On the transfer of dense gases from a forest sub-canopy into the wind field above

The need to anticipate high risk of exposure following the generation of a cloud of cold and dense gas beneath a forest canopy introduces a need to examine the processes facilitating exchange with the air above the canopy. It is the concentrations in this above-canopy air that are used to initialize many dispersion models and these concentrations are lower than would be expected if no canopy were present. In the lack of direct experimental studies of trans-canopy dilution in a forested environment, results from a variety of related experiments are used here to illustrate the complexity of the problem and to derive a first-order assessment of the extent of expected dilution. The focus is on the dense gas clouds following accidents involving liquid chlorine, ammonia and carbon dioxide. It is concluded that the dilution would result in above-canopy concentrations between 2% and 50% of average sub-canopy levels, depending on site-specific circumstances. It is also concluded that the relative importance of the various contributing processes is such that detailed incorporation of them in dispersion models would constitute an unjustified complexity unless definitive experimental observations are available. Instead, the consequences of the issues now considered might best be accommodated by reducing predicted downwind concentrations by a factor of about ten with a corresponding extension of the duration of the dispersion event.