Air Quality Impact Analyses

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 actual ground-level concentrations of pollutants at nearby receptors, such as homes, schools, and businesses. Emission levels can be used for a screening approach to show that more detailed analysis is 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 is the Environmental Protection Agency’s (EPA’s) guideline dispersion model for performing stationary source impact assessments. AERMOD can be used to calculate pollutant concentrations at selected downwind receptor locations based on emission rates, exhaust parameters, terrain characteristics, and meteorological inputs. In addition, if necessary for satisfactory short-term air quality or health risk characterization, modeling can also be used to assess impacts from construction activities, specifically to represent the use of diesel-fueled off-road equipment.

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 heights, stack diameters, exhaust temperatures, exhaust velocities, 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. For these data, Yorke works with the project proponent or Lead Agency staff to identify the data needed for the analysis at the start of the project. We also review the information for completeness and reasonableness. Using publicly available data, nearby worker and residential receptors and other sensitive receptors are identified. Sensitive receptors include schools, hospitals, daycare centers, and other locations where populations sensitive to pollution may be found. This information, along with the meteorological data, emissions, and source characterizations, is used as input to the AERMOD program.

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 uses the AERSCREEN model to conduct incremental air quality dispersion modeling analysis for the proposed project. AERSCREEN is a time-saving, 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 CO, NOx, PM10, and PM2.5 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. 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 very large projects such as a new power plant, additional modeling analyses, including long-range impacts using CalPuff, 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 the projected impacts would be less than significant with mitigation incorporated.

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