Comment on acp-2022-336

Abstract

Although water-soluble organic carbon (WSOC) in the cryosphere can significantly influence the global carbon cycle and radiation budget, WSOC in the snowpack has received little scientific attention to date. This study reports the fluorescence characteristics, absorption properties, and radiative effects of WSOC based on 34 snow samples collected from sites in northeastern China. Sampling sites were divided into five groups, comprising southeastern Inner Mongolia (SEIM), northeastern Inner Mongolia (NEIM), the south of northeastern China (SNC), the north of northeastern China (NNC), and the Changbai Mountain area (CBM). Together, these groups represent a significant degree of regional WSOC variability, with concentrations ranging from 0.50 ± 0.19 to 5.70 ± 3.68 μg g−1 (mean = 3.59 ± 3.19 μg g−1). We then identified the three principal fluorescent components of WSOC as (1) a high‑oxygen humic-like component (HULIS-1) of terrestrial origin, (2) a low‑oxygen humic-like component (HULIS-2) of mixed origin, (3) and a protein-like component (PRLIS) derived from autochthonous microbial activity. In SEIM, a region dominated by desert and exposed soils, the WSOC content exhibits the highest humification index (HIX) but the lowest fluorescence (FI) and biological (BIX) indices; the fluorescence signal is mainly attributed to HULIS-1, and thus implicates soil as the primary source. By contrast, the HIX (FI and BIX) value was the lowest (highest) and PRLIS most intense in the remote grasslands and forested areas of NEIM, suggesting a primarily biological source. For SNC and NNC, both of which are characterized by intensive agriculture and industrial activity, the fluorescence signal is dominated by HULIS-2 and the HIX, FI, and BIX values are all moderate, indicating mixed origins for WSOC (anthropogenic activity, microbial activity, and soil). We also observed that, throughout northeastern China, the light absorption of WSOC is dominated by HULIS-1, followed by HULIS-2 and PRLIS. The contribution of WSOC to albedo reduction (average concentration 3.6 μg g−1) in the ultraviolet–visible (UV–vis) band is approximately half that of black carbon (BC: average concentration 0.6 μg g−1); radiative forcing is 3.8 (0.8) W m−2 in old (fresh) snow, equating to 19 % (17 %) of the radiative forcing of BC. These results indicate that WSOC has a profound impact on snow albedo and the solar radiation balance.