AirWare On-line Reference Manual
PBM: Photochemical Box Model
PBM is a simple numerical air quality model that simulates photochemical smog at an urban scale. The model has been developed by the US EPA, Atmospheric Modeling Branch, Meteorology and Assessment Division, at Research Triangle Park, North Carolina. PBM is also one of the central models in the AirWare air quality management information system.
The original EPA code has been integrated in the AirWare system with a fully interactive graphical user interface, including an embedded rule-based expert system for the editing of the required input data. Due to the simplicity of the box model's physical structure PBM is well suited for rapid assessment, and can be coupled with a discrete multi-criteria optimisation scheme to design and evaluate ozone control strategies.
Implementation note: as of Release R5.4, PBM is being replaced with a low-resolution version of more versatile 3D model CAMx (with the latest CB05 photochemistry), replacing the single box representation with a set of 9 to 25 coupled boxes. Both the original PBM and the low-resolution CAMx are used for the emission control optimization using the discrete multi-criteria DSS DCM as a post-processor for multi-criteria analysis. versionPBM is best suited to treat low wind conditions, in presence of sun light. Data requirements for the PBM include meteorology, air quality, and emission data. The urban area is represented by one or a a set of cells, within which chemical reactions are computed in particular to evaluate the concentrations of hydrocarbons and ozone. It may deal with point, line and area sources. The pollutants considered are basically nitrogen oxydes and hydrocarbons.
The Photochemical Box Model is a stationary single-cell model with a variable height designed to provide volume- integrated hour averages of 03 (ozone) and other photochemical smog pollutants of interest for an urban area for a single day of simulation.
The PBM is most appropriate for typical ozone episodes: for application in air stagnation conditions with light and variable winds (generally under 2 m/s). Horizontal dimensions of the box are typically on the order of 10-50 km; the vertical dimension may vary between 0.1 and 2 km. The depth of the mixed layer, or depth of the PBM domain, also follows a diurnal pattern; it can be optionally specified as following a non-linear growth curve.
The PBM assumes that emission sources are homogeneously distributed across the surface face of the box volume and that the volume is well mixed at all times. Atmospheric diffusion and wind shear are neglected.
Chemical reactions are simulated using kinetic mechanism that includes diurnal variation of photolytic rate constants. The PBM contains a 63-step chemical kinetic mechanism, developed by Dr. Demerjian of EPA. The reactions include 37 reactive species and 8 hydrocarbon classes. The reaction set is coupled between species.
Model simulation typically begins at 0500h, local standard time (LST) and continues throughout the day, typically ending at or just before sunset, with a maximum duration of 18 hours. The default period of simulation has been designed to start near sunrise and end in the late afternoon/early evening period, after the maximum O3 (ozone) concentration has typically been reached. The period of simulation may be specified as longer than the default, but not exceed 18 hours.
The user must provide the PBM with initial species concentrations, hourly inputs of wind speed, source emission fluxes of CO, NOx (nitrogen oxide) THC (hydrocarbons), reactivity classes, and boundary species concentrations. Values of measured solar radiation and mixed layer depth may be specified at sub-hourly intervals throughout a simulation.
Kenneth L. Schere and Kenneth L. Demerjian U.S. Environmental Protection Agency Environmental Sciences and Research Laboratory Office of Research and Development Research Triangle Park, NC 27711
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