Laurent Deguillaume

Potential impact of microbial activity on the oxidant capacity and organic carbon budget in clouds.

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.

Sensitive determination of glyoxal, methylglyoxal and hydroxyacetaldehyde in environmental water samples by using dansylacetamidooxyamine derivatization and liquid chromatography/fluorescence

In this study we improved the dansylacetamidooxyamine (DNSAOA)-LC-fluorescence method for the determination of aqueous-phase glyoxal (GL), methylglyoxal (MG) and hydroxyacetaldehyde (HA). As derivatization of dicarbonyls can potentially lead to complex mixtures, a thorough study of the reaction patterns of GL and MG with DNSAOA was carried out. Derivatization of GL and MG was shown to follow the kinetics of successive reactions, yielding predominantly doubly derivatized compounds. We verified that the bis-DNSAOA structure of these adducts exerted only minor influence on their fluorescence properties. Contrary to observations made with formaldehyde, derivatization of GL, MG and, to a lesser extent of HA, was shown to be faster in acidic (H(2)SO(4)) medium with a maximum of efficiency for acid concentrations of ca. 2.5 mM. Concomitant separation of GL, MG, HA and of single carbonyls was achieved within 20 min by using C(18) chromatography and a gradient of CH(3)CN in water. Detection limits of 0.27, 0.17 and 0.12 nM were determined for GL, MG and HA, respectively. Consequently, low sample volumes are sufficient and, unlike numerous published methods, neither preconcentration nor large injection volumes are necessary to monitor trace-level samples. The method shows relative measurement uncertainties better than ±15% at the 95% level of confidence and good dynamic ranges (R(2)>0.99) from 0.01 to 1.5 μM for all carbonyls. GL, MG and HA were identified for the first time in polar snow samples, but also in saline frost flowers for which unexpected levels of 0.1-0.6 μM were measured. Concentrations in the 0.02-2.3 μM range were also measured in cloud water. In most samples, a predominance of HA over GL and MG was observed.

Effect of iron dissolution on cloud chemistry: from laboratory measurements to model results

This study investigates the influence of iron dissolution from aerosol particles on cloud chemistry and presents improvements in modeling of the associated multiphase processes. Iron redox species are important pollutants; they are very reactive in clouds especially through their interactions with HXOy compounds. Solid phase iron is transferred into the aqueous–phase by dissolution. The rate of dissolution of iron drives its concentration in the solid and aqueous–phases and has been determined in laboratory based on dissolution experiments using an urban particles standard. The parameterization of the iron dissolution rate and iron redox speciation as a function of time, stemming from the experimental data, was implemented in the cloud chemistry model M2C2 (Model of Multiphase Cloud Chemistry). A continental cloud event with simplified microphysical properties was simulated. Two simulations were performed: (1) using a predefined fixed iron content, and (2) using iron content values evolving temporally resulting from the dissolution parameterization. Numerical results show that the iron speciation (i.e. the partitioning between iron oxidation states) is driven by the chemical reactivity and not by dissolution. The oxidative capacity of the atmospheric cloud water decreases when dissolution is included in the model. The flux of OH radicals produced is positively correlated to the concentration of iron present that continuously increases with time throughout the simulation. Thus, ignoring the iron dissolution kinetics results in overestimations of net OH production, by factors of 1.7 and 1.2 after 5 minutes and 2 hours of the simulation, respectively. This study concludes that the consideration of the dissolution process is potentially important in estimating radical concentrations and hence cloud oxidative capacity.

Active microorganisms thrive among extremely diverse communities in cloud water

Clouds are key components in Earth’s functioning. In addition of acting as obstacles to light radiations and chemical reactors, they are possible atmospheric oases for airborne microorganisms, providing water, nutrients and paths to the ground. Microbial activity was previously detected in clouds, but the microbial community that is active in situ remains unknown. Here, microbial communities in cloud water collected at puy de Dôme Mountain’s meteorological station (1465 m altitude, France) were fixed upon sampling and examined by high-throughput sequencing from DNA and RNA extracts, so as to identify active species among community members. Communities consisted of ~103−104 bacteria and archaea mL-1 and ~102−103 eukaryote cells mL-1. They appeared extremely rich, with more than 28 000 distinct species detected in bacteria and 2 600 in eukaryotes. Proteobacteria and Bacteroidetes largely dominated in bacteria, while eukaryotes were essentially distributed among Fungi, Stramenopiles and Alveolata. Within these complex communities, the active members of cloud microbiota were identified as Alpha- (Sphingomonadales, Rhodospirillales and Rhizobiales), Beta- (Burkholderiales) and Gamma-Proteobacteria (Pseudomonadales). These groups of bacteria usually classified as epiphytic are probably the best candidates for interfering with abiotic chemical processes in clouds, and the most prone to successful aerial dispersion.

Siderophores in Cloud Waters and Potential Impact on Atmospheric Chemistry: Production by Microorganisms Isolated at the Puy de Dôme Station

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.

Atmospheric Aqueous-Phase Photoreactivity: Correlation Between the Hydroxyl Radical Photoformation and Pesticide Degradation Rate in Atmospherically Relevant Waters

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(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.

