Insights into tropical cloud chemistry at Reunion Island (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 at Reunion Island (Indian Ocean). Fourteen cloud samples have been collected along the slope of this mountainous island. A comprehensive chemical characterization of these samples is performed, including inorganic ions, metals, oxidants, and organic matter (organic acids, sugars, amino acids, carbonyls, and low-soluble 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. However, the non-sea salt fraction of sulphate 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 mgC L−1), with levels reaching up to 62 mgC 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 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. Additionally, several VOCs (oxygenated and low-soluble VOCs) were analysed in both gas and aqueous phases. Significant levels of biogenic low-soluble VOCs were detected in the aqueous phase, indicating the cloud-terrestrial vegetation exchange. Cloud scavenging of VOCs is assessed and compared to Henry’s law equilibrium to evaluate potential super or sub saturation conditions. The evaluation reveals the supersaturation of low-soluble VOCs from both natural and anthropogenic sources. Our results depict even higher supersaturation of terpenoids, suggesting their importance in the aqueous phase chemistry in highly impacted tropical areas.

Box Model Intercomparison of Cloud Chemistry

“…Discrepancies between observations and model are likely due to the low initial OC aerosol concentrations taken from the WRF‐Chem simulation, in which chemical transformations in aerosols were inadvertently omitted. It should also be noted that a large fraction (35%–85%) of the organic content in cloud and fog water is usually not characterized on a molecular level (Dominutti et al., 2021; Herckes et al., 2013). Therefore, its formation processes cannot be included in models.…”

High Resolution Dynamical Analysis of Volatile Organic Compounds (VOC) Measurements During the BIO‐MAÏDO Field Campaign (Réunion Island, Indian Ocean)

“…This threshold has already been used in Dominutti et al. (2021). Figure 2 shows the Corine Land Cover‐2018 map and land use categories detailed in the register.…”

“…This depletion can be mostly explained by the decrease of solar radiation and temperature, affecting the biogenic emission of isoprene (Monson et al., 1992; Rinne et al., 2002), and by the oxidation processes of this compound leading to consumption and decrease. A recent study had shown the presence of low‐soluble VOCs, such as isoprene, in cloud water at Réunion Island (Dominutti et al., 2021). Even though the transfer from gas to the aqueous phase represents a minor sink due to isoprene’s solubility, this mechanism could also affect the depletion observed.…”

“…As suggested previously, the cloud formation inducesa decrease of the intensity of solar radiation and then, isoprene emissions. Then, in a second step, it can induce a transfer of gas‐phase VOCs into cloud droplets (Dominutti et al., 2021). Indeed, isoprene is the major VOC present in cloud droplet (55.4% of total VOC measured).…”

“…The register spatial resolution is around 70 m. The calculation of the relative land cover footprint is based on trajectory points located below 500 m above ground level since air masses located above this altitude are not very sensitive to surface emissions. This threshold has already been used in Dominutti et al (2021). Figure 2 shows the Corine Land Cover-2018 map and land use categories detailed in the register.…”