Large-eddy simulation (LES) results are used to study two different turbulent flows in the atmospheric boundary layer with reacting chemical species. Previous LES studies of a binary irreversible reaction have shown that the chemistry could be influenced by the convective boundary layer turbulent structure. Key processes such as turbulent mixing, radiation, deposition and entrainment are investigated in order to determine their effect on the spatial distribution and evolution of reacting species. Moreover, the chemical mechanisms considered allow us to study the departure from chemical equilibrium due to the boundary layer processes.
In the first case, the essential reactions to form and to deplete ozone are studied in a dry convective boundary layer. The chemical mechanism covers a wide range of time scales. It is found that the volume-averaged concentrations calculated with the LES are in close agreement with a box-model, i.e. reactants are homogeneously mixed
in all the domain. However, model results differ when a reactant is heterogeneously emitted at the surface or when an increase of the chemical reaction rate produces a large segregation of the species and in consequence the chemical reaction is retarded.
Clouds severely perturb the ultraviolet radiation field and consequently have a direct impact on the photodissociation rates. The combined effect of turbulent mixing and radiation in a smoke cloud is simulated in the second case. The simulation does not take evaporative cooling and aqueous phase into account. The following aspects are studied: the segregation of species in the cloud, the formation of a concentration gradient for the photodissociated species
and the entrainment of species at cloud top.
The understanding of the above mentioned processes is fundamental to determine the quantity of species that are formed or reacted away in the atmospheric boundary layer and the exchange of species between this layer and the free troposphere
Symposium on Interdisciplinary Issues in Atmospheric Chemistry