Heterogenous and Multiphase Reactions

Reactions taking place on the surface of solid or liquid particles and inside liquid droplets play an important role in the middle atmosphere, especially in the lower stratosphere where sulfate aerosol particles and polar stratospheric clouds (PSCs) are observed. The nature, properties and chemical composition of these particles are described in Chapters 5 and 6. Several parameters are commonly used to describe the uptake of gas-phase molecules into these particles (1) the sticking coefficient s which is the fraction of collisions of a gaseous molecule with a solid or liquid particle that results in the uptake of this molecule on the surface of the particle (2) the accommodation coefficient a which is the fraction of collisions that leads to incorporation into the bulk condensed phase, and (3) the reaction probability 7 (also called the reactive uptake coefficient) which is the fraction of collisions that results in reactive loss of the molecule (chemical reaction). Thus, the accommodation coefficient a represents the probability of reversible physical uptake of a gaseous species colliding with a surface, while the reaction probability 7 accounts for reactive (irreversible) uptake of trace gas species on condensed surfaces. This latter coefficient represents the transfer of a gas into the condensed phase and takes into account processes such as liquid phase solubility, interfacial transport or aqueous phase diffusion, chemical reaction on the surface or inside the condensed phase, etc. 

The loss rate of an atmospheric molecule resulting from the uptake by liquid or solid particles is generally expressed by a pseudo-first-order equation 

Liquid and solid particles observed in the atmosphere are generally a mixture of water, sulfuric acid and nitric acid with potentially the presence of traces of other chemical compounds like HC1 (see Table 5.9 in Section 5.7). The determination of the pseudo first-order rate coefficient kx (see Eq. (2.65)) for uptake by such particles in the stratosphere requires an accurate estimate of the surface area density available and of the reaction probability involved. 

The mean surface area density A can be calculated if the size distribution of the particles present in the atmosphere is known. For spherical particles 

The reaction probability 7 depends in general on the physical nature and chemical composition of the particle and on temperature. The composition of the liquid or solid particles is derived from thermodynamic considerations. The value of 7 for different conditions is determined in the laboratory. In the case of liquid sulfuric acid aerosols, for most reactions, 7 depends on the weight percentage W (wt%) of H2SO4 in the aerosol. W can be expressed as a function of temperature T(K) and water vapor pressure Ph2o (hPa) by (Steele and Hamill, 1981 Hanson et al, 1994) 

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