Alkaline dust deposition to foliage surfaces likely enhances the dry deposition velocity of SO₂: 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 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 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 in influencing atmospheric acidity and trace gas removal.