Mixing state of refractory black carbon aerosol in the South Asian outflow over the northern Indian Ocean during winter

Abstract

Regional climatic implications of aerosol black carbon (BC), which has a wide variety of anthropogenic sources in large abundance, are well recognized over South Asia. Significant uncertainties remain in its quantification due to a lack of sufficient information on the microphysical properties (its concentration, size, and mixing state with other aerosol components) that determine the absorption potential of BC. In particular, the information on the mixing state of BC is extremely sparse over this region. In this study, the first observations of the size distribution and mixing state of individual refractory black carbon (rBC) particles in the South Asian outflow to the south-eastern Arabian Sea and the northern and equatorial Indian Ocean regions are presented based on measurements using a single particle soot photometer (SP2) aboard the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB-2018) ship during winter 2018 (16 January to 13 February). The results revealed significant spatial heterogeneity of BC characteristics. The highest rBC mass concentrations (∼938±293 ng m−3) with the highest relative coating thickness (RCT; the ratio of BC core to its coating diameters) of ∼2.16±0.19 are found over the south-east Arabian Sea (SEAS) region, which is in the proximity of the continental outflow. As we move to farther oceanic regions, though the mass concentrations decreased by nearly half (∼546±80 ng m−3), BC still remained thickly coated (RCT∼2.05±0.07). The air over the remote equatorial Indian Ocean, which received considerable marine air masses compared to the other regions, showed the lowest rBC mass concentrations (∼206±114 ng m−3) with a moderately thick coating (RCT∼1.73±0.16). Even over oceanic regions far from the landmass, regions that received the outflow from the more industrialized east coast/the Bay of Bengal had a thicker coating (∼104 nm) compared to regions that received outflow from the west coast and/or peninsular India (∼86 nm). Although different regions of the ocean depicted contrasting concentrations and mixing state parameters due to the varied extent and nature of the continental outflow as well as the atmospheric lifetime of air masses, the modal parameters of rBC mass–size distributions (mean mass median diameters ∼ 0.19–0.20 µm) were similar over all regions. The mean fraction of BC-containing particles (FBC) varied in the range of 0.08–0.12 (suggesting significant amounts of non-BC particles), whereas the bulk mixing ratio of coating mass to rBC mass was highest (8.31±2.40) over the outflow regions compared to the remote ocean (4.24±1.45), highlighting the role of outflow in providing condensable material for coatings on rBC. These parameters, along with the information on the size-resolved mixing state of BC cores, throw light on the role of sources and secondary processing of their complex mixtures for coatings on BC under highly polluted conditions. Examination of the non-refractory sub-micrometre aerosol chemical composition obtained using the aerosol chemical speciation monitor (ACSM) suggested that the overall aerosol system was sulfate-dominated over the far-oceanic regions. In contrast, organics were equally prominent adjacent to the coastal landmass. An association between the BC mixing state and aerosol chemical composition suggested that sulfate was the probable dominant coating material on rBC cores.