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4.6 Air Quality v and into the neighboring Southeast Desert Air Basin. Summer transport of pollutants into the region from upwind sources sometimes contributes to ozone formation. Conversely,emissions may impact downwind communities. Winter air quality is influenced by regional storms carrying moisture from the Pacific Ocean, with periods of calm winds between storms. During winter months, winds occasionally originate from the south end of the Valley and flow in a north- northwesterly direction. Also,during winter months,the Valley experiences light, variable winds, 6. less than 10 mph. Low wind speeds, combined with low inversion heights, create a winter climate conducive to high concentrations of certain air pollutants(e.g., PMto and CO). Temperature inversions are formed when the vertical dispersion of air pollutants is inhibited. As a rule of ` thumb, air temperatures usually decrease with an increase in altitude.A reversal of this atmospheric state, where the air temperature increases with height, is termed an inversion. Air above and below an inversion does not mix because of differences in air density,thereby restricting air pollutant dispersal. Regulatory Context Regulation of air pollution is achieved through both federal and state ambient air quality standards and emission limits for individual sources of air pollutants. An "ambient air quality standard" represents the level of air pollutant in the outdoor(ambient) air necessary to protect public health. These ambient standards do not apply to indoor environments. As required by the federal Clean Air Act, the U.S.EPA has identified criteria pollutants and established National Ambient Air Quality Standards(NAAQS or national standards)to protect public health and welfare. NAAQS have been established for ozone,CO, nitrogen dioxide(NO2), sulfur dioxide(SO2), PMto,particulate matter less than 2.5 microns (PM2.5),and lead(Pb). These pollutants are called"criteria" air pollutants because standards have been established for each of them to meet specific public health and welfare criteria. The NAAQS are defined as the maximum acceptable concentration that may be reached, but not exceeded more than once per year. California has adopted more stringent ambient air quality standards for most of the criteria air pollutants(California Ambient Air Quality Standards or CAAQS). The pollutants of greatest concern in the project area are CO, ozone, and Mo. State and federal ambient air quality standards are summarized in Table 4.6-1, which also provides a brief discussion of the related health effects and principal sources for each pollutant. Ozone Ozone is a respiratory irritant and an oxidant that increases susceptibility to respiratory infections L and that can cause substantial damage to vegetation and other materials. Ozone is not emitted directly into the atmosphere,but is a secondary air pollutant produced in the atmosphere through a complex series of photochemical reactions involving reactive organic gases (ROG) and NO.. ROG and NO. are known as precursor compounds for ozone. Significant ozone production generally requires ozone precursors to be present in a stable atmosphere with strong sunlight for approximately three hours. Ozone is a regional air pollutant because it is not emitted directly by sources,but is formed downwind of sources of ROG and NOx under the influence of wind and 6. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.6-3 ESA/203015 Draft Environmental Impact Report May 2006