The dual carbon isotope signatures and optical properties of carbonaceous aerosols have been investigated simultaneously for the first time in the South Asian outflow during an intensive campaign at the Maldives Climate Observatory on Hanimaadhoo (MCOH) (February and March 2012). As one component of the Cloud Aerosol Radiative Forcing Dynamics Experiment, this paper reports on the sources and the atmospheric processing of elemental carbon (EC) and water-soluble organic carbon (WSOC) as examined by a dual carbon isotope approach. The radiocarbon (Δ 14 C) data show that WSOC has a significantly higher biomass/biogenic contribution (86 ± 5%) compared to EC (59 ± 4%). The more 13 C-enriched signature of MCOH-WSOC (À20.8 ± 0.7‰) compared to MCOH-EC (À25.8 ± 0.3‰) and megacity Delhi WSOC (À24.1 ± 0.9‰) suggests that WSOC is significantly more affected by aging during long-range transport than EC. The δ 13 C-Δ 14 C signal suggests that the wintertime WSOC intercepted over the Indian Ocean largely represents aged primary biomass burning aerosols. Since light-absorbing organic carbon aerosols (Brown Carbon (BrC)) have recently been identified as potential contributors to positive radiative forcing, optical properties of WSOC were also investigated. The mass absorption cross section of WSOC (MAC 365 ) was 0.5 ± 0.2 m 2 g À1 which is lower than what has been observed at near-source sites, indicating a net decrease of WSOC light-absorption character during long-range transport. Near-surface WSOC at MCOH accounted for~1% of the total direct solar absorbance relative to EC, which is lower than the BrC absorption inferred from solar spectral observations of ambient aerosols, suggesting that a significant portion of BrC might be included in the water-insoluble portion of organic aerosols.

[1] Water-soluble organic carbon (WSOC) is typically a large component of carbonaceous aerosols with a high propensity for inducing cloud formation. The sources of WSOC, which may be both of primary and secondary origins, are in general poorly constrained. This study assesses the concentrations and dual-carbon isotope ( 14 C and 13 C) signatures of South Asian WSOC during a 15-month continuous campaign in [2008][2009]. Total suspended particulate matter samples were collected at Sinhagad (SINH) India and at the Maldives Climate Observatory at Hanimaadhoo (MCOH). Monsoon-driven meteorology yields significant WSOC concentration differences between the dry winter season (0.94 AE 0.43 mg m À3 MCOH and 3.6 AE 2.3 mg m À3 SINH) and the summer monsoon season (0.10 AE 0.04 mg m À3 MCOH and 0.35 AE 0.21 mg m À3 SINH). Radiocarbon-based source apportionment of WSOC not only shows the dominance of biogenic/biomass combustion sources but also a substantial anthropogenic fossil-fuel contribution (17 AE 4% MCOH and 23 AE 4% SINH). Aerosols reaching MCOH after long-range over-ocean transport were enriched by 3-4% in d 13 C-WSOC relative to SINH. This is consistent with particle-phase aging processes influencing the d 13 C-WSOC signal in the South Asian regional receptor atmosphere.Citation: Kirillova, E. N., A. Andersson, R. J. Sheesley, M. Krusa˚, P. S. Praveen, K. Budhavant, P. D. Safai, P. S. P. Rao, and Ö . Gustafsson (2013), 13 C-and 14 C-based study of sources and atmospheric processing of water-soluble organic carbon

Climate-warming brown carbon aerosols lose warming capacity during atmospheric transport.