To attempt to understand this problem, certain concepts and definitions need to be laid out at the beginning to better understand the complexity of this subject.

Air pollution, optics, radiative transfer methodology, information on atmospheric construction, along with models and statistics are all utilized in this chapter to show the complexity of the interaction of ultraviolet UV radiation with air pollution. Because of the nature of UV radiation and its interaction with the atmosphere and its constituents, certain parameters and their known effects on the UV will be explained first so that the reader gets a perspective of the variations that the role of air pollution can play in the earth's UV field. UV radiation, while making up only 1.5% of the total radiation that the earth receives, is the most energetic. Its shorter wavelengths make it highly susceptible for interception by almost anything in its path. Since the earth's climate is constantly changing, the UV that reaches the surface is also changing.

11.1.1 Factors Affecting UV Flux at the Earth's Surface

• Solar zenith angle

• Stratospheric ozone

• Atmospheric density

• Air pollution (gases and aerosols) Solar Zenith Angle

If one goes from New York to the Bahamas for a winter vacation, there is a known increase in the UV just from the change in latitude, and thus, the solar zenith angle (SZA) (Fig. 11.1). The same occurs on a summer's day where the solar angle (dependant on latitude) can change dramatically. Ultraviolet radiation around solar noon has its shortest path length to the surface and thus, its greatest intensities. To further increase the UV exposure, one only needs to go to a higher altitude in a lower latitude situation. Here, there is less atmosphere to scatter and absorb the UV and generally, as in the case of Mauna Loa, HI, a very clean environment in which to calibrate the very monitors that measure the surface UV in the more polluted regions of the earth. Ultraviolet-B (UV-B) radiation (280 nm - 320 nm)

Radiometer-measures irradiance Figure 11.1 Solar zenith angle

-measures irradiance

Radiometer-measures irradiance Figure 11.1 Solar zenith angle

-measures irradiance comprises roughly 1.5% of the total extraterrestrial solar irradiance and 0.5% of incident irradiance at the earth's surface. Ultraviolet-A (UV-A) radiation encompasses the wavelength region from 320 nm - 400 nm. Ultraviolet radiation has both a daily component that resembles the bell shape distribution curve and a seasonal component that can be expressed with a combination of sine and cosine functions dependant on the Julian date. The daily component's shape is dependant on the wavelength. The shorter wavelengths of 300 nm - 310 nm are generally not observed until the sun has risen past 9:00 a.m. local time in the summer. The longer wavelengths 360 nm - 400 nm exist in the daylight sky from sunrise until sunset.

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