The Impact of Subsonic Aircraft Emission on Upper Troposphere/Lower Stratosphere Composition and Radiative Forcing: An Update

Emissions from subsonic aircraft can have potential impact on radiatively-important gas phase and particulate components in the upper troposphere/lower stratosphere and their precursors. These include CO2,O3, CH4, water, sulfate and soot aerosols, contrails and cirrus clouds. In particular, emissions of NOx induces production of O3 (positive radiative forcing), and increases in OH which lead to decreases in CH4 (negative radiative forcing). A thorough assessment of these and other effects was carried out by IPCC in 1999, resulting in modeled ozone increases of 5-10% at cruise altitudes, and decreases in methane of about 2% globally averaged, resulting in radiative forcings of about 0.02 and -0.01 W m-2, respectively, for emissions in 1992. At that time, most participating models did not include a complete treatment of both tropospheric and stratospheric chemistry and dynamics. We present calculations carried out by the Global Modeling Initiative Chemical Transport model, which includes complete chemistry and dynamics from the ground up to about 70 km. We adopt the same 1992 aircraft and boundary conditions as in IPCC (1999), in order to determine the impact of improved models on the early results. To estimate the effect of dynamic variability, we force the CTM with meteorological fields obtained from the Goddard/Geos Coupled Chemistry-Climate Model, for extreme El Nino and La Nina conditions. Results for the impact on UT/LS ozone and methane, as well as radiative forcing for these species, are discussed