Global Sensitivity of Tropospheric Ozone to Precursor Emissions in Clean and Present-Day Atmospheres: Insights from AerChemMIP Simulations

Abstract. Ozone (O3) is a Short-lived Climate Forcer (SLCF) that contributes to radiative forcing, influences secondary aerosol formation, and indirectly affects the atmospheric lifetime of methane, a major greenhouse gas. This study investigates the sensitivity of global O3 to precursor gases in a clean atmosphere, where hydroxyl (OH) radical characteristics are approximately uniform globally, using data from the PiClim experiments of the Aerosols and Chemistry Model Intercomparison Project (AerChemMIP) within the CMIP6 framework. We also evaluate the O3 simulation capabilities of four Earth system models (CESM2-WACCM, GFDL-ESM4, GISS-E2-1-G, and UKESM1-0-LL). Our analysis reveals that the CESM and GFDL models effectively capture seasonal O3 cycles and consistently simulate vertical O3 distribution. In contrast, the GISS and UKESM models effectively replicate the positive correlation between tropospheric O3 and temperature but exhibit lower sensitivity to natural precursor sources than anthropogenic ones. While all models successfully simulate O3 responses to anthropogenic precursor emissions, CESM and GFDL do not capture tropospheric O3 forcing from natural NOx emissions (e.g., from lightning). The sensitivities of O3 to its natural precursors (NOx and VOCs) in GISS and UKESM models are also very low. These findings refine our understanding of O3 sensitivity to natural precursors in clean atmospheres and provide insights for improving O3 predictions in Earth system models.