Photochemical Smog Essay

Causes of Photochemical Smog

The initial step in photochemical reactions is the absorption of a photon by an atom, molecule, free radical or ion. The result of this absorption is strongly dependent on the energy, in other word the wavelength of the photon. Visible and ultraviolet portion of the light is required to start the photochemical reactions. The absorption can generate dissociation, internal rearrangement, flourescence, or excited species. Photochemical smog can occur in any environment where there are large and continuous emissions of primary air pollutants. However specifics of climate and geography play an important role in the persistence and severity of the photochemical smog. Two factors influencing the formation of photochemical smog:

(a) Topography: Wind moves air masses both horizontally and vertically, dispersing pollutants and reducing

These conditions include: (a) Precipitation can reduce photochemical smog as the pollutants are washed out of the atmosphere with the rainfall.(b) Winds can transfer photochemical smog away, replacing it with fresh air. However, the problem may arise in distant areas that receive the pollution. (c)Temperature inversions can enhance the severity of a photochemical smog episode. If a temperature inversion is developed, the pollutants can be trapped near the Earth's surface. Inversions can last from a few days to several weeks. The atmosphere temperature directly affects the reaction rates and some emission rates.

(4) Topography is another important factor influencing on how severe a smog event can become. Communities situated in valleys are more susceptible to photochemical smog because the hills and mountains surrounding them tend to reduce the air flow, allowing for pollutant concentrations to rise. valleys are sensitive to photochemical smog because relatively strong temperature inversions can frequently develop in these