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Air Quality Technical Reports

The California Environmental Quality Act (CEQA) brings together many important aspects of project planning, and in some cases can affect project design. For industrial projects in particular, preparing an Air Quality Technical Report that is appended to the CEQA Mitigated Negative Declaration (MND) or Environmental Impact Report (EIR) is a common practice. Yorke Engineering, LLC is familiar with preparing air quality documents and technical analysis to support the CEQA process.

Yorke staff routinely perform analyses to assess a project’s potential impacts to air quality, public health, odors, and/or noise in support of CEQA documents. These analyses typically consist of some or all of the following components: background air quality conditions and regulatory discussions, emissions estimates [including criteria pollutants, toxic air contaminants (TACs), and greenhouse gases (GHGs)], air dispersion modeling, health risk assessments (HRA), GHG emission impacts on the climate, and/or odor analyses. The technical report(s) provide documentation of the methodologies used and results obtained in comparison to significance criteria for each type of impact analyzed.

> Emissions Calculations

Air Pollutant, TAC, and GHG emissions quantification is a central component of any air-related inventory analysis. Emissions from project construction and operation generally reflect the information provided in the project description. For large projects, emissions from proposed alternatives may also need to be estimated. The estimated quantities of criteria pollutants and GHGs emitted by a project, in combination with other factors, demonstrate whether a project could have a significant impact on air quality, including health risks or odors.
In the past 10 years, regulations have increasingly required projects to inventory future GHG emissions, primarily due to fossil fuel combustion. As a result, methods have been established to provide a defensible quantification approach to determine climate change impacts related to GHG emissions. Yorke has experience with current emissions estimation techniques and models, including EPA emission factors, CalEEMod, EMFAC, OFFROAD, and other custom emissions models.

> Air Quality Impact Analyses

In many cases, the potential for a project to have significant impacts on air quality can be determined based on the mass emissions estimates alone, as mass-based significance criteria have been defined for most areas in California. However, for more complex projects, projects which could have a significant localized impact, and/or projects where the mass-based significant criteria may be too conservative, air dispersion modeling is the primary analysis method used. Dispersion modeling can predict ground-level concentrations based on a source emission rate to determine compliance with ambient air quality standards. The results can also be used as inputs for HRA calculations, which determine health risks based on predicted concentrations of individual TACs combined with health risk factors. Yorke has used various dispersion modeling functions – from screening to refined – to estimate downwind air pollutant concentrations and applied these results to air quality impact assessment. The level of HRA and the methods used to evaluate air toxic emissions can vary considerably depending on the toxicity of the air pollutants being emitted, the pollutant mass emission rate, and the distance to the nearest receptors.

> Significance Criteria and Determinations

The role of CEQA is to evaluate potential environmental impacts and inform the permitting agencies and the public of these impacts during the progressive stages of project planning and approval. Agencies have adopted “significance thresholds” to be used by project proponents to assist the permitting and approval agencies with determining the scale of impacts related to the proposed action. Air quality and climate-related CEQA thresholds are unique compared to other environmental impacts because they are typically numeric, which provides a quantitative boundary for evaluating project significance. Air quality and GHG emissions thresholds may be based on mass emission rates, health risk impact levels, air pollutant concentrations, or performance standards. Once the significance of the impact is mathematically determined, the Air Quality Technical Report is prepared. If the impacts are determined to be significant, then additional work may be needed to reduce the impacts by applying further mitigation measures until the project can be shown to have a less than significant impact on air quality or climate change.


Yorke specializes in air quality regulations and project planning in California, benefitting project developers with high-quality air technical analyses leading to defensible CEQA documents. If your project requires local California Air District permitting, it is also more cost-efficient and saves time for one team to complete both the air quality section of the CEQA documents and the air permit applications, since there is significant overlap in the air quality analysis required. We also support our customers with:

  • Presentations to CEQA Lead Agencies;
  • Consistent CEQA and Air Permitting Analyses;
  • California Air District Relationships;
  • Teamwork and Solid Communication;
  • Streamlined Emissions Analysis; and
  • Improved Design Mitigation and Control Features.

Main Office:

31726 Rancho Viejo Rd. Suite 218
San Juan Capistrano, CA 92675

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