NUMERICAL MODELING OF THE TURBULENT-DIFFUSION AND CHEMISTRY OF NOX, O3, ISOPRENE, AND OTHER REACTIVE TRACE GASES IN AND ABOVE A FOREST CANOPY

A coupled diffusion-chemistry model was developed for the turbulent tr ansport of reactive trace gases in and above a forest canopy. The one- dimensional model was used to study daytime vertical profiles of gaseo us concentrations and fluxes and the influences of vertical distributi ons of solar irradiation and uptake and emission at leaves and the for est floor. The upper boundary of the model was extended to the top of the atmospheric boundary layer to allow adequate coupling at the atmos phere-forest interface. To study the effects of biogenic nonmethane hy drocarbons, chemical reactions for isoprene and its atmospheric oxidat ion products were linked with reactions for inorganic species and the oxidation of CO and CH4. Isoprene emission rates at various heights in the canopy were calculated as a function of local radiation, temperat ure, and leaf density distribution. Photolysis rates for photochemical reactions were allowed to vary with height according to the change in solar irradiation in the canopy. Vertical profiles Of O3, NO, NO2, NO (x), NO(y), OH, HNO3, H2O2, and isoprene concentrations and fluxes sim ulated for a specified deciduous forest were examined with a single se t of measured and computed daytime micrometeorological conditions. Res ults show that for strongly depositing gases like O3, HNO3, and H2O2, deposition velocities appear to be insensitive to chemistry and to hav e a profile similar to those predicted for a nonreactive case (simulat ion without chemistry), although the fluxes are influenced by concentr ation changes caused by chemistry. Simulated profiles of isoprene conc entration and flux agree closely with results for the nonreactive case , largely because of the dominant effects of emission and turbulent mi xing. Chemical reactions have the most important influence on profiles of NO, NO2, and NO(x) concentrations and fluxes. With a small and rep resentative NO emission forced at the forest floor, NO concentration d ecreases quickly with height near the ground and falls to a minimum va lue near the middle of the canopy because the chemical transformation of NO is fast while photodecomposition of NO2 is weak inside the canop y. As a result, the NO2 concentration becomes higher inside the canopy than above, and an upward NO2 flux occurs near the canopy top despite NO2 deposition in the canopy. The total flux of NO(x) near the canopy top appears to be downward because of strong downward NO flux. The fl ux of NO(y) above the canopy is almost invariant with height as chemic al interchanges create no net effect on the total nitrogen flux, altho ugh a large flux divergence caused by dry deposition occurs inside the canopy.