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4.1 – Air Quality Draft Environmental Impact Report February 2021 14800 W. Schulte Road Logistics Center 4.1-3 Precipitation, Humidity, and Fog Precipitation and fog can result in the reduction or increase in some pollutant concentrations. For instance, O3 needs sunlight for its formation, and clouds and fog can block the required solar radiation. In addition, wet fogs can cleanse the air during winter as moisture collects on particles and deposits them on the ground. Fog with less moisture content, however, can contribute to the formation of secondary ammonium nitrate particulate matter. The winds and unstable air conditions experienced during the passage of winter storms result in periods of low pollutant concentrations. Between winter storms, high pressure and light winds allow cold, moist air to pool on the San Joaquin Valley floor, resulting in strong low-level temperature inversions and very stable air conditions, which can lead to Tule fog. Wintertime conditions favorable to fog formation are also conditions favorable to high concentrations of particulate matter. Urban Heat Island Effect The “urban heat island” refers to the effect of urbanized areas on surface and air temperature compared to their rural surroundings. Buildings, roads, and other “hardscape” create an island of higher temperatures within the regional landscape. As described by the U.S. Environmental Protection Agency (EPA), “[u]rban heat islands are caused by development and the changes in radiative and thermal properties of urban infrastructure as well as the impacts buildings can have on the local microclimate—for example tall buildings can slow the rate at which cities cool off at night. Heat islands are influenced by a city’s geographic location and by local weather patterns, and their intensity changes on a daily and seasonal basis” (EPA 2008). The term is generally used to refer to community -wide effects, particularly for large metropolitan cities. The potential adverse effects of the urban heat island effect include increased energy consumption, elevated emissions of air pollutants and greenhouse gases (GHGs), compromised human health and comfort, and impaired water quality. Increased temperatures due to the urban heat island effect may also lead to increased energy consumption, which has implications for air quality and GHG emissions. In addition to energy-related increases in air emissions, elevated air temperatures increase the rate of ground -level O3 formation. Communities have adopted various strategies to deal with these environmental impacts, such as increasing vegetation and using more energy-efficient building materials. These strategies are often part of more general energy savings or “sustainability” practices and are not identified as “urban heat island effect” mitigation, but nevertheless they provide the benefits of reducing surface and atmospheric heat islands. Pollutants and Effects Criteria Air Pollutants Criteria air pollutants are defined as pollutants for which the federal and state governments have esta blished ambient air quality standards, or criteria, for outdoor concentrations to protect public health. The federal and state standards have been set, with an adequate margin of safety, at levels above which concentrations could be harmful to human health and welfare. These standards are designed to protect the most sensitive persons from illness or discomfort. Pollutants of concern include O3, NO2, carbon monoxide (CO), sulfur dioxide (SO2), particulate matter with an aerodynamic diameter less than or equal to 10 microns (PM10), particulate matter with an aerodynamic diameter less than or equal to 2.5 microns (PM2.5), and lead. ROGs (also referred to as volatile organic compounds [VOCs])1 and NOx are also important because they are precursors to O3. These pollutants, as well as toxic air 1 The SJVAPCD threshold is set for ROG. However, ROG and VOC are generally considered equivalent for CEQA analyses; as such, ROG and VOC are used interchangeably in this analysis.