Characteristics of ice nucleating particles in the marine atmosphere from the South China Sea to the eastern Indian Ocean

Marine aerosols can act as ice nucleating particles (INPs) and thus influence cloud microphysical properties, water cycle, and global climate. The low concentrations and high variability of INPs in the marine atmosphere lead to difficulties in their measurement and characterization and lack of observational data. In particular, there is a large gap on atmospheric INPs over tropical oceans, especially the Indian Ocean, which may cause large uncertainties in the simulation of atmospheric INPs, resulting in radiation flux errors and thus affecting the climate sensitivity in models. In order to characterize atmospheric INPs over tropical oceans, airborne total suspended particles (TSP) and rainwater samples were collected during a cruise from the South China Sea to the eastern Indian Ocean during April to June 2021. Using the ice nucleation detection device (TJU-INA) combined with multiple treatments, the levels of total INPs and INP compositions including organic, nanoscale (<0.22 mm), biological and bacterial INPs in TSP and rainwater samples were measured, and the sources and influencing factors of INPS were investigated. Organic INPs dominated INPs (71.7%), and nanoscale INPs also accounted for a large fraction (57.6%) of INPs in marine aerosols during the cruise, with slightly higher proportions in marine areas closer to continents. The concentrations of total, nanoscale and organic INPs were higher in the South China Sea closer to the continent, straits, and the eastern Indian Ocean near Sri Lanka, while the concentrations were lower in the open areas of the eastern Indian Ocean. Carbonaceous components emitted from the continents strongly affected the levels of total and nanoscale INPs. Biological INPs were generally higher near the coast and lower in the open ocean. However, high wind speeds in the eastern Indian Ocean south of the equator likely produced more sea spray aerosols, resulting in higher concentrations of biological INPs than in other open ocean areas. Scavenging of airborne particles by wet deposition likely led to reduction in airborne INPs. After conversion, the spatial distribution of the levels of INPs in rainwater were comparable to that of INPs in marine aerosols. Organic INPs were also dominant in rainwater, which were likely affected by marine biological activities. However, the proportion of nanoscale INPs in rainwater was only half that in aerosols, while INPs larger than 0.2 μm were more abundant in rainwater, maybe because rainfall removed larger particles more easily. Additionally, Cyanobacteria were probably important contributors to ice nucleation activity in rainwater.