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Sci Adv. 2019 Jan 30;5(1):eaau8066. doi: 10.1126/sciadv.aau8066. eCollection 2019 Jan.

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

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Department of Environmental Science and Analytical Chemistry (ACES), Bolin Centre for Climate Research, Stockholm University, Stockholm 10691, Sweden.
Maldives Climate Observatory at Hanimaadhoo (MCOH), Hanimaadhoo, Republic of the Maldives.
Centre for Atmospheric and Oceanic Sciences and Divecha Centre for Climate Change, Indian Institute of Sciences (IISC), Bangalore 560012, India.
Atmospheric Composition Unit, Finnish Meteorological Institute (FMI), Helsinki 00560, Finland.
Servicio Meteorológico Nacional (SMN), C1425 CABA, Argentina.
Department of Chemistry, University of Dhaka, Dhaka 1000, Bangladesh.
Indian Institute of Tropical Meteorology (IITM), New Delhi 110008, India.
Maldives Meteorological Services (MMS), Hulhule 22000, Republic of Maldives.


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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