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Grand Avenue Widening Project Environmental Impact Report Section 3.0 3.4 AIR QUALITY Based on the findings of the IS, the proposed Grand Avenue widening may or would result in potentially significant adverse air quality impacts, specifically related to the potential violation of, or contribution to the violation of, a defined ambient air quality standard and the potential exposure of sensitive receptors to pollutants. The analysis in the following sections focuses on the existing conditions in the study area, the analysis methodology, thresholds of significance, the potential air quality impacts of the Grand Avenue widening alternatives related to the ambient air quality standards (AAQS) and sensitive receptors, and mitigation as needed. The analysis of the potential air quality impacts of the proposed Grand Avenue widening was conducted by Mestre Greve Associates as documented in the Air Quality Assessment for the Grand Avenue Widening City of Santa An (Mestre Greve Associates, July 20, 2001). The findings of this technical analysis are summarized in the following sections. The complete air quality technical report is included in Appendix E of this EIR. 3.4.1 EXISTING SETTING RELATED TO AIR QUALITY Climate The climate in and around the Grand Avenue project area, as with all of southern California, is controlled largely by the strength and position of the subtropical high pressure cell over the Pacific Ocean. It maintains moderate temperatures and comfortable humidities, and limits precipitation to a few storms during the winter, wet season. Temperatures are normally mild, except during the summer months which commonly experience substantially higher temperatures. Temperatures of 100 degrees Fahrenheit (F) have been recorded during summer months in recent years. The annual average temperature in the South Coast Air Basin (Basin) is approximately 75 degrees F. Winds in the project area are almost always driven by the dominant land/sea breeze circulation system. Regional wind patterns are dominated by daytime on -shore sea breezes. At night, the wind generally slows and reverses direction, traveling towards the sea. During the transition period from one wind pattern to the other, the dominant wind direction rotates into the south and causes a minor wind direction maximum from the south. The frequency of calm winds, defined as winds of less than two miles per hour, is less than 10 percent. Therefore, there is little stagnation in the vicinity of the project segment of Grand Avenue, especially during busy daytime traffic hours. Southern California frequently has temperature inversions which inhibit the dispersion of pollutants. Ground based or radiation inversions are most severe during clear, cold, early winter mornings. Under conditions of a ground based inversion, very little mixing or turbulence occurs, and high concentrations of primary pollutants may occur near major roads. Elevated inversions act as a lid or upper boundary and restrict vertical mixing. Dispersion is not restricted below an elevated inversion. Mixing heights for elevated inversions are lower and more persistent in the summer. Low summer inversions put a lid over the Basin and are responsible for the high levels of ozone (03) observed during summer months in the Basin. F.-IPROJ-ENMGrand eirWew Text - GrandlSection 3 SplitlSection 3.4.doc Page 3.4-I