• Select receptor site locations : 

− The locations at which concentrations are estimated are known as receptors. Generally receptors are located along the roadway where the general public is likely to have continuous access. 

• Select dispersion models and calculate concentrations at selected intersection(s) : 96 

− For quantitative hot-spot analyses the EPA-recommended models for CO and PM$_{10 }$ and PM$_{2.5 }$ are CAL3QHC and AERMOD/ CAL3QHCR, respectively. CAL3QHCR is recommended for CO when a CAL3QHC analysis with conservative assumptions indicates a potential to exceed the NAAQS. 

• Determine background concentrations : 

− Background concentrations are typically obtained from a representative background monitoring site not affected by the intersection(s) of interest. 

• Tabulate results : 

− Representative background concentrations should be added to the predicted worst-case project-level concentration and compared to the NAAQS. If project does not conform, consider mitigation or control measure(s). 

## 7.2.4. Roadway Dispersion Modeling Results 

The last step of the process f or conducting a quantitative CO and/or PM$_{10 }$ and PM$_{2.5 }$ hot-spot analysis is to document the analysis and results. For both project -level air quality requirements under the CAA and those under the NEPA, it is always necessary to complete emissions and air quality modeling on the project/action and compare the resulting values to the relevant NAAQS. Table 7-3 ( CO Intersection Hot-Spot Results ) and Table 7-4 ( PM Intersection Hot-Spot Results ) provide a sample format for reporting CO, PM$_{10}$, and PM$_{2.5 }$ impacts of roadway traffic from a hot-spot analysis, respectively. As shown, CO results are typically presented in ppm and PM$_{10 }$ and PM$_{2.5 }$results in µg/m$^{3 }$and all results include background concentrations. 

