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Tropospheric ozone is an extremely important atmospheric compound as it oxidises, and thus removes, chemical compounds in the atmosphere. One important source in the troposphere are anthropogenic emissions. At elevated concentrations ozone presents a pollutant that is harmful to human health and ecosystems, and it acts as a short-lived greenhouse gas in the upper troposphere. Based on previously developed community models to describe the spatial and temporal variation of tropospheric ozone, this highly controversially discussed Opinion article proposes the establishment of a conceptual, simplified, but widely accepted tropospheric ozone model to facilitate communication between different communities concerned with different ozone impacts, and between ozone research and policy. This step could pave the way for a coordinated global action to improve air quality standards worldwide in an efficient and cost-effective manner.

After a period of near-zero growth of atmospheric methane, a major greenhouse gas, its growth rates have increased, with values in 2020 and 2021 exceeding all prior values since the beginning of systematic measurements in 1983. This recent acceleration, particularly during the covid-19 years 2020 and 2021, raises the question whether the methane increase was due to stronger sources or reduced sinks in the troposphere. A reduction of the tropospheric abundance of the hydroxyl radical (OH), the reaction of which is the main tropospheric methane sink, could be plausibly explained by global-scale reductions in nitrogen oxides due to pandemic-related industry shutdowns. Using an inversion scheme, Feng et al. demonstrate that such a reduction only accounts for about 34% in 2020 and 10% in 2021 of the observed methane rise. Instead, the authors attribute increased methane emissions to hydrological anomalies and microbial sources over the tropics, i.e., Eastern Africa and tropical South America, and temperate North America. The study demonstrates the importance of simultaneously accounting for changes in methane emissions and sinks for an improved quantitative understanding of the evolution of the methane concentrations to assess the role of greenhouse gases in climate change and tropospheric composition.