A significant increase in H2SO4 aerosol concentration coincidental with an enhancement in NO was detected above 10 km pressure altitude during a cross-corridor flight out of Shannon on October 23, 1997. The source of this aerosol is ascribed to commercial aircraft operations in flight corridors above 10 km, because (1) a stable atmosphere prevented vertical air mass exchanges and thus eliminated surface sources, (2) air mass back trajectories documented the absence of remote continental sources, and (3) temperature profiler data showed the tropopause at least one kilometer above flight altitude throughout the flight, thus excluding stratospheric sources. Particle volatility identified (60--80)% H2SO4, ≈20% (NH4)2SO4 and ≈10% nonvolatile aerosol in the proximity of flight corridors, and (10--30)% H2SO4, up to 50% (NH4)2SO4, and (40--60)% nonvolatile aerosols in air that was not affected by aircraft operations below 10 km. The newly formed H2SO4 particles did not measurably affect surface area and volume of the background aerosol due to their small size, hence did not influence radiative transfer directly. ¿ 1999 American Geophysical Union