Volcanic Ash a Sink for Atmospheric Trace Species? A Laboratory Study of SO2 and O3 Uptake by Ash

Abstract:

The impacts of volcanic activity on atmospheric chemistry have dominantly been viewed in relation to sulphur and halogen gas and aerosol emissions, while volcanic ash has been largely overlooked. However, solid particles in the atmosphere are increasingly recognised to be important in providing surfaces for heterogeneous reaction with trace gases such as SO₂, NO_x, O₃ and organic compounds. Average annual emissions of ash, the <2 mm aluminosilicate particles generated during explosive eruptions, correspond to a surface area roughly equivalent to one-third to one-half of Earth’s geometric surface area. Despite the substantial surface presented by airborne ash particles, interactions between ash and gases at ambient temperature have seldom been investigated. Laboratory studies with volcanic ash similar to those conducted with mineral dust from arid and semi-arid regions are much needed to understand and quantify the kinetics and mechanisms involved in heterogeneous reactions. Addressing this gap in knowledge is fundamental to better assess the capacity of ash emissions to affect atmospheric chemistry. We determined the initial uptake coefficient (γ_M) and the total uptake capacity (N_i^M) for gaseous SO₂ and O₃ by a compositional range of ash and glass powders in a Knudsen flow reactor. The volcanic materials exhibited γ_SO2 and N_i^SO2 values ranging from 10^-3 to 10^-2 and from 10¹¹ to 10¹³ molecules cm^-2, respectively. The solids samples also showed γ_O3 and N_i^O3 values ranging from 10^-3 to 10^-2 and from 10¹² to 10¹³ molecules cm^-2, respectively. Results of sequential exposure trials (SO₂ then O₃, O₃ then SO₂) suggest that SO₂ and O₃ do not compete for surface sites on the aluminosilicate materials, although O₃ may participate in redox reactions with surface adsorbed sulphur species, enhancing the total capacity for O₃ uptake by the solid. Differences in reactivity of the samples towards SO₂ and O₃ may be interpreted in light of variations in types and abundances of chemical functional groups on the ash and glass materials, as dictated by their unique surface generation and alteration histories. Extrapolation of our results to quantify the potential SO₂ and O₃ sink generated by ash emissions from a large explosive eruption will be presented.