The effects of ambient temperature and relative humidity on particle formation in the jet regime of commercial aircrafts: A modelling 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 emission of S(VI) (SO3, H2SO4) has been assumed. The phase change of the H2O/H2SO4 coverage of the soot particles and hence its impact on particle growth has been investigated by variation of the ambient temperature (219 K Ta 233 K) and the H2O content of the surrounding air (0.02 mbar pH2O 0.10 mbar), corresponding to relative humidities between 0.1 and 1.8. At Ta=219 K and pH2O=0.10 mbar the ice particles grown on combustion soot reach diametres of 4.5 μm on the plume axis and of 5.7 μm in its outer region. With increasing temperature, however, the particle diameters decrease. Moreover, above a threshold temperature which is dependent on ambient H2O vapour pressure, the modelled particle sizes drop to values below 62 nm, corresponding to liquid particles grown by heterogeneous nucleation of H2O and H2SO4 on liquid H2O/H2SO4 soot surface coverages. This is caused by the fact that the freezing condition is no longer fulfilled. The simultaneous formation of H2O/H2SO4 droplets by homogeneous nucleation shows no significant dependence on pH2O and the diametres of the droplets do not exceed values of 3.5 nm.