Sangeetha Pulapaka

Smog is a synchrony of two words - smoke and fog. Smog can be of two types - industrial or winter smog (e.g. London smog) and photochemical or summer smog (e.g. Los Angeles smog).

Photochemical smog occurs in the troposphere, the lower portion of our

atmosphere. Ground-level ozone, the primary component of photochemical smog, is the most prevalent pollutant that has been known to cause a serious air pollution problem in many developed countries over the past few decades. Photochemical smog is a widespread  phenomenon in many population centers of the

World. The components of photochemical smog that are the most damaging to plants and detrimental to human health are the photochemical oxidants. These oxidants include ozone (O_{3}), peroxyacetyl nitrate (PAN), peroxybenzoyl nitrate (PBN), hydrogen peroxide (H_{2}O_{2}), formic acid (HCOOH), and other trace substances. They are collectively termed photochemical oxidants with ozone and

PAN, and are present in the highest concentrations. In addition, the aerosols formed during the chemical reactions cause a marked reduction in visibility with a brownish cast in the atmosphere [13]. PAN in photochemical smog can irritate the eyes, causing them to water and sting.

Certain conditions are required for the formation of photochemical smog. These

conditions include:  

(1) Emission rates of the sources of nitrogen oxides (NOx) and volatile organic

compounds (VOC). High concentrations of these two substances are associated with industrialization and transportation, which create these pollutants through fossil fuel combustion.  

(2) The time of day is a very important factor influencing on the amount of

photochemical smog.

(3) Meteorological factors the formation of photochemical smog. These conditions include:  

• Precipitation can reduce photochemical smog as the pollutants are washed out of the atmosphere with the rainfall.  

• Winds can transfer photochemical smog away, replacing it with fresh air. However, the problem may arise in distant areas that receive

the pollution.  

• 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 nfluencing 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. In addition, valleys are sensitive to photochemical smog because relatively strong temperature inversions can frequently develop in these areas