Footnote(s): (a) Each city contains several ozone monitoring stations. The data presented are an average of exceedance days across all ozone monitoring stations in each city. Melbourne averages only consider stations with data available for at least 74% of days in a given year.
Source(s): NSW Department of Environment, Climate Change and Water Air quality; Qld Department of Environment and Resource Management Resource Centre; Victoria Environment Protection Authority Air quality.
OZONE AND SMOG
Ozone is formed when oxides and nitrogen react with sunlight in the atmosphere. It is a natural part of the upper levels of the atmosphere where it absorbs harmful UV rays, preventing the rays from reaching the earth's surface.
Near the ground, ozone is a secondary pollutant, often formed by the reactions of primary pollutants. These pollutants arise mainly from vehicle emissions, stationary combustion sources, and industrial and domestic use of solvents. In high concentrations though, ozone can irritate the nose, airways and lungs, and can also damage plants.
For most Australian towns and cities, the level of ozone in the air does not exceed the National Environment Protection Measure (NEPM) standard. Even large cities like Melbourne and Brisbane averaged less than two days when levels exceeded the NEPM standard in each year from 1998 to 2008.
Of Australia’s three most populous cities (Sydney, Melbourne & Brisbane), Sydney recorded the most days exceeding the four-hour ozone NEPM standard between 1998 and 2009. Sydney’s warm temperatures in 2001 were reflected in the number of ozone exceedance days in that year (21 days). Ozone exceedances are associated with meteorological features such as high pressure systems, channelling effects induced by topography, and the sea breeze. Sydney usually records more than five days per year when the standard is exceeded, partly due to the topography of the Sydney Basin.
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