The potential role of organic compounds in the chemistry of wet aerosols and raindrops was investigated by Graedel and Weschler [1981] already twenty years ago. They presented a conceptual model of an aqueous droplet containing an insoluble core, an aqueous solution of both organic and inorganic species, and an organic surface film with “any polar portion of the organic molecule preferentially oriented toward the core as the result of solution bondings.” This model represented a complex and heterogeneous system in which the organic compounds would produce important consequences on the chemical and physical properties of the droplets. In particular, a potential for photochemical reactions in the aqueous phase and air-solution interface effects was evidenced among the processes involving organic species.

Over the last two decades, only soluble organic molecules derived from the gas phase have commonly been measured in cloudwater. Formic, acetic and other lightweight carboxylic acids, formaldehyde and other gas phase-derived carbonyls were reported to constitute the main dissolved organic compounds [Grosjean and Wright, 1983; Munger et al., 1984; Winiwarter et al., 1988; Collett et al., 1990; Facchini et al., 1992]. Important liquid-phase reactions involving these compounds were also studied as, for example, the formation of bisulfite-formaldehyde adducts [Boyce and Hoffmann, 1984; Dong and Dasgupta, 1986] and the photochemical formation of formic acid in clouds [Jacob, 1986]. All these reactions have implications in the process of acidification of precipitation.

The remaining part of organic carbon in the aerosol was considered mostly insoluble in cloudwater and, therefore, the possible effects of organic compounds on cloud chemistry were those connected to the formation of insoluble organic films [Gill et al., 1983] or some catalytic soot–metal effects on aqueous-phase SO₂ oxidation [Jacob and Hoffmann, 1983].

For these reasons, the above model of aqueous droplets proposed by Graedel and Weschler [1981] remained more of a conceptual frame rather than a study system supported by observations.

Organic compounds in cloud water

In the early 1980s, a pioneer paper appeared [Likens et al., 1983] reporting on the composition of organic compounds in precipitation. These authors measured total organic carbon (TOC) concentration in precipitation samples and found that the dissolved organic fraction (DOC) was very high: ca. 80% of the TOC measured. Particulate organic carbon (POC) plus dissolved macromolecular compounds (> 1000 dalton) accounted for 50-60% of the TOC. Carboxylic acids, alhehydes, polysaccharides and tannin-lignin comprised most of the remaining carbon. These results suggested that the major fraction of organic carbon in precipitation was derived from particle scavenging rather than from the gas phase.

Over the last ten years, considerable time and effort has been devoted to the characterization of the organic composition of aerosols and cloudwater. However, the difficulties encountered in the analysis of the polar, water soluble organic species, the ones important for the aerosol-cloud interaction, have represented a technical limit to understanding the role of organic compounds in cloud chemistry [Jacobson et al., 2000]. The traditionally used analytical technique for organic compound speciationsolvent extraction followed by gas chromatography coupled to mass spectrometry (GC–MS) analysisfails to identify most of the water soluble organic compounds (WSOC), since this method is not aimed at analyzing very polar species. Typically, less than 5% of the total mass of aerosol WSOC was resolved into individual species [see e.g., Rogge et al., 1993]. These difficulties were evidenced in a review by Saxena and Hildemann [1996, and references therein]: their paper investigated theoretically the characteristics of WSOC and identified the specific classes of compounds that had either been detected or that were likely to contribute to aerosol WSOC on the basis of their thermodynamic properties. Mono- and di-carboxylic acids, carbonyls, alcohols and some other classes of compounds, e.g., polyols, amino acids and other multifunctional organic compounds were indicated as potential candidates for the organic composition of aerosol WSOC and cloudwater.

Recently, a new class of macromolecular polycarboxylic acids has been detected in aerosol samples [Mukai and Ambe, 1996; Havers et al., 1998; Zappoli et al., 1999], accounting for a significant fraction of the aerosol WSOC [Facchini et al., 1999a; Kiss et al., 2001; Decesari et al., 2001]. This class of macromolecular compounds has physical and chemical properties similar to those of humic (or fulvic) acids, the main constituent of dissolved organic carbon in natural waters [Stumm and Morgan, 1981], and for this reason they are sometimes referred to in the literature as HULIS [humic-like substances]. Combustion processes, in particular biomass burning, have been indicated as possible sources of HULIS [Mukai and Ambe, 1986; Zappoli et al., 1999; Mayol-Bracero et al., 2001].

