Origin, size distribution, and hygroscopic properties of marine aerosols in the southwestern Indian Ocean: results of six campaigns of shipborne observations

Abstract. This study presents observations of marine aerosols made during six ship-based campaigns in the southwestern Indian Ocean in 2021 and 2023. A set of aerosol measurement instruments is used to study the spatial and temporal variability in the number and size distribution of marine aerosols, the concentration of cloud condensation nuclei (CCN), and the hygroscopic properties of aerosols (kappa–Köhler parameter, κ). It has been shown that the number of submicron aerosols measured varies much more significantly (ranging from 100 to over 3000 cm−3) than the number of CCN (60 to 500 cm−3 at 0.4 % supersaturation). As a result, the κ values obtained show considerable variability, ranging from 0.05 to 0.7. Four distinct scenarios are examined to elucidate some of these variations: (1) the predominance of pristine air masses in the eastern regions of the subtropical Indian Ocean, with highly variable κ values sensitive to the low aerosol concentration measured in this area; (2) the predominance of polluted air masses in the Mozambique Channel, with weakly hydrophilic aerosols; (3) a precipitation and storm event in the southern Indian Ocean, with highly variable κ values; and (4) a new particle formation event in the open ocean, with an increase in κ values as the newly formed particles grow to Aitken mode particles. The size distribution of the sampled marine aerosols was analyzed according to the origin of the air masses. In general, a shift of the Aitken and accumulation modes toward larger aerosol sizes was observed for continental and subtropical air masses in the Indian Ocean due to aging. Conversely, the modes shifted toward smaller sizes for air masses in the southern Indian Ocean due to higher primary marine emissions. Aerosols are more hydrophobic for continental air masses (κ ∼ 0.1), more hydrophilic and variable over the subtropical Indian Ocean (κ ranging from 0.2 to 0.6), and intermediate (κ ∼ 0.2) over the southern Indian Ocean. The κ of the subtropical Indian Ocean increases with wind intensity, while it remains stable in the southern Indian Ocean. This effect is attributed to the high proportion of primary organic matter, which is due to the important concentration of nanophytoplankton in the southern Indian Ocean. It has been shown that primary organic aerosols act as surfactants, thus counterbalancing the highly hydrophilic properties of NaCl.