Airborne measurements of the size distribution of submicron aerosols over the Arabian Sea during ARMEX – Phase I

Abstract

Airborne measurements of the concentration and size distribution of submicron aerosols are made from 6 to 24 August 2002 over the Arabian Sea during the Arabian Sea Monsoon Experiment (ARMEX). Results show that the aerosol concentrations at 0.9 km altitude are larger if the mixing layer depth is more than this height. Further, values of aerosol concentration at 5.1 – 6 kms altitude are  one to three orders of magnitude higher if the air mass is being advected from the central Arabian Sea or the Arabian deserts  in comparison to the values when the air mass is being advected up from the deserts of the northwest India. Aerosol size distributions at 0.9 km level are bimodal or trimodal with a maximum at 133 nm in accumulation mode and the minima at 75 and 480 nm.  Size distributions tend to become log-normal at 5.1 – 6 km altitudes. Results suggest that the  processes of aerosol coagulation and gravitational  settling mainly determine the evolution of modal nature of size distributions observed at lower altitudes over sea. Processes leading to modification of size distribution are also discussed.