Simulation of turbulent mixing effects on chemical reactions in the convective boundary layer.

摘要

The goal of this research is to quantitatively account for incomplete mixing of interacting chemical species when one species is emitted from ground level and the other species is mixed downward into the top of the daytime atmospheric boundary layer. Top-down and bottom-up (TDBU) diffusion of two species in an inhomogeneous convective boundary layer (CBL) are simulated by means of 1-D vertical turbulent chemical kinetics model (TKM). The concentration field splitting method (CSM) with a phenomenal extent of reaction and asymmetric convective model (ACM) schemes are implemented. Because of the lack of suitable experimental data, results of the TKM are compared with a box model (BM), with those of the conventional linear chemical kinetics model (LKM) ignored incomplete mixing of reaching species, and with those of a conventional turbulent kinetics model (CTKM) based on the CSM with a conventional extent of reaction.; The TKM simulations were performed to study the effects of convective turbulent mixing on chemical reaction rates in vertically asymmetric turbulence of the CBL. Important findings from those simulations include the following: (1) The results in the TKM prove that the effects of incomplete mixing (segregation of species) on the chemical reaction rates are not important in predicting in the mean concentrations of species in any realistic photochemical system in the convective boundary layer. (2) The TKM and the CTKM simulations provide almost the same results of species concentrations and segregations, under both equilibrium and non-equilibrium conditions, even though the chemical reactions and the dispersion effects are coupled and evolve simultaneously in space and time. (3) The CSM scheme works well with both the conventional extent of reaction and the phenomenal extent of reaction if the complex chemistry is used, the overall reaction rates are determined in the CBL, and the species fluxes from the boundary layers are not opposite. (4) In the TKM, the segregation effects and the concentrations are very sensitive to chemical characteristics (initial concentration and reaction rate constants) but less sensitive to the physical characteristics (deposition flux, exchange flux, turbulent timescales) and numerical resolution. They are also very sensitive to emission fluxes. (5) The segregation effects are most sensitive to the reaction rate constants, while all concentrations varied sensitively with the parameters groups at the surface except the sensitivity of ozone concentration is small in all cases tested.