Variability of aerosol size-resolved composition at an Indian coastal site during the Indian Ocean Experiment (INDOEX) intensive field phase

International audienceIn March 1999, that is to say, during the Indian Ocean Experiment (INDOEX)Intensive Field Phase, various aerosol characteristics have been measured on the campus ofGoa University (Goa, India: 15 240N, 74 480E). This site, located on the western coast ofIndia, is at the exit of the continent during the winter northeastern monsoon, and thevariability of the aerosol characteristics measured here will certainly reflect the variabilityof aerosol properties over large INDOEX areas. Concentrations in total suspended matter(TPM) and fine particles (PM1) were determined by means of a two-stage sampler. Theyvaried greatly during the measurement period (e.g., between 30 and 120 mg/m3 for theTPM) but were found to be, on the whole, of the same order of magnitude as thosemeasured far downwind of the Indian subcontinent, namely at Kaashidhoo ClimateObservatory (KCO, the Maldives). The aerosol mass composition determined bycombining XRF analysis (for inorganic elements) and thermal analysis (for carbonaceouscomponents) of filter samples shows that mineral dust (MD) and sea salt (SS) dominate thismass composition. For instance, temporary inputs of MD from distant desert sourcesexplain the observed TPM variations. In contrast, aerosol number concentrations and sizeresolved composition determined by using optical particle counters, microscopeobservation, and low- or high-pressure stage impactors, reveal that in terms of numbers,black carbon (BC) is by far the dominant species in the submicron mode. These particlesare frequently coated by less absorbing matter, presumably a mixture of sulfate andcondensed organic compounds. By using the time variability of the K/BC ratio, it is alsoshown that the origin of carbonaceous components varies during the measurement period.In early March, 70% of BC is produced by fossil fuel combustion and 30% by biomassburning, but the contribution of biomass burning increases during the period of theexperiment until the previous proportions are reversed around March 25. Comparison ofthese results with other INDOEX measurements indicates that the spatial gradient observedfor aerosol concentrations and composition in the northern part of the Indian Ocean are of thesame order of magnitude as the temporal variability observed at Go