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Differences across the ITCZ in the chemical characteristics of the Indian Ocean MBL aerosol during INDOEX

by M. Norman, C. Leck, H. Rodhe
Atmospheric Chemistry and Physics ()


The water soluble inorganic part of the sub-micrometer aerosol was\nmeasured from two research vessels over the Indian Ocean during the\nwinter monsoon season (February and March) as part of the INDOEX project\nin 1998 and 1999. Additional measurements were made of gas phase SO2\nfrom one of the vessels in 1999. All samples collected north of the\nInter Tropical Convergence Zone, ITCZ, were clearly affected by\ncontinental, anthropogenic sources. A sharp transition occurred across\nthe ITCZ with concentrations of nss-SO42-, NH4+ and nss-K+ being lower\nby a factor of 7-15, > 20 and > 40, respectively, on the southern side\nof the ITCZ. The contribution from DMS to the sub-micrometer nss-SO42-\nwas estimated to be up to 40% in clean air north of the ITCZ but less\nthan 10% in polluted air originating from India. South of the ITCZ\nvirtually all nss-SO42- was likely to be derived from oxidation of DMS.\nThe concentration of SO2 decreased rapidly with distance from the Indian\ncoast, the molar ratio SO2/nss-SO42- reaching values below 5% after 35\nh travel time over the ocean. Surprisingly, MSA, which is derived from\nDMS, also showed higher concentrations in the sub-micrometer aerosol\nnorth of the ITCZ than south of it. This could be explained by the\nlarger sub-micrometer surface area available north of the ITCZ for the\ncondensation of MSA. South of the ITCZ a major part of the MSA was found\non the super-micrometer particles. An analysis based on the air\ntrajectories showed that systematic variation in the observed\nconcentrations was associated with variations in the transport from\nsource regions. For example, differences in time since air parcels left\nthe Arabian or Indian coasts was shown to be an important factor for\nexplaining the substantial differences in absolute concentrations.

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