Measurement report: Molecular composition, optical properties, and radiative effects of water-soluble organic carbon in snowpack samples from Northern Xinjiang, China

Abstract. Water-soluble organic carbon (WSOC) in the cryosphere has important impact on the biogeochemistry cycling and snow/ice surface energy balance through changes in the surface albedo. This work reports on chemical characterization of WSOC in 28 representative snowpack samples collected across regional area of northern Xinjiang, northwestern China. We employed multi-modal analytical chemistry techniques to investigate both bulk and molecular-level composition of WSOC and its optical properties, informing the follow-up radiative forcing (RF) modeling estimates. Based on the geographic differences and proximity of emission sources, the snowpack collection sites were grouped as urban/industrial (U), rural/remote (R), and soil-influenced (S) sites, for which average WSOC total mass loadings were measured as 1968 ± 953 ng g−1 (U), 885 ± 328 ng g−1 (R), and 2082 ± 1438 ng g−1 (S), respectively. The S sites showed the higher mass absorption coefficients at 365 nm (MAC365) of 0.94 ± 0.31 m2 g−1 compared to those of U and R sites (0.39 ± 0.11 m2 g−1 and 0.38 ± 0.12 m2 g−1, respectively). Bulk composition of WSOC in the snowpack samples and its basic source apportionment was inferred from the Excitation-Emission Matrices and the Parallel Factor analysis featuring relative contributions of two humic-like (HULIS-1 and HULIS-2) and one protein-like (PRLIS) components with ratios specific to each of the S, U, and R sites. Additionally, a sample from site 120 showed unique pollutant concentrations and spectroscopic features remarkably different from all other U, R, and S samples. Molecular-level characterization of WSOC using high-resolution mass spectrometry (HRMS) provided further insights into chemical differences among four types of samples (U, R, S, and 120). Specifically, much more reduced S-containing species with high degree of unsaturation and aromaticity were uniquely identified in U samples, suggesting an anthropogenic source. Aliphatic/proteins-like species showed highest contribution in R samples, indicating their biogenic origin. The WSOC components from S samples showed high oxygenation and saturation levels. A few of unique CHON and CHONS compounds with high molecular weight were detected in the 120 sample, which might be anthraquinone derivatives from plant debris. Modeling of the WSOC-induced RF values showed warming effects of 0.04 to 0.59 W m−2 among different groups of sites, which contribute up to 16 % of that caused by BC, demonstrating the important influences of WSOC on the snow energy budget.