Photolytic Degradation of Water‐Soluble Organic Carbon in Snowmelts

 

Submitter

Laskin, Alexander — Purdue University

Area of research

Cloud-Aerosol-Precipitation Interactions

Journal Reference

Zhou Y, C West, D Calderon‐Arrieta, M Misovich, A Hettiyadura, H Wen, T Shi, J Cui, W Pu, X Wang, and A Laskin. 2024. "Photolytic Degradation of Water‐Soluble Organic Carbon in Snowmelts: Changes in Molecular Characteristics, Brown Carbon Chromophores, and Radiative Effects." Journal of Geophysical Research: Atmospheres, 129(16), e2024JD040755, 10.1029/2024JD040755.

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 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.