cycles of tropospheric ozone in the Congo Basin.

Tropospheric ozone (O 3) is a secondary pollutant and a greenhouse gas with a positive radiative forcing. Many studies have documented its negative impacts on plant growth and human health. Historically, studies have focused on determining levels of exposure in mid- and high-latitude regions. In the tropics, high O 3 concentrations are expected due to large concurrent and future precursor emissions. In Africa, seasonal biomass burning (from both natural and anthropogenic fires) during the dry season plays a crucial role in O 3 precursor production. However, O 3 observational studies in tropical Africa are currently missing. To fill this major knowledge gap, we established in November 2019 a continuous monitoring of near-surface O 3 in the Congo Basin at the Yangambi research centre in the Democratic Republic of the Congo. Using this unique dataset in the heart of the second-largest tropical forest in the world, we assessed the ability of current remote sensing products to capture the magnitude and temporal dynamics of in situ tropospheric O 3 concentrations, especially O 3 concentration variation between dry and wet seasons until March of 2022. We compared near-surface atmospheric O 3 measurements collected in Yangambi and four different reanalysis products: European Centre for Medium-Range Weather Forecasts Reanalysis (ECMWF) v5 (ERA5), Copernicus Atmospheric Monitoring Service reanalysis (CAMSRA), Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) and Japanese Reanalysis (JRA-55). The results show that reanalysis products overestimated the magnitude of near-surface O 3 across the region with a mean bias of 27.3 ppbv, 19.9 ppbv, 10.8 ppbv and 1.0 ppbv for ERA5, CAMSRA, MERRA-2 and JRA-55, respectively. ERA5 and CAMSRA reanalysis were the only products able to capture, to some extent, the observed annual variation, showing higher O 3 concentrations during dry season months, despite the inability to reproduce the daily cycle of near-surface O 3. • The first-ever dataset of near-surface ozone was collected in the core of the African tropical forests. • It fills a critical gap in a relevant area as ozone has been thought to reach large concentrations in central Africa. • Access the ability of state-of-the-art reanalysis products to capture the magnitude and temporal dynamics of in situ O3. • The reanalysis products fail to reproduce the seasonality and the daily cycles of tropospheric ozone. [ABSTRACT FROM AUTHOR]