Chemical characterization of aerosols in the South Asian outflow over the northern Indian Ocean: latitudinal gradients and ultrafine particle events†
Chemical properties of the continental outflow to the northern Indian Ocean are investigated using shipborne measurements carried out as a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB-2018) experiment during winter 2018. The organic carbon (OC) and elemental carbon (EC) showed high values (OC = 4.8 ± 2.1 μg m−3 and EC = 2.0 ± 0.6 μg m−3) over the northern Indian Ocean and relatively lower values (OC = 1.20 ± 0.50 μg m−3 and EC = 0.82 ± 0.53 μg m−3) over the equatorial Indian Ocean. The relative contribution of organic matter (OM) to the sub-micron mass loading also decreased from southeastern Arabian Sea (40%) to the equatorial Indian Ocean (23%). The short-term increase in OC, the OC/EC ratio, OM and the OM/sulfate ratio during ultrafine particle bursts over the remote equatorial Indian Ocean indicate the possibility of prominent sources of marine organic compounds. In the southeastern Arabian Sea, the mass concentration of all aerosol species showed a decreasing trend towards the open ocean, where the magnitude of this latitudinal decrease was relatively higher for OC and OM compared to EC and sulfate. The latitudinal variation in the OM/sulfate ratio showed a reduction from 1.20 ± 0.11 to 0.47 ± 0.15, which is further supported by the similar latitudinal decrease in the OC/EC ratio. These observations indicate the possible loss of organic aerosols, which might be due to ageing during long-range transport. The synthesis of earlier measurements over South Asia and surrounding oceanic regions also shows a relative decrease in the organic mass concentration, which changes the organics-rich nature of the South Asian aerosol system during transport.