Control of chemical reactions by convective turbulence in the boundary layer

The influence of convective turbulence on chemical reactions in the atmospheric boundary layer is studied by means of direct numerical simulation (DNS). An archetype of turbulent reacting flows is used to study the reaction zones and to obtain a description of the turbulent control of chemical reactions. Several simulations are carried out and classified using a turbulent Damkohler number and a Kolmogorov Damkohler number. Using a classification based on these numbers, it is shown that it is possible to represent and to solve adequately all relevant scales of turbulence and chemistry by means of DNS. The simulations show clearly that the reaction zones are located near the boundaries where the species are introduced. At the lower boundary of the convective boundary layer, the reaction takes place predominantly in the core of the updrafts, whereas in the upper part of the domain the chemical reaction is greatest in the center of the downdrafts. In the bulk of the boundary layer the chemical reaction proceeds very slowly, due to the almost complete segregation of the chemical species. From the point of view of chemistry, the mixing across the interface between updrafts and downdrafts in the bulk of the convective boundary layer plays only a minor role. The amount of chemical reaction in relation to the degree of turbulence is quantified by the introduction of an effective Damkohler number. This dimensionless number explicitly takes into account the reduction of the reaction rate due to the segregation of the chemical species. It is shown that the number approaches an asymptotic value that is O(1) for increasingly fast reaction rates. This shows explicitly that the timescale of the chemical reactions is limited by the integral turbulent timescale. It is suggested how a parameterization could be used to include this effect into one-dimensional atmospheric models.