PHYSICOCHEMISTRY OF AIRCRAFT-GENERATED LIQUID AEROSOLS, SOOT, AND ICEPARTICLES - 2 - COMPARISON WITH OBSERVATIONS AND SENSITIVITY STUDIES

Results from a coupled microphysical-chemical-dynamical trajectory box model have been compared to recent in situ observations of particles generated in the wake of aircraft. Sulfur emissions mainly cause the f ormation of ultrafine volatile particles in young aircraft plumes (mea n number radius similar to 5nm). Volatile particle numbers range betwe en 10(16) and 10(17) per kg fuel burnt for average to high fuel sulfur levels, exceeding typical soot emission indices by a factor of 10-100 . Model results come into closer agreement;with observations when chem i-ions from fuel combustion are included in the aerosol dynamics. Ice particles (mean number radius < 1 mu m) in young contrails mainly nucl eate on.water-activated exhaust aerosols. Homogeneous freezing and soo t-induced heterogeneous freezing are competitive processes leading to ice formation, depending on the temperature and level of oxidized sulf ur species. There is evidence that soot triggers freezing even for low fuel sulfur contents, suggesting a sulfur-independent water activatio n pathway. Metal particles emitted by jet engines and entrained ambien t aerosols may contribute to the formation of larger (> 1 mu m) crysta ls. Contrails with larger crystals would also form without soot and su lfur emissions. The lifecycle of cirrus clouds can be modified by exha ust aerosols.