Smog Air Pollution: What Is It and How Does It Form?

Those of us who live in large cities are familiar with smog air pollution; the familiar brown haze resting over the city on a still day. We need to know what smog contains and where it comes from, as well as the health and environmental impacts of this hallmark of industrialization.

There are two categories of smog to be covered: classic smog and the more recently described photochemical smog.

In its most primitive and basic form, smog air pollution is the result of the burning of fossil fuels. It has several major components:

1) Smoke, which is tiny particles of ash, is released from the smokestacks of coal fired power stations. Coal power plants are now on the whole designed to trap this ash, called fly ash, which can be used to great effect in the concrete industry. These tiny particles contain not only carbon residue, but also silicon dioxide, calcium oxide and traces of heavy metals. If inhaled these can pose significant health risks. The silicon dioxide in the fly ash alone can cause lesions, scarring and inflammation of the lungs.

2) Sulfur Dioxide. Sulfur is present in all fossil fuels and is released as Sulfur Dioxide when the fuels are burned. Sulfur Dioxide reacts with oxygen gas to give Sulfur Trioxide. This then can react with water to to give Sulfuric Acid (H2SO4). Another possibility is that the Sulfur Dioxide reacts immediately with water to give Sulfurous Acid (H2SO3).

Either way, the acid produced is highly reactive and capable of causing significant damage to crops, soil, buildings and more. Several industrialized nations now employ technology that captures approximately a third of Sulfur Dioxide from the emission gases of power stations, greatly reducing the problems of acid rain.

Photochemical Smog

This is a far more noxious mixture of chemicals than classic smog air pollution. Significant inroads have been made to reduce the main contributors to classic smog. Such efforts in relation to photochemical smog are still in their early stages.

Photochemical smog air pollution is a mixture of various chemicals that react with sunlight to produce new chemicals. This is where the name comes from; photo means light and chemical means chemical, or product of a chemical reaction. The chemical reactions involved are complex and while they are important, we need to know the source of these pollutants and their effects.

The chemicals involved need to be addressed separately. The three main ingredients are Nitrous Oxides, volatile organic compounds and Ozone. Carbon Monoxide is a toxic byproduct of fossil fuel combustion but is considered separate to photochemical smog.

Nitrous Oxides These result from the reaction of Oxygen gas (O2) with Nitrogen gas (N2) which typically occurs in the exhausts of fossil fuel powered engines. The formation of oxides of nitrogen is unavoidable since Nitrogen gas makes up 70% of our atmosphere and is present in all naturally occurring air samples. The formulae for these oxides vary from NO, NO2, NO3(2-) to some more exotic forms. While these chemicals are dangerous on their own, it is their ability to produce ground-level ozone that is the major concern.

Volatile Organic Compounds This impressive name simply compounds that evaporate easily. We know that petrol evaporates far more quickly than water and therefore it is said to be more volatile. The most volatile organic compounds (VOCs) are the smallest members of the alkane series, to which Methane belongs. Methane is also a Greenhouse gas produced by cows and hydroelectric energy power plants, and has been established as a Greenhouse gas more than 20 times as effective at trapping the sun's heat than the much better known Carbon Dioxide properties.

These VOCs react with Nitrous Oxides in the presence of sunlight to produce the last of our noxious chemicals, Ozone.

Ozone Ozone is an allotrope (pure form variation) of Oxygen and has the chemical formula of O3. We have all heard about Ozone depletion in the upper atmosphere causing problems, so the production of this gas at ground level may not seem like such a bad thing.

Unfortunately Ozone is a respiratory irritant and also a very effective Greenhouse agent. It is produced by the reaction between the nitrous oxides and VOCs discussed above. Ozone is also produced in significant quantities by the operation of lawn mowers.

From this we can understand that smog air pollution, whether it is the classic or photochemical variety, is a significant threat to our health, which can be extended to the health of the environment in general. The best way to combat this issue is to reduce our energy use and be careful with how we go about our lives; even something as simple as walking to the shop instead of driving will have a positive effect on the buildup of smog and our health and future prospects.