Ozone (O3) makes up approximately one part in three million of all of the gases in the atmosphere. It is the triatomic form of oxygen, meaning it is made up of three oxygen atoms. If all the ozone contained in the atmosphere from the ground level up to a height of 60 km could be assembled at the earth's surface, it would comprise a layer of gas only about 3 mm thick, and weigh some 3000 million tonnes.
How does tropospheric ozone relate to stratospheric ozone?
It does not! In the stratosphere (upper atmosphere) ozone is our protector. It protects both plants and animals from the sun's harmful ultraviolet rays. In the troposphere (lower atmosphere) however, ozone is a troublemaker being both a pollutant and a greenhouse gas.
In the troposphere it can be harmful to health. Here, ozone is formed by a very different process. Oxides of nitrogen and reactive organic compounds react together in the presence of sunlight. Motor vehicles are the major contributor of oxides of nitrogen and reactive organic compounds, called "primary pollutants", in most cities. Industry also makes significant contributions. There are also natural sources of reactive organic compounds. Chemical reactions amongst the mixture of gases in the atmosphere also produce other secondary pollutants such as aldehydes and fine particles. This mixture of ozone and other secondary pollutants is often called photochemical smog.
During the summer months when temperatures are high and the weather is calm, chemical reactions amongst the mixture of gases in the atmosphere increase and more ozone is formed.
National standards and legislation
As ozone is toxic at high concentrations, standards have been set to protect human health and ensure protection of vegetation. The standard for tropospheric ozone (or photochemical oxidants as ozone) contained in the National Environmental Protection Measure (NEPM) for ambient air quality is:
- 0.10 parts per million (ppm) - one hour standard
- 0.08 ppm - four hour standard
Monitoring for ozone in Tasmania has been very limited, but has confirmed that ozone levels are very low. Given that the vehicle fleet is fairly small (approximately 604,000) and our summer temperatures remain fairly low, it is considered that ozone does not constitute a major problem in our cities.
Ambient air sampling for ozone was conducted in Hobart between 1994 and 1995, covering a single summer period. Monitoring was conducted at Moonah, approximately 6 km to the north-west of the Hobart CBD. Measurements reported for the site revealed a maximum ozone concentration of approximately 30% of the NEPM one hour Standard.
The effects of increased tropospheric ozone
At low concentrations ozone is colourless and odourless but at higher concentrations it has a pungent odour and is bluish in colour. Ozone is irritating to the nose and throat and is explosive. Ozone is toxic at high concentrations because it reacts strongly with other molecules.
High tropospheric ozone concentrations may cause eye irritation. People suffering allergic disorders, such as asthma and hay fever, chronic disorders such as bronchitis, or cardiac conditions, are at risk when ozone concentrations exceed the acceptable level. Overseas studies indicate that elevated tropospheric ozone levels can reduce crop yields of citrus fruits, grapes, potatoes, and soybeans. Prolonged exposure to smog also tends to crack rubber, weaken synthetic fibres and fade paints and dyes.
What to do to help
If a car is well tuned and maintained it is likely to emit between 9-25% less pollution into the atmosphere than a similar poorly maintained vehicle. In addition, a well maintained car is between 1.5% and 5.0% more fuel-efficient. The pollutants that cars emit include carbon monoxide, oxides of nitrogen, hydrocarbons and particulates. Catalytic converters in cars help reduce the amount of pollutants being emitted and, therefore, also reduce the amount of secondary pollutants, such as ozone, being formed. Catalytic converters contain the rare metals platinum, palladium and rhodium. These substances promote the conversion of hydrocarbons, carbon monoxide and oxides of nitrogen in the vehicles exhaust to water, carbon dioxide and nitrogen.
Limiting your car use is also an effective way to improve air quality.