Insights into tropical cloud chemistry in Réunion (Indian Ocean): results from the BIO-MAÏDO campaign

Abstract

Abstract. We present here the results obtained during an intensive field campaign conducted in the framework of the French “BIO-MAÏDO” (Bio-physico-chemistry of tropical clouds at Maïdo (Réunion Island): processes and impacts on secondary organic aerosols' formation) project. This study integrates an exhaustive chemical and microphysical characterization of cloud water obtained in March–April 2019 in Réunion (Indian Ocean). Fourteen cloud samples have been collected along the slope of this mountainous island. Comprehensive chemical characterization of these samples is performed, including inorganic ions, metals, oxidants, and organic matter (organic acids, sugars, amino acids, carbonyls, and low-solubility volatile organic compounds, VOCs). Cloud water presents high molecular complexity with elevated water-soluble organic matter content partly modulated by microphysical cloud properties. As expected, our findings show the presence of compounds of marine origin in cloud water samples (e.g. chloride, sodium) demonstrating ocean–cloud exchanges. Indeed, Na+ and Cl− dominate the inorganic composition contributing to 30 % and 27 %, respectively, to the average total ion content. The strong correlations between these species (r2 = 0.87, p value: < 0.0001) suggest similar air mass origins. However, the average molar Cl-/Na+ ratio (0.85) is lower than the sea-salt one, reflecting a chloride depletion possibly associated with strong acids such as HNO3 and H2SO4. Additionally, the non-sea-salt fraction of sulfate varies between 38 % and 91 %, indicating the presence of other sources. Also, the presence of amino acids and for the first time in cloud waters of sugars clearly indicates that biological activities contribute to the cloud water chemical composition. A significant variability between events is observed in the dissolved organic content (25.5 ± 18.4 mg C L−1), with levels reaching up to 62 mg C L−1. This variability was not similar for all the measured compounds, suggesting the presence of dissimilar emission sources or production mechanisms. For that, a statistical analysis is performed based on back-trajectory calculations using the CAT (Computing Atmospheric Trajectory Tool) model associated with the land cover registry. These investigations reveal that air mass origins and microphysical variables do not fully explain the variability observed in cloud chemical composition, highlighting the complexity of emission sources, multiphasic transfer, and chemical processing in clouds. Even though a minor contribution of VOCs (oxygenated and low-solubility VOCs) to the total dissolved organic carbon (DOC) (0.62 % and 0.06 %, respectively) has been observed, significant levels of biogenic VOC (20 to 180 nmol L−1) were detected in the aqueous phase, indicating the cloud-terrestrial vegetation exchange. Cloud scavenging of VOCs is assessed by measurements obtained in both the gas and aqueous phases and deduced experimental gas-/aqueous-phase partitioning was compared with Henry's law equilibrium to evaluate potential supersaturation or unsaturation conditions. The evaluation reveals the supersaturation of low-solubility VOCs from both natural and anthropogenic sources. Our results depict even higher supersaturation of terpenoids, evidencing a deviation from thermodynamically expected partitioning in the aqueous-phase chemistry in this highly impacted tropical area.

Analysis of CO<sub>2</sub>, CH<sub>4</sub>, and CO surface and column concentrations observed at Réunion Island by assessing WRF-Chem simulations

“…The atmospheric transport of the inverse model was calculated using the FLEXPART (FLEXible PARTicle dispersion model) Lagrangian dispersion model (Verreyken et al, 2019) coupled with the Meso-NH mesoscale model (Lac et al, 2018) at a resolution of 500 m and 60 vertical levels. FLEXPART-Meso-NH was run backward in time to calculate the sourcereceptor relationships between MA and the surface sources from the CO measurements at MA, from 4 April to 3 May 2019, during the BIO-MAIDO (Bio-physico-chemistry of tropical clouds at Maïdo, Réunion Island) campaign (Dominutti et al, 2022). The ODIAC CO 2 emission inventory was used a priori to benefit from its native spatial resolution of major urban areas.…”

Assessing the efficiency of water-soluble organic compound biodegradation in clouds under various environmental conditions

“…Formic acid could be efficiently consumed in warm clouds, at relatively high temperatures (10–20 °C), conditions typically encountered in tropical environments. 53 The recent study by Franco et al 54 highlighted the important role of clouds in formic acid production via multiphase reactivity. Their modeling investigations indicated overestimated concentrations of formic acid in the tropics; various reasons were given in this work to explain this statement (especially the elevated isoprene emissions) but biological activity (especially at night) could strongly reduce formic acid levels in these regions.…”

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

“…These differences could be related to the presence of cloud events, which were more intense during Period 2 (Figure S9). 37 The evaluation of campaign periods also integrates the backward trajectories' analysis results. Figure 3 shows the average daily (3 to 16 UTC, 7 to 20 LT) backward trajectories (12 h) obtained during the two periods of the campaign.…”

“…A comprehensive chemical analysis was performed on collected cloudwater, evaluating their inorganic (ions, metals) and organic (amino acids, organic acids, sugars, dissolved organic matter, carbonyls, and low solubility volatile organic compounds) composition. 37 The MO is situated close to the summit on the western part of the island at an altitude that allows for free troposphere (FT) chemical characterization at nighttime as well as boundary layer (BL) chemical characterization during daytime. Large-scale air mass dynamics influence the atmosphere at Reúnion island, chiefly characterized by Hadley Cell-induced westerlies in the free troposphere 38 and southeasterly trade winds near the ground.…”