IN-SITU MEASUREMENTS OF ENHANCED CRYSTAL NUMBER DENSITIES IN CIRRUS CLOUDS CAUSED BY AIRCRAFT EXHAUST

Concurrent measurements of crystal number densities and concentrations of absorbing material contained in cirrus crystals reveal statistical ly higher crystal number densities in areas of the cloud where absorbi ng material exceeds 0.01 mu g m(-3). This enhancement in crystal numbe r density ranged between a factor 1.6 and 2.8 depending on the aerosol number density. The absorption signature appeared as banded structure s 2 to 8 km wide, which suggests aged aircraft plumes as the source of absorbing material. In the altitudes of the most frequently used flig ht levels, the concentration of absorbing material in crystals exceede d 0.01 mu g m(-3) during more than 40% of the time in cirrus clouds.