Air pollution causes atmospheric effects including reductions in visibility, changes in urban climatological characteristics, increased frequency of rainfall and attendant meteorological phenomena, changes in the chemical characteristics of precipitation, reductions in stratospheric ozone levels, and global warming (Godish 1991). The latter three effects can be considered from a macro (large-scale) perspective and are addressed in Section 5.5.
Particulate matter can reduce visibility and increase atmospheric turbidity. Visibility is defined as the greatest distance in any direction at which a person can see and identify with the unaided eye (1) a prominent dark object against the sky at the horizon in the daytime, and (2) a known, preferably unfocused, moderately intense light source at night. In general, visibility decreases as the concentration of particulate matter in the atmosphere increases. Particle size is important in terms of visibility reduction, with sizes in the micron and submicron range of greatest importance. Turbidity in ambient air describes the phenomena of back scattering of direct sunlight by particles in the air, thus reducing the amount of direct sunlight
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