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

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.

Characterization of particulate matter and black carbon over Bay of Bengal during summer monsoon: results from the OMM cruise experiment.

Total and size-segregated particulate matter (PM) and black carbon (BC) concentrations over the Bay of Bengal (BoB) have been measured in the summer monsoon (August-September 2014) onboard a scientific cruise conducted as a part of the Ocean Monsoon and Mixing (OMM) experiment. Role of long-range transport and prevailing meteorology in producing the observed spatio-temporal features is inferred by synthesizing the results of in situ observations in conjunction with the wind components from Modern Era Retrospective Analysis for Research and Applications (MERRA), rainfall data from Tropical Rainfall Measuring Mission (TRMM), surface BC concentration and BC Aerosol Optical Thickness (AOTBC) from MERRA2 and HYSPLIT back trajectory and dispersion model analysis. Mean values of total PM and BC mass concentrations are observed to be ~ 21.4 µgm-3 and ~ 393ngm-3 respectively. The study has revealed significant influence of monsoon rainfall (over the measurement locations and regions through which transport occurred) on the concentration of both PM and BC over northern BoB. Results also indicate transport of aerosols with significant anthropogenic fraction, from the land regions at west to the BoB. A comprehensive analysis showed that while an eastward wind (westerly) from the Indian mainland produced an increase in PM2.5 over northern BoB, a southerly wind, mostly from the Indian Ocean, caused a decrease in concentration of PM2.5. Spectral variation of absorption coefficients of aerosols reveals that most of the BC over BoB is associated with fossil fuel combustion. Prevailing strong surface-level convergence (associated with a low-level anticyclone) resulted in accumulation and consequent enhancement of aerosol concentration over central and northern BoB during the study period. In addition, horizontal flow rates estimated across western boundary of BoB using AOTBC from MERRA2 for 10 years revealed an increasing trend in BC transport from the mainland leading to a gradual buildup in BC concentration over the regions of BoB.