Morphological properties of soot particles deposited on the Greenland Ice Sheet: Implications for albedo and melting

Abstract. Black carbon (soot) can be transported long distances and act as a climate forcing agent in cryosphere environments. Soot consists of aggregates of carbon-rich primary particles, known as spherules, that form complex fractal structures. While the radiative forcing effect of soot is well documented in the atmosphere, it remains mostly unconstrained on bare ice surfaces, in part due to limited data quantifying surface concentrations and detailing the morphological characteristics of soot particles deposited in remote cryosphere environments. Here, we characterize the structure, composition, and concentration of soot being deposited on snow and ice in the Dark Zone of SW-Greenland Ice Sheet. The soot particles had an average maximum aggregate length of 487 ± 342 nm and an average area-equivalent diameter of 332 ± 231 nm. The spherules exhibited the onion-like structure typical of soot, with an average diameter of 42 ± 21 nm and were comprised mainly of amorphous carbon interspersed with graphitic domains (3 – 7 lattice planes, average d-spacing: 0.39 ± 0.03 nm). Calculated fractal dimensions (1.80 – 2.28 ± 0.21) were consistent with combustion products. In-situ aerosol monitoring at the site identified persistent, low concentrations (2016: 14 ± 42 ng·m^-3, 2017: 10 ± 14 ng·m^-3) of airborne soot, which accumulates on snow and ice over time. Corresponding laboratory measurements highlighted that both organic (OC) and elemental carbon (EC) are deposited in snowfall, with organic carbon being the dominant fraction. EC concentrations increased from fresh snow to weathered snow and to bare ice samples, and were 300-fold higher on ice surfaces hosting dense algal blooms compared to clean ice surfaces. Model outputs suggest that this accumulated EC can locally cause an estimated average albedo reduction of 0.012 ± 0.006, approximately 15% of the total impact of algal blooms and EC combined. These estimates could be improved through the development of models that better incorporate soot morphological properties and mixing with other surface particles, such as data presented in this study. Such advancements will help constrain the radiative forcing effects of soot in cryosphere environments, with implications for ice and snow albedo and melting.