Gaseous deposition to snow: 2. Physical‐chemical model for SO₂ deposition

Abstract

A diffusion/reaction model of SO 2 uptake by snow containing liquid water is used to examine the snowpack processes controlling accumulation of dissolved S(IV) and S(VI). Surface deposition velocity v d (defined as overall accumulation rate divided by surface SO 2 concentration) depends on the amount of liquid water in the snow, the time scale considered, the rate of S(IV) to S(VI) conversion in the aqueous phase, and the SO 2 concentration. In the absence of any oxidation, v d for dense snow (specific gravity 0.4) with a moderate liquid water mass fraction (X m = 0.01) and SO 2 concentration of 20 ppbv (parts per billion by volume) is calculated to be 0.026 cm s −1 after 6 hours. Deposition velocity increases by a factor of 3.2 for each tenfold increase in X m and by a factor of 2.5–3.2 for each tenfold decrease in time. SO 2 penetration into the snowpack is about 5 cm for the 6‐hour case. In the presence of air containing 30 ppbv ozone and otherwise identical conditions, 80% of the accumulated sulfur is as S(VI), and the calculated v d is 0.036 cm −1 . A further tenfold increase in ozone concentration gives only a 70% increase in v d . A similar dependence is calculated for oxygen (catalyzed by Fe(III)) as oxidant, but that for hydrogen peroxide is much stronger and almost linear. A tenfold increase in SO 2 concentration (with ozone at 30 ppb) results in a 2.3‐fold decrease in v d . Wet snow with a liquid water mass fraction of about 0.1 gives a deposition velocity at 20 ppb SO 2 of 0.12 cm s −1 , with a penetration of only 2 cm. Calculated and observed uptakes of SO 2 by snow are in good agreement.