Lightning NO_x, a key chemistry–climate interaction: impacts of future climate change and consequences for tropospheric oxidising capacity

Abstract

Abstract. Lightning is one of the major natural sources of NOx in the atmosphere. A suite of time-slice experiments using a stratosphere-resolving configuration of the Unified Model (UM), containing the UK Chemistry and Aerosols sub-model (UKCA), have been performed to investigate the impact of climate change on lightning produced NOx (LNOx) and to highlight its critical impacts on photochemical ozone production and the oxidising capacity of the troposphere. Two Representative Concentration Pathway (RCP) scenarios (RCP4.5 and RCP8.5) are explored. LNOx emissions are simulated to increase in a year-2100 climate by 33% (RCP4.5) and 78% (RCP8.5) in response to changes in convection. The total tropospheric chemical odd oxygen production (P(Ox)) increases linearly with total LNOx emissions and consequently, the tropospheric ozone burden also increases by 29 ± 4 Tg(O3) (RCP4.5) and 46 ± 4 Tg(O3) (RCP8.5). We thus show that, through changes in LNOx, the effects of climate change counteract the simulated mitigation of the ozone burden, which results from reductions in ozone precursor emissions as part of air quality controls projected in the RCP scenarios. Without the driver of increased LNOx, our simulations suggest that the net effect of climate change would be to lower free tropospheric ozone. In addition, we identify large climate-change induced enhancements in the concentration of the hydroxyl radical (OH) in the tropical upper troposphere (UT), particularly over the Maritime Continent, primarily as a consequence of larger LNOx emissions. The OH enhancement in the tropics increases oxidation of both methane (with feedbacks onto chemistry and climate) and very short-lived substances (VSLS) (with implications for stratospheric ozone depletion). We emphasise that it is important to improve our understanding of LNOx in order to gain confidence in model projections of future climate.