A COMPARISON OF 2 PHOTOCHEMICAL-REACTION MECHANISMS USING MASS-BALANCE AND PROCESS ANALYSIS

Output from air quality models typically consists of model species con centrations given as a function of time or location. When two differen t photochemical reaction mechanisms are used in the same air quality m odel, predictions of species concentrations may differ. Explaining why the models predict differently is difficult using only the typical ou tput. We have developed a process analysis method in which a complete mass balance is used to quantify characteristic values of the reacting system. We have modified both Lagrangian and Eulerian air quality mod els to calculate the integrated rate, i.e. the mass throughput for eac h chemical reaction and each physical process. The integrated rates ar e written to an external file, and a post processing program is used t o perform a detailed mass balance on the model simulation. Using the m ass balance information, we compute characteristic values that describ e the reacting system. In this paper we describe the method and give a n example analysis comparing two photochemical reaction mechanisms, th e Carbon Bond IV and the SAPRC90 mechanism in a Lagrangian model simul ation. This analysis method significantly increases our ability to exp lain the predictions of air quality models. In addition the method rev eals several significant reactivity parameters that can be used to com pare different reaction mechanisms and that suggest additional measure ments that might distinguish between the different representations.