Quantifying Light-Absorbing Aerosol Snow Darkening Using Cryogenic Snow Generation and Integrating Sphere Spectrophotometry

The deposition of light-absorbing aerosols like dark brown carbon (d-BrC) accelerates cryospheric melt, yet accurately modeling this radiative forcing is hindered by a lack of empirical data. To address this gap, we developed a novel, low-footprint laboratory snow synthesis and deposition apparatus. This system couples the cryogenic generation of synthetic snow with controlled aerosol dry deposition, allowing spectral albedo reductions to be quantified via an integrating sphere spectrophotometer. The setup was rigorously validated using Cabojet, a highly absorbing black carbon (BC) proxy, achieving high-fidelity optical closure with the Snow, Ice, and Aerosol Radiative (SNICAR) model (root-mean-square error < 0.022) and establishing a 165 parts per billion (ppb) detection limit of BC in snow. Applying this validated methodology to nebulized d-BrC tarballs revealed that the dry deposition of ∼1000 ppb d-BrC drives a visible broadband albedo decrease of 0.06. This apparatus offers a highly controlled, empirical platform to ground-truth theoretical radiative forcing calculations for diverse, real-world cryospheric contaminants. Further, by introducing a modular and rapid laboratory setup, these experiments overcome the limitations of outdoor field studies and resource-intensive cold rooms.