A SVAT SCHEME FOR NO, NO2, AND O-3 - MODEL DESCRIPTION AND TEST-RESULTS

A soil/vegetation/atmosphere transfer (SVAT) scheme for determining th e dry deposition and/or emission fluxes of NO, NO2, and O-3 in the atm ospheric surface layer over horizontally uniform terrain covered with fibrous canopy elements is presented and discussed. This transfer sche me is based on the micrometeorological ideas of the transfer of moment um, heat and matter near the Earth's surface, where chemical reactions between these trace gases are included. The fluxes are parameterized by first-order closure principles. The uptake processes by vegetation and soil are described in accord with Deardorff (1978). The SVAT schem e requires only routine data of wind speed, dry- and wet-bulb temperat ures, short wave and long wave radiation, and the concentrations of O- 3 and nitrogen species provided by stations of monitoring networks. Fi rst model results indicate that the dry deposition fluxes of NO, NO2, and O-3 are not only influenced by meteorologi cal and plant-physiolog ical parameters, but also by chemical reactions between these trace sp ecies and by NO emission from the soil. Furthermore, a small displacem ent in the concentrations of NO, NO2, and O-3 within in the range of t he detection limits of the chemical sensors can produce large discrepa ncies in the flux estimates, which are manifested here by the shift fr om height-invariant fluxes substantiated by the photostationary state to strongly height-dependent fluxes caused by the departure from that state. Especially in the case of these nitrogen species the widely use d 'big leaf' multiple resistance approach, which is based on the const ant flux approximation seems to be inappropriate for computing dry dep osition fluxes and deposition velocities.