Vertical and horizontal distributions of the aerosol number concentration and size distribution over the northern Indian Ocean

Airborne measurements of the aerosol number concentration and size distribu tion were conducted over the northern Indian Ocean during the Indian Ocean Experiment (INDOEX) in February-March 1999. Vertical profiles of the aeroso l number concentration demonstrate elevated concentrations of nucleation mo de particles in a layer between 8 and 12.5 kin altitude. By using a novel c ombination of back trajectory information and cloud top temperatures retrie ved from satellite images, it is shown that these particles most likely ori ginated from the outflow of large convective clouds. Accumulation mode part icles observed at these altitudes show indications for cloud processing. Th e aerosol size distributions observed in the layer between 4 and 8 kin alti tude show characteristics of an aged aerosol. In this layer the accumulatio n mode particle number concentration presents a minimum, a low variability, and very few particles larger than 0.7 mum diameter. The aerosol in the ma rine boundary layer can be characterized by high number concentrations of s ubmicron and accumulation mode particles, which gradually decrease with dis tance 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 distr ibution and can therefore not be explained by coagulation, but is likely du e to precipitation and entrainment of free tropospheric air.