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What makes air pollution increase?

The concentration of air pollution in our environment depends on both the amount of pollution produced and the rate at which pollutants disperse. This depends largely on wind (both strength and direction). In areas where the wind is very strong, pollution is dispersed and blown away. In areas where there is little or no wind, air pollution accumulates and concentrations can be high.

However, local factors such as topography (hills and mountains), proximity to the coast, building height and time of the year all affect local wind conditions and can play a role in increasing air pollution levels.

In summer and winter, two common types of air pollution frequently develop. These are outlined below.

Summer smog

On still, sunny days in summer when there is little wind, certain volatile organic compounds (VOCs) and nitrogen oxides (NO2, NOx) react in sunlight to form photochemical smog and ozone (O3). This type of air pollution is common in large cities.

Winter smog

In winter inversion layers can develop on still evenings and mornings. These trap pollution near the ground and pollutant concentrations can build up to levels that affect your health. As the sun rises, warming the air in the morning, inversion layers usually break down so that pollutants can be dispersed. Sometimes inversion layers can stay for up to two or three days, but in New Zealand this is uncommon. Inversion layers occur frequently in Christchurch and many South Island towns during winter.

Last updated: 9 January 2008