Extensive measurements of cloud condensation nuclei (CCN) and condensation nuclei (CN) concentrations in the South Asian outflow to the northern Indian Ocean were carried out on board an instrumented research vessel, as part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) during the winter season (January–February?2018). Measurements include a north–south transect across the South Asian plume over the northern Indian Ocean and an east–west transect over the equatorial Indian Ocean (～2°S), which is far away from the continental sources. South Asian outflow over the northern Indian Ocean is characterized by the high values of CCN number concentration (～5000cm?3), low CCN activation efficiency (～25%) and a steep increase in CCN concentration with the increase in supersaturation. In contrast, low CCN concentration (～1000cm?3) with flat supersaturation spectra was found over the equatorial Indian Ocean. The CCN properties exhibited significant dependence on the geometric mean diameter (GMD) of the aerosol number size distribution, and CCN activation efficiency decreased to low values (20%) at the time of new-particle formation events over near-coastal and remote oceanic regions. The analysis of the activation efficiencies for the “similar” aerosol size distributions over the northern Indian Ocean indicated the primary role of aerosol number size distribution on CCN activation efficiency. The dependence of CCN properties and activation efficiency on size-segregated aerosol number concentration, especially during the ultrafine (100nm) particle events, is investigated in detail for the first time over the region.
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