Underestimated Enhancement of Surface Ozone By Deep Convection

Nitrogen oxides (NO_x) produced by lightning are not only an essential source of upper tropospheric NO_x and ozone (O₃) but also affect surface O₃. The contribution of lightning NO_x to surface O₃ was generally estimated by comparing model simulations with and without lightning NO_x emissions. However, this approach is significantly influenced by the vertical redistribution of lightning NO_x by convective updraft and downdraft. By comparing surface O₃ concentrations before and after deep convection occurrence, we find the Regional chEmical and trAnsport Model (REAM) cannot reproduce the observed enhancement of surface ozone by thunderstorms during nighttime in summer, which is mainly attributed to the underestimated downdraft and turbulent mixing caused by thunderstorms. By using improved convective downdraft mass fluxes and diffusion coefficients during thunderstorms and applying a new convective transport parameterization, REAM can capture the observed surface ozone enhancement by thunderstorms at night in the contiguous United States. We then use the updated model configuration to examine the impact of lightning NO_x on surface ozone by comparing simulations with and without lightning NO_x. The results show much larger contributions of lightning NO_x to surface ozone than previous estimations. The study significantly improves our understanding of how deep convection affects surface ozone and has critical implications for background ozone estimation.