Revisiting the high tropospheric ozone over Southern Africa: overestimated biomass burning and underestimated anthropogenic emissions

Abstract. Tropospheric ozone over Southern Africa is particularly high and causes tremendous health risks and crop yield losses. It has been previously attributed to the influence by biomass burning (BB), with a minor contribution from anthropogenic emissions. However, due to the lack of measurements for ozone and its precursors, the modeled impacts of BB and anthropogenic emissions on tropospheric ozone were not well evaluated in Southern Africa. In this study, we combined the nested GEOS-Chem simulation with a horizontal resolution of 0.5° × 0.625° with rare multiple observations at the surface and from space to quantify tropospheric ozone and its main drivers in Southern Africa. Firstly, BB emissions from current different inventories exhibit similar peaks in summer season but also have large uncertainties in Southern Africa (e.g., uncertainty of a factor of 2–3 in emitted NO_x). The model-satellite comparison in fire season (July–August) in 2019 shows that using the widely used GFED4.1 inventory, the model tends to overestimate by 87 % compared to OMI NO₂, while the QFED2 inventory can greatly reduce this model bias to only 34 %. Consequently, the modeled tropospheric column ozone (TCO) bias was reduced from 14 % by GFED4.1 to 2.3 % by QFED2; the simulated surface MDA8 ozone was decreased from 74 ppb by GFED4.1 to only 56 ppb by QFED2. This suggests a highly overestimated role of BB emissions in surface ozone if GFED4.1 inventory is adopted. The model-observation comparison at the surface shows that the global CEDSv2 anthropogenic inventory tends to underestimate anthropogenic NO_x emissions in typical Southern African cities by a factor of 2–10 and even misrepresented anthropogenic sources in some areas. That means that urban ozone and PM_2.5 concentrations in Southern Africa may be strongly underestimated. For example, a ten-fold increase in anthropogenic NO_x emissions can change ozone chemistry regime and increase PM_2.5 by up to 50 µg m⁻³ at the Luanda city. Furthermore, we also find that the newly TROPOMI can already capture the urban NO₂ column hotspots over low-emission regions like Southern Africa while this is unavailable from the OMI instrument, highlighting the critical role of high-quality measurements in understanding atmospheric chemistry issues over Southern Africa. Our study presents a quantitative understanding of the key emission sources and their impacts over Southern Africa that will be helpful not only to formulate targeted pollution controls, but also to enhance the capability in predicting future air quality and climate change, which would be beneficial for achieving a healthy, climate-friendly, and resilient development in Africa.