affect the plume's spread in the y and z directions and are determined by the characteristics of the area surrounding the site. In heavily developed areas such as cities, the urban heat island and structures affect the surrounding atmosphere, increasing turbulence and air temperature. In the country, the foliage and undeveloped land reduce longwave radiation and generate less turbulence. The amount of turbulence generated by each of these locations affects how the plume is dispersed in the atmosphere.

Irwin (1978) recommends two methods for categorizing the surrounding area as urban or rural. One technique relies on a methodology developed by Auer (1978), which characterizes the land use within a 3-km radial area. The second technique is based on a population density threshold (750 people/sq km) within a 3-km area. The Auer method is typically the preferred approach; it would identify a large industrial plant with storage yards (i.e., a steel mill) as an industrial (urban) site instead of an unpopulated rural site. Table 5.8.6 lists the Auer land use categories.

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