The significance of species segregation for Amazonian chemistry

Tropical rain forest chemistry is driven by the exchange of biogenic compounds, dynamic processes like turbulent mixing and the diurnal variability of the atmospheric boundary layer. The segregation of species due to inefficient turbulent mixing has recently been recognized as a possible relevant dynamic influence that could explain the large discrepancies between observations and simulations of reactive trace gas concentrations over the tropical rain forest. The inability of turbulence to mix the species leads to correlated or anti-correlated accumulations of the reacting species, therefore modifying the boundary layer mean reactivity. In order to make observations comply with large scale model results, chemists currently assume ad hoc values of the intensity of segregation that correspond to a substantial decrease in the isoprene-OH reaction rate. We will present numerical experiments with a large-eddy simulation (LES) model that are based on typical dynamic and chemistry conditions in the Amazonian rain forest. With a chemistry module coupled to the LES, we examine the main processes that control the species segregation. Our results indicate that the influence of segregation is negligible in a typical situation for the Amazonian rain forest characterized by uniform emission conditions. Finally we will present the potential enhancement of segregation due to spatially heterogeneous surface emissions. In these simulations, spatial segregation is induced by introducing heterogeneities in the surface properties: a cold and wet forested patch characterized by high isoprene emissions is alternated with a warm and dry patch, representing pasture with relatively low biogenic emissions.