13C‐ and 14C‐based study of sources and atmospheric processing of water‐soluble organic carbon (WSOC) in South Asian aerosols

Abstract

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 ± 0.43 µg m−3 MCOH and 3.6 ± 2.3 µg m−3 SINH) and the summer monsoon season (0.10 ± 0.04 µg m−3 MCOH and 0.35 ± 0.21 µg m−3 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 ± 4% MCOH and 23 ± 4% SINH). Aerosols reaching MCOH after long‐range over‐ocean transport were enriched by 3–4‰ in δ13 C‐WSOC relative to SINH. This is consistent with particle‐phase aging processes influencing the δ13 C‐WSOC signal in the South Asian regional receptor atmosphere.