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

In March 1999, that is to say, during the Indian Ocean Experiment (INDOEX) Intensive Field Phase, various aerosol characteristics have been measured on the campus of Goa University (Goa, India: 15° 24′N, 74° 48′E). This site, located on the western coast of India, is at the exit of the continent during the winter northeastern monsoon, and the variability of the aerosol characteristics measured here will certainly reflect the variability of 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. They varied greatly during the measurement period (e.g., between 30 and 120 μg/m 3 for the TPM) but were found to be, on the whole, of the same order of magnitude as those measured far downwind of the Indian subcontinent, namely at Kaashidhoo Climate Observatory (KCO, the Maldives). The aerosol mass composition determined by combining XRF analysis (for inorganic elements) and thermal analysis (for carbonaceous components) of filter samples shows that mineral dust (MD) and sea salt (SS) dominate this mass composition. For instance, temporary inputs of MD from distant desert sources explain the observed TPM variations. In contrast, aerosol number concentrations and size resolved composition determined by using optical particle counters, microscope observation, 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 particles are frequently coated by less absorbing matter, presumably a mixture of sulfate and condensed organic compounds. By using the time variability of the K/BC ratio, it is also shown 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 biomass burning, but the contribution of biomass burning increases during the period of the experiment until the previous proportions are reversed around March 25. Comparison of these results with other INDOEX measurements indicates that the spatial gradient observed for aerosol concentrations and composition in the northern part of the Indian Ocean are of the same order of magnitude as the temporal variability observed at Goa.