Morphological features and mixing states of soot-containing particles in the marine boundary layer over the Indian and Southern oceans

Abstract

Abstract. Mixing states of soot-containing aerosol particles constitute important information for the simulation of climatic effects of black carbon in the atmosphere. To elucidate the mixing states and morphological features of soot-containing particles over remote oceans, we conducted on-board observations over the southern Indian Ocean and the Southern Ocean during the TR/V Umitaka-maru UM-08-09 cruise, which started from Benoa, Indonesia, on 1 December 2008 via Cape Town, South Africa, and which terminated in Fremantle, Australia, on 6 February 2009. The light absorption coefficients of size-segregated particles (< 0.5 and < 1.0 µm diameter) and aerosol number concentrations (0.1–0.5 µm diameter) were measured to assist direct aerosol sampling. Size-segregated aerosol particles were collected for chemical analysis using ion chromatography. For transmission electron microscopy (TEM) analyses using water-dialysis methods, dried submicrometer aerosol particles were collected using a cascade impactor. We analyzed 13 TEM samples. Results of water-dialysis analysis demonstrate that most particles were water-soluble. However, for all TEM samples, particles were rarely found (2.1 % of particles on a TEM sample at a maximum) containing insoluble residuals with the characteristic soot shape. For samples collected over the Indian and Southern oceans at latitudes less than 62∘ S, some (20–35 %) soot-containing particles were found as bare soot. For samples collected near the Antarctic coast (65–68∘ S, 38–68∘ E), all soot-containing particles were mixed with water-soluble materials. Furthermore, 56 % of soot-containing particles had a satellite structure formed by the impact of droplets such as sulfuric acid. Chemical analysis of submicrometer particles near the Antarctic coast revealed high concentrations of non-sea-salt (nss) SO42- and CH3SO3-, suggesting that aged soot-containing particles were transformed by soluble materials derived from dimethyl sulfide (DMS) oxidation. The obtained information of soot at various remote ocean areas is expected to be useful to understand long-range transport processes and to improve simulations of global soot concentration.