Effects of lightning and convection on changes in tropospheric ozone due to NOx emissions from aircraft

A global 3-dimensional chemical tracer model (CTM) has been used to calculate the impact on tropospheric ozone caused by NOx emissions from today’s fleet of subsonic aircraft (0.52 Tg(N)/ yr). Uncertainties in the magnitude and distribution in the ozone perturbation due to uncertainties in lightning and deep convection as sources of upper tropospheric NOx have been studied. Three sets of two CTM experiments have been performed, with and without emissions from aircraft. A reference set with normal convection and 12 Tg(N)/ yr from lightning, a set with reduced lightning source (5 Tg(N)/ yr), and one set with reduced convective activity (67% lower mass fluxes of air). A zonally homogeneous increase in upper tropospheric ozone north of 40°N, reaching a maximum of 5–6 ppbv during May was found in the reference case. The largest effect of lower NOx emissions from lightning was a 50–70% higher enhancement of ozone due to aircraft at northern mid- and high-latitudes during summer. This was caused by lower background concentrations of NOx and therefore more effcient ozone production. Reduced convective mixing lead to a 40% increased enhancement in aircraft induced ozone at northern mid-latitudes and 150% enhancement in the tropics. In this case background NOx levels were higher in the upper troposphere, giving a decreased ozone production efficiency of NOx from aircraft. This was however, more than compensated for by a decreased downward mixing of ozone produced by emissions from aircraft.