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## E.2.6 Select Dispersion Models 

CAL3QHC and CAL3QHCR are the two EPA-approved mobile source dispersion models used to predict CO and PM (i.e., PM $_{10 }$and PM$_{2.5}$) concentrations, respectively, at sensitive receptor locations adjacent to highway and roadway intersections 107 . Information on EPA’s recommended models is detailed in EPA’s Guideline on Air Quality Models mentioned above. 

The CAL3QHC model is an effective tool for predicting hourly CO emissions due to vehicles operating under free -flow conditions and from idling vehicles under stop- and-go conditions (or queuing conditions) near signalized intersection s. The model’s inputs include roadway geometries, traffic data, receptor locations, meteorological conditions, and vehicular emission rates (estimated using either EPA’s MOVES or California’s EMFAC). Additionally, it incorporates intersection-specific parameters and detailed signal information (e.g., signal timing and intersection lane as signments) to predict 1-hour and 8-hour CO concentrations at near-by sensitive receptors. 

The CAL3QHCR is the refined, yet independent version of CAL3QHC that has the capability to predict 1-hour, 8-hour, 24-hour, and annual concentrations, with the use of a full year of hourly meteorological and traffic data; thus CAL3QHCR is better suited for PM hot-spot analyses. Notably, when using the CAL3QHCR for PM hot -spot analysis highway and roadway intersection projects, its queuing algorithm should not be used. 108 CAL3QHCR should be used for CO when a CAL3QHC analysis with conservative assumptions indicates a potential exceed the NAAQS. 

Additionally, EPA has also approved AER MOD as a recommended air quality model completing PM hot -spot analyses for different types of transportation projects (e.g., parking facilities). Refer to EPA’s Transportation Conformity Guidance for Quantitative Hot -Spot Analyses in PM 2.5 and PM 10 Nonattainment and Maintenance Areas mentioned above, for complete information on selecting an appropriate air quality model. 

The CAL3QHC and CAL3QHCR model inputs and their respective parameters are provided in Table E-5 ( CAL3QHC and CAL3QHCR Model Input Data ) and further discussed in this section. 

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