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

Abstract. Lightning is one of the major natural sources of NO_x in the atmosphere. A suite of time slice experiments using a stratosphere-resolving configuration of the Unified Model (UM), containing the United Kingdom Chemistry and Aerosols sub-model (UKCA), has been performed to investigate the impact of climate change on emissions of NO_x from lightning (LNO_x) 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. LNO_x is simulated to increase in a year-2100 climate by 33% (RCP4.5) and 78% (RCP8.5), primarily as a result of increases in the depth of convection. The total tropospheric chemical odd oxygen production (P(O_x)) increases linearly with increases in total LNO_x and consequently, tropospheric ozone burdens of 29 ± 4 Tg(O₃) (RCP4.5) and 46 ± 4 Tg(O₃) (RCP8.5) are calculated here. By prescribing a uniform surface boundary concentration for methane in these simulations, methane-driven feedbacks are essentially neglected. A simple estimate of the contribution of the feedback reduces the increase in ozone burden to 24 and 33 Tg(O₃), respectively. We thus show that, through changes in LNO_x, 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 LNO_x, 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 greater LNO_x. 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 LNO_x in order to gain confidence in model projections of composition change under future climate.