A Methodology to Quantify the Geographic Origin of Secondary Pollutants

This work explores the application of a hybrid photochemical model (HY-SPLIT CheM) for the identification of the location of primary sources contributing to the formation of secondary pollutants such as ozone. This model incorporates a unique feature that allows it to identify source regions of pollutants undergoing non-linear chemical interactions. The simulation relies on a Lagrangian approach to compute transport, dispersion and deposition; and uses an Eulerian framework to represent chemical transformations of different air contaminants. The entire pollutant mass at each emission source is distributed among a number of lagrangian "particles", each of which may be thought of as a capsule containing the various chemical species. At each time-step these particles are advected, dispersed, and deposited throughout the simulation domain. The concentration of each chemical species within a cell is calculated by dividing the sum of the particle masses of a particular chemical compound by the volume of the corresponding concentration grid cell in which the particles reside, so uniform mixing inside each cell is assumed. The time evolution of the system due to chemical interactions is then computed by solving a stiff set of differential equations that represent the chemical changes taking place within each cell. The resulting concentrations are then utilized to calculate the change in mass of the chemical species for all the particles within the cell. The source apportionment is accomplished by tagging each particle to its source location. Hence, the relative mass of a secondary pollutant formed at a particular receptor point is assigned to the corresponding points of origin. As an example, we present a case study for which considerable measurement data are available. Furthermore, high ozone concentrations are modeled and the source-receptor relationship is utilized to determine the position and the fractional contribution of each of the primary pollutant sources responsible for ozone generation for different receptor locations.