Chemical characterization of pollution layers over the tropical Indian Ocean: Signatures of emissions from biomass and fossil fuel burning

We have performed airborne measurements of atmospheric trace gases and aero sol composition on the National Center for Atmospheric Research C-130 resea rch aircraft over the tropical Indian Ocean during the Indian Ocean Experim ent (INDOEX) intensive field phase in February and March 1999. Gases measur ed included acetone, acetonitrile, sulfur dioxide, and carbon monoxide. The aerosol composition was analyzed for water-soluble ions, and black and org anic carbon. South of the Intertropical Convergence Zone (ITCZ), we sampled pristine air originating from the remote southern Indian Ocean. North of t he ITCZ, signatures of heavy pollution were evident over large areas of the Indian Ocean. Heavy pollution was present in the marine boundary layer as well as in the free troposphere at altitudes up to almost 4000 m. Outflow f rom the Indian subcontinent as well as from other source regions (Arabian S ea, Southeast Asia) could be identified by back trajectory calculations usi ng the Hybrid Single Particle Lagrangian Integrated Trajectory model. The h ighest pollutant concentrations were observed in a free tropospheric pollut ion layer ("residual layer"), which originated from the Indian continental boundary layer. High mixing ratios of acetonitrile (up to 0.8 ppb) and subm icron aerosol potassium (up to 0.6 ppb) indicate an important contribution from biomass or biofuel burning sources. On the other hand, high mixing rat ios of sulfur dioxide (up to 1.5 ppb) and aerosol sulfate (up to 3 ppb) ind icate the influence of fossil fuel burning. During most flights the contrib utions from these two sources were well mixed within the same air mass, sug gesting that the sources on the ground are also close to each other. This i s consistent with the assumption that biomass is mainly burnt as biofuel fo r domestic use in populated areas, where fossil fuel is also used. The rati os dX/dCO (X=acetone, acetonitrile, sulfur dioxide, potassium, or sulfate) measured during the flights indicate that most of the CO in the continental outflow is due to biomass or biofuel burning, whereas the majority of the aerosols results from fossil fuel burning.