Marine aerosols can act as ice nucleating particles (INPs) and thus influence cloud microphysical properties, water cycle, and global climate. However, 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 particle (TSP) 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 µm), biological (heat sensitive) and bacterial (lysozyme sensitive) INPs 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. Biological INPs were generally higher near the coast and lower in the open ocean. However, high wind speeds (exceeding 8 m s^–1) 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. At the organic molecular level, carboxylic-rich alicyclic molecules and aliphatic/peptide-like molecules among CHON molecules may promote ice nucleation. Additionally, some nitrogen- and sulfur-containing carboxylic-rich alicyclic molecules, lipid and aliphatic/peptide-like molecules that are possibly ice-nucleation active, were probably related to microbial activity. The relative abundance of potential ice nucleation active bacteria likely affected the initial freezing temperature of aerosol samples. Cyanobacteria may be an important contributor to atmospheric INPs from the South China Sea to the eastern Indian Ocean.

Ice nucleating particles,Organic molecular composition,Ice-nucleation active microorganisms,Indian Ocean,South China Sea