In an attempt to further our understanding of turbulent mixing on atmospheric chemistry occurring above and within a plant canopy, we present an investigation of the transport of decaying scalars released from a forest canopy. Of particular interest are biogenic hydrocarbons, such as isoprene, which play important roles in photochemistry, but whose atmospheric lifetimes are short. Observations have indicated that these species are not consistently well mixed within the Convective Boundary Layer (CBL). Typically, the atmospheric layers within forest canopies are stably stratified and thus biogenic compounds can be retained within the canopy. Intermittent passage of canopy scale coherent structures may be a major source of the variability in the CBL profiles of these compounds. CBL photochemical models typically assume the CBL is well-mixed for simplicity, while at the same time, canopy-meteorologists typically study conserved scalars. We make an attempt to investigate the relationship between turbulent mixing and chemical loss.
To perform this investigation, we have included eight scalars into the large-eddy simulation (LES) code presented previously by Patton (1997). The canopy emits each of these scalars at an equivalent and constant rate that varies with depth within the horizontally homogeneous forest. The first of these scalars is a conserved quantity. The next six of these scalars are allowed to decay at an exponential rate with lifetimes that vary from two hours down to ten seconds. The last scalar is allowed to decay based upon a simplified reaction between an isoprene-like compound and the hydroxyl radical.
Previous studies have performed quadrant analyses of the fluxes of conserved scalars above and within a forest canopy. However, the effects of chemical loss have not been considered. Using quadrant analysis we investigate the fractional contribution to the total scalar flux by intermittently occurring vigorous departures from the mean scalar flux. For flows within and just above a forest canopy, our investigation of conserved vs. non-conserved species reveals that the fluxes of species with decay-timescales similar to the turnover time of the canopy scale eddies are distinctly different from those species that decay either extremely fast or slow.