FOOTPRINT CONSIDERATIONS IN BOREAS

Comparisons of observations of concentration or flux from platforms at various heights, such as tower and aircraft, must take into account d ifferences in the location and extent of upwind surface source or sink areas which affect the individual observations, with their physical a nd biological characteristics. Such ''footprint'' estimates are based on solutions of the diffusion/advection equation which have not previo usly been evaluated over a boreal ecosystem. In order to adjust an ana lytical footprint model within the surface layer above forest canopies typical for the Boreal Ecosystem-Atmosphere Study (BOREAS) sites, 29 tracer gas release experiments were carried out between August 30 and September 9, 1994, at three tower sites in the northern study area (NS A). Sulphur hexafluoride (SF6) was released from point sources at vari ous upwind distances from the towers under various meteorological, ter rain, and release conditions. Wind, temperature, and stability paramet ers, during each release period, were used as input into calculations of vertical concentration profiles sampled at the towers, based on a t hree-dimensional diffusion model. Predictions of concentration profile s, or back calculation of source strength from observed profiles, were in good agreement with observed concentrations or actual release rate s, respectively. The diffusion model was then used to compute footprin t estimates for stable to unstable conditions, for tower and aircraft- based observation platforms. They show spatially constrained footprint s in the surface layer, due to effective vertical coupling, so observa tions from towers and low-flying aircraft must be expected to be very site specific, and scaling up to larger areas will have to be done wit h careful consideration of surface mosaics. Our study also included fo otprint estimates made for airborne observations above the surface lay er, based on large-eddy simulations over ''BOREAS-like'' terrain, for boundary layer structures comparable to those observed in BOREAS. They document the progressive decoupling of airborne observations from the surface features at these heights.