The optical characteristics and sources of chromophoric dissolved organic matter (CDOM) in seasonal snow of northwestern China
Chromophoric dissolved organic matter (CDOM) plays an important role in the global carbon cycle and energy budget but is rarely studied in seasonal snow. A field campaign was conducted across northwestern China from January to February 2012, and surface snow samples were collected at 39 sites in Xinjiang and Qinghai provinces. Absorption and fluorescence spectroscopies, along with chemical analysis, were used to investigate the optical characteristics and potential sources of CDOM in seasonal snow. The abundance of CDOM, shown as the absorption coefficient at 280 nm, aCDOM(280), and the spectral slope from 275 to 295 nm (S275−295) ranged from 0.15 to 10.57 m−1 and 0.0129 to 0.0389 nm−1. The highest average aCDOM(280) (2.30±0.52 m−1) was found in Qinghai, and the lowest average S275−295 (0.0188±0.0015 nm−1) indicated that the snow CDOM in this region had a strongly terrestrial characteristic. The lower values of aCDOM(280) were found at sites located to the north of the Tianshan Mountains and northwestern Xinjiang along the border of China (0.93±0.68 m−1 and 0.80±0.62 m−1). Parallel factor (PARAFAC) analysis identified three types of fluorophores that were attributed to two humic-like substances (HULIS, C1 and C2) and one protein-like material (C3). C1 was mainly from soil HULIS, C3 was a type of autochthonously labile organic matter, while the potential sources of C2 were complex, including soil, microbial activity, anthropogenic pollution, and biomass burning. Furthermore, the regional variations of sources for snow CDOM were assessed by analyses of chemical species (e.g., soluble ions), fluorescent components, and air mass backward trajectories combined with satellite-derived active-fire locations.