Clouds form due to the condensation of water vapor onto atmospheric aerosol particles in air that has become supersaturated. About 60% of the Earth's surface is covered by clouds at any given time, even though clouds occupy only 7% of the troposphere's total volume [Lelieveld et al., 1989; Pruppacher and Jaenicke, 1995] and the volume fraction of liquid water in clouds rarely exceeds 10^–6. Although this volume fraction represents a fairly small value, clouds have a major influence on the entire atmosphere.

In fact, clouds redistribute trace compounds emitted at the Earth's surface in the vertical from the boundary layer to the free troposphere and, in some cases, to the stratosphere. They interact with incoming solar radiation and long wave radiation emitted by the Earth, thus affecting both the atmosphere's photochemistry and the Earth's radiation budget. Clouds produce precipitation, an efficient mechanism for removing trace components from the atmosphere. Finally, they constitute an ideal reaction medium, allowing chemical transformations that would otherwise not take place in the gas phase or would proceed at much slower rates. For example, model studies [e.g., Langner and Rodhe, 1991] have shown that, on a global scale, more than 70% of the global oxidation of SO₂ to SO₄^2– occurs within cloud droplets.

Chemical constituents in the liquid phase of clouds derive from the incorporation of the soluble species contained in aerosol particles on which cloud droplets nucleate or which are scavenged by the droplets themselves, and from the dissolution of trace gases within the droplets. The different species introduced in cloud droplets can then react in the liquid phase to form other products.

The notion that chemical reactions in a cloud's aqueous phase might be important for atmospheric chemistry was advanced by Junge and Ryan [1958], who called attention to the role of ammonia and the importance of the oxidation of dissolved sulfur dioxide catalyzed by heavy-metals. Chameides and Davis [1982] pointed out the effects on cloud chemistry of free radicals scavenged from the gas phase and/or produced within the droplets.

Two families of chemical species are key participants in the cloud liquid phase chemical reactions: sulfur compounds and organic compounds. While much work has been done on the liquid phase reactions of sulfur species in clouds [see, e.g., Warneck, 1991], the organic chemistry within cloud droplets is still largely unknown. Little knowledge is available even on the actual organic composition of cloud water [Saxena and Hildemann, 1996].

Traditionally, gas phase atmospheric chemistry and cloud chemistry have been two fields of research that have proceeded in parallel for many years, without too many interactions. The atmosphere is a multiphase system, however, where, in addition to the gas phase, the solid (dry aerosol) and liquid phases (wet aerosol, cloud and precipitation elements) coexist; chemical processes in one phase cannot be properly assessed without a comprehensive knowledge of the processes within the multi-phase system. For example, Lelieveld and Crutzen [1990, 1991] drew attention to the effects of clouds on the tropospheric O₃ concentration. Although the quantitative estimations of this study have been questioned [Liang and Jacob, 1997], the importance of addressing atmospheric chemistry processes in a multiphase context clearly emerges from this study and others that followed.

Another relevant issue in cloud chemistry was stressed by Ogren and Charlson [1992], who showed that the chemical composition of cloud droplets is size-dependent. Model results [Hegg and Larson, 1990] have shown, for example, in-cloud S(IV) to S(VI) conversion rates that are three to thirty times higher when explicit droplet size-dependent chemistry is introduced, compared to a bulk approach. While most current cloud models take into account the size-resolved chemistry within cloud droplets, there is still a lack of experimental data for the comparisons with model results or for an investigation of the processes leading to the detected chemical inhomogeneities across the droplet size spectrum [Noone et al., 1988; Collett et al., 1995; Laj et al., 1998; Bower et al., 2000]. In fact, it has been shown that the solute concentration in cloud droplets can either increase or decrease with increasing droplet size [Schell et al., 1997].

It should be pointed out that most current knowledge on cloud chemistry is related to warm (liquid phase) clouds. Ice processes (mixed- and ice-clouds) and their impact on cloud chemistry remain largely unknown, and this topic constitutes an important challenge for the future, since the presence of the ice phase characterizes most clouds, at least over the mid-latitude regions of the globe.

This issue of the IGAC Newsletter provides an overview of some key issues in cloud chemistry and related atmospheric processes. D. Hegg describes how clouds impact aerosol populations. H. Herrmann and co-workers survey S(IV) to S(VI) aqueous phase oxidation by both radical and non-radical pathways. M.C. Facchini reviews current understanding of the organic chemistry of cloud-water and its effect on cloud properties. M. Barth describes how the effects of clouds on atmospheric trace constituents are treated in numerical models. Finally, K. Noone discusses the so-called indirect radiative effect of aerosols, a term which denotes the process(es) by which clouds influence the Earth's radiative budget.

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