The {open_quotes}footprint{close_quotes} area influencing a high-volume air sampler

A recent question posed by air sampling analysts is how to calculate the source area that contributes to the collection of material by a high-volume (HV) air sampler. In micrometeorology studies theoretical and numerical approaches have been used to determine the ``footprint`` of an atmospheric flux from the ground that influences a sensor at any given height. Over the course of a few days or more, the influence of atmospheric stability on a high volume air sampler run can be neglected. For surfaces that have dense vegetative cover, the boundary layer is displaced upward by the vegetation height and the roughness is proportional to the vegetation height. It can be concluded that for typical surface conditions, the HV air sampler is 90% influenced by material flux from the surface less than 145--175 m upwind. For rougher surfaces, the upwind bounded distances are shorter. There may be some very specialized cases (e.g., empty parking lots) where the upwind bounded distances are much greater.