FOOTPRINT ESTIMATION FOR SCALAR FLUX MEASUREMENTS IN THE ATMOSPHERIC SURFACE-LAYER

The flux footprint is the contribution. per unit emission, of each element of a surface area source to the vertical scalar flux measured at height z(m); it is equal to the vertical flux from a unit surface point source. The dependence of the flux footprint on crosswind location is shown to be identical to the crosswind concentration distribution for a unit surface point source; an analytic dispersion model is used to estimate the crosswind-integrated flux footprint. Based on the analytic dispersion model, a normalized crosswind-integrated footprint is proposed that principally depends on the single variable z(BAR)/z(m), where zBAR is a measure of vertical dispersion from a surface source. The explicit dependence of the crosswind-integrated flux footprint on downwind distance. thermal stability and surface roughness is contained in the dependence of z(BAR) on these variables. By also calculating the flux footprint with a Lagrangian stochastic dispersion model, it is shown that the normalized flux footprint is insensitive to the analytic model assumption of a self-similar vertical concentration profile.