Sources and light absorption of water-soluble organic carbon aerosols in the outflow from northern China

Abstract

High loadings of anthropogenic carbonaceous aerosols in Chinese air influence the air quality for over one billion people and impact the regional climate. A large fraction (17-80 %) of this aerosol carbon is water-soluble, promoting cloud formation and thus climate cooling. Recent findings, however, suggest that water-soluble carbonaceous aerosols also absorb sunlight, bringing additional direct and indirect climate warming effects, yet the extent and nature of light absorption by this water-soluble "brown carbon" and its relation to sources is poorly understood. Here, we combine source estimates constrained by dual carbon isotopes with light-absorption measurements of water-soluble organic carbon (WSOC) for a March 2011 campaign at the Korea Climate Observatory at Gosan (KCOG), a receptor station in SE Yellow Sea for the outflow from northern China. The mass absorption cross section at 365 nm (MAC(365)) of WSOC for air masses from N. China were in general higher (0.8-1.1 m(2) g(-1)), than from other source regions (0.3-0.8 m(2) g(-1)). However, this effect corresponds to only 2-10% of the radiative forcing caused by light absorption by elemental carbon. Radiocarbon constraints show that the WSOC in Chinese outflow had significantly higher fraction fossil sources (30-50 %) compared to previous findings in S. Asia, N. America and Europe. Stable carbon (delta C-13) measurements were consistent with aging during long-range air mass transport for this large fraction of carbonaceous aerosols.