Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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3.2. Aerosol Emission and Formation in Aircraft Plumes
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Aircraft jet engines directly emit aerosol particles and condensable gases such as water vapor (H 2 O), sulfuric acid (H 2 SO 4 ), and organic compounds, which lead to the
formation of new, liquid (volatile) particles in the early plume by gas-to-particle conversion (nucleation) processes. Other gas-phase species and charged molecular
clusters (chemi-ions, or CIs) are also generated at emission, including nitric acid (HNO 3 ) and nitrous acid (HNO 2 ). Emission and formation of H 2 SO 4 depend on fuel
sulfur content, or sulfur emission index [EI(S)], and the conversion fraction of fuel sulfur to H 2 SO 4 . Formation of HNO 3 and HNO 2 depends on reactions of nitrogen
oxides (NO x = NO + NO 2 ) with hydroxyl radicals (OH). Particle formation depends on mixing of exhaust gases with ambient air, plume cooling rate, plume chemistry,
and ambient aerosol properties. Soot particles formed during fuel combustion and emitted metallic particles constitute the solid (nonvolatile) particle fraction present in
exhaust plumes. Under certain thermodynamic conditions, emitted water vapor condenses and freezes to form water-ice particles, thereby producing a condensation
trail (contrail). These line clouds evaporate rapidly if the ambient humidity is low but may change the size and chemical composition of the remaining liquid aerosol
particles. If the humidity is above ice saturation, contrails persist and grow through further deposition of ambient water.
An invisible aerosol trail is always left behind cruising aircraft. Aerosol and contrail formation processes in an aging plume determine the number, surface area, and
mass of particles that are formed per mass of fuel consumed. Exhaust particle properties change in the presence of a contrail. Exhaust particle morphology and
surface properties and aircraft-induced perturbations of background aerosol surface areas (Section 3.3) are of central importance for ozone changes caused by
heterogeneous chemical reactions (Chapters 2 and 4). Particle number and freezing probability are key for the formation of ice (cirrus) clouds after passage of an
aircraft in a region where otherwise no clouds would form (Section 3.4). Finally, aviation-produced aerosol can directly or indirectly influence the radiation budget of the
atmosphere (Section 3.6 and Chapter 6). For recent reviews see Schumann (1996a), Fabian and Kärcher (1997), Friedl (1997), and Brasseur et al. (1998).
The following subsections provide a description of volatile aerosol precursors and the formation of volatile aerosol particles, a characterization of emitted soot and
metal particles, a review of contrail and ice formation, and a discussion of the mutual interactions between these particle types. Comments on reducing the impact of
aerosols are given in Section 3.7.4.
3.2.1. Volatile Aerosol Precursors
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