Photochemical Smog

Historically, the term smog referred to a mixture of smoke and fog, hence the name smog. The industrial revolution has been the central cause for the increase in pollutants in the atmosphere over the last three centuries. Before 1950, the

majority of this pollution was created from the burning of coal for energy generation, space heating, cooking, and transportation. Under the right conditions, the smoke and sulfur dioxide produced from the burning of coal can

combine with fog to create industrial smog. In high concentrations, industrial smog can be extremely toxic to humans and other living organisms. London is world famous for its episodes of industrial smog. The most famous London smog event occurred in December, 1952 when five days of calm foggy weather created a toxic atmosphere that claimed about 4000 human lives. Today, the use of other fossil fuels, nuclear power, and hydroelectricity instead of coal has greatly reduced the occurrence of industrial smog. However, the burning of fossil fuels like gasoline can create another atmospheric pollution problem known as photochemical smog.

Photochemical smog is a condition that develops when primary pollutants (oxides of nitrogen and volatile organic compounds created from fossil fuel combustion) interact under the influence of sunlight to produce a mixture of hundreds of different and hazardous chemicals known as secondary pollutants. Development of photochemical smog is typically associated with

Specific climatic conditions and centers of high population density. Cities like Los Angeles, New York, Sydney, and Vancouver frequently suffer episodes of photochemical smog.

One way in which the production of photochemical smog is initiated is through the photochemical reaction of nitrogen dioxide (NO2) to form ozone. There are many sources of photochemical smog, including vehicle engines (the number one cause of photochemical smog), industrial emissions, and area sources (the loss of vapors from small areas such as a local service station, surface coatings and thinners, and natural gas leakage).

Vehicle engines, which are extremely numerous in all parts of the world, do not completely burn the petroleum they use as fuel. This produces nitrogen dioxide which is released through the vehicle exhaust along with a high concentration of

hydrocarbons. The absorption of solar radiation by the nitrogen dioxide results in the formation of ozone (O3). Ozone reacts with many different hydrocarbons to produce a brownish-yellow gaseous cloud which may contain numerous chemical

compounds, the combination of which, we call photochemical smog.

Both types of smog can greatly reduce visibility. Even more importantly, they pose a serious threat to our health. They form as a result of extremely high concentrations of pollutants that are trapped near the surface by a temperature inversion. Many of the components which make up these smog’s are not only respiratory irritants, but are also known carcinogens.

There are many conditions for the development of photochemical smog:

1. A source of nitrogen oxides and volatile organic compounds.

2. The time of day is a very important factor in the amount of photochemical smog present.

• Early morning traffic increases the emissions of both nitrogen oxides (NOx)

and Peroxyacetyl Nitrates (PAN) as people drive to work.

• Later in the morning, traffic dies down and the nitrogen oxides and volatile organic compounds begin to react forming nitrogen dioxide, increasing

its concentration.

• As the sunlight becomes more intense later in the day, nitrogen dioxide is broken down and its by-products form increasing concentrations of ozone.

• At the same time, some of the nitrogen dioxide can react with the volatile organic compounds (VOCs) to produce toxic chemicals.

• As the sun goes down, the production of ozone is halted. The ozone that

remains in the atmosphere is then consumed by several different reactions.

3. Several meteorological factors can influence the information of photochemical smog. These conditions include :

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

• Winds can blow photochemical smog away replacing it with fresh air.