THE EFFECTS OF AMBIENT-TEMPERATURE AND RELATIVE-HUMIDITY ON PARTICLE FORMATION IN THE JET REGIME OF COMMERCIAL AIRCRAFT - A MODELING STUDY

The formation of contrails by H2O/H2SO4 condensation on activated soot particles in the jet regime of a B 747 airliner has been investigated by modelling studies. Emission indices of 0.05 g kg(-1) fuel and 1.0 g kg(-1) fuel for soot and SO2, respectively, were chosen. No direct e mission of S(VI) (SO3, H2SO4) has been assumed. The phase change of th e H2O/H2SO4 coverage of the soot particles and hence its impact on par ticle growth has been investigated by variation of the ambient tempera ture (219 K less than or equal to T-a less than or equal to 233 K) and the H2O content of the surrounding air (0.02 mbar less than or equal to p(H2O) less than or equal to 0.10 mbar), corresponding to relative humidities between 0.1 and 1.8. At T-a = 219 K and p(H2O) = 0.10 mbar the ice particles grown on combustion soot reach diametres of 4.5 mu m on the plume axis and of 5.7 mu m in its outer region. With increasin g temperature, however, the particle diameters decrease. Moreover, abo ve a threshold temperature which is dependent on ambient H2O vapour pr essure, the modelled particle sizes drop to values below 62 nm, corres ponding to liquid particles grown by heterogeneous nucleation of H2O a nd H2SO4 on liquid H2O/H2SO4 soot surface coverages. This is caused by the fact that the freezing condition is no longer fulfilled. The simu ltaneous formation of H2O/H2SO4 droplets by homogeneous nucleation sho ws no significant dependence on p(H2O) and the diametres of the drople ts do not exceed values of 3.5 nm. (C) 1998 Elsevier Science Ltd. All rights reserved.