Clouds: A Transient and Stressing Habitat for Microorganisms

In this chapter, we synthesized the current knowledge about clouds as ecosystems which have been discovered very recently. First, we briefly described the cloud habitat. Cloud physics chemistry and microphysics are described, showing that this environment is extreme. Microorganisms are exposed to a dynamic medium changing extremely rapidly (evaporation/condensation of the cloud droplets, quick temperature and pressure changes, freeze/thaw cycle) and also to chemical stresses (strong oxidants, acidic pHs and toxics). Then the life cycle of microorganisms in the atmosphere is detailed showing that cloud is a transient habitat: microorganisms are aerosolized, transported in the air, integrated in cloud droplets and deposited back to the ground with precipitation. Finally the cloud microbiome is described; it appears that it remains largely unknown and based mainly on culture techniques. In the second part of the chapter, the abilities of these microorganisms to survive in this stressing environment are described in details. Microbes can adapt their metabolism as it was shown that the majority of the community is metabolically active and that they metabolize organic compounds in cloud water. They have also developed general strategies that help resisting to atmospheric constraints, such as the production of extracellular polymeric substances and pigments, or the formation of spores. Finally they can respond to specific stresses such as oxidative, osmotic and temperature stresses thanks to protecting metabolites such as osmo- and thermo-protectants, anti-oxidants or by using specific enzymes.

Improving the characterization of dissolved organic carbon in cloud water: Amino acids and their impact on the oxidant capacity

Improving our understanding of cloud chemistry depends on achieving better chemical characterization (90% of the organic carbon [OC] fraction remains uncharacterized) and, consequently, assessing the reactivity of this complex system. In this manuscript, we report for the first time the concentrations of 16 amino acids (AAs) in 25 cloud water samples. The concentrations of individual AAs ranged from a few nM up to ~2.0 μM, and the average contribution of AAs corresponded to 9.1% (4.4 to 21.6%) of the dissolved OC (DOC) concentration. Considering their occurrence and concentrations, AAs were expected to represent an important hydroxyl radical (HO•) sink in aqueous cloud samples. In this work, we estimated that approximately 17% (from 7 to 36%) of the hydroxyl radical-scavenging ability of the DOC could be attributed to the presence of AAs, whereas comparing the AAs suggested that an average of 51% (from 22 to 80%) of their reactivity with HO• could account for the presence of tryptophan. These results clearly demonstrate that the occurrence and reactivity of AAs must be considered to better estimate the chemical composition and oxidant capacity of the cloud aqueous phase.

Molecular Characterization of Cloud Water Samples Collected at the puy de Dôme (France) by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Cloud droplets contain dynamic and complex pools of highly heterogeneous organic matter, resulting from the dissolution of both water-soluble organic carbon in atmospheric aerosol particles and gas-phase soluble species, and are constantly impacted by chemical, photochemical, and biological transformations. Cloud samples from two summer events, characterized by different air masses and physicochemical properties, were collected at the Puy de Dôme station in France, concentrated on a strata-X solid-phase extraction cartridge and directly infused using electrospray ionization in the negative mode coupled with ultrahigh-resolution mass spectrometry. A significantly higher number (n = 5258) of monoisotopic molecular formulas, assigned to CHO, CHNO, CHSO, and CHNSO, were identified in the cloud sample whose air mass had passed over the highly urbanized Paris region (J1) compared to the cloud sample whose air mass had passed over remote areas (n = 2896; J2). Van Krevelen diagrams revealed that lignins/CRAM-like, aliphatics/proteins-like, and lipids-like compounds were the most abundant classes in both samples. Comparison of our results with previously published data sets on atmospheric aqueous media indicated that the average O/C ratios reported in this work (0.37) are similar to those reported for fog water and for biogenic aerosols but are lower than the values measured for aerosols sampled in the atmosphere and for aerosols produced artificially in environmental chambers.

Modeling Formation of SOA from Cloud Chemistry with the Meso-NH Model: Sensitivity Studies of Cloud Events Formed at the Puy de Dôme Station

The majority of the organic fraction of aerosols is suspected to be of secondary origin. However, the sources, chemical composition and formation mechanisms of secondary organic aerosols (SOA) remain one of the least understood processes relevant to the atmosphere. Laboratory experiments, in situ measurements and numerical simulations recently highlighted the formation of SOA through aqueous chemistry in humid aerosol particles and in cloud droplets. However, there is still a need to evaluate the relative relevance of SOA formation through aqueous chemistry in comparison to classical gas to particles conversion pathways. Cloud resolving model (CRM) allows simulating the complex aerosols-cloud-chemistry interactions. Meso-NH (Mesoscale Non-Hydrostatic atmospheric model) CRM model includes a cloud chemistry module and an aerosol module, which allows representing the formation of SOA due to aqueous phase reactivity. The model is applied on a cloud event observed at the puy de Dome Mountain. Comparing simulations with or without the cloud chemistry module activated assesses the contribution of the cloud chemistry in the SOA formation. Results show a significant contribution of aqueous phase reactivity in the formation of SOA downstream the Mountain.