Uptake and hydration of sulfur dioxide on dry and wet hydroxylated silica surfaces: a computational study

We present a first-principle molecular dynamics study on the uptake and hydration of sulfur dioxide on the dry and wet fully hydroxylated surfaces of (0001) alpha-quartz, which are a proxy for suspended silica dust in atmosphere. The average adsorption energy for SO2 is ~10 kcal/mol on both dry and wet surfaces. The adsorption is driven by hydrogen bonds formation between SO2 and the interfacial hydroxyl groups (on dry silica), or with water molecules (in the wet case). In the dry system, we report an additional electrostatic interaction between the interfacial hydroxyl oxygen and the sulfur atom, which further stabilize the adsorbate. On dry silica, the interfacial hydroxyl group coordinates SO2 yielding to a surface bounded bisulfite (Si-SO3H) complex. On the wet surface, SO2 reacts with water forming bisulfite (HSO3-), and the latter remains solvated inside the adsorbed water layer. The hydration barrier for sulfur oxide is 1 kcal/mol and 3 kcal/mol on the dry and wet silica, respectively, while for the backward reaction (i.e., bisulfite to SO2) the barrier is 6 kcal/mol on both surfaces. The modest backward barrier rationalizes earlier experimental findings showing no SO2 uptake on silicates. These results underline the importance of the surface hydroxylation and/or of adsorbed water layers for the SO2 uptake and its hydration on silica. Moreover, the hydration to bisulfite may prevent direct SO2 photochemistry and be an additional source of sulfate: this is especially relevant in atmosphere subject to high level of suspend mineral dust, intense solar radiation and atmospheric oxidizers.