The concentration and number size distribution measurements of the Marine boundary layer aerosols over the Indian Ocean

Abstract

The measurements of total number concentration and number size distribution of aerosols in size ranges of 16-700 nm and 0.5-20 μm diameters made from 14°N to 56°S in the Indian ocean during January 23 to March 31, 2004, are reported. The average values of total number (mass) concentration of micrometer aerosols (0.5-20 μm) are 7.2 ± 3 cm^-3 (8.89 μg m^-3) from 14°N to the Inter-Tropical Convergence Zone (ITCZ), 4.6 ± 2 cm^-3 (4.9 μg m^-3) in the ITCZ, 3.0 ± 1.4 cm^-3 (5.78 μg m^-3) in the trade wind region from 8°-30°S, 3.8 ± 2.6 cm^-3 (7.9 μg m^-3) from 30°-40°S, and 5.8 ± 3.5 cm^-3 (9.65 μg m^-3) in the roaring forties from 40°-56°S. Latitudinal distribution of such aerosols shows that their number concentration is minimum (0.5 cm^-3) at 11°S and increases on either side of this location. Size distributions of micrometer aerosols in all latitudinal belts show a maxima in coarse mode at 0.5-1.5 μm diameter. The correlation coefficient in the aerosol concentration-wind speed relations of these aerosols is observed to differ in different latitudinal belts and has maximum value in the belt of strongest winds. Also, diurnal variations of the average aerosol concentration and wind speed show some similarity in belts of strong winds. Total number concentration of submicrometer (16-700 nm) particles is also minimum in the southern trade wind region and their size distribution is bimodal with maxima in Aitken mode (~ 50 nm) and accumulation mode (~ 130 nm). South of the ITCZ, concentrations of both, total number and Aitken mode particles normally increase with latitude. The increase in Aitken mode particles at high latitudes is large enough for the Aitken mode maxima to superimpose the accumulation mode maxima. Observations suggest that addition of Aitken particles is so fast that process of coagulation is not able to reach equilibrium to develop a distinct accumulation maxima. Observations show that pristine air of the trade wind region in the Southern Hemisphere can be advected to mid-latitudes along the ridge developed between anticyclonic and cyclonic systems.