Cloud droplets solutes

Cloud nucleation also has chemical consequences. The soluble material of the CCN introduces solute into cloud droplets which, in many instances, is a major and even dominant ingredient of cloud and rainwater. A simple but useful expression for the amount of solute from CCN is... 

There are several basic physical-chemical principles involved in the ability of aerosol particles to act as CCN and hence lead to cloud formation. These are the Kelvin effect (increased vapor pressure over a curved surface) and the lowering of vapor pressure of a solvent by a nonvolatile solute (one of the colligative properties). In Box 14.2, we briefly review these and then apply them to the development of the well-known Kohler curves that determine which particles will grow into cloud droplets by condensation of water vapor and which will not. 

As we have seen in Chapter 9, there are a variety of dissolved solutes in atmospheric particles, which will lower the vapor pressure of droplets compared to that of pure water. As a result, there is great interest in the nature and fraction of water-soluble material in atmospheric particles and their size distribution (e.g., Eichel el al., 1996 Novakov and Corrigan, 1996 Hoffmann et al., 1997). This vapor pressure lowering effect, then, works in the opposite direction to the Kelvin effect, which increases the vapor pressure over the droplet. The two effects are combined in what are known as the Kohler curves, which describe whether an aerosol particle in the atmosphere will grow into a cloud droplet or not under various conditions. 

Assumption of solubility equilibrium (Henry s law) for reagent gases in cloud droplets. Henry s law coefficients (H) of solute gases that do not react with water, e.g., O3, H2O2, and PAN, can be measured by conventional techniques. A variety of techniques have been applied to determination of Hj with fairly consistent results (7). 

From the above outline, the mass-transport problem is seen to consist of coupled boundary value problems (in gas and aqueous phase) with an interfacial boundary condition. Cloud droplets are sufficiently sparse (typical separation is of order 100 drop radii) that drops may be treated as independent. For cloud droplets (diameter 5 ym to 40 pm) both gas- and aqueous-phase mass-transport are dominated by molecular diffusion. The flux across the interface is given by the molecular collision rate times an accommodation coefficient (a 1) that represents the fraction of collisions leading to transfer of material across the interface. Magnitudes of mass-accommodation coefficients are not well known generally and this holds especially in the case of solute gases upon aqueous solutions. For this reason a is treated as an adjustable parameter, and we examine the values of a for which interfacial mass-transport limitation is significant. Values of a in the range 10 6 to 1 have been assumed in recent studies (e.g.,... 

For a consideration of wet deposition mechanism it is useful to make a distinction between processes transferring material to cloud droplets before they begin their descent as a raindrop, known collectively as rain-out and processes transferring material to falling raindrops known as washout, There are five mechanisms [8] by which particulate and gaseous compounds may be captured by cloud or rain drop diffusiophoresis, brownian diffusion, impact and interception, solution and oxidation of gaseous species (notably SO2 and NO2) and the cloud condensation nuclei (CCN) pathway. 

Equilibrium concentrations of gas phase SO2 and dissolved sulphur species in the same oxidation state (SO2, HSO s) are reached in a few seconds for droplets smaller than 100 pm diameter. However, for the time scales involved in rainout, oxidation of S(IV) to S(VI) dominates this mechanism for the accumulation of S by cloud droplets. The oxidation may proceed through the variety of mechanisms previously described ranging from catalytic reactions in solution often assisted by the presence of NHj,... 

In cloud droplets in remote regions the metal concentrations are likely to be low. Here more typically the reaction proceeds with oxidants such as dissolved hydrogen peroxide (or other atmospheric peroxides) and ozone. Hydrogen peroxide is an especially important droplet phase oxidant, because the gas is very soluble in water so can dissolve from the atmosphere. Additionally, it is readily produced within droplets in the atmosphere via photochemical processes. Oxidation by hydrogen peroxide is also significant, because the reaction is faster in acidic solutions, which means that the oxidation process does not become much slower as droplets become more acidic with the production of sulfuric acid. This oxidation can be represented as... 

The unique detection principle of evaporative lightscattering detectors involves nebulization of the column effluent to form an aerosol, followed by solvent vaporization in the drift tube to produce a cloud of solute droplets (or particles), and then detection of the solute droplets (or particles) in the light-scattering cell. 

In addition to solute from CCN, clouds contain dissolved gases (e.g. SO2, NH3, HCHO, H2O2, HNO3, and many more). In turn, some of these may react in the cloud droplets to form other substances which subsequently can appear in rainwater. Finally, falling raindrops can collect other materials (e.g. large dust particles) on their way to the Earth s surface. Thus, rainwater composition does not uniquely reflect the chemistry of the CCN. 

