Numerical modeling of the turbulent diffusion and chemistry of NOx, O3, isoprene, and other reactive trace gases in and above a forest canopy

Abstract

A coupled diffusion‐chemistry model was developed for the turbulent transport of reactive trace gases in and above a forest canopy. The one‐dimensional model was used to study daytime vertical profiles of gaseous concentrations and fluxes and the influences of vertical distributions of solar irradiation and uptake and emission at leaves and the forest floor. The upper boundary of the model was extended to the top of the atmospheric boundary layer to allow adequate coupling at the atmosphere‐forest interface. To study the effects of biogenic nonmethane hydrocarbons, chemical reactions for isoprene and its atmospheric oxidation 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, temperature, 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, NOx, NOy, OH, HNO3, H2O2, and isoprene concentrations and fluxes simulated for a specified deciduous forest were examined with a single set of measured and computed daytime micrometeorological conditions. Results show that for strongly depositing gases like O3, HNO3, and H2O2, deposition velocities appear to be insensitive to chemistry and to have a profile similar to those predicted for a nonreactive case (simulation without chemistry), although the fluxes are influenced by concentration changes caused by chemistry. Simulated profiles of isoprene concentration and flux agree closely with results for the nonreactive case, largely because of the dominant effects of emission and turbulent mixing. Chemical reactions have the most important influence on profiles of NO, NO2, and NOx concentrations and fluxes. With a small and representative NO emission forced at the forest floor, NO concentration decreases quickly with height near the ground and falls to a minimum value near the middle of the canopy because the chemical transformation of NO is fast while photodecomposition of NO2 is weak inside the canopy. 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 NOx near the canopy top appears to be downward because of strong downward NO flux. The flux of NOy above the canopy is almost invariant with height as chemical interchanges create no net effect on the total nitrogen flux, although a large flux divergence caused by dry deposition occurs inside the canopy.