Changes in the concentration and size-distribution of the sub-micron particles associated with the sea- and land-breezes at a coastal station

Surface measurements of the size-distribution of submicron aerosol particle s in the range of 0.003 to 1 mu m diameter have been made on 6-8 January 19 98, at Thiruvananthapuram during an inter-comparison campaign of the Indian Ocean experiment (INDOEX), The results are studied with respect to the set ting-in of the sea- and land-breezes at the station. Observations show an i ncrease of up to an order of magnitude in aerosol concentrations of all siz e categories with the setting-in of the land breeze at 1800-1900 IST. High concentrations of aerosol particles prevail throughout the period of the la nd breeze at night-time. Aerosol concentrations remarkably decrease at abou t 1000 IST with the arrival of much cleaner air with the sea breeze. During the land breeze at night-time, the size-distributions of aerosol particles is bimodal with the maximum at 0.075 and 0.024 mu m diameters, During the sea breeze, especially in the afternoon, the maximum in the accumulation mo de shifts to a slightly higher size (0.133 mu m) and the maximum in the nuc leation mode seems to shift to a smaller size (0.013 mu m or smaller). The size-distribution curves during the daytime are mostly open-ended at the sm all particle-size end. The enhanced coagulation of aerosol particles and th e gas-to-particle conversion processes have been proposed to explain the sh ift of maxima in the accumulation mode and the enhanced generation of small particles in the nucleation mode in the afternoon, respectively.