Changing optical properties of Black Carbon and Brown Carbon aerosols during long-range transport from the Indo-Gangetic Plain to the equatorial Indian Ocean

Abstract. Atmospheric aerosols strongly influence the global climate by their light absorption (e.g., black carbon, BC, brown carbon, BrC) and scattering (e.g., sulfate) properties. This study presents simultaneous measurements of ambient aerosol light absorption properties and chemical composition from three large-footprint South Asian receptor sites during the South Asian Pollution Experiment (SAPOEX) in December-March 2018. The BC mass absorption cross-section (BC-MAC₆₇₈) values increased from 3.5 ± 1.3 at the Bhola Climate Observatory-Bangladesh (i.e., located at exit outflow of Indo-Gangetic Plain) to 6.4 ± 1.3 at the two regional receptor observatories at Maldives Climate Observatory-Hanimaadhoo (MCOH) and Maldives Climate Observatory-Gan (MCOG). This likely reflects a coating-enhancement effect due to ageing of the aerosols during long-range transport. At the same time, the BrC-MAC₃₆₅ decreased by a factor of three from the IGP exit to the equatorial Indian Ocean, likely due to photochemical bleaching of organic chromophores. The high chlorine-to-sodium ratio at the near-source-region BCOB suggests a significant contribution of chorine from anthropogenic activities. This particulate Cl^- has the potential to convert into Cl-radicals that can affect the oxidation capacity of the polluted air. Moreover, Cl^- is shown to be near-fully consumed during the long-range transport. The results of this synoptic study over the large South Asian scale have significance for understanding the ageing effect of the optical and chemical properties of aerosols as the pollution from the Indo-Gangetic Plain disperses over regional scales.