The chemical lifetime of reactants in the atmosphere can vary within a wide range of timescales (Table 1). In the convective boundary layer (CBL), the so-called long lived species are well mixed. For reactants with a short chemical lifetime or with a lifetime of the same order of magnitude as the turnover time of the convective boundary layer, approximately 10-20 minutes, the chemical transformations can be limited by the turbulent mixing.

Table 1: Typical timescales of transformation of chemical species and of some atmospheric phenomena (Vilà-Guerau de Arellano et al., 2003).

Figure 1: Schematic representation of the driving processes of the CBL growth (Vinuesa and Vilà-Guerau de Arellano, 2005).

In fact, two main processes govern the structure and the characteristics of the convective boundary layer over land and with a clear sky. First, the heat release in form of sensible surface heat flux warms the boundary layer at the surface. It drives upward motions of warm air. Second, the entrainment of warm air from the free troposphere contributes to the heating of the ABL (see figure 1). As a result of these thermal motions, compounds introduced in the CBL are segregated and they require a time larger than the characteristic turbulent time to be uniformly mixed.

Since the chemistry requires a complete mixing up to molecular level, the segregation of species due to turbulent characteristics can affect the reaction rate and can lead to a slowing down of the reactivity of chemical compounds. In figure 2, the colored fields represent the place in the CBL where a second-order irreversible reaction between two reactants transported in opposite directions occurs. 0ne can notice that close to the surface, the reaction takes place in the updrafts and that close to the top of the CBL, it occurs in the downdraft. In this case, the turbulent structure of the CBL is clearly controlling the chemical transformations. Moreover, the turbulence is inefficient to mix homogeneously the reactants leading to a heterogeneous distribution of the scalar in the CBL. As a result, a regional chemical model, that is assuming homogeneous mixing, will over-estimated the reactant rates leading to an under-estimation of pollutant concentrations.

Figure 2: Vertical velocity (updrafts and downdrafts in solid and dashed lines respectively) and reaction zone for chemical species transported in opposite directions and involved in a second-order reaction (Molemaker and Vilà-Guerau de Arellano, 1998; Vilà-Guerau de Arellano and Lelieveld, 1998).

Figure 3: Scalar dimensionless flux profiles: potential temperature, inert scalar, slow and fast reactants in solid, dotted, dashed, green and red lines, respectively (Vinuesa and Vilà-Guerau de Arellano, 2003).

Figure 3 shows the vertical profiles of scalar dimensionless fluxes that consist of a resolved part and a sub-grid scale contribution modelled as a diffusion process. Notice that the concentration scale used to make the dimensionless profiles is calculated as the ratio of the emission flux of A to the convection velocity scale. Within the boundary layer, the profiles of inert scalars (temperature and bottom-up scalar for the inert chemical case) have a linear shape. For reactive scalars, the profiles deviate from this shape. In fact, these deviations become more significant for faster chemistry; they are larger for the fast reactant than for the slow one. These deviations are due to the fact that chemistry acts as a sink term in the flux budget. As the chemical contribution to fluxes increases with the reaction rate, the deviations will increase with the reaction rate.

Currently, our research aims at:

References:

• Molemaker J., and J. Vilà-Guerau de Arellano (1998), Control of chemical reactions by convective turbulence in the boundary layer. Journal of the Atmospheric Sciences 55, 568-579.

• Vilà-Guerau de Arellano J., A. Dosio, J.-F. Vinuesa, A.A.M Holtslag, and S. Galmarini (2004), The dispersion of chemically reactive species in the convective boundary layer. Meteorology and Atmospheric Physics, 87, 23-28.

• Vilà-Guerau de Arellano J., and J. Lelieveld (1998), Chemistry in the atmospheric boundary layer. In: Clear and Cloudy Boundary Layers (eds. Holtslag, A.A.M and Duynkerke, P.G.). Royal Netherlands Academy of Arts and Sciences, P.O. Box 19121, 1000 GC Amsterdam, The Netherlands, 267-286.

• Vinuesa J.-F., and J. Vilà-Guerau de Arellano (2003), Fluxes and (co-)variances of reacting scalars in the convective boundary layer. Tellus, 55B, 935-949.

• Vinuesa J.-F., and J. Vilà-Guerau de Arellano (2005), Introducing effective reaction rates to account for the inefficient mixing of the convective boundary layer. Atmospheric Environment, 39, 445-461.

• Vinuesa J.-F., and F. Porté-Agel (2005), A dynamic similarity subgrid model for chemical transformations in large-eddy simulation of the convective atmospheric boundary layer. Geophysical Research Letters, 32, L03814. available here

• Vinuesa J.-F., F. Porté-Agel, S. Basu, and R. Stoll (2005), Subgrid-scale transport of reacting scalars in Large-Eddy Simulations of atmospheric boundary layers, submitted