Atmospherically important heterogeneous process, example

Thus, it is important to differentiate between the perturbation effects caused on the atmosphere by light-induced heterogeneous processes on aerosols from those caused by photochemical gas-phase reactions induced or inhibited by aerosol light scattering. In the following subsections several examples of the former type will be discussed. The enhancement of homogeneous photolysis rates and the problem of ozone depletion will not be discussed, as they are outside the scope of the present chapter. 

Heterogeneous processes involving natural liquid or solid aerosols play an important role in the chemistry of the Earth s atmosphere. For example, they can provide the downstream flux of many atmospheric compounds via sedimentation in the form of the absorbed or adsorbed species, facilitate recombination of free radicals, as well as provide topochemical, thermal catalytic and photocatalytic chemical reactions in the atmosphere. 

This highly hygroscopic molecule readily combines with water molecules to form an acid aerosol droplet. Other aerosols are formed by nucleation around mineral particles injected as a result of volcanic activity. Under very cold conditions, such as at the poles in winter, these aerosols freeze to form polar stratospheric ice clouds (PSCs), the surfaces of which provide a substrate for important heterogeneous catalytic processes. An example of this is the well-known ozone hole effect. This arises because the steady state concentration of O3 is sustained by the series of reactions (5.1) and (5.21)-(5.25). As already discussed, the sink mechanism (5.24) and (5.25) requires the presence of catalyst X, of which Cl atoms are nowadays the most important, and which are provided, such as reaction (5.26), mainly by the photolysis of CFCs present at trace levels in the upper atmosphere and much of the Cl is temporarily locked up into the reservoir species such as HCl and ClOx-... 

The database regarding the gas-phase degradation of the acid anhydrides is limited to only a pair of Cl-atom rate coefficient measurements, as is summarized in table VI-H-1. Gas-phase destruction of these species is likely controlled by reaction with OH, for which no data are presently available. However, on the basis of estimates from structure-activity relationships (Kwok and Atkinson, 1995 Kwok et al 1996b), these rate coefficients are likely quite small. For example, rate coefficients of 12, 6.5, and 0.9 (all in units of 10 cm molecule" s ) can be estimated for formic anhydride [HC(0)OCHO], formic acetic anhydride [HC(0)0C(0)CH3], and acetic anhydride [CH3C(0)0C(0)CH3], respectively, implying lifetimes for reaction of OH of about 10, 18, and 130 days for these species, respectively. Heterogeneous processes (e.g., hydrolysis to acetic acid on aerosol) seem likely to be important in the atmospheric removal of the anhydrides. 

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