Aviation and the Global Atmosphere

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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 H2SO4/H2O coating. Upon further cooling, volatile liquid H2SO4/H2O 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).

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