Latitudinal and Size Segregated Compositional Variability of Aerosols over the Indian and Southern Ocean during 2010 Austral Summer

Abstract

To characterize the spatial variations in size segregated aerosols, samples were collected over the Indian and Southern oceans (7°N to 68°S) during the Indian expedition to Southern Ocean (January–February 2010). The chemical and mass concentrations of ions were measured in nine size classes (0.4–> 10 µm) of aerosols through ion-exchange chromatographic system and gravimetric estimates, respectively. Mass concentrations of coarse aerosols increase towards the Antarctic coast which is well correlated with the increasing humidity suggesting growth of particles. The study area was found to be predominately impacted by sea salt aerosols with a sea salt load of 91%. F^-, Cl^- and NO₃^- reflected anthropogenic impact within the fine mode aerosols but the marine influence dominated the coarse mode. Methanesulphonic (MSA) was predominantly of biogenic origin but a substantially low MSA/nssSO₄^2- ratio suggested that DMS contribution to the total nssSO₄^2- concentration was low. NH₄⁺ concentrations showed a shift from fine to coarse particles, with a trimodal distribution over the Southern Ocean. Our study reports a significant Cl^- depletion in the aerosols and the degree of Cl^- deficit was size-dependent, increasing with decreasing particle sizes. The neutralization ratios suggested differential behavior of ions with respect to the prevailing meteorological conditions. The coarse mode aerosols were neutralized by ammonia leading to the formation of NH₄NO₃ particles. The fine modes were predominately composed of NH₄HSO₄ and its formation is favored by the high humidity and foggy conditions over the study area. Factor analysis support that the Southern Indian Ocean (10°S to 59°S) had the highest loading of anthropogenic and sea salt aerosols, which lead to the formation of secondary aerosols through gas to particle transformations. The rocky Antarctic coastline, act as a source of coarse crustal aerosols over the Southern Ocean (60°S to 65°S).