Impact of NOx emissions from lightning on mid-tropospheric ozone concentrations in the North Hemisphere: a modelling study

The present study assesses the impact of the nitrogen oxides emissions from lightning (LiNOx) on mid-tropospheric ozone concentrations using simulations with the CHIMERE model, through a detailed evaluation of simulated tropospheric ozone (O₃) with respect to observations. LiNOx emissions have been implemented in the model (CHIMERE v2020r1). The chemistry-transport model CHIMERE is coupled online with the Weather Research and Forecasting (WRF) meteorological model. Two simulations have been carried out for year 2018, (i) including LiNOx emissions and (ii) without LiNOx emissions, in CHIMERE to examine the impact of LiNOx on the pollutant concentration in comparison to that without LiNOx emissions. In this study the simulations are performed over the northern hemisphere at a horizontal resolution of 100 km x 100 km.

The experiments show an increase in surface-level O₃ by 4-8 ppbv over most part of northern hemisphere, while a large increase by 10-20 ppbv is observed over parts of south America and Africa due to inclusion of LiNOx. The increase in simulated O₃ is high (10-20 ppbv) at middle troposphere in comparison to surface and upper troposphere. We have compared the model outputs to radiosonde measurements (World Ozone and Ultraviolet Radiation Data Centre), showing that including the NOx emissions from lightning substantially improves the realism of model simulations, significantly reducing bias and error when compared to measurements. This is particularly true in the middle troposphere. These results show that, for hemispheric or global studies, it is very important to include a realistic representation of lightning NOx emissions, because they critically influence ozone concentrations, but also the concentrations of OH, and therefore the lifetime of many greenhouse and trace gases such as methane.