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4.1 – Air Quality Draft Environmental Impact Report February 2021 14800 W. Schulte Road Logistics Center 4.1-2 The vertical dispersion of air pollutants in San Joaquin Valley can be limited by the presence of persistent temperature inversions. 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. A temperature inversion can act like a lid, restricting vertical mixing of air above and below an inversion because of differences in air density and thereby trapping air pollutants below the inversion. The subtropical high-pressure cell is strongest during spring, summer, and fall and produces subsiding air, which can result in temperature inversions. Most of the surrounding mountains are above the normal height of summer inversions (1,500–3,000 feet). Wintertime high-pressure events can often last many weeks, with surface temperatures often lowering into the 30s°F. During these events, fog can be present and inversions are extremely strong. These wintertime inversions can inhibit vertical mixing of pollutants to a few hundred feet (County of San Joaquin 2014). Wind Patterns Wind speed and direction play an important role in dispersion and transport of air pollutants. Winds in San Joaquin Valley most frequently blow from the northwesterly direction, especially in the summer. The region’s topographic features restrict air movement and channel the air mass toward the southeastern end of San Joaquin Valley. Marine air can flow into the SJVAB from the Sacramento–San Joaquin River Delta and over Altamont Pass and Pacheco Pass, where it can flow through San Joaquin Valley, over the Tehachapi Pass, into the Mojave Desert Air Basin. The Coastal Range and the Sierra Nevada are barriers to air movement to the west and east, respectively. A secondary but significant summer wind pattern is from the southeasterly direction and can be associated with nighttime drainage winds, prefrontal conditions, and summer monsoons. During winter, winds can be very weak, which minimizes the transport of pollutants and results in stagnation events. Two significant diurnal wind cycles that occur frequently in San Joaquin Valley are the sea breeze and mountain - valley upslope and drainage flows. The sea breeze can accentua te the northwest wind flow, especially on summer afternoons. Nighttime drainage flows can accentuate the southeast movement of air down San Joaquin Valley. In the mountains during periods of weak synoptic scale winds, winds tend to be upslope during the day and downslope at night. Nighttime and drainage flows are pronounced during the winter when flow from the easterly direction is enhanced by nighttime cooling in the Sierra Nevada. Eddies can form in the valley wind flow and can re-circulate a polluted air mass for an extended period (County of San Joaquin 2014). Temperature, Sunlight, and Ozone Production Solar radiation and temperature are particularly important in the chemistry of ozone (O3) formation. The SJVAB averages over 260 sunny days per year. Photochemical air pollution (primarily O3) results from atmospheric reactive organic gases (ROGs) and nitrogen dioxide (NO2) under the influence of sunlight. O3 concentrations are very dependent on the amount of solar radiation, especially during late spring, summer, and early fall. O3 levels typically peak in the afternoon. After the sun goes down, the chemical reaction between oxides of nitrogen (NOx) and O3 begins to dominate. This reaction tends to reduce O3 concentrations in the metropolitan areas through the early morning hours. At sunrise, NOx tends to peak, partly due to low levels of O3 at this time, and also due to the morning commuter vehicle emissions of NOx. Reaction rates generally increase with temperature, which results in greater O3 production at higher temperatures. However, extremely hot temperatures can “lift” or “break” the inversion layer. Typically, if the inversion layer remains intact, O3 levels peak in the late afternoon. If the inversion layer breaks and the resultant afternoon winds occur, O3 levels peak in the early afternoon and decrease in the late afternoon as the contaminants are dispersed or transported out of the SJVAB. O3 levels are low during winter periods when there is much less sunlight to drive the photochemical reaction (County of San Joaquin 2014).