A SVAT scheme for NO, NO2, and O-3 - Model description and test results

A soil/vegetation/atmosphere transfer (SVAT) scheme for determining the dry deposition and/or emission fluxes of NO, NO2, and O-3 in the atmospheric surface layer over horizontally uniform terrain covered with fibrous canopy elements is presented and discussed. This transfer scheme is based on the micrometeorological ideas of the transfer of momentum, 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 scheme requires only routine data of wind speed, dry- and wet-bulb temperatures, short wave and long wave radiation, and the concentrations of O-3 and nitrogen species provided by stations of monitoring networks. First model results indicate that the dry deposition fluxes of NO, NO2, and O-3 are not only influenced by meteorologi cal and plant-physiological parameters, but also by chemical reactions between these trace species and by NO emission from the soil. Furthermore, a small displacement in the concentrations of NO, NO2, and O-3 within in the range of the detection limits of the chemical sensors can produce large discrepancies in the flux estimates, which are manifested here by the shift from 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 used 'big leaf' multiple resistance approach, which is based on the constant flux approximation seems to be inappropriate for computing dry deposition fluxes and deposition velocities.