NOx Emissions from Aircraft: Its Impact on the Global Distribution of CH4 and O3 and on Radiative Forcing

Abstract

Model studies of the impact of aircraft emissions on atmospheric ozone, and on the methane lifetime have been performed, using a global 3-D CTM (The Oslo CTMl). The calculated changes in the global distribution of ozone and methane have been used to calculate Radiative Forcing (RF) of the current and future fleets (2015 and 2050) of subsonic aircraft. The calculations show that ozone perturbation from aircraft emission occur predominantly in the Northern Hemisphere at mid and high latitudes. Maximum increase in ozone is found in the upper troposphere in the 10 to 12 km height region. The annual average future increases in the region of maximum increase, is estimated to be approximately 12 ppb in 2015. Scenarios for NOx emissions in 2050 are less certain, however; calculations give a near linear increase in global ozone perturbation for increasing NOx, in the future over the range of NOX, emissions assumed to be realistic until 2050. Calculations with NOx, emissions well beyond this range (50 % higher than the high NOx, emission) showed non-linear ozone response with less ozone increase per NOx increase than in the cases with lower NOx emission from aircraft. There is a reduction in the methane lifetime due to enhanced OH from NOx emissions. The change in the global methane lifetime is estimated to be -1.3 % in 1992 from aircraft emissions, increasing to -3.9 % in 2050. Estimates of RF from CH4 and O3 due to aircraft emissions are of opposite sign and of similar magnitude, which makes it difficult to give reliable estimates of the net impact. The regional patterns in RF for the two compounds are however highly different. It is therefore likely that the radiative impact from NOx, emissions could be larger than what can be obtained from global average RFs.