Photolytic degradation of water‐soluble organic carbon in snowmelts
Submitter
Laskin, Alexander — Purdue University
Area of research
Cloud-Aerosol-Precipitation Interactions
Journal Reference
Science
Melted samples of the seasonal snow collected from northwestern China were exposed to ultraviolet radiation to investigate the photolytic transformations of water‐soluble organic carbon (WSOC). Molecular characteristics and chemical composition of WSOC and its light-absorbing constituents (termed as brown carbon, BrC) were investigated using an advanced high‐resolution mass spectrometery platform.
Impact
Different sources of BrC exhibit varying photo-reactivity under UV irradiation, with snowmelt samples containing soil and plant-derived WSOC showing greater changes compared to snowmelt samples containing anthropogenic WSOC, which are more resistant. Low-polarity BrC chromophores are highly sensitive to light and become more polar after photolysis. The study highlights that current models may overestimate BrC's snow-darkening effects due to discrepancies between laboratory-derived kinetic parameters and real-world observations.
Summary
Seasonal snow is one of the largest terrestrial cryosphere components: its 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 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, and the 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.