Aerosol indirect effect on tropospheric ozone via lightning

Tropospheric ozone (O 3 ) is a pollutant and major greenhouse gas and its radiative forcing is still uncertain. Inadequate understanding of processes related to O 3 production, in particular those natural ones such as lightning, contributes to this uncertainty. Here we demonstrate a new effect of aerosol particles on O 3 production by affecting lightning activity and lightning‐generated NO x (LNO x ). We find that lightning flash rate increases at a remarkable rate of 30 times or more per unit of aerosol optical depth. We provide observational evidence that indicates the observed increase in lightning activity is caused by the influx of aerosols from a volcano. Satellite data analyses show O 3 is increased as a result of aerosol‐induced increase in lightning and LNO x , which is supported by modle simulations with prescribed lightning change. O 3 production increase from this aerosol‐lightning‐ozone link is concentrated in the upper troposphere, where O 3 is most efficient as a greenhouse gas. In the face of anthropogenic aerosol increase our findings suggest that lightning activity, LNO x and O 3 , especially in the upper troposphere, have all increased substantially since preindustrial time due to the proposed aerosol‐lightning‐ozone link, which implies a stronger O 3 historical radiative forcing. Aerosol forcing therefore has a warming component via its effect on O 3 production and this component has mostly been ignored in previous studies of climate forcing related to O 3 and aerosols. Sensitivity simulations suggest that 4–8% increase of column tropospheric ozone, mainly in the tropics, is expected if aerosol‐lighting‐ozone link is parameterized, depending on the background emission scenario. We note, however, substantial uncertainties remain on the exact magnitude of aerosol effect on tropospheric O 3 via lightning. The challenges for obtaining a quantitative global estimate of this effect are also discussed. Our results have significant implications for understanding past and projecting future tropospheric O 3 forcing as well as wildfire changes and call for integrated investigations of the coupled aerosol‐cloud‐chemistry system.