Photolytic Degradation of Water-Soluble Organic Carbon in Snowmelts: Changes in Molecular Characteristics, Brown Carbon Chromophores, and Radiative Effects

Abstract

Water-soluble organic carbon (WSOC) deposited in ambient snowpack play key roles in regional carbon cycle and surface energy budget, but the impacts of photo-induced processes on its optical and chemical properties are poorly understood yet. In this study, melted samples of the seasonal snow collected from northern Xinjiang, northwestern China, were exposed to ultraviolet (UV) radiation to investigate the photolytic transformations of WSOC. Molecular characteristics and chemical composition of WSOC and its brown carbon (BrC) constituents were investigated using high-performance liquid chromatography interfaced with a photodiode array detector and a high-resolution mass spectrometer. Upon illumination, formation of nitrogen- and sulfur-containing species with high molecular weight was observed in snow samples influenced by soil- and plant-derived organics. In contrast, the representative sample collected from remote region showed the lowest molecular diversity and photolytic reactivity among all samples, in which no identified BrC chromophores decomposed upon illumination. Approximately 65% of chromophores in urban samples endured UV irradiation. However, most of BrC composed of phenolic/lignin-derived compounds and flavonoids disappeared in the illuminated samples containing WSOC from soil- and plant-related sources. Effects of the photochemical degradation of WSOC on the potential modulation of snow albedo were estimated. Apparent half-lives of WSOC estimated as albedo reduction in 300–400 nm indicated 0.1–0.4 atmospheric equivalent days, which are shorter than typical photolysis half-lives of ambient biomass smoke aerosol. This study provides new insights into the roles of WSOC in snow photochemistry and snow surface energy balance.

Key Points

• S- and N-containing species with high molecular weight formed in the illuminated snowmelt samples containing soil and plant organics

• Brown carbon (BrC) in the remote snowpack are photo-resistant, however, those in the samples influenced by soil and plant organics are photo-labile

• The photochemical half-lives of snow water-soluble organic carbon showed 0.1–0.4 atmospheric-relevant days, which are shorter than that of biomass smoke aerosol

Plain Language Summary

Seasonal snow is one of the largest terrestrial cryosphere components, its water-soluble organic carbon (WSOC) constituents are highly dynamic and significantly affect the snowpack energy balance and carbon cycles. In this study, photolysis experiments were conducted in the laboratory to investigate the photochemical characteristics of WSOC in snowmelt collected from northwestern China. Impacts of ultraviolet (UV) light on the molecular composition of WSOC, its brown carbon (BrC) constituents, and the radiative effects of WSOC were analyzed. The results showed that WSOC and its BrC components in the samples collected from remote areas were less affected by the UV irradiation compared to the urban samples. The samples influenced by soil and plants were more photo-active, majority of the chromophores disappeared upon irradiation, along with significant formation of nitrogen- and sulfur-containing organic compounds. Furthermore, we found that WSOC in snowmelts have shorter lifetimes than the biomass burning aerosol. This study showed that photo-induced processes substantially influence the molecular composition and radiative properties of WSOC in snowpack across the arid regions of northwestern China, where seasonal snow is the primary fresh water resource.