High sulfur dioxide deposition velocities measured with the flux–gradient technique in a boreal forest in the Alberta Oil Sands Region

The emission of SOinline-formula₂ from the Athabasca Oil Sands Region (AOSR) has been shown to impact the surrounding forest area. Recent studies using aircraft-based measurements have demonstrated that deposition of SOinline-formula₂ to the forest is at a rate many times higher than model estimates. Here we use the flux–gradient method to estimate SOinline-formula₂ deposition rates at two tower sites in the boreal forest downwind of AOSR SOinline-formula₂ emissions. We use both continuous and passive sampler measurements and compare the two techniques. The measurements infer SOinline-formula₂ deposition velocities ranging from 2.1–5.9 cm sinline-formula⁻¹ (when corrections are applied). There are uncertainties associated with the passive sampler flux–gradient analysis, primarily due to an assumed Schmidt number, a required assumption of independent variables, and potential wind effects. We estimate the total uncertainty as inline-formula± 2 cm sinline-formula⁻¹. Accounting for these uncertainties, the range of measurements is approximately double the previous aircraft-based measurements (1.2–3.4 cm sinline-formula⁻¹) and more than 10 times higher than model estimates for the same measurement periods (0.1–0.6 cm sinline-formula⁻¹), suggesting that SOinline-formula₂ in the AOSR has a much shorter lifetime in the atmosphere than is currently predicted by models.