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

We present a first-principles molecular dynamics study on the uptake and hydration of sulfur dioxide on the dry and wet fully hydroxylated surfaces of (0001) α-quartz, which are a proxy for suspended silica dust in the atmosphere. The average adsorption energy for SO₂is about −10 kcal mol⁻¹on both dry and wet surfaces. The adsorption is driven by hydrogen bond formation between SO₂and 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 stabilizes the adsorbate. On dry silica, the interfacial hydroxyl group coordinates to SO₂yielding a surface bound bisulfite (Si-SO₃H) complex. On the wet surface, SO₂reacts with water forming bisulfite (HSO₃⁻), and the latter remains solvated inside the adsorbed water layer. The hydration barrier for sulfur dioxide is 1 kcal mol⁻¹and 3 kcal mol⁻¹on dry and wet silica, respectively, while for the backward reaction (i.e., bisulfite to SO₂) the barrier is 6 kcal mol⁻¹on both surfaces. The modest backward barrier rationalizes earlier experimental findings showing no SO₂uptake on silica. These results underline the importance of the surface hydroxylation and/or adsorbed water layers for the SO₂uptake and its hydration on silica. Moreover, the hydration to bisulfite may prevent direct SO₂photochemistry and be an additional source of sulfate; this is especially relevant in atmospheres subject to a high level of suspended mineral dust, intense solar radiation and atmospheric oxidizers.