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

A global 3-dimensional chemical tracer model (CTM) has been used to study the impact on tropospheric ozone caused by NOx emissions from today’s fleet of subsonic aircraft (0.52 Tg(N)/yr), due to uncertainties in sources of upper tropospheric NOx from lightning and deep convection.\n\nThree 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. A zonally homogeneous increase in upper tropospheric ozone north of 40° N, reaching 5-6 ppbv during May was found in the reference case. \n\nReduced emissions from lighting lead to 50-70 % higher enhancement of ozone at northern mid-latitudes during summer, due to lower background concentrations of NOx and more efficient ozone production. Reduced convective mixing lead to a 40% increased enhancement in aircraft induced ozone at northern mid-latitudes and 150% in the tropics. In this case background NOx levels were higher in the upper troposphere, but the decreased ozone production efficiency was compensated by decreased downward mixing of ozone produced by emissions from aircraft.