Abstract
Abstract. 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 (∼5000 cm−3), low CCN activation efficiency (∼25 %) and a steep increase in CCN concentration with the increase in supersaturation. In contrast, low CCN concentration (∼1000 cm−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 (<100 nm) particle events, is investigated in detail for the first time over the region.
Highlights
Aerosol–climate interaction is one of the major uncertain components of the Earth–atmosphere system, which includes several pathways like aerosol–radiation, aerosol–cloud, aerosol–cryosphere and aerosolbiosphere interactions having significant radiative forcing (IPCC, 2013; Rosenfeld et al, 2014; Li et al, 2016)
The SEAS2 region, which is mostly affected by the advection from the Bay of Bengal region, showed high cloud condensation nuclei (CCN) concentrations of up to 2500 cm−3 indicating the widespread influence of South Asian outflow
The influence of continental outflow on the CCN characteristics over the marine atmospheric boundary layer extending from 15◦ N to 2◦ S close to the Indian subcontinent were investigated
Summary
Aerosol–climate interaction is one of the major uncertain components of the Earth–atmosphere system, which includes several pathways like aerosol–radiation (scattering and absorption of solar radiation), aerosol–cloud (modification of cloud properties due to aerosols), aerosol–cryosphere (snow albedo reduction due to aerosol deposition) and aerosolbiosphere interactions having significant radiative forcing (IPCC, 2013; Rosenfeld et al, 2014; Li et al, 2016). Due to the favourable prevailing wind system, these continental aerosols are being transported over the northern Indian Ocean (Arabian Sea and Bay of Bengal), and the effects of these anthropogenic aerosols on regional climate have been the major scientific theme for the several field campaigns and modelling studies carried out during the last 2 decades (Moorthy et al, 2009; Ramanathan et al, 2001; Lelieveld et al, 2001; Ackerman et al, 2000; Nair et al, 2013). The present study focusses on wintertime measurements of the CCN concentrations at different supersaturations, along with simultaneous measurements of the aerosol properties when the entire northern Indian Ocean was under the influence of continental outflow from South Asia. All the instruments were calibrated prior to the campaign following the standard protocols
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