You, the young generation of today face many issues that can have a great impact on your lives. Hopefully air pollution, and especially photochemical smog, has gained your consideration. Photochemical smog poses easily the biggest threat to clean air in Australia and other major cities around the world.

What Is It?

Photochemical smog is a complex mixture of chemicals, of which some are harmless and others that are highly reactive and very dangerous to plants, animals, and humans. It is characterized by reduced visibility, a yellow brown haze, and the presence of irritating substances in the atmosphere. It is formed when oxides of nitrogen (NOx) and hydrocarbons, such as volatile organic compounds (VOCs), react together in the presence of sunlight (at a minimum air temperature of 18 degrees Celsius) to form excess levels of ground level ozone. In contrast to the beneficial effects of stratosphere ozone which plays an essential role in protection from the harmful effects of ultraviolet radiation, ground level ozone can have an adverse effect on human health, vegetation (particularly agriculture), and buildings. Ozone requires 3 to 5 hours to develop and requires calm sunny weather to peak.

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The Villains

VOCs and oxides of nitrogen (NOx) are precursors of photochemical smog. Several oxides of nitrogen are found in urban air and nitric oxide (NO) and nitrogen dioxide (NO2) are the most common of these. The sources of these are power generators, industrial processes, solid waste incinerators, and predominantly motor vehicle emissions. Motor vehicles emit 70 percent of the total NOx and 75 per cent of human caused NOx in South East Queensland. VOCs (e.g. benzene, formaldehyde, chlorinated hydrocarbons etc) are produced from evaporation of petrol, solvents, oil based paints, hydrocarbons from the petrochemical industry, and motor vehicle emissions. Vehicles emit 52 percent of the VOCs present in the Queensland airspace. Interestingly, recent CSIRO research found that plants release highly reactive carbons that can add significantly to photochemical smog problems. However, it can be clearly seen that areas with high traffic densities are more prone to photochemical smog. All you need is a nice sunny day.


So how exactly does photochemical smog form?

In an unpolluted atmosphere, nitrogen oxide (NO) destroys ozone (O3) to form nitrogen dioxide (NO2) and oxygen (O2)

Reaction 1: NO+O3 ? NO2 + O2

In an area with higher levels of nitrogen dioxides (caused by vehicles emissions) sunlight can break the nitrogen dioxides (NO2) down to nitrogen oxide (NO).

Reaction 2: NO2 + sunlight ? NO + O

The oxygen atom formed in the above reaction bonds with one of the abundant oxygen molecules (O2) to form ozone (O3)

Reaction 3: O + O2 ? O3

On a sunny day it seems as if all the reactions cancel each other out. Reaction 1 takes care of the by products from reactions 2 and 3, limiting the amount of ozone produced. Hydrocarbons control the rate of smog formation. Sunlight causes reactive hydrocarbons to produce many compounds that also oxidize NO to NO2 but without destroying ozone. In urban air the hydrocarbons react this way, increasing production of NO2 and then production of ozone through reaction 2.Ozone absorbs ultraviolet light and through a reaction forms a chemical called hydroxyl radical. This chemical is the “cleaning agent” in the air and cleans it by reacting with the hydrocarbons to remove it. Some of the hydrocarbons are converted into the toxic PAN (Peroxyacetyl nitrate) one of the major irritants in photochemical smog. If there are more hydrocarbons in the air, there will be higher levels of PAN.

Ozone Concentration (ppm)

(8-hour average, unless noted)

Air Quality Index


Air Quality


0.0 to 0.064

0 to 50


0.065 to 0.084

51 to 100


0.085 to 0.104

101 to 150

Unhealthy for

Sensitive Groups

0.105 to 0.124

151 to 200


0.125 (8-hr.) to 0.404 (1-hr.)

201 to 300

Very Unhealthy

Effects of Photochemical Smog Ozone makes up 85 percent of photochemical smog and is hence a major indicator of the level of smog. Ground level ozone concentrations are measured nationwide (CSIRO uses Airtrak pollution measurement system and a process called chemiluminescence).These concentrations, which are available at the government websites, can be linked to particular descriptors of air quality (see Table1).

The ozone and nitrogen dioxides found in the photochemical smog can do a lot to harm human health:

> They can reduce lung function

> They can aggravate asthma

> They can inflame and damage the lining of the lung

> They can aggravate chronic lung diseases such as bronchitis

> They can irritate the eyes

> They can cause sore throats, inflammation, and discharges in the nasal passages

What You Can Do To Stop It

There are a number of ways in which you can help reduce the formation and build up of photochemical smog.

> Drive less. Try walking, cycling or using public transport. This will considerably reduce emissions of nitrogen oxides and VOCs.

> Make sure your car is well tuned. Studies show that properly tune cars produce significantly less nitrogen oxide emissions

> Make sure that you tightly seal the lids of chemical products – such as solvent, garden chemicals, or household cleaners – to keep evaporation to a minimum.

> Use unleaded petrol where possible

> Drive at or under speed limits. Fuels consumption decreases by 20 – 25 percent when a vehicle is driven at slower speeds

> Drive smoothly. Fast aggressive driving uses 35 percent more fuel and create more air pollution.

Photochemical smog cannot be completely eradicated but letting it get out of hand can prove fatal, especially for you, the future people of tomorrow.


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