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Cloud/fog droplets comprise a myriad of chemical compounds and are living environments in which microorganisms are present and active. These chemical and biological elements can evolve in various ways within the cloud system, and the aqueous transformation of chemicals contributes to atmospheric chemistry. In situ cloud studies are fundamental in this sense, because they enable us to study the variability in cloud chemical composition as a function of environmental conditions and assess their potential for transforming chemical compounds. To achieve this objective, cloud water collectors have been developed in recent decades to recover water from clouds and fogs using different designs and collection methods. In this study, a new active ground-based cloud collector was developed and tested for sampling cloud water to assess the cloud microbiology and chemistry. This new instrument, BOOGIE, is an easy mobile sampler for cloud water collection with the objective of being cleanable and sterilisable, respecting chemical and microbial cloud integrity, and presenting an efficient collection rate of cloud water. Computational fluid dynamics simulations were performed to theoretically assess the capture of cloud droplets by this new sampler. Few turbulences have been observed inside the collector and a 50 % collection efficiency cutoff of 10 µm has been estimated. The collector was deployed at Puy de Dôme station under cloudy conditions for evaluation. The water collection rates were measured at 156 ± 52 mL h-1 for a collection of 17 cloud events; considering the measured liquid water content, the sampling efficiency of this new collector has been estimated at 87.2 ± 8.6 % over the same set of cloud events. BOOGIE was compared with other active cloud collectors commonly used by the scientific community (Cloud Water Sampler and Caltech Active Strand Cloud Collector version 2). Four cloud events were collected; the three samplers presented similar collection efficiencies (between 79 % and 88 % on average). The measured ionic composition was comparable even if differences were highlighted between collectors, the consequence of different designs, and the intrinsic homogeneity in the chemical composition within the cloud system. [ABSTRACT FROM AUTHOR]

Within cloud water, microorganisms are metabolically active and, thus, are expected to contribute to the atmospheric chemistry. This article investigates the interactions between microorganisms and the reactive oxygenated species that are present in cloud water because these chemical compounds drive the oxidant capacity of the cloud system. Real cloud water samples with contrasting features (marine, continental, and urban) were taken from the puy de Dôme mountain (France). The samples exhibited a high microbial biodiversity and complex chemical composition. The media were incubated in the dark and subjected to UV radiation in specifically designed photo-bioreactors. The concentrations of H2O2, organic compounds, and the ATP/ADP ratio were monitored during the incubation period. The microorganisms remained metabolically active in the presence of •OH radicals that were photo-produced from H2O2. This oxidant and major carbon compounds (formaldehyde and carboxylic acids) were biodegraded by the endogenous microflora. This work suggests that microorganisms could play a double role in atmospheric chemistry; first, they could directly metabolize organic carbon species, and second, they could reduce the available source of radicals through their oxidative metabolism. Consequently, molecules such as H2O2 would no longer be available for photochemical or other chemical reactions, which would decrease the cloud oxidant capacity. [ABSTRACT FROM AUTHOR]

In the present study, we investigated the correlation between the hydroxyl radical formation rate (R˙OH) and the degradation of a pesticide (mesotrione) in synthetic cloud water solutions and in two real atmospheric cloud waters collected at the top of puy de Dôme station (France). Using terephthalic acid as the hydroxyl radical chemical probe, we established the linear correlation between the photogenerated hydroxyl radical under polychromatic wavelengths and the pesticide degradation rate: ( m s−1) = (1.61 ± 0.15) × 10−1 R˙OH( m s−1). Moreover, the formation rate of hydroxyl radical in two natural cloud waters was estimated considering H2O2 and NO3− and the difference between the predicted values and those experimentally obtained could be attributed to the presence of other photochemical sources: iron-complexes and total organic matter. The organic constituents could play a dual role of sources and scavengers of photoformed hydroxyl radicals in the aqueous phase. [ABSTRACT FROM AUTHOR]

Abstract: The kinetics of biodegradation of methanol and formaldehyde in phosphate buffer at pH 7 by 4 bacterial strains (Pseudomonas spp., Bacillus sp. and Frigoribacterium sp.) isolated from cloud water at the puy de Dôme mountain have been investigated using 1H and 13C NMR spectroscopy. We showed that biodegradation occurred at 5°C and 17°C, respectively average and summertime temperature considered within the cloud system at this site. They ranged from 10−19 to 10−21 mol cell−1 s−1 both at 5 and 17°C for formaldehyde, and from 10−21 to 10−23 mol cell−1 s−1 at 5 and 17°C for methanol. Metabolic intermediates were identified, with notably production of C3 compounds (glycerol, 1,2- and 1,3-propanediol) from formaldehyde by the strain Bacillus sp. In order to evaluate to which extent microbiological oxidation of organic compounds has to be considered as an alternative route to radical chemistry in cloud water, the biodegradation rates measured were compared with rates related to the reactivity of organic species with free radicals OH (daytime chemistry) and NO3 (nighttime chemistry) under two cloud situations (urban and remote cases). Clearly, measured biological and chemical reaction rates were in the same range of magnitude and their relative contribution varies according to the scenarios we tested, including the temperature of the clouds (5 or 17°C), the category of the clouds (urban and remote) and the diurnal cycle (day and nighttime). Except for the degradation of methanol at 5°C in remote clouds, our results show that biotransformation processes could be the main sink for C1 compounds in liquid clouds (T ≥5°C≡“warm cloud”) during the night and both in polluted and non polluted clouds. [Copyright &y& Elsevier]

