The ultrafine particles over the southern mid-latitude Indian Ocean — wind-generated or advected down with subsidence?

Concentration and number size distribution of aerosol particles from 4.4 to 168 nm diameter are measured at ten latitudinal positions in the Indian Ocean during the Pilot Expedition to Southern Ocean (PESO) from January 23 to March 31, 2004. Total number concentration of particles is minimum in the southern tropical trade wind region and maximum in the roaring forties. Number size distributions follow the power law over the northern Indian Ocean but are monomodal with the maxima lying between 35 and 60 nm over the southern Indian Ocean. Particles of diameter as small as 5 nm are observed in concentrations of ∼ 103 cm− 3 in the roaring forties (e.g. ∼ 40–50°S). Concentration of particles of < 50 nm diameter increases with wind speed. Thus, in addition to the ultrafine and Aitken mode particles being transported from the free troposphere to the marine boundary layer with the subsidence following cold frontal passages in the southern mid-latitudes, the wind-generated particles significantly contribute to the ultrafine aerosols down to 5 nm diameter in the roaring forties. Observations indicate that while the wind-generated particles dominate at ∼ 45°S, the particles associated with the subsidence dominate at ∼ 38°S.