Experimental investigation of SO2 uptake in snow

We conducted a series of bench-top experiments to examine accumulation of acidity in temperate snowpacks due to the deposition of SO2. To achieve this, SO2 at concentrations of 45–85 ppbv was allowed to diffuse into containers of snow at 0°C for varying lengths of time, from 6 to 46 h. Depth profiles of dissolved total sulfur (S(IV)+S(VI)) in the snow were measured and modeled with a coupled diffusion and chemical reaction transport model. Measured deposition velocities decreased from 0.06 to 0.03 cm s−1 for increasing exposure times of 6 to 46 h. Simulations matched measured total sulfur depth profiles for liquid–water mass fractions in the snow of 0.07 for 6 h, increasing to 0.15 for 46 h of exposure. Multi-parametric sensitivity analysis indicates that deposition velocity is extremely sensitive to liquid water mass fraction. Modeling also shows that the decrease in deposition velocity with exposure time can be attributed to reduced solubility of SO2 as the pH in the liquid water mass fraction decreased. These results suggest that clean snow is the most effective scavenger of SO2.