A two-layer model of the atmosphere indicating the effects of mixing between the surface layer and the air aloft

Abstract

A two-layer air pollution model is developed for the case when inter-layer mixing is continuous, but slower than intra-layer vertical mixing. The model was designed to examine the implications of inter-layer transport for ground-level impacts. The model equations, consisting of two coupled partial differential equations, are solved numerically by the method of characteristics. In the case of first order chemical reactions, the zero and first order moments are computed in order to determine the locations of the centroids of pollutant mass. The model indicated that a pulse of pollutants emitted at ground level can result in bifurcation of a pollutant plume due to the wind directional shear between the two atmospheric layers. The residual concentrations due to the transport between layers can move upwind, downwind or remain stationary with respect to the ground level winds, depending on the speed and orientation of the winds aloft. Several specific cases are described, and, in each case, the behavior of the residual plume would not be expected from the surface layer winds that are normally used to predict the transport and dispersion of pollutants. In addition, it appears that the characteristic exchange rate between the air aloft and the surface mixing layer is large enough that the winds aloft must be considered when describing long range transport or the multi-day impacts of a pollutant.