Airborne measurements of the aerosol number concentration and size distribution were conducted over the northern Indian Ocean during the Indian Ocean Experiment (INDOEX) in February-March 1999. Vertical profiles of the aerosol number concentration demonstrate elevated concentrations of nucleation mode particles in a layer between 8 and 12.5 km altitude. By using a novel combination of back trajectory information and cloud top temperatures retrieved from satellite images, it is shown that these particles most likely originated from the outflow of large convective clouds. Accumulation mode particles observed at these altitudes show indications for cloud processing. The aerosol size distributions observed in the layer between 4 and 8 km altitude show characteristics of an aged aerosol. In this layer the accumulation mode particle number concentration presents a minimum, a low variability, and very few particles larger than 0.7 μm diameter. The aerosol in the marine boundary layer can be characterized by high number concentrations of submicron and accumulation mode particles, which gradually decrease with distance from the Indian subcontinent. The particle loss rate is equivalent to 10--16 cm-3 per hour. This decrease takes place over the whole size distribution and can therefore not be explained by coagulation, but is likely due to precipitation and entrainment of free tropospheric air. ¿ 2001 American Geophysical Union