The uptake of SO2 on Saharan dust: a flow tube study

[摘要] The uptake of SO₂ onto Saharan mineral dust from the Cape Verde Islandswas investigated using a coated wall flow tube coupled to a massspectrometer. The rate of loss of SO₂ to the dust coating was measuredand uptake coefficients were determined using the measured BET surface areaof the sample. The uptake of SO₂, with an initial concentration between(2-40)x10¹⁰molecule cm^-3 (0.62-12 µTorr), was foundto be strongly time dependent over the first few hundred seconds of anexperiment, with an initial uptake γ_0,BET of (6.6±0.8)x10^-5 (298 K), declining at longer times. The amount ofSO₂ adsorbed on the dust samples was measured over a range of SO₂ concentrations and mineral dust loadings. The uptake of SO₂was found to be up to 98% irreversible over the timescale of theseinvestigations. Experiments were also performed at 258 K, at a relativehumidity of 27% and at 298 K in the presence of ozone. The initial uptakeand the amount of SO₂ taken up per unit area of BET dust surface wasthe same within error, irrespective of the conditions used; however thepresence of ozone reduced the amount of SO₂ released back into thegas-phase per unit area once exposure of the surface ended. Multiple uptakesto the same surface revealed a loss of surface reactivity, which did notreturn if the samples were exposed to gas-phase water, or left under vacuumovernight. A mechanism which accounts for the observed uptake behaviour isproposed and numerically modelled, allowing quantitative estimates of therate and amount of SO₂ removal in the atmosphere to be estimated.Removal of SO₂ by mineral dust is predicted to be significant at highdust loadings.

[学科分类] 大气科学