Simulations with the fully coupled climate-chemistry model E39/C suggest that the 1990 aircraft NO x emissions contributed substantially to the Northern Hemisphere NO x (30-40%) and ozone (3-4%) tropospheric burdens. Ozone production rates are increased by air traffic NO x emissions in the mid- and upper troposphere, whereas ozone loss rates are increased in the lower troposphere but decreased at cruise altitudes. The latter reduction results from increased tropospheric NO and NO 2 concentrations and a change in the OH:HO 2 ratio at cruise altitudes. Sensitivity studies showed that feedback processes between chemical species and dynamics are not altered significantly by air traffic. However, the results are sensitive to the lightning NO x emission patterns, the vertical resolution of the model at tropopause altitudes, model domain, and maximum flight level.