In view of the difficulty of pursuing individual compound speciation of WSOC employing traditional GC-MS techniques, Decesari et al. [2000a] have proposed a new approach for the analysis of WSOC in aerosol and cloudwater, which aims to characterise the main functional groups and chemical properties of WSOC. The procedure is based on a combination of: a) chromatographic separation; b) TOC analysis; and c) organic functional group analysis. The complex WSOC mixture is separated into three main classes of compounds classified according to their acidic character: i) neutral/basic compounds; ii) mono- and di-carboxylic acids; iii) polycarboxylic acids. The TOC of each of the three fractions is determined, and organic functional group analysis is then performed on the separated fractions by means of Proton Nuclear Magnetic Resonance (PNMR). In the case of aerosol and fog water samples from the Po Valley, Italy, the neutral/basic fraction was found to be composed mainly of polyols and sugars, while mono- and di-carboxylic acids were mainly hydroxylated aliphatic compounds and polyacids were highly unsaturated compounds, both aliphatic and aromatic, with a smaller content of hydroxyl groups. It is suggested that the structure of the latter class of compounds may correspond to the above mentioned HULIS [Mayol-Bracero et al., 2001].

Figure 1. Average concentration of the main classes of inorganic and organic water-soluble species in fog water samples collected in the Po Valley, Italy. Data are expressed in mg m-3, calculated by multiplying the measured liquid phase concentrations by the fog liquid water content [Decesari et al., 2000b].

Decesari et al. [2000b] further investigated a number of Po Valley fog samples and found that the above three classes of organic compounds account for an average 88% of the total WSOC within the samples (Figure 1). Acidic compounds (mono/dicarboxylic acids and polyacids) are the dominant WSOC, accounting for an average 59% of soluble organic species; while neutral compounds account for ca. 30% of the total WSOC. Although the inorganic ions represent an average 75% of total fog water soluble mass, the WSOC concentration is by no means negligible.

While the above characterization of WSOC supplies a less detailed picture compared to individual compound speciation, it certainly provides more comprehensive information for modeling purposes and is particularly helpful when aerosol chemical mass closure is pursued, since the procedure accounts for almost 90% of WSOC.

Organic cloud condensation nuclei and cloud microphysics

Over the last few years, the contribution of organic aerosols to the mass of cloud condensation nuclei [CCN] has been investigated by several authors [Novakov and Penner, 1993; Rivera-Carpio et al., 1996; Corrigan and Novakov, 1999]. In particular, Novakov and Corrigan [1996] reported that pure organic smoke aerosols from cellulose combustion are efficient CCN without being associated to inorganic salts. Penner et al. [1996] pointed out that organic compounds derived from biomass burning may also act as CCN, thus influencing indirect aerosol forcing. The hygroscopic properties of atmospheric particles containing organic and inorganic compounds and their ability to act as CCN have been extensively investigated by the Tandem Differential Mobility Analyzer (TDMA) technique, in both laboratory and field experiments [Saxena et al., 1995; Cruz and Pandis, 1996; Hansson et al., 1998]. Other methods like electrodynamic balance [Andrew and Larson, 1993; Peng and Chan, 2001] or molecular-controlled semiconductor resistor [Rudich et al., 2000] have been used to study the interaction of organic compounds with water vapor. The results of these studies have shown that organic compounds can either enhance or inhibit water absorption and that the hygroscopicity is strongly dependent on the specific organic mixture (the different origin of the particles, in the case of real samples).

It is known that some organic compounds present in aerosols are surface active and that their presence in CCN particles can significantly affect the surface tension of cloud droplets [Gill et al., 1983; Capel et al., 1990; Shulman et al., 1996]. Facchini et al. [1999b] have shown that the lowering of surface tension due to organic compounds dissolved within cloud droplets may influence droplet nucleation and growth, hence affecting the droplet population. Facchini et al. [2000] have also shown that, in the case of Po Valley fog water, among the three kinds of compounds classified according to the procedure of Decesari et al. [2000], the polycarboxylic acid fraction exhibited the most pronounced surface active behavior: four times higher than the mono- and dicarboxylic acids and one order of magnitude higher than the neutral compounds.

