The effects of the canopy medium on dry deposition velocities of aerosol particles in the canopy sub-layer above forested ecosystems

Understanding how the leaf area density (a(z)) and its depth integrated value, the leaf area index (LAI), modify dry deposition velocities (Vd) of aerosol particles within the canopy sub-layer is needed for progressing on a plethora of aerosol related problems in climate change, air quality, and ecosystem service evaluation. Here, the interplay between a(z) (and LAI) of tall and densely forested canopies, the flow dynamics, and Vd are explored via model calculations. A multi-layer size-resolving deposition model (hereafter referred to as MLM) is coupled to a second-order closure model (WS77), which are then used to explore a subset of the manifold of a(z) and LAI variations and their concomitant effects on the relationship between Vd and particle diameter (dp). The combined MLM-WS77 calculations are evaluated against Vd measurements collected above a Scots pine stand in Hyytiälä (southern Finland) in which a(z) was experimentally manipulated via forest thinning. Three key findings are derived from these model calculations: (1) at a given LAI, a near-constant a(z) yields the lowest Vd for a given dp class, (2) when the foliage is concentrated in the upper layers of the canopy, increasing LAI predictably increases Vd at a given dp, though some saturation occurs thereafter, but (3) suppressing turbo-phoresis leads to an opposite conclusion, decrease of Vd with LAI increase, for a dp class between 0.5 and 5 μm. Comparison between the combined MLM-WS77 calculations and a recently proposed pipe-flow analogy formulation that includes turbo-phoresis are also presented.