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Tropospheric ozone is harmful to both human health and vegetation, currently causing hundreds of thousands of premature deaths, and damage to crops and ecosystems worth billions of euros annually worldwide. While there are no direct measurements of pre-industrial ozone, these were very likely substantially lower than typical present-day concentrations. Tropospheric ozone is not directly emitted but is rather formed in-situ through photochemical reactions involving emitted precursors, with a relatively small amount also transported downwards from the stratospheric ozone layer. Production of tropospheric ozone requires two chemically distinct precursors: oxides of nitrogen (NOx) and reactive carbon (RC). Anthropogenic emissions of both precursors have increased substantially since pre-industrial times; anthropogenic NOx emissions are currently estimated to be about six times higher than natural emissions, while the biosphere remains a significant source of RC. Using a global model of atmospheric chemistry with a module for attribution of ozone to both NOx and RC precursors, we show that present-day production of tropospheric ozone is dominated by the combination of anthropogenic sources of NOx and biogenic emissions of RC, especially during the summer when the biosphere is a large source of RC. Over the winter months, anthropogenic RC builds up due to slower photochemistry and contributes to a secondary peak in ozone during the springtime when photochemistry resumes. Methane, which is emitted from both natural and anthropogenic sources, is among the least reactive types of RC in the atmosphere but contributes substantially to tropospheric ozone formation due to its high atmospheric concentration.
