FIRST-ORDER CHEMISTRY IN THE SURFACE-FLUX LAYER

We have discussed the behavior of a non-conserved scalar in the statio nary, horizontally homogeneous, neutral surface-flux layer and, on the basis of conventional second-order closure, derived analytic expressi ons for flux and for mean concentration of a gas, subjected to a first -order removal process, The analytic flux solution showed a clear devi ation from the constant flux, characterizing a conserved scalar in the surface-flux layer. It decreases with height and is reduced by an ord er of magnitude of the surface flux at a height equal to about the typ ical mean distance a molecule can travel before destruction. The predi cted mean concentration profile, however, shows only a small deviation from the logarithmic behavior of a conserved scalar. The solution is consistent with assuming a flux-gradient relationship with a turbulent diffusivity corrected by the Damkohler ratio, the ratio of a characte ristic turbulent time scale and the scalar mean lifetime. We show that if we use only first-order closure and neglect the effect of the Damk ohler ratio on the turbulent diffusivity we obtain another analytic so lution for the profiles of the flux and the mean concentration which, from an experimental point of view, is indistinguishable from the firs t analytic solution. We have discussed two cases where the model shoul d apply, namely NO which, by night, is irreversibly destroyed by inter action with mainly O-3 and the radioactive Rn-220. Only in the last ca se was it possible to find data to shed light on the validity of our p redictions. The agreement seemed such that a falsification of our mode l was impossible. It is shown how the model can be used to predict the surface flux of Rn-220 from measured concentration profiles.