Continuous measurements of nitrogen oxide and ozone were performed from a commercial airliner during 1 year at cruising altitudes below and above the tropopause, The upper tropospheric nitrogen oxides distribution was found to be strongly influenced by large-scale plumes extending about 100 to 1300 kilometers along the flight track. The plumes were frequently observed downwind of thunderstorms and frontal systems, which most probably caused;upward transport of polluted air from the continental boundary layer or nitrogen oxide production in lightning strokes, or both, Particularly in summer, average ozone concentrations in the plumes were enhanced compared to the tropospheric background Levels.