Process Analysis (PA) is an accounting system that tracks the quantitative effects of the individual chemical and physical processes that combine to produce the predicted hourly species concentrations within a CTM simulation. Typically Eulerian grid models, such as the CCTM, produce output concentration fields that are solutions of systems of partial differential equations for the time-rate of change in species concentrations due to a series of individual physical and chemical processes and then combining these results to obtain a cumulative hourly concentration. PA tracks the mass throughput of these individual processes and provides quantitative information about how each process impacted the predicted hourly species concentrations. PA is an optional configuration in CMAQ that is implemented by compiling the CCTM with include files that define PA configuration options. A PA-instrumented version of the CCTM outputs additional files that contain hourly concentrations by physical and/or chemical process for selected model species.

PROCAN is the PA preprocessor in CMAQ. PROCAN inputs a configuration file that is used to define the model species and processes to track using PA and outputs three include files that are used when compiling the CCTM. Figure 2-8 is schematic of PA in the CMAQ modeling system.