SCICHEM, CMAQ, and CAMx Single-Source Secondary Pollutants: A Model Intercomparison

Recent updates to USEPA regulatory guidelines require the assessment of near-field secondary pollutants, such as ozone (O₃) and fine particulate matter (PM_2.5) from new or modified sources under certain circumstances. USEPA has supplied a “Tier 1” look-up table approach. The preferred “Tier 2” demonstration approach is to use photochemical models with a complete treatment of gas-phase and aerosol chemistry. Such models include photochemical grid models (PGMs), as well as Lagrangian puff models with the requisite treatment of processes governing the formation of secondary pollutants from their precursors. SCICHEM is an open-source reactive puff model that includes treatments for gas, aerosol, and aqueous-phase chemistry that are comparable to those in PGMs. The dispersion component of SCICHEM is based on the SCIPUFF (Second-order Closure Integrated Puff) Gaussian puff model. SCICHEM is less computationally expensive to run than PGMs to calculate the formation of downwind ozone and PM_2.5 from a single source or a small number of sources. This presentation describes a model inter-comparison study of secondary pollutant concentrations from a coal-fired power plant stack calculated by SCICHEM and by the two most commonly used PGMs, CMAQ and CAMx. Hypothetical stacks were located in 4 separate regions of the U.S., with distinct differences in chemical environments and geographical features. The single-source impacts calculated by the PGMs are based on zero-out simulations (i.e., with and without the hypothetical source) as well as with the source apportionment tools available in CAMx. SCICHEM directly predicts secondary single-source impacts in a single simulation. Annual simulations were conducted with each of the three models to compare the modeled frequency distributions of source impacts over the 4 regions, including visibility impacts in Class I areas in each region.