This work constitutes the first large report on aerobic cultivable microorganisms present in cloud water. Seven cloud-event samples were collected at the Puy de Dôme summit, and cultivation was performed leading to the isolation of 71 bacterial, 42 fungal and 15 yeast strains. Most of the fungi isolated were of Cladosporium or Trametes affiliation, and yeasts were of Cryptococcus affiliation. Bacteria, identified on the basis of their 16S rRNA gene sequence, were found to belong to Actinobacteria, Firmicutes, Proteobacteria (Alpha, Beta and Gamma subclasses) and Bacteroidetes phyla, and mainly to the genera Pseudomonas, Sphingomonas, Staphylococcus, Streptomyces, and Arthrobacter. These strains appear to be closely related to some bacteria described from cold environments, water (sea and freshwater), soil or vegetation. Comparison of the distribution of Gram-negative vs. Gram-positive bacteria shows that the number of Gram-negative bacteria is greater in summer than in winter. Finally, a very important result of this study concerns the ability of half of the tested strains to grow at low temperatures (5 degrees C): most of these are Gram-negative bacteria, and a few are even shown to be psychrophiles. On the whole, these results give a good picture of the microbial content of cloud water in terms of classification, and suggest that a large proportion of bacteria present in clouds have the capacity to be metabolically active there. This is of special interest with respect to the potential role of these microorganisms in atmospheric chemistry.

Abstract. Long-term monitoring of the chemical composition of clouds (73 cloud events representing 199 individual samples) sampled at the puy de Dôme (pdD) station (France) was performed between 2001 and 2011. Physicochemical parameters, as well as the concentrations of the major organic and inorganic constituents, were measured and analyzed by multicomponent statistical analysis. Along with the corresponding back-trajectory plots, this allowed for distinguishing four different categories of air masses reaching the summit of the pdD: polluted, continental, marine and highly marine. The statistical analysis led to the determination of criteria (concentrations of inorganic compounds, pH) that differentiate each category of air masses. Highly marine clouds exhibited high concentrations of Na+ and Cl−; the marine category presented lower concentration of ions but more elevated pH. Finally, the two remaining clusters were classified as "continental" and "polluted"; these clusters had the second-highest and highest levels of NH4+, NO3−, and SO24−, respectively. This unique data set of cloud chemical composition is then discussed as a function of this classification. Total organic carbon (TOC) is significantly higher in polluted air masses than in the other categories, which suggests additional anthropogenic sources. Concentrations of carboxylic acids and carbonyls represent around 10% of the organic matter in all categories of air masses and are studied for their relative importance. Iron concentrations are significantly higher for polluted air masses and iron is mainly present in its oxidation state (+II) in all categories of air masses. Finally, H2O2 concentrations are much more varied in marine and highly marine clouds than in polluted clouds, which are characterized by the lowest average concentration of H2O2. This data set provides concentration ranges of main inorganic and organic compounds for modeling purposes on multiphase cloud chemistry.

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