A total of 450 bacteria and yeast strains isolated from cloud waters sampled at the puy de Dôme station in France (1465 m) were screened for their ability to produce siderophores. To achieve this, a high-throughput method in 96-well plates was adapted from the CAS (chrome azurol S) method. Notably, 42% of the isolates were siderophore producers. This production was examined according to the phyla of the tested strains and the type of chelating functional groups (i.e., hydroxamate, catechol, and mixed type). The most active bacteria in the clouds belong to the γ-Proteobacteria class, among which the Pseudomonas genus is the most frequently encountered. γ-Proteobacteria are produced in the majority of mixed function siderophores, such as pyoverdines, which bear a photoactive group. Finally, siderophore production was shown to vary with the origin of the air masses. The organic speciation of iron remains largely unknown in warm clouds. Our results suggest that siderophores could partly chelate Fe(III) in cloud waters and thus potentially impact the chemistry of the atmospheric aqueous phase. [ABSTRACT FROM AUTHOR]

Abstract: Currently, cloud chemistry models are including more detailed and explicit multiphase mechanisms based on laboratory experiments that determine such values as kinetic constants, stability constants of complexes and hydration constants. However, these models are still subject to many uncertainties related to the aqueous chemical mechanism they used. Particularly, the role of oxidants such as iron and hydrogen peroxide in the oxidative capacity of the cloud aqueous phase has typically never been validated against laboratory experimental data. To fill this gap, we adapted the M2C2 model (Model of Multiphase Cloud Chemistry) to simulate irradiation experiments on synthetic aqueous solutions under controlled conditions (e.g., pH, temperature, light intensity) and for actual cloud water samples. Various chemical compounds that purportedly contribute to the oxidative budget in cloud water (i.e., iron, oxidants, such as hydrogen peroxide: H2O2) were considered. Organic compounds (oxalic, formic and acetic acids) were taken into account as target species because they have the potential to form iron complexes and are good indicators of the oxidative capacity of the cloud aqueous phase via their oxidation in this medium. The range of concentrations for all of the chemical compounds evaluated was representative of in situ measurements. Numerical outputs were compared with experimental data that consisted of a time evolution of the concentrations of the target species. The chemical mechanism in the model describing the “oxidative engine” of the H x O y /iron (H x O y = H2O2, HO2 /O2 − and HO ) chemical system was consistent with laboratory measurements. Thus, the degradation of the carboxylic acids evaluated was closely reproduced by the model. However, photolysis of the Fe(C2O4)+ complex needs to be considered in cloud chemistry models for polluted conditions (i.e., acidic pH) to correctly reproduce oxalic acid degradation. We also show that iron and formic acid lead to a stable complex whose photoreactivity has currently not been investigated. The updated aqueous chemical mechanism was compared with data from irradiation experiments using natural cloud water. The new reactions considered in the model (i.e., iron complex formation with oxalic and formic acids) correctly reproduced the experimental observations. [Copyright &y& Elsevier]

Abstract: Recent studies showed that living microorganisms, including bacteria, fungi and yeasts, are present in the atmospheric water phase (fog and clouds) and their role in chemical processes may have been underestimated. At the interface between atmospheric science and microbiology, information about this field of science suffers from the fact that not all recent findings are efficiently conveyed to both scientific communities. The purpose of this paper is therefore to provide a short overview of recent work linked to living organisms in the atmospheric water phase, from their activation to cloud droplets and ice crystal, to their potential impact on atmospheric chemical processes. This paper is focused on the microorganisms present in clouds and on the role they could play in atmospheric chemistry and nucleation processes. First, the life cycle of microorganisms via the atmosphere is examined, including their aerosolization from sources, their integration into clouds and their wet deposition on the ground. Second, special attention is paid to the possible impacts of microorganisms on liquid and ice nucleation processes. Third, a short description of the microorganisms that have been found in clouds and their variability in numbers and diversity is presented, emphasizing some specific characteristics that could favour their occurrence in cloud droplets. In the last section, the potential role of microbial activity as an alternative route to photochemical reaction pathways in cloud chemistry is discussed. [Copyright &y& Elsevier]