Evaluation of the Sources, Precursors, and Processing of Aerosols at a High-Altitude Tropical Site.

This work presents the results from a set of aerosol- and gas-phase measurements collected during the BIO-MAÏDO field campaign in Réunion between March 8 and April 5, 2019. Several offline and online sampling devices were installed at the Maïdo Observatory (MO), a remote high-altitude site in the Southern Hemisphere, allowing the physical and chemical characterization of atmospheric aerosols and gases. The evaluation of short-lived gas-phase measurements allows us to conclude that air masses sampled during this period contained little or no anthropogenic influence. The dominance of sulfate and organic species in the submicron fraction of the aerosol is similar to that measured at other coastal sites. Carboxylic acids on PM10 showed a significant contribution of oxalic acid, a typical tracer of aqueous processed air masses, increasing at the end of the campaign. This result agrees with the positive matrix factorization analysis of the submicron organic aerosol, where more oxidized organic aerosols (MOOAs) dominated the organic aerosol with an increasing contribution toward the end of the campaign. Using a combination of air mass trajectories (model predictions), it was possible to assess the impact of aqueous phase processing on the formation of secondary organic aerosols (SOAs). Our results show how specific chemical signatures and physical properties of air masses, possibly affected by cloud processing, can be identified at Réunion. These changes in properties are represented by a shift in aerosol size distribution to large diameters and an increased contribution of secondary sulfate and organic aerosols after cloud processing.

Contribution of microbial activity to carbon chemistry in clouds.

The biodegradation of the most abundant atmospheric organic C1 to C4 compounds (formate, acetate, lactate, succinate) by five selected representative microbial strains (three Pseudomonas strains, one Sphingomonas strain, and one yeast strain) isolated from cloud water at the puy de Dôme has been studied. Experiments were first conducted under model conditions and consisted of a pure strain incubated in the presence of a single organic compound. Kinetics showed the ability of the isolates to degrade atmospheric compounds at temperatures representative of low-altitude clouds (5 degrees C and 17 degrees C). Then, to provide data that can be extrapolated to real situations, microcosm experiments were developed. A solution that chemically mimicked the composition of cloud water was used as an incubation medium for microbial strains. Under these conditions, we determined that microbial activity would significantly contribute to the degradation of formate, acetate, and succinate in cloud water at 5 degrees C and 17 degrees C, with lifetimes of 0.4 to 69.1 days. Compared with the reactivity involving free radicals, our results suggest that biological activity drives the oxidation of carbonaceous compounds during the night (90 to 99%), while its contribution accounts for 2 to 37% of the reactivity during the day, competing with photochemistry.

Impact of radical versus non-radical pathway in the Fenton chemistry on the iron redox cycle in clouds.

Modeling studies have shown that the Fenton reaction of iron(II) with H2O2 can contribute, in a significant amount, to OH radicals production in cloud droplets. However, the destruction mechanism of hydrogen peroxide by iron(II) is still uncertain. Two reaction pathways for the first step of Fenton chemistry have been advanced: a radical pathway which considers an OH radical production and a non-radical pathway considering ferryl ion production. The aim of this work is to evaluate the impact of these two possible reaction pathways for Fenton chemistry on the iron redox cycle in cloud droplets. For this purpose, the numerical model of multiphase chemistry M2C2 has been applied to a rural chemical scenario representative of continental conditions. This study highlights that the iron redox cycling is driven by Fenton reaction whatever Fenton mechanism is considered. The ferryl ion chemistry becomes significant in the iron redox cycling when this species is considered as an active intermediate in Fenton chemistry and under night time conditions the iron redox chemistry is controlled by the ferryl ion reactivity. The partitioning of iron between its two main oxidation states (+II and +III) in cloud droplets, which is the indicator of the iron oxido-reduction potential, does not change significantly between the two cases. However, for the non-radical case, the ferryl ion concentration is up to four orders of magnitude higher than the OH concentration highlighting its potential role in oxidative capacity of cloud droplets.