Photo-enhanced uptake of SO2 on Icelandic volcanic dusts

Abstract

Iceland is the largest volcanic dust (v-dust) desert on Earth, with an estimated area of 22 000 km². In addition, Iceland is one of the most active aeolian areas in the world, with frequent high-velocity winds resuspending v-dust from the ground into the atmosphere. Suspended v-dust particles can then be transferred over thousands of kilometers, reaching as far as central Europe. Once released into the atmosphere, v-dust particles can interact or react with atmospheric pollutants. In this study, we investigate the heterogeneous reactivity of sulfur dioxide (SO₂), one of the most prominent gases released by volcanic eruptions, with Icelandic v-dust particles, and the influence of relevant atmospheric parameters, such as relative humidity (RH) and ultraviolet (UV) light flux, on this reactivity. The experiments are conducted at atmospheric pressure in a coated-wall flow-tube reactor coupled with an SO₂ analyzer. To quantify the heterogeneous processes, we determine the initial number of SO₂ molecules taken up by dust, N_S, and the steady-state uptake coefficient, γ_ss, of SO₂ on different v-dusts. N_S increases with RH and with the photon flux characterized by the photolysis rate of NO₂ in the setup, J_NO2. The photo-enhanced removal of SO₂ is also found to depend on the surface elemental composition of v-dust particles, and an empirical parametrization of the photo-induced effect is proposed to account for the most important environmental factors, leading to the general expression: N_S,light/N_S,dark = 6.1 × (1 + 7.76 × 10⁻² × RH) × (1 + 480.5 × J_NO2) × (Ti/Si). The steady-state uptake coefficients of SO₂ are in the 10⁻⁸ range, once normalized to the specific surface area of v-dust. RH and UV light influence the value of γ_ss, but to a lesser extent than they influence N_S. Our results suggest that the photo-induced heterogeneous uptake of SO₂ on v-dust particles may provide a significant sink of sulfur in volcanic clouds, and should be taken into account in atmospheric modeling.