Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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2.1.3. Effects of Aircraft Aerosol Emissions
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There is strong laboratory and observational evidence for the importance of aerosols in atmospheric photochemical processes such as lower stratospheric ozone
depletion (WMO-UNEP, 1995), tropospheric SO 2 oxidation (Calvert et al., 1985), and tropospheric soluble trace gas removal. A large number of additional chemical
processes involving aerosols has been postulated; however, a lack of information on the abundance, nature, and reactivity of the various aerosol types (e.g., dust,
soot, sulfate, nitrate, organic) has hindered an accurate assessment. In this section, we summarize the connections between aerosol processes and ozone chemistry
in the UT and LS. In addition, we assess the nature of aircraft-derived aerosol impacts on ozone abundances at aircraft cruising altitudes.
2.1.3.1. Aerosols and Tropospheric Ozone Chemistry
Reactions occurring on the surface of solid and liquid aerosols (i.e., heterogeneous reactions) or inside aqueous aerosols (i.e., homogeneous bulk reactions) can lead
to a decrease in the production of ozone (reactions 1-5) by catalyzing the removal of NO x and HO x . Generally, heterogeneous reactions counteract reactions involving
free radical species. From the ozone production perspective, removal of active species can occur by irreversible deposition of HO x , NO x , or NO x and HO x source
species or by conversion of more reactive nitrogen- and hydrogen-containing species into less reactive ones. Conversion of N 2 O 5 into HNO 3 on sulfate or ice particles
is the best established example of the latter mechanism. Wet deposition of HNO 3 , hydrogen peroxide (H 2 O 2 ), and other soluble acids is an example of the former
mechanism. Because removal of active species can also occur by gas-phase mechanisms, the importance of this aerosol chemistry depends on the relative rates of
the gas and aerosol processes. The rates of heterogeneous processes depend on the available aerosol surface area as well as on their size distribution. As
summarized in Chapter 3, typical mid-latitude upper tropospheric soot and sulfate aerosol surface areas are 10 µm2 cm-3 (see also Pueschel et al., 1997). Much larger
water-ice surface areas, on the order of 104 µm2 cm-3 , are found inside young contrails (Petzold et al., 1997) and natural cirrus clouds (Dowling and Radke, 1990).
Aircraft impact studies have motivated laboratory investigations of heterogeneous soot reactions. Interest in reactions occurring on soot has also increased in response
to suggestions, based on analysis of field observations (Hauglustaine et al., 1996; Jacob et al., 1996; Lary et al., 1997), that aerosol-assisted conversion of HNO 3 to
NO x may occur under some unknown conditions. Processes like the HNO 3 -to-NO 2 conversion, which result in a reduction of the oxidation state of an atmospheric
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