A method for computing single trajectories representing boundary layer transport

For many purposes it is necessary to represent the transport of air pollutants by a single trajectory. Examples are models which simulate the physical and chemical processes within a box moving along a trajectory or the receptor-oriented statistical modelling of air pollutants. Especially within the planetary boundary layer strong vertical wind shear poses serious problems for describing transport processes with a single trajectory. This paper describes a new method for calculating boundary layer trajectories which uses a pollutant mass-weighted transport vector. For evaluating this vector it is assumed that. the pollutant is evenly distributed within the mixed layer. During periods of reduced mixed layer depth (e.g. nighttime) a residual layer may form above the mixed layer and most of the pollutant is transported aloft. However, this residual layer transport is only of interest if the mixed layer height rises again and pollutants are fumigated back to ground-level. Thus, estimation of the most appropriate pollutant transport depth requires the knowledge of mixing heights along the whole trajectory from the starting point to the receptor point. As this information is not a priori available, an iterative scheme was developed which uses the mixing heights along a preliminary trajectory as an input for an improved guess of the final trajectory. Experiments have shown that this method converges very fast. The computation of a first guess trajectory with fixed transport depth and one iteration is sufficient.