Figure 2. Köhler curves calculated for three aerosol dry sizes and two different aerosol chemical compositions. The dotted lines correspond to a purely inorganic aerosol composition with surface tension equal to that of pure water, while the solid lines represent an inorganic + organic aerosol composition and variable surface tension. The concentration for inorganic and organic species and the surface tension depression were retrieved from measurements of real aerosol and fog water samples collected in the Po Valley, Italy [Mircea et al., 2001].

The effect of organic composition and surface tension depression on cloud droplet formation is shown in Figure 2, which illustrates for three different aerosol dry sizes the equilibrium growth predicted by the Köhler theory [Köhler, 1936] of CCN with two different chemical compositions; i) a purely inorganic composition with constant surface tension equal to that of pure water and ii) organic + inorganic composition and variable surface tension as a function of WSOC concentration [Mircea et al., 2001]. The effect of organic composition on CCN activation becomes evident by comparing the two families of curves in the vicinity of the critical radius: the decrease in critical supersaturation produces a difference of roughly 20% in the calculated droplet number concentration (or a decrease of ca. 6% in the estimated effective radius). This example shows that organic CCN composition and surface tension effects should be taken into account in cloud models for describing cloud formation and evolution. On the other hand, it has also been shown that slightly soluble organic compounds may inhibit cloud droplet growth [Shulman et al., 1996; Laaksonen et al., 1997], although the lack of measurements on real cloud/aerosol organic chemical composition and of solubility data prevents a more thorough analysis on this point.

Emerging issues concerning organic compounds in clouds

The presence of many different classes of polyfunctional organic compounds in clouds with a wide range of molecular weights opens new scenarios in cloud chemistry.

In the first place, it can be speculated that the polar water-soluble organic species in cloudwater not only derive from gas phase scavenging or the dissolution of polar products of gas-to-particle-conversion, but that they also derive from the scavenging of carbonaceous particles from combustion sources oxidized in the atmosphere [Chugthai et al., 1991; Lary et al.,1999]. The oxidation of carbonaceous surfaces leads to the formation of polar groups such as carboxylates, which cause the aerosol to become more hydrophilic and hence effective as CCN.

Lary et al. [1999] also pointed out that, once in cloud, the carbonaceous material (both the soluble part and the insoluble core) can be photo-reactive. Furthermore, Anastasio et al. [1997] have suggested that non-phenolic aromatic carbonyl compounds and phenols can be a source of hydrogen peroxide in the aqueous phase via photochemical reactions. Aromatic carbonyls and phenols are formed by biomass burning processes and the same kind of moieties are present in lignin photo-degradation products and in the HULIS.

The tendency of natural humic substance to form stable complexes with metal ions is well known [Stumm and Morgan, 1981] and it is possible that similar processes also occur within cloud droplets containing HULIS. Another important property of polycarboxylic acids in natural waters is their colloid behavior, and they tend to precipitate in the presence of Ca²⁺ and Mg²⁺ ions [Stumm and Morgan, 1981]. Such processes may also be important within cloud droplets, influencing the partitioning of material from the aqueous phase to the insoluble phase, in analogy to the chemistry of natural waters.

Beside the fact that surfactants are present in cloud water [Facchini et al., 2000], their role in cloud chemistry is completely unknown, as also are their likely sources. These complex materials may operate as concentration or transport agents for other non polar organic compounds or for hydrous oxide clays [P. Brimblecombe, personal communication; see also the URL.

The limitation of reliable chemical methodologies to fully elucidate the complex composition of organic aerosol and cloudwater and the fact that the organic composition cannot be attributed to a few major species alone (unlike inorganic compounds) make the choice of a reference inorganic + organic composition to be used in numerical simulations and in laboratory experiments very challenging [Blando and Turpin, 2000], and the choice of model compounds to represent the whole WSOC mass should be done with caution. In addition, it is necessary to acquire experimental and theoretical information on other important thermodynamic properties of WSOC, such as actual solubility and water activity, in order to obtain the necessary input parameters for correct model simulations of cloud physical and chemical processes.

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