Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
species, are of particular interest because they proceed counter to the overall tendency of the troposphere to act as an oxidizing medium. For the specific HNO 3
example, the reduction mechanism operates at the expense of the oxidation of the soot substrate and would promote production of ozone by increasing levels of active
NO x .
Some of the most important heterogeneous reactions on soot involving NO x and its reservoirs, which are of likely importance in the UT and have been studied in the
laboratory, are presented below. In view of the fact that the number concentration of nonvolatile particles-the majority of which are presumed to be soot-in a young
contrail is on the order of 104 particles cm -3 (Brasseur et al., 1998), we start our discussion with heterogeneous oxidation-reduction reactions that may occur on soot.
Heterogeneous kinetic studies involving "soot" have been performed on a variety of substrates, encompassing materials as diverse as commercially available
amorphous carbon, carbonaceous material ("active carbon" or "carbon black"), and soot from hydrocarbon diffusion flames that use fuels such as hexane, toluene,
ethylene, and acetylene. Therefore, comparisons of results obtained on different substrates should be made with caution. The heterogeneous reaction of NO 2 with
soot may be represented by the following reactions (Tabor et al., 1993, 1994; Rogaski et al., 1997; Gerecke et al., 1998):
followed by the thermal decomposition of the oxygen adduct [soot . O]:
NO 2 + soot NO + [soot . O] (19a)
[soot . O] CO, CO 2 (19b)
NO 2 + soot + H 2 O HONO + [soot . OH] (19c)
NO 2 + H(ads) HONO (19d)
where [soot.O] and [soot.OH] represent surface sites on soot that have been oxidized, hence deactivated by the heterogeneous reaction. It is not clear yet if soot
participates as a reducing agent (reaction 19c) or simply as a reservoir of hydrogen (H(ads), reaction 19d) in its reaction with NO 2 to yield HONO.
Reactions 19a and 19c correspond to a reduction-oxidation reaction in which soot is the reducing agent. Soot is oxidized in the process and releases CO and CO2 upon heating, although the primary oxidation product [soot.O] has not been characterized on a molecular level. It is thought that the surface of soot will be modified in
the oxidation process, resulting in the accumulation of multiple functional groups bearing oxygen (Chughtai et al., 1990).
The branching ratio between reactions 19a and 19c or 19d depends on the soot sampling location as well as the fuel used to produce the soot, albeit to a minor extent.
In general, amorphous carbon does not generate HONO, whereas soot sampled early in its formation process may give rise to a HONO yield of up to 90% compared
to the NO 2 taken up (Gerecke et al., 1998). Measurements within exhaust plumes have established the presence of nitrous acid (HNO 2 ) and HNO 3 at concentrations
well above background (Arnold et al., 1992, 1994). The data indicate that about 0.6% of the NO x is converted to HNO 2 and HNO 3 . More recent data on exhaust
plumes of five B-747s and one DC-10 increase this efficiency to 1-5% of the NO x (Brasseur et al., 1998). Therefore, care must be exercised when HONO
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