Aerosol size distributions in the north and south Indian ocean during the northeast monsoon season

Abstract

Measurements of aerosol size distributions (3- to 1000-nm diameter) were made over the Indian Ocean (15°N, 75°E to 70°S, 11°E) during the onward (December 12, 1996 to January 6, 1997) and return (March 9 to April 5, 1997) cruises of the XVI Indian Scientific Expedition to the Antarctic. Observations show that during the January to April period, the environment over the Indian Ocean undergoes a transition from a relatively clean to a fairly polluted one. Our observations of the large concentrations and the North-to-South positive gradient of aerosol concentration over the northern Indian Ocean strongly support the transport of aerosols and trace gases with the seasonal northeasterly winds from the Indian subcontinent to the Indian Ocean. The results also indicate the production of new particles over the oceanic region by the gas-to-particle conversion processes. During this season, because of the persistent northeasterly winds and the shift of the Inter-Tropical Convergence Zone (ITCZ) to the Southern Hemisphere in this region, the air pollutants over the northern Indian Ocean are carried into the Southern Hemisphere with the cross-equatorial flow and reach up to the southern limit of the ITCZ. Some pockets of very high aerosol concentrations have been observed in and around the ITCZ. The nucleation mode particles are observed in great abundance up to 30°S. From the Indian coast to 5°N, aerosol particles are observed to have bimodal size distributions with a maximum in accumulation mode at 133-nm diameter and a minimum for nucleation/Aitken mode particles of <74 nm in diameter. The size distributions gradually change to trimodal ones with another minimum appearing for the coarse mode particles>422 nm in the 5°N-Equator belt and are generally trimodal in the Southern Hemisphere. Transport of the nucleation mode particles from the free troposphere to the marine boundary layer (MBL) associated with the large-scale subsidence and the subsequent North-to-South transport of these and continental aerosols with the seasonal northeasterly surface winds over the Indian Ocean is proposed to explain the high concentration of the nucleation mode particles observed up to the ITCZ. Size distributions change to Junge's power-law type in regions of high aerosol concentration inside the ITCZ. Comparatively small concentrations with almost uniform size distributions that are typical of the pristine air in the Southern Hemisphere are observed from 42°S to 56°S. Some high peaks in aerosol concentration that can be associated with the low-pressure systems that encircle Antarctica are observed in the 60–70°S latitudinal belt. Further, our observations strongly demonstrate the effect of wind direction on the land-to-ocean transport of the atmospheric aerosols.