Air Quality Impact Analyses (AQIAs) can range from calculation of a project’s construction and operational emissions, to the use of air dispersion modeling to predict ground-level concentrations of pollutants at nearby receptors, such as homes, schools, and businesses.

Emission levels can be used in a screening approach to show that more detailed analyses are not needed. However, if dispersion modeling is needed for the AQIA, there are both screening and refined approaches, depending on the level of analysis required.

Dispersion modeling may be needed to determine compliance with ambient air quality standards and/or to provide inputs for health risk assessment (HRA) calculations. This modeling can be done with either a screening model, such as AERSCREEN, or a refined model, like AERMOD. AERMOD and AERSCREEN are the Environmental Protection Agency’s (EPA’s) guideline dispersion models for performing stationary source impact assessments and ensuring regulatory alignment.

AERMOD can be used to calculate pollutant concentrations at selected downwind receptor locations, based on emission rates, exhaust parameters, terrain characteristics, and meteorological inputs. Modeling can also be used to assess air quality or health risk impacts from construction activities, with emissions from diesel-fueled off-road equipment as the primary concern.

To perform air dispersion modeling using AERMOD, individual source parameters for the permitted sources need to be incorporated. For point sources, the required data include stack height, stack diameter, exhaust temperature, exhaust velocity, building dimensions, and other parameters affecting downwind dispersion. Non-point sources are modeled as either volume or area sources as appropriate.

A receptor grid must also be established. At the start of the project, Yorke works with the project proponent or Lead Agency staff to identify the data needed for the analysis. We also review the information for completeness and reasonableness. Using publicly available data, nearby worker, residential, and sensitive receptors are identified. Sensitive receptors include schools, hospitals, daycare centers, and other locations where populations sensitive to pollution may be found.

If the project has few sources and minimal emissions, use of a screening approach may be sufficient due to inherent conservative assumptions. In those cases, Yorke employs AERSCREEN to conduct air quality dispersion modeling analysis for the proposed project. AERSCREEN is a timesaving, single-source version of AERMOD which does not require site-specific terrain and meteorological data. If the screening approach yields less than significant impacts, then the analysis is complete, and refined modeling with AERMOD is not necessary.

In addition to demonstrating that the project will not cause or contribute to exceedances of the California or National Ambient Air Quality Standards (CAAQS, NAAQS), other project-specific impacts may need to be addressed. For instance, in the South Coast Air Quality Management District (SCAQMD), localized significance thresholds (LSTs) may need to be addressed for carbon monoxide (CO), nitrogen oxides (NO_x), particulate matter with a diameter of 10 micrometers or less (PM₁₀), and fine particulate matter with a diameter of 2.5 micrometers or less (PM_2.5) emissions.

If the daily active area for the project is 5 acres or less, the assessment may use a simple emissions rate “lookup table” published by the SCAQMD. In addition, if the proposed project is determined to exceed SCAQMD LSTs and/or mass emissions-based significance thresholds for construction or operation, additional mitigation measures for the construction or operational phases of the project can be included in the analysis.

For large projects – such as a new power plant – additional modeling analyses, including long-range impacts using CalPuff or other specialized models, may be needed. Plume impact, regional visibility, and/or acid deposition modeling may also be required.

All modeling results are compared to the Lead Agency-defined CEQA significance thresholds to determine if the proposed project has the potential to cause a significant adverse impact. If so, then additional mitigation can be identified and included in the modeling analysis, and the mitigated emissions can be analyzed to determine if impacts would be less than significant with mitigation incorporated.