Figure 3-1: Aerosol and contrail formation processes in an aircraft plume
and wake as a function of plume age and temperature. Reactive sulfur gases, water
vapor, chemi-ions (charged molecules), soot aerosol, and metal particles are emitted
from the nozzle exit planes at high temperatures.
H2SO4 increases as a result
of gas-phase oxidation processes. Soot particles become chemically activated by
adsorption and binary heterogeneous nucleation of SO3 and
H2SO4 in the presence
of H2O, leading to the formation of a partial liquid
coating. Upon further cooling, volatile liquid
droplets are formed by binary homogeneous nucleation,
whereby chemi-ions act as preferred nucleation centers. These particles grow in
size by condensation and coagulation processes. Coagulation between volatile particles
and soot enhances the coating and forms a mixed H2SO4/H2O-soot aerosol, which
is eventually scavenged by background aerosol particles at longer times. If liquid
H2O saturation is reached in the plume, a contrail forms. Ice particles are created
in the contrail mainly by freezing of exhaust particles. Scavenging of exhaust
particles and further deposition of H2O leads to an increase of the ice mass.
The contrail persists in ice-supersaturated air and may develop into a cirrus
cloud. Short-lived and persistent contrails return residual particles into the
atmosphere upon evaporation. Scavenging time scales are highly variable and depend
on exhaust and background aerosol size distributions and abundances, as well as
on wake mixing rates (see Section 3.3).