Relative to the levels of the species we have been considering, water vapor is at a high concentration in the atmosphere. Liquid water, in the form of clouds and fog, is frequently present. Small water droplets can themselves be viewed as microscopic chemical reactors where gaseous species are absorbed, reactions take place, and species evaporate back to the gas phase. Droplets themselves do not always leave the atmosphere as precipitation more often than not, in fact, cloud droplets evaporate before coalescing to a point where precipitation can occur. In terms of atmospheric chemistry, droplets can both alter the course of gas-phase chemistry through the uptake of vapor species and act as a medium for production of species that otherwise would not be produced in the gas phase or would be produced by different paths at a lower rate in the gas phase (Fig. 10). Concentrations of dissolved species in cloud, fog, and rain droplets are in the micromolar range, and therefore one usually assumes that the atmospheric aqueous phase behaves as an ideal solution. 

The importance of the reaction of S(IV) with dissolved oxygen in the absence of any metal catalysts (iron, manganese) has been a controversial issue. Solutions of sodium sulfite in the laboratory oxidize slowly in the presence of oxygen (Fuller and Crist 1941 Martin 1984). However, observations of Tsunogai (1971) and Huss et al. (1978) showed that the rate of the uncatalyzed reaction is negligible. The observed rates can be explained by the existence of very small amounts of catalyst such as iron (concentrations lower than 0.01 pM) that are extremely difficult to exclude. It is interesting to note that for real cloud droplets there will always be traces of catalyst present (Table 7.5), so the rate of an uncatalyzed reaction is irrelevant (Martin 1984). 

Atmospheric aerosols at high relative humidities are aqueous solutions of species such as ammonium, nitrate, sulfate, chloride, and sodium. Cloud droplets, rain, and so on are also aqueous solutions of a variety of chemical compounds. 

Aerosol particles in the atmosphere contain a variety of volatile compounds (ammonium, nitrate, chloride, volatile organic compounds) that can exist either in the particulate or in the gas phase. We estimate in this section the timescales for achieving thermodynamic equilibrium between these two phases and apply them to typical atmospheric conditions. The problem is rather different compared to the equilibration between the gas and aqueous phases in a cloud discussed in the previous section. Aerosol particles are solid or concentrated aqueous solutions (cloud droplets are dilute aqueous solutions), they are relatively small, and aqueous-phase reactions in the aerosol phase can be neglected to a first approximation because of the small liquid water content. 

The classic Kohler formulation does not consider the cases of solutes that are not completely soluble or soluble gases, both of which influence the solute effect term in (17.27). In such cases cloud droplets can exist at S < 1 since B/D > A/Dp. 

This equation describes the growth/evaporation rate of an atmospheric droplet. The numerator is the driving force for the mass transfer of water, namely, the difference between the ambient saturation SVt00 and the equilibrium saturation for the droplet (or equivalently the water vapor saturation at the droplet surface). The equilibrium saturation includes, as we saw in Section 17.2.4, the contributions of the Kelvin effect (first term in the exponential) and the solute effect (second term in the exponential). When the ambient saturation exceeds the equilibrium saturation, the cloud droplets grow and vice versa. The numerator is qualitatively equivalent to the term cTO 0C - r))1 in (17.60). The first term in the denominator corresponds to the diffusivity of water vapor (compare with (17.60)), while the second accounts for the temperature difference between the droplet and its surroundings. Note that if no heat were released during condensation, AHV = 0, and this term would be zero. 

Cloud droplets are typically far more acidic than precipitation droplets collected at the ground. In essence cloud drops are small and have not been subjected to the dilution associated with growth to the size of raindrops, snowflakes, and so on, nor the neutralization as sociated with the capture of surface-derived NH3 and alkaline particles held in layers at lower altitudes. Interception of these droplets therefore provides a route by which concentrated solutions of sulfate and nitrate can be transferred to foliage in high-elevation areas that are exposed to clouds. Only limited areas of the eastern part of the United States are frequently exposed to such deposition, but for these sensitive areas cloud interception is an important acid deposition pathway. 

It is important to note that few simplifications have been considered in application of Henry s law (sometime also called Henry-Dalton s law) the validity of the ideal gas equation, ideal diluted solution and that the partial molar volume of the dissolve gas is negligible compared with that in the gas phase. However, the range of its validity in the climate system is appreciable. It is clear that during heterogeneous nucleation (CCN to cloud droplet formation) Henry s law is not valid. Absolute values of solubility cannot be found from thermodynamic considerations. Nevertheless, general rules are valid for all gases ... 

In some cases when oxidising conditions are required, milder oxidants may be needed, because the hydroxyl radical can react with the solute forming adducts as well as via electron transfer. Hydroxyl radicals can be converted into milder (one-electron) oxidants by the addition of halides, thiocyanate or azide ions (reactions 8.15-8.17). In fact, halide radical reactions occur in atmospheric chemistry, particularly in urban cloud droplets, as well as in marine water radical reactions [29]. 

Kohler theory describes cloud droplet activation and growth from soluble particles as an equilibrium process [171], The Kohler equation takes into account two competing effects the Raoult or solute, effect which tends to decrease the equilibrium vapor pressure of water over the growing droplet, and the Kelvin, or curvature effect, which serves to increase the equilibrium vapor pressure. The Kohler curve O ig. 3) for a growing droplet describes the equilibrium saturation ratio of water as a function of droplet size and several parameters inherent to the aerosol particle [171, 172] ... 

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