This work identifies an important and hitherto unsuspected controlling factor influencing the rate of deposition of atmospheric gases to foliage surfaces. We show base cation-containing dust that settles on foliage can raise the pH of thin water films on leaves, significantly increasing sulphur dioxide dry deposition rates (often by more than 1 cm s^-1), and lowering concentrations (by up to 60%), near sources of alkaline dust emissions, changing sulphur deposition fluxes' spatial distribution.