Alkaline dust deposition to foliage surfaces likely enhances the dry deposition velocity of SO ₂ : an investigation in the Alberta Oil-Sands Region using the GEM-MACH air-quality model

Abstract. We examine the potential impact of alkaline particle deposition on foliage and its influence on sulphur dioxide dry deposition velocities, using a new theoretical development, a high resolution air-quality model, and comparisons to observations. Our study domain encompasses the Athabasca Oil Sands Region, an industrial area where base cation-bearing fugitive dust from open-pit mining and forest fires coexists with elevated SO2 emissions from large stacks. Our pH-modulated dry deposition scheme links thin aqueous films on foliage to local chemical conditions, including alkaline dust accumulation. We present the mechanism's theoretical basis, along with a simplified algorithm predicting foliage water pH and linked to SO2 deposition velocities. We predict enhanced SO2 deposition, due to increased leaf surface pH from dust co-deposition near major dust sources, often by more than 1 cm s−1. These result in dry deposition fluxes 2.5 to 10 times greater than in the absence of these effects – consistent with estimates from aircraft studies. The enhanced deposition reduces surface SO2 concentrations by up to 60 % near sources, improves agreement with continuous monitoring data, and reduces the normalized mean bias at several stations. Taylor diagram statistics show improved model temporal variability performance. Further from sources of base-cation-containing dust, aqueous films on foliage remain acidic, reducing SO2 deposition velocity and increasing concentrations. We make specific recommendations for new observation data which would reduce formulation uncertainties. The findings have broad implications for global SO2 budgets, given the significant role of wind-blown mineral dust and forest fire base cation emissions in influencing atmospheric acidity and trace gas removal.