The oxidation rate and residence time of sulphur dioxide in the arctic atmosphere

The Arctic air mass contains gaseous and particulate compounds that originate mainly from fossil fuel combustion at mid-latitudes and especially from the Eurasian sector. Observations of the temporal variation of SO₂, SO₄²⁻ and V concentrations in the North American and Norwegian Arctic are presented. At Igloolik, Canada, 3–7-day average SO₂ concentrations ranged from 2.3 to 4.3 μg m⁻³ in February to much lower values in spring and fall. The most probable cause of similar strong variations in the ratio of SO₄⁻² to V observed throughout the North American Arctic is a seasonally varying SO₂ oxidation rate. Interpreted in the light of a Lagrangian transport model, observations indicate that the mean SO₂-oxidation rate between Eurasian sources and the North American Arctic is 0.1 % h⁻¹ in early December, 0.04 % h⁻¹ in late February, and 0.1–0.2% h⁻¹ in early April. The residence time of SO₂, controlled not only by chemical conversion to sulphate but also by dry deposition, is 14–20 days in late fall, 16–32 days at mid-winter and 10–19 days in April. The estimated rates of SO₂ oxidation cannot be explained by photochemical oxidation mechanisms at least when reactive hydrocarbons are ignored.