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 NO_x 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 NO_x 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 NO_x 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 NO_x 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 NO_x levels were higher in the upper troposphere, giving a decreased ozone production efficiency of NO_x from aircraft. This was however, more than compensated for by a decreased downward mixing of ozone produced by emissions from aircraft.