Background aerosol concentration derived from the atmospheric electric conductivity measurements made over the Indian Ocean during INDOEX

. Measurements of the atmospheric electric conductivity on board ORV Sagarkanya during her three cruises over the Indian Ocean (17øN to 20øS, 57øE to 79øE) during the periods of December to March 1996-1997, 1998, and 1999 are reported. The results show that the values of atmospheric conductivity over the southern hemisphere are 2 to 3 times of that over the northern Indian Ocean and the north-to-south gradients extend up to the Intertropical Convergence Zone (ITCZ) and have large interseasonal and intraseasonal variations. The values of electric conductivity have been used to calculate the aerosol concentrations. The latitudinal variations of the aerosol concentration have been observed to have positive gradients from the Indian coastline to the ITCZ, and the gradients are different during the three cruises. The aerosol concentrations attain their pristine level only at 15ø-20øS in this season. Because of the large interseasonal variability of the aerosol concentration observed over the northern Indian Ocean, it is concluded that estimating any secular change in the background aerosol pollution may be a futile exercise in this area.

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