Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
aerosols consist of a wide variety of constituents, including water, sulfuric acid, soot, mineral dust, sea salt, and organic particles.
Soot, sulfuric acid ("sulfate"), and water-ice particles are the main condensed-phase species found in the exhaust of jet aircraft. As mentioned above, all of these
species are present in the background atmosphere, and all derive from natural and other non-aircraft anthropogenic sources. Consequently, the effect of aircraft
aerosol emissions on atmospheric photochemistry, to a first approximation, is to increase the soot, sulfate, and water-ice surface areas available for heterogenous and
multiphase processes discussed in the previous subsections. Large-scale aerosol loading from aircraft can be estimated roughly from knowledge of fleet emission
rates and average residence time of particles deposited at given altitudes. A detailed discussion of aircraft particle loading appears in Chapter 3. For the purposes of
this chapter, it suffices to note that aircraft soot and sulfur emissions are thought to be significant at cruise altitudes, whereas estimates of the perturbation from aircraft
H 2 O remain highly uncertain.
Beyond the simple aircraft aerosol loading approximation, chemistry occurring inside aircraft plumes carries the potential to produce volatile aerosols of significantly
different heterogeneous reactivity relative to typical background aerosols. For instance, the water content of H 2 SO 4 particles will vary over a wide range in an aircraft
plume and wake, with concomitant effects on the rates of reactions 27-32. In addition, under cold lower stratospheric conditions, aircraft plume production of HNO 3 -rich
liquid aerosol may propogate to the larger scale and contribute to the formation of solid PSCs, hence enhance processing of inactive chlorine species (Kärcher, 1997).
Finally, evidence is mounting that volatile aerosols formed under conditions of low fuel sulfur content may contain significant amounts of light fuel-bound organic
constituents (Kärcher et al., 1998b). The heterogeneous reactivity of such organic-containing aerosols is unknown and must await further in situ aerosol
characterization.
Significant differences may also exist between aircraft-derived and ambient background soot particles. As discussed in Section 2.1.3.1, soot surfaces may act as
reaction catalysts or consumables, depending on their properties. In addition, the surface properties likely determine the extent to which reduction-oxidation reactions
occur on a particular soot particle. At present there is virtually no information on the properties of aircraft-derived or ambient background carbonaceous particles upon
which to base an evaluation.
2.1.3.4. Net Effects on Ozone
Based on our current understanding of heterogeneous chemistry, aircraft sulfate and water-ice particles will lower ozone concentrations in the UT and LS relative to
what they would be if aircraft emissions contained only NO x . This process occurs because the particles remove the ozone precursors HO x and NO x in the UT and
liberate ozone-destroying ClO x in the LS. The heterogeneous chemistry occurring on soot is much less well understood, so its role in atmospheric chemistry is much
harder to define. However, because particle abundances of sulfate in the LS are much greater than those of soot, we can conclude with some confidence that present
aircraft soot emissions are having little impact on stratospheric ozone, provided that their primary impact is on partitioning between NOy and NO x .
The extent to which aircraft aerosols offset the effects of aircraft NO x emissions on atmospheric ozone depends on a variety of chemical and dynamical factors. To
quantify this balance, we will need atmospheric models that combine representations of aerosol microphysics, gas and heterogeneous chemistry, and atmospheric
dynamics.
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