The nature of photochemical smog

Photochemical air pollution consists of a complex mixture of gaseous pollutants and aerosols, some of which are photochemically produced. Among the gaseous compounds are the oxidizing species ozone, nitrogen dioxide, and peroxyacyl nitrate  

The member of this series most commonly found in the atmosphere is peroxy-acetyl nitrate (PAN) 

The three compounds 03, N02, and PAN are often grouped together and called photochemical oxidant. 

Photochemical smog comprises mixtures of particulate matter and noxious gases, similar to those that occurred in the typical London-type pea-soup smog. The London smog was a mixture of particulates and oxides of sulfur, chiefly sulfur dioxide. But the overall system in the London smog was 

In dealing with the heterogeneous gas-liquid-solid mixture characterized as photochemical smog, it is important to realize from a chemical, as well as a biological, point of view that synergistic effects may occur. 

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There is basically little difference between the mechanism of photochemical smog formation and the naturally occurring photo-oxidation of terpenes. The former, associated with a greater emission intensity of ozone precursors, however, leads to higher concentrations of those intermediates responsible for the undesirable effects of photochemical air pollution. 

One of the key culprits in the formation of photochemical smog is NO (nitric oxide), a colorless but reactive gas. Its nominal concentration in our atmosphere is quite low, and there are natural sources of nitric oxide about which we can do little. It is the anthropogenic nitric oxide about which we are most concerned, as it is a pollutant over which we may have some control. Where does this nitric oxide come from You are probably aware that an... 

VOCs have a very important role in the atmosphere. They are emitted by natural and anthropogenic processes and their presence and distribution can have a marked influence on the environment. In the troposphere, VOCs from anthropogenic sources play a major role in the formation of photochemical smog. However, biogenic VOCs (B VOCs) from the world s forests and oceans dominate over VOCs from anthropogenic sources except in the densely populated areas. The role of VOCs in the workplace environment is another growing area of interest. In recent times, there has been a concerted effort in the application of analytical techniques that can directly monitor VOCs in the environment, that are easy to apply and that are both fast and accurate. 

Ozone A molecule made up of three atoms of oxygen. It occurs naturally in the stratosphere and provides a protective layer shielding the Earth from harmful ultraviolet radiation. In the troposphere, it is a chemical oxidant, a greenhouse gas and a major component of photochemical smog. 

Tropospheric ozone, a primary constituent of photochemical smog, is naturally present at concentrations on the order of 20-40 parts per billion (ppb). However, elevated levels of ground-level ozone are now fotmd virtually worldwide, reaching in some cities concentrations of up to 10 times the natural background. 

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