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

Atmospheric Chemistry and Physics, Vol. 22 (2022)

Keywords
Authors
  • P. A. Dominutti
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • P. Renard
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • M. Vaïtilingom
  • Laboratoire de Recherche en Géosciences et Energies, EA 4539, Université des Antilles, 97110 Pointe-à-Pitre, France
  • A. Bianco
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • J.-L. Baray
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • J.-L. Baray
  • Observatoire de Physique du Globe de Clermont-Ferrand, UAR 833, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • A. Borbon
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • T. Bourianne
  • Centre National de Recherches Météorologiques (CNRM), Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • F. Burnet
  • Centre National de Recherches Météorologiques (CNRM), Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • A. Colomb
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • A.-M. Delort
  • Institut de Chimie de Clermont-Ferrand, CNRS, SIGMA Clermont, Université Clermont Auvergne, 63178 Aubière, France
  • V. Duflot
  • Laboratoire de l'Atmosphère et des Cyclones (LACy), UMR 8105, Université de la Réunion-CNRS-Météo-France, Saint-Denis de La Réunion, France
  • S. Houdier
  • Institut des Géosciences de l'Environnement, UMR 5001, CNRS, IRD, Université Grenoble Alpes, Grenoble, France
  • J.-L. Jaffrezo
  • Institut des Géosciences de l'Environnement, UMR 5001, CNRS, IRD, Université Grenoble Alpes, Grenoble, France
  • M. Joly
  • Institut de Chimie de Clermont-Ferrand, CNRS, SIGMA Clermont, Université Clermont Auvergne, 63178 Aubière, France
  • M. Leremboure
  • Institut de Chimie de Clermont-Ferrand, CNRS, SIGMA Clermont, Université Clermont Auvergne, 63178 Aubière, France
  • J.-M. Metzger
  • Observatoire des Sciences de l'Univers de La Réunion (OSUR), UAR 3365, Saint-Denis de la Réunion, France
  • J.-M. Pichon
  • Observatoire de Physique du Globe de Clermont-Ferrand, UAR 833, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • M. Ribeiro
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • M. Rocco
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • P. Tulet
  • Laboratoire de l'Atmosphère et des Cyclones (LACy), UMR 8105, Université de la Réunion-CNRS-Météo-France, Saint-Denis de La Réunion, France
  • P. Tulet
  • Laboratoire d'Aérologie, UMR 5560 (Université de Toulouse, CNRS, IRD), Toulouse, France
  • A. Vella
  • Institut des Géosciences de l'Environnement, UMR 5001, CNRS, IRD, Université Grenoble Alpes, Grenoble, France
  • M. Leriche
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • M. Leriche
  • Centre pour l'étude et la simulation du climat à l'échelle régionale (ESCER), Département des sciences de la terre et de l'atmosphère, Université du Québec à Montréal, Montréal, Canada
  • L. Deguillaume
  • Laboratoire de Météorologie Physique, UMR 6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • L. Deguillaume
  • Observatoire de Physique du Globe de Clermont-Ferrand, UAR 833, CNRS, Université Clermont Auvergne, 63178 Aubière, France

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.

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