Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aircraft also occasionally introduce hydrocarbons by jettisoning fuel at low altitudes in the troposphere. Most of the fuel evaporates while it falls to the ground
(Quackenbush et al., 1994), which leads to a small increase of hydrocarbons in this region. Because of the small amounts of fuel released in this way, no essential
impacts on atmospheric aerosols are expected.
3.2.2. Volatile Particles
3.2.2.1. Basic Processes
Volatile particles form in the exhaust plume of an aircraft as a result of nucleation processes associated with the emission of aerosol precursors (Hofmann and Rosen,
1978). Typical aerosol parameters in a young plume are included in Table 3-1 for reference. The newly formed particles grow by condensation (uptake of gaseous
species) and coagulation (particles collide and attach) in the expanding plume. Coagulation processes involving charged particles originating from CI emissions are
more effective because charge forces enhance collision rates. These processes are schematically presented in Figure 3-1. Key processes at young plume ages are
determined mainly from the results of simulation models because of the lack of suitable plume measurements.
Previous models (Miake-Lye et al., 1994; Kärcher et al., 1995; Zhao and Turco, 1995; Brown et al., 1996b; Taleb et al., 1997; Gleitsmann and Zellner, 1998a,b) show
that particles form when the condensing species, primarily H 2 SO 4 /H 2 O, reach concentrations critical for binary homogeneous nucleation in the expanding and cooling
exhaust gas. Because concentrations fall below nucleation thresholds as the plume further dilutes, the amount of H 2 SO 4 in the early stages of the plume (< 1 s)
controls the formation of new volatile particles. More recent models emphasize the role of CI emissions in volatile particle formation and growth (Yu and Turco, 1997,
1998a,b; Kärcher, 1998a; Kärcher et al., 1998a). Figure 3-2 shows the size distributions of exhaust particles at a plume age of 1 s, as inferred from models and a few
measurements. In the radius range below 10 nm, volatile particles containing H 2 SO 4 and H 2 O dominate the overall distribution. Model analyses of near-field particle
measurements strongly suggest that the volatile particle size distribution exhibits a bimodal structure (Yu and Turco, 1997), with smaller particles formed by the
aggregation of homogeneously nucleated clusters of hydrated H 2 SO 4 molecules (neutral mode) and larger particles formed by rapid scavenging of charged molecular
clusters by CIs (ion-mode). Only particles from the ion mode are expected to grow beyond the smallest detectable sizes (radius ~2 to 3 nm) of particle counters. Soot
and ice contrail particles are significantly larger than the volatiles. An approximate stratospheric size distribution is shown for comparison. In contrast to soot and ice
particles (see below), volatile particle spectra are mainly derived from numerical simulation models because of the lack of size-resolved, in situ particle measurements
in the nanometer size range. However, the use (in field measurements) of multiple particle counters with different lower size-detection limits allows derivation of the
mean sizes of observable particles as a function of fuel sulfur content and plume age (Kärcher et al., 1998b; Yu et al., 1998), thereby providing strong criteria to test
the validity of model results.
Table 3-1: Summary of number mean radius, number density, and surface area density for sulfate and soot particles in aircraft plumes and in the
background atmosphere, and for ice particles in contrails and cirrus. Flight levels of subsonic (supersonic) aircraft are in the 10-12 km (16-20 km)
range. Also included are estimates of zonal mean perturbations to sulfate and soot properties caused by the 1992 aircraft fleet.
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