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4. ENVIRONMENTAL ANALYSIS r 4.6 AIR QUALrrY r Wind speed and direction play an important role in dispersion and transport of air pollutants. During summer periods airflow in the region is primarily influenced by marine air that enters through the Carquinez Straits. Winds usually originate out of the north end of the San Joaquin Valley and flow in a south-southeasterly direction through the Valley,through the Tehachapi Pass 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,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., PM 10 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. r. PM10 dust emissions in the SJVAB do not follow conventional assumption that wind erosion is the dominant factor. Average wind velocity is the lowest in the nation for an area so large. r. Winds normally exceed erosive velocity levels at a site for only 30 to 50 days per year and sometimes less. Over 75 percent of the winds with enough velocity to cause erosion occur in the spring and summer season when PM10 levels in the ambient air is among the lowest. This suggests that these winds are effective in dispersing PM10 concentrations and/or transporting PM10 out of the SJVAB. Although PM10 originating from or going to other air basins, referred to as pollutant transport, has not been definitively quantified. PM10 monitoring data in the SJVAB are most severe during the fall and winter periods when wind speed and direction are not conducive to interregional transport. ` Other factors such as soil type and soil moisture content prevent these winds from entraining a large amount of PM10 during this period. Erosive winds are defined as having a velocity of 13 miles per hour at a height of one foot above the ground or 18 miles per hour at a height of ` approximately 33 feet above the ground. Erosive wind speeds can be much lower for some soil conditions. 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 RMC Pacific Vernalis Quany Mining and Reclamation Project 4.6-3 FSA 1203015 Administrative Draft EIR March 2004