Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
A number of heterogeneous reactions are important to various degrees on the surfaces of ice particles (PSC type II), HNO 3 /H 2 O aerosols (PSC type Ia), and saturated
ternary solutions of water, HNO 3 , and H 2 SO 4 (PSC type Ib). Rate parameters for heterogeneous reactions depend complexly on temperature, pressure, and humidity
conditions. Consequently, modeling of stratospheric reactions must explicitly address differences in conditions between poles and mid-latitudes and between the
tropopause and the LS. The most important heterogeneous reaction in relation to ozone depletion on a global scale is the heterogeneous hydrolysis of N 2 O 5 (reaction
20) (Hofmann and Solomon, 1989; Solomon et al., 1996; Kotamarthi et al., 1997). This and the following reactions are likely to be enhanced in the presence of
increased atmospheric particulates such as contrails and aircraft-induced cirrus clouds, which correspond to PSC type II in the LS from a chemical point of view.
Chlorine and bromine activation on PSC type Ia and Ib as well as on PSC type II (ice) takes place according to the following reactions:
ClONO 2 + H 2 O(s) HOCl + HNO 3 (27)
ClONO 2 + HCl(s) Cl2 + HNO 3 (28)
HOCl + HCl(s) Cl2 + H 2 O (29)
BrONO 2 + H 2 O(s) HOBr + HNO 3 (30)
BrONO 2 + HCl(s) BrCl + HNO 3 (31)
HOBr + HCl(s) BrCl + H 2 O (32)
Reactions of N2O5 with HCl(s) and HONO with HCl(s) (reactions 23 and 26) are also possible (IUPAC, 1997a; JPL, 1997). Because of the strong decrease in the
solubility of HCl with increasing acidity of H2SO4 solutions-thus with increasing stratospheric temperature-reactions 28, 29, 31, and 32 will be most important at high
latitudes (Portmann et al., 1996). Accordingly, these bimolecular reactions will proceed fastest on type II PSC surfaces in the polar vortex or on cirrus cloud particles in
the tropopause region (Borrmann et al., 1996; Solomon et al., 1997). In addition, the hydrolysis reaction of ClONO2 (reaction 27) is less efficient than the
corresponding one involving BrONO 2 (reaction 30), on account of thermochemical differences between the reactions (Allanic et al., 1997; Barone et al., 1997; Oppliger
et al., 1997). One consequence of this difference is that reaction 27 is prone to surface saturation on solid substrates, whereas reaction 30 is a strong and continuous
source of HOBr, thereby making BrONO 2 a relatively labile reservoir compound.
2.1.3.3. Aircraft Aerosols and Heterogeneous Reactivity
Heterogeneous reactions are important in defining ozone removal rates in the stratosphere and troposphere. Stratospheric and tropospheric aerosol particles differ
markedly in composition, lifetime, and size distribution. In the stratosphere, the global background aerosol consists predominantly of sulfate; water-ice and
carbonaceous particles are minor components on the global scale, although PSCs that contain water-ice can be important on smaller geographic scales. Tropospheric
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