Photochemical smog is a type of air pollution characterised by the presence of high concentrations of ozone (O₃) and secondary pollutants such as nitrogen dioxide (NO₂), peroxyacetyl nitrate (PAN), and volatile organic compounds (VOCs) in the lower atmosphere. It forms through a complex series of chemical reactions involving pollutants emitted from various sources and sunlight.

Primary pollutants: The two major primary pollutants, nitrogen oxides and VOCs, combine to change in sunlight in a series of chemical reactions, outlined below, to create what are known as secondary pollutants.

Secondary pollutants: The secondary pollutant that causes the most concern is the ozone that forms at ground level. While ozone is produced naturally in the upper atmosphere, it is a dangerous substance when found at ground level. Many other hazardous substances are also formed, such as peroxyacetyl nitrate (PAN). 

What are the major sources of photochemical smog?

While nitrogen oxides and VOCs are produced biogenically (in nature), there are also major anthropogenic (man-made) emissions of both. Natural emissions tend to be spread over large areas, reducing their effects, but man-made emissions tend to be concentrated close to their source, such as a city.

Biogenic sources: In nature, bushfires, lightning and the microbial processes that occur in soil generate nitrogen oxides. VOCs are produced from the evaporation of naturally-occurring compounds, such as terpenes, which are the hydrocarbons in oils that make them burn. Eucalypts have also been found to release significant amounts of these compounds.

Anthropogenic sources: Nitrogen oxides are produced mainly from the combustion of fossil fuels, particularly in power stations and motor vehicles. VOCs are formed from the incomplete combustion of fossil fuels, from the evaporation of solvents and fuels, and from burning plant matter—such as backyard burning and wood-burning stoves. In Adelaide in 2000, an estimated 66% of nitrogen oxides (NO and NO₂) came from motor vehicles, and a further 20% from fuel combustion. Motor vehicles contributed 44% of VOC emissions, and area sources including petrol and solvent evaporation contributed 33%. 

How is smog formed?

Below is a simplified explanation of the chemistry of smog formation.

Nitrogen dioxide (NO2) can be broken down by sunlight to form nitric oxide (NO) and an oxygen radical (O): 

Oxygen radicals can then react with atmospheric oxygen (O2) to form ozone (O3): 

Ozone is consumed by nitric oxide to produce nitrogen dioxide and oxygen: 

Harmful products, such as PAN, are produced by reactions of nitrogen dioxide with various hydrocarbons (R), which are compounds made from carbon, hydrogen and other substances: 

The main source of these hydrocarbons is the VOCs. Similarly, oxygenated organic and inorganic compounds (ROx) react with nitric oxide to produce more nitrogen oxides: 

The significance of the presence of the VOCs in these last two reactions is paramount. Ozone is normally consumed by nitric oxide, as in reaction 3. However, when VOCs are present, nitric oxide and nitrogen dioxide are consumed as in reactions 4 and 5, allowing the build up of ground level ozone. 

How location and weather can have an effect?

Topography:The topography of the area surrounding a city can vastly influence the formation of photochemical smog. Because of the restriction of air movement, a city in a valley can experience problems that a city on an open plain may not.

Meteorology: Normally the layer of air closest to the earth’s surface is warmer than the air higher in the atmosphere because the heat of the sun is re-radiated (warmed by the earth’s surface). The higher level cool air sinks and is then warmed and displaced upwards in a convection cycle (Figure 1). This condition is called ‘unstable’ and helps to carry pollutants upwards, where they are dispersed and diluted. This cycle is usually assisted by higher wind speeds.  However, when the opposite occurs—a temperature inversion—cities can experience prolonged periods of photochemical smog. 

Here are some actions you can take to help minimise the pressures on our environment

1. Keep your motor vehicle regularly serviced and the tyres inflated to the manufacturer's specifications. This will ensure the car is running efficiently and not emitting excessive pollutants.

2. When you trade in your old car, replace it with a fuel-efficient, low emission car. Check out the fuel consumption label, which now has to be displayed on new cars. For commercial vehicles (3.5 tonnes gross vehicle mass and over), the Alternative Fuels Conversion Program is available.

3. Instead of using a car, try riding a bike or walking, and use buses, trams or trains whenever you can.

4. Use energy efficient appliances. Look for the Energy Star logo when buying a computer, printer or scanner, TV, VCR, audio or DVD product; or the Energy Rating on the next air conditioner, clothes dryer, washing machine, dishwasher, fridge or freezer you purchase.

5. If renovating or building, use energy-efficient designs and materials.

6. Turn off unnecessary electrical appliances at the power point wherever possible. 

7. Generate you own green power—investigate the State Government's Solar Hot Water Rebate, Photo-voltaic Rebate and the Remote Renewable Power Generation grants.

8. Limit your wood fires at home. Wear warmer clothes as your first action to keep warm. If you are buying a slow combustion wood heater, make sure it meets the Australian Standard AS4013 and is installed correctly, according to AS2918.

9. Schools can get involved with Air watch, a program for primary and secondary schools where students can become pollution watchdogs in their local areas. 

10. Make it your goal to purchase 'green power'—power generated from clean, renewable energy sources.

How can we reduce the occurrence of photochemical smog?

The most effective way of reducing the amount of secondary pollutants created in the air is to reduce emissions of both primary pollutants.

Reduction of nitrogen oxide: The main method of lowering the levels of nitrogen oxides is by a process called ‘catalytic reduction’, which is used in industry and in motor vehicles. For example, a catalytic converter fitted to a car’s exhaust system will convert much of the nitric oxide from the engine exhaust gases to nitrogen and oxygen. In Australia, all motor vehicles built after 1985 must be fitted with catalytic converters. Nitrogen is not in the actual fuels used in motor vehicles or power stations; it is introduced from the air when combustion occurs. Using less air in combustion can reduce emissions of nitrogen oxides. Temperature also has an effect on emissions—the lower the temperature of combustion, the lower the production of nitrogen oxides. Temperatures can be lowered by using processes such as two stage combustion and flue gas recirculation, water injection, or by modifying the design of the burner.

Reduction of VOCs :There are various ways to reduce VOC emissions from motor vehicles. These include the use of liquefied petroleum gas (LPG) or compressed natural gas (CNG) rather than petrol, decreasing distances vehicles travel by using other modes of transport, such as buses and bikes, and implementing various engine and emission controls now being developed by manufacturers.