A sensitive method for measuring atmospheric concentrations of sulfur dioxide

A new sensitive method for measuring atmospheric concentrations of sulfur dioxide is presented. Samples are obtained using the mist chamber, which collects highly water-soluble gases with high efficiency, and concentrates them in a small volume of water. Particles are removed from the sampled air stream with a teflon filter, before it enters the mist chamber. After collection, the pH of the water is raised above pH 10 using sodium carbonate, then hydrogen peroxide (H₂O₂) is added to oxidize sulfur that may be present in the sulfur (IV) oxidation state, to sulfate. After a reaction time of at least 16 hours, the sulfate concentration is measured by ion chromatography. From the sulfate concentration, the water volume used in the mist chamber, and the volume of air sampled, the atmospheric concentration of SO₂ is computed. The method is not sensitive to other atmospheric sulfur gases such as DMS, SC₂, H₂S, COS, or MSH. The estimated overall precision of the method is 10%. The detection limit at the present stage of technique development is approximately 20 ppt (parts per trillion, or 10^-12 mol · mol^-1) for a 45 minute sampling time, with lower concentrations being detectable with lower precision.