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

[摘要] The water soluble inorganic part of the sub-micrometer aerosol was measured from tworesearch vessels over the Indian Ocean during the winter monsoon season (February andMarch) as part of the INDOEX project in 1998 and 1999. Additional measurements weremade of gas phase SO₂ from one of the vessels in 1999. All samples collected north of the Inter Tropical Convergence Zone,ITCZ, were clearly affected by continental, anthropogenic sources. A sharp transitionoccurred across the ITCZ with concentrations of nss-SO₄^2-,NH₄⁺ and nss-K⁺ being lower by a factor of7-15, >20 and >40, respectively, on the southern side of the ITCZ. Thecontribution from DMS to the sub-micrometer nss-SO₄^2- was estimated to be up to40% in clean air north of the ITCZ but less than 10% in polluted air originating from India. Southof the ITCZ virtually all nss-SO₄^2- was likely to be derived from oxidation of DMS. Theconcentration of SO₂ decreased rapidly with distance from the Indian coast, the molar ratioSO₂/nss-SO₄^2- reaching values below 5% after35 h travel time over the ocean. Surprisingly, MSA, which is derived from DMS, also showed higher concentrations in thesub-micrometer aerosol north of the ITCZ than south of it. This could be explained by thelarger sub-micrometer surface area available north of the ITCZ for the condensation ofMSA. South of the ITCZ a major part of the MSA was found on the super-micrometerparticles. An analysis based on the air trajectories showed that systematic variation in the observed concentrations was associated with variations in thetransport from source regions. For example, differences in time since air parcels left theArabian or Indian coasts was shown to be an important factor for explaining the substantialdifferences in absolute concentrations.

[学科分类] 大气科学