Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Hagen et al., 1998; Miake-Lye et al., 1998; Pueschel et al., 1998). The values deduced from
these measurements range from the minimum value of 0.4% to more than 20%. Some of
the indirect analyses may be affected by uncertainties (possibly about 20%) regarding the
sulfur content in the fuels. Other experimental uncertainties are associated with these
determinations, and the range of aircraft engines and operating parameters adds to the
observed variability.
Generalization of these results cannot be done with confidence because of limited empirical
knowledge of the S(VI) conversion fraction and emission levels of CIs representative of the
exhaust as it enters the atmosphere. Part of the observed volatile aerosol may be
composed of HNO 2 (Zhang et al., 1996; Kärcher, 1997) or NMHCs (Kärcher et al., 1998b),
or it may result from unrecognized sulfur oxidation reactions (Danilin et al., 1997; Miake-Lye
et al., 1998). Uptake of gaseous SO 2 and subsequent heterogeneous oxidation to sulfate in
the new volatile particles is likely small (Kärcher, 1997), but the possibility of other
condensational or aqueous-phase growth mechanisms has not yet been fully explored. On
the other hand, CI emissions of about 1017/kg fuel are consistent with observations (see
Section 3.2.1.3), although they need to be confirmed by further measurements at the engine
exit. The best estimate for the S-to-H2SO4 conversion fraction in young plumes is 5%, with
an estimated variability, or uncertainty range, of 1 to 20%. Combustion models predict a
range of sulfur conversion up to about 10% and a potential for slightly higher values
because of turbine blade cooling effects (Sections 3.2.1.2 and 7.6). Low conversion
fractions of 2% are sufficient to explain observed volatile particle concentrations in the
young plume behind the ATTAS when the effects of CIs are taken into account in simulation
models of plume chemistry and microphysics (Kärcher et al., 1998a,b; Yu and Turco, 1998a,
b; Yu et al., 1998). In contrast, the Concorde observations can be explained by assuming
that about 20% of the fuel sulfur is converted to SO3 before leaving the engine exit in such
simulations (Yu and Turco, 1997).
In situ measurements detailing particle volatility and size distributions such as those
included in Figure 3-3 have involved relatively young plumes. Further observations in aging
plumes (> 1 h) as they dilute with the background atmosphere are currently lacking. Without
detailed observations of the microphysical evolution and chemical composition of volatile
exhaust particles from the engine exhaust plume to the global scale, important uncertainties
remain in assessing the potential global impact of exhaust products on chemistry and
cloudiness.
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Figure 3-3: (a) Emission indices of detectable volatile particles in
number per kg fuel measured in situ in plumes of various subsonic
aircraft and the supersonic Concorde. (b) Same as (a), but for the
emission indices of soot particles.
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