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 nature-identical 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 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 set up, these experiments overcome the limitations of outdoor field studies and resource-intensive cold rooms.