Yolo-Solano AQMD

The Yolo-Solano Air Quality Management District covers Yolo County and the northeastern part of Solano County, reaching from Vacaville in the west to Rio Vista in the south. Its CEQA thresholds are in the Handbook for Assessing and Mitigating Air Quality Impacts, adopted July 11, 2007, an update to the district's 1996 handbook. The handbook calls itself an advisory document for lead agencies, consultants and applicants, and says its thresholds are the district's recommendations, adopted by its Governing Board. The district's CEQA and Land Use page, last modified February 17, 2023, still links the 2007 handbook. It says better models exist today and that the district supports using the existing thresholds, with a clarification for greenhouse gases.

Thresholds for construction and operation

Table 1, Thresholds of Significance for Criteria Pollutants of Concern, applies the same values to construction and to operations:

Ozone precursors are compared as annual tons and PM10 as daily pounds. Appendix B gives the basis. The 10 ton figure is the no net increase level of Rule 3.20, Ozone Transport Mitigation, which caps what a stationary source may add; the handbook applies the same number to projects and treats VOC and ROG as equivalent. The 80 lbs/day figure is the level at which the district's New Source Review program requires Best Available Control Technology for PM10. Appendix B.4 reads the CO standard as 9 ppm over 8 hours and 20 ppm over 1 hour.

Table 1 has no row for PM2.5 or SOx. Appendix B.3 says no PM2.5 threshold was proposed because EPA had not yet proposed a PM2.5 designation for the district.

Screening for operations

Table 2, Project Size That May Exceed District Thresholds for ROG, NOx, and PM10, lists sizes by land use for analysis years 2007 and 2010, built with URBEMIS version 8.7. It leaves out construction and industrial uses, and the handbook says its values are not absolute thresholds. A project more than 10 percent under the listed size may be assumed to need no quantification of ozone precursor emissions; toxics, asbestos and odors still need analysis.

Both analysis years have passed, and the district's page now recommends the online version of CalEEMod (see what CalEEMod is and how to read its report).

Carbon monoxide

A project may cause a CO violation if its traffic study shows peak-hour level of service on a nearby street or intersection falling to an unacceptable level, typically E or F, or substantially worsening an existing LOS F, which includes delay growing by 10 seconds or more. If either traffic criterion remains after traffic mitigation, the applicant may use the analysis in section 4.7.2 of the Caltrans Transportation Project-Level Carbon Monoxide Protocol, followed by dispersion modeling such as CALINE-4 if that analysis shows potential significance. The handbook says not to use the protocol's quantitative screening, built on EMFAC 7F, and sets district background CO at 1 ppm for one hour and 0 ppm for eight hours.

Toxic air contaminants

Section 3.2.1, based on the district's Risk Management Policy, sets two thresholds for exposure to toxics from stationary sources: a cancer probability for the Maximally Exposed Individual of 10 in one million or more, and a Hazard Index of 1 or greater. The handbook proposed no threshold for mobile sources.

For siting, Figure 2 reprints Table 1-1 of the Air Resources Board's Air Quality and Land Use Handbook (April 2005), with distances such as 500 feet from a freeway and 1,000 feet from a major service and maintenance rail yard. New sensitive receptors inside those distances are considered at elevated risk, and the lead agency should estimate the health risk; Section 5.5.5 calls a health risk assessment with a dispersion model advisable.

Odor

The odor threshold rests on the general nuisance rule, Health and Safety Code section 41700 and District Rule 2.5. A project that would put receptors and a known odor source, such as a wastewater plant, landfill or composting facility, within one mile of each other gets a full analysis. Significance is judged from the district's complaint records: a project near an existing source is potentially significant when it would be at least as close as another site that has already had significant odor problems from it. The district does not accept sealed buildings, filtered air or disclosure statements in place of buffer zones or technical controls.

Construction dust

Even projects under the PM10 threshold should use dust best management practices. Table 5, Construction Dust Mitigation Measures, rates each: watering active sites at least twice daily, 50 percent; at least 2 feet of freeboard on haul trucks, or covering them, 90 percent; covering inactive storage piles, up to 90 percent; sweeping streets when soil is carried out, 14 percent; and 6 inches of gravel on accesses for 100 feet from the paved road, 42 to 52 percent. Two measures on the same source do not add.

Cumulative impacts and plan consistency

A project that is individually significant is also cumulatively significant (Section 3.2.2), and Appendix B.5 says ozone precursor or PM10 emissions above the thresholds are cumulatively significant unless offset. A general plan can claim a less than significant impact only if it is consistent with the district's Air Quality Attainment Plan and the State Implementation Plan. Section 5.5.6 also calls for cumulative toxics analysis that identifies nearby sources and, where initial screening indicates that dispersion modeling is appropriate, includes all toxics sources in the vicinity that may affect receptors. Cumulative CO modeling includes traffic from reasonably foreseeable projects.

Greenhouse gases

The 2007 handbook sets no greenhouse gas threshold; Section 5.5.10 recommends at least a qualitative discussion for sizable projects. The district's 2023 page says it still has no GHG significance threshold and recommends evaluating climate impacts for every CEQA project. The district treats GHG impacts as cumulative: the question is whether a project's added direct and indirect emissions are cumulatively considerable. Where the lead agency's jurisdiction has a Climate Action Plan, or GHG goals and policies in its general plan, the analysis rests on those. Where it has neither, the district recommends weighing the project's total emissions against the AB 32 and Scoping Plan goals and later state goals, or against thresholds other jurisdictions have set.

What this means for a report

A Yolo-Solano report compares ROG and NOx in tons per year and PM10 in pounds per day against Table 1, for construction and for operations, and tests CO against the state standard. A project over a threshold has a significant impact and, by Section 3.2.2, a significant cumulative one. Section 6 lists the mitigation the district considers feasible; the lead agency decides feasibility. There is no PM2.5 or GHG number to compare against. The thresholds are the 2007 handbook's; the district's 2023 page carries its current word on models and greenhouse gases. For how district thresholds work in general, see what a CEQA air quality report is measured against.

Where the thresholds are published

Yolo-Solano Air Quality Management District, Handbook for Assessing and Mitigating Air Quality Impacts, adopted July 11, 2007: Section 3.0 and Table 1, Thresholds of Significance for Criteria Pollutants of Concern; Sections 3.2.1, 3.2.2, 3.2.3 and 3.2.5; Section 4.1 and Table 2, Project Size That May Exceed District Thresholds for ROG, NOx, and PM10; Figure 2, ARB Recommended Minimum Separations; Sections 5.5.4, 5.5.5, 5.5.7 and 5.5.10; Sections 6.0 and 6.1 and Table 5, Construction Dust Mitigation Measures; Section 6.7; Appendix B, Justification for Thresholds of Significance. The district's CEQA and Land Use page, last modified February 17, 2023.

• CEQA and Land Use
• Handbook for Assessing and Mitigating Air Quality Impacts