While visitors to New Zealand are impressed by the clean air and absence of haze, they are sometimes less impressed by the strong winds they encounter in many parts of the country. Yet the two things are related. The prevailing and largely unpolluted westerly winds which often buffet New Zealand have a cleansing effect, dispersing air contaminants before they can become too concentrated (see Figure 6.2).
When the winds die away, however, serious air pollution can occur in some places. Christchurch, which is very prone to air pollution problems, is a good example. The city is situated in a geographical 'basin' on the east coast of the South Island. The Port Hills lie on one side, and the Canterbury Plains slope up towards the Southern Alps on the other. In winter, the city often experiences what is known as a 'temperature inversion' with a layer of warm air trapping cooler air, and the pollution from cars and domestic fires-underneath.
Another reason for New Zealand's generally clean air is the relative sparseness of its human population and the small number of 'smokestack' industries compared to European countries. The country's main revenue comes from outdoor industries such as agriculture, horticulture, forestry, fishing, and tourism. Except for isolated incidents of spray drift or controlled burn-offs, air pollutants generated by these industries are generally low and localised. By world standards, then, New Zealand has low concentrations of most urban air pollutants such as smoke, photochemical smog, and sulphur dioxide.
However, it is important to recognise that the levels of most air pollutants are constantly changing. This is due to the physical and chemical characteristics of the contaminants, the location of individual sources relative to the observer or monitoring instrument, variations in source strengths, and the prevailing weather conditions. Because these factors can vary considerably, depending on time and place, the following points should be borne in mind when generalising about New Zealand's air quality:
Pollutant levels can show marked variations throughout each day, and from one day to the next. This can be due to source variations (e.g. vehicle flows, domestic heating patterns) or changes in weather conditions.
Pollutant levels will often show a seasonal variation, with higher levels usually recorded during the winter months. This is partly due to poorer atmospheric dispersion conditions during winter and the increased combustion-related pollutants associated with domestic heating. The seasonal variation also applies to ozone, which needs warm temperatures to form, and wind-blown dust, which is often more of a problem during drier summer months.
Levels of air pollutants generally decline with distance from a source (i.e. distance from roads, residential areas, or factories). However, their effects may be widespread (e.g. photochemical smog, visibility).
Winds are generally predominant from a westerly direction and are particularly strong through Cook and Foveaux Straights, in the southern North Island and across the South Island. Along the eastern coast of the South Island there is predominance of northerly or southerly winds.
Source: McKenzie (1958)
The shape of the surrounding land and/or buildings can also influence air quality. For example, valleys often tend to trap air while flat or hilly locations allow it to disperse. In a city, air pollution from motor vehicles is often worst where dispersion is inhibited by a 'street canyon' effect. In New Zealand, therefore, areas such as Christchurch, Dunedin, and the Hutt Valley experience periods of relatively high air pollution because of their locations, local topography, and prevailing weather conditions, particularly during the winter months.
Most parts of Wellington, and areas such as New Plymouth and Invercargill, are thought to be less affected because of their frequent and higher average wind speeds throughout the year. Auckland should be less vulnerable because of its maritime location, but this is offset to some extent by the much greater numbers and densities of pollution sources, especially motor vehicles.
Taken together, these factors mean that any summary of air quality can only be made in fairly general terms. Individual results may also depend on the specific methods used for measuring, collecting samples and averaging results over different time periods (e.g. 1, 8, or 24 hours).