Atmospherically important heterogeneous

There is a growing literature [17,18] on theoretical attempts at a basic understanding of the chemistry and kinetics of these atmospherically important heterogeneous processes. For the most part, however, the emphasis is on experimental determination of both mechanism and rate data. There have been several techniques developed in this pursuit, many of which have been outlined previously [19,20]. In this paper, the Knudsen cell technique is described in some detail, since the experiments we wish to describe have been performed using this apparatus. 

THEORETICAL CHEMISTRY FOR HETEROGENEOUS REACTIONS OF ATMOSPHERIC IMPORTANCE. 

Heterogeneous reactions. Knowledge of atmospherically relevant heterogeneous reactions is far from complete. Important reactions probably still remain to be identified and their rates and mechanisms determined. Just as ignorance of heterogeneous chemistry contributed to the failure of stratospheric ozone models to anticipate the formation of the antarctic ozone hole, much still is to be discovered and learned about the role of heterogeneous reactions in the troposphere. 

An important heterogeneous reaction in the atmosphere is that of gaseous N205 with water molecules on the surface of atmospheric particles ... 

We have previously considered the possibility of an interaction between NO3 and O3. Detailed analysis of stopped-flow experiments yielded a value for the rate coefficient of between 0.6 and 1.0xl0 cm molecule" s However, this process might be heterogeneous, and we therefore suggest the upper limit given in the table. It seems unlikely that the reaction could be of atmospheric importance during the time during which substantial concentrations of NO3 and O3 coexist. 

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-... 

THEORETICAL CHEMISTRY FOR HETEROGENEOUS REACTIONS OF ATMOSPHERIC IMPORTANCE. THEHCl +CTONO2 REACTION ON ICE. 

Combining k(OH HC(O)Cl) < 3.2 x 10 cm molecule" s at 298 K with a diurnal average [OH] = 1.0 x 10 molecule cm provides a lower limit of 40 days for the atmospheric lifetime of HC(0)C1 with respect to reaction with OH radicals. Heterogeneous decomposition of HC(0)C1 into HCl and CO is observed in experimental studies on a timescale of minutes to hours (Libuda et al., 1990 Niki et al., 1980). Computational studies have shown that the activation barrier for homogeneous decomposition of HC(0)C1 is large, approximately 41 kcal mol (Francisco and Zhao, 1992 Phillips et al., 2005) and this reaction is not important. Heterogeneous decomposition. 

Presently, there is a general consensus that heterogeneous catalytic processes play an important role in environmental issues regarding their high selectivity towards the removal of undesired side products, such as atmospheric pollutants, in comparison with that obtained from non-catalysed processes. However, such a benefit could be disputed in the future with the implementation of severe restrictions on standard emission of those atmospheric pollutants, particularly nitric oxide, which is a very challenging aspect. 

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