Source footprint considerations in the determination of volatile organic compound fluxes from forest canopies

Above-canopy sampling of trace gases to determine volatile organic compound (VOC) emissions should be interpreted in terms of footprint considerations. This can be accomplished by defining the upwind canopy areas effectively sampled under the given wind and stability conditions. Using solutions of the advection–diffusion equation adjusted through controlled tracer gas (SF ) release experiments in the Boreal Ecosystem Atmosphere 6 Study of 1994, sampling of VOC concentrations and fluxes over a forest whose VOC sources are heterogeneous (Camp Borden) are studied. Analysis demonstrates that the variability observed in measured VOC fluxes could be accounted for by varying numbers of randomly distributed clumps of emitter species within a varying footprint. It suggests that heterogeneity of the forest canopy, in terms of source distribution of VOCs, has to be explicitly considered in estimates of source strength from above-canopy concentration/flux sampling