Smog days in Melbourne

Air pollution incidents in Melbourne differ in character depending on the season:

• In summer the main problem is photochemical smog.
• Winter episodes are characterised by poor visibility due to airborne particles.

Smog can have a brownish tinge due to the presence of particles together with nitrogen dioxide. Later, the colour may change to white due to the presence of sulfate and nitrate aerosols.

Melbourne is fortunate that its prevailing winds and weather systems move quickly, tending to disperse pollution. However, under certain light wind conditions, the air and the pollutants it contains is recirculated, creating more smog.

Summer smog

The main pollutants involved in producing summer smog are hydrocarbons and oxides of nitrogen which come from cars and factories. In the warmer months (October to March), sunlight causes this mixture of primary pollutants to react chemically, producing photochemical smog. The resultant smog contains secondary pollutants such as ozone, aldehydes and fine particles, acting like a blanket in the air on still days.

Ozone

Ozone, the principal constituent of summer smog, is used to indicate smog levels.

A summer smog event is defined when the ozone level over one hour reaches 0.10 parts per million (ppm) at any monitoring station.

This is the national standard for air quality for ozone. The Ambient Air Quality National Environment Protection Measure (NEPM) was made by the Commonwealth Government in June 1998 and adopted in Victoria in February 1999 in the State Environment Protection Policy (Ambient Air Quality). The standards and goals for ozone were were designed to protect human health and will ensure protection of vegetation.

High 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 ozone levels can reduce crop yields of citrus fruits, grapes, potatoes, and soy beans. Prolonged exposure to smog also tends to crack rubber, weaken synthetic fibres and fade paints and dyes.

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Winter smog

During the cooler months (April to September), Melbourne is more affected by the accumulation of fine particles.

The main sources of these airborne particles are domestic wood heaters and open fireplaces, industrial emissions, motor vehicles and open burning of agricultural stubble and grass, as well as fuel reduction burning in forested areas. Other sources, including barbecues, lawn mowers and road dust, also add to airborne particle levels.

The main constituents of winter smog are those fine particles which scatter sunlight and cause a reduction in visibility. The State Environment Protection Policy (Ambient Air Quality) contains an air quality objective for visibility-reducing particles - a winter smog event is defined when (dry-air) visibility becomes less than 20 kilometres, for at least one daylight hour, as recorded by at least two monitoring stations.

As well as reducing visibility, these particles can affect health, particularly the respiratory system. People at risk during summer smog episodes are also at risk during extended winter episodes.

Airborne particles also contribute to the soiling of buildings and fabrics. Particle deposits on electrical insulators may result in a short circuit or flashover at electrical contacts. This can cause fuses to blow and create a fire risk.

A primary pollutant (carbon monoxide, sulfur dioxide or PM₁₀) that exceeds an objective as recorded by at least two monitoring stations is also defined as smog event.

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