Photochemical degradation affects the light absorption of water-soluble brown carbon in the South Asian outflow

Sanjeev Dasari; August Andersson; Srinivas Bikkina; Henry Holmstrand; Krishnakant Budhavant; Sreedharan Satheesh; Eija Asmi; Jutta Kesti; John Backman; Abdus Salam; Deewan Singh Bisht; Suresh Tiwari; Zahid Hameed; Örjan Gustafsson;
2019 | Sci. Adv. | 5, no. 1, eaau8066
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Light-absorbing organic aerosols, known as brown carbon (BrC), counteract the overall cooling effect of aerosols on Earth’s climate. The spatial and temporal dynamics of their light-absorbing properties are poorly constrained and unaccounted for in climate models, because of limited ambient observations. We combine carbon isotope forensics (δ13C) with measurements of light absorption in a conceptual aging model to constrain the loss of light absorptivity (i.e., bleaching) of water-soluble BrC (WS-BrC) aerosols in one of the world’s largest BrC emission regions—South Asia. On this regional scale, we find that atmospheric photochemical oxidation reduces the light absorption of WS-BrC by ~84% during transport over 6000 km in the Indo-Gangetic Plain, with an ambient first-order bleaching rate of 0.20 ± 0.05 day−1 during over-ocean transit across Bay of Bengal to an Indian Ocean receptor site. This study facilitates dynamic parameterization of WS-BrC absorption properties, thereby constraining BrC climate impact over South Asia.

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