The concept of species age in photochemical modeling

This paper introduces the concept of species age, which allows one to separate transport and chemistry in a photochemical model. This separation is an approximation that can lead to substantial decreases in the computational burden of photochemical modeling. Thus, the method can be used to simulate a large number of cases to select a few for more detailed study with a comprehensive photochemical model. The concept of species age can also be used to probe observations directly to draw inferences on the impact of precursor emission changes at specific sources on observed ozone levels. Species age is computed using a conservation equation derived in this paper. The pollutants are initially transported within the airshed as inert species for the required simulation time. These inert concentrations are then allowed to react over the appropriate species ages to yield ozone concentrations. Computational efficiency is obtained by performing chemistry as a post-processing step only at the receptors of interest. The usefulness of the species age concept has been evaluated by comparing results from a model that uncouples transport and chemistry to those from a comprehensive photochemical model applied to an ozone episode observed during the Lake Michigan Oxidant study in 1991. The initial results are very encouraging.