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National environmental standards: Airsheds

Maps showing fine particle levels in airsheds

Frequently asked questions on air quality

Although New Zealand’s air quality is mostly good, there are times when the air in many urban areas is affected by pollution. Air pollution can affect people’s health so national environmental standards have been introduced to set acceptable levels for air quality. The pollutant most commonly affecting our urban air is smoke and soot (fine particles called PM₁₀).

Identifying airsheds

In September 2005 regional councils and unitary authorities identified 42 areas where air quality could reach levels higher than the national air quality standards. These areas are called airsheds.

Airsheds have been identified based on councils knowledge of existing air quality data and the location of significant sources and factors that affect the spread of pollution (such as local geography and weather). These airsheds have been published in the New Zealand Gazette.

All but one of the forty-two airsheds were gazetted because of known or potential concerns about levels of smoke and soot PM₁₀) at certain times of the year. The remaining airshed at Marsden Point in Northland has the potential to breach the sulphur dioxide and PM₁₀ standard.

Gazetted airsheds include areas where the PM₁₀ standard is regularly breached each year, and a smaller number that have the potential to breach the standard unless carefully managed.

Seven gazetted airsheds do not appear to breach the standard, but monitoring to date is inconclusive, or emissions are expected to increase in the airshed. A further six airsheds lack any historical monitoring and have been gazetted because of their potential to breach the standard.

Councils have taken a number of approaches to defining airsheds in their region. An airshed may be based on one of the following;

urban areas where pollution levels may be high
a portion of the region that contain a number of urban areas
a number of separate, but similar, urban areas grouped together.

The path towards complying with the PM₁₀ standard

The PM₁₀ standard sets a maximum concentration of 50 microgrammes per cubic metre (50µg/m³) over a 24 hour period. Levels above the standard, when concentrations are unacceptably high, are known as exceedences. The PM₁₀standard permits only one exceedence in a 12 month period.

Councils managing airsheds that breach the standard must plan for how to improve air quality. Air quality will be tracked along a Straight or Curved Line Path; a target set by councils that tightens every year until the standard is met by 1 September 2013.

If PM₁₀ levels track on or above the Straight or Curved Line Path councils will not grant resource consents for any process that makes the levels worse. However, the resource consent could be granted if the proposed emissions are offset by reductions elsewhere in the airshed.

After 2013, no resource consents to discharge PM₁₀ will be granted if the air quality standard is still being breached.

Monitoring air quality in airsheds

In order to track air quality in airsheds councils carry out monitoring. Monitoring is carried out where pollution levels are highest in an airshed. It is impractical to measure air quality everywhere as the air quality standards require ‘high tech’ round-the-clock monitoring.

To comply with the air quality standards many councils have had to upgrade monitoring equipment. Equipment that typically measured one day in three has been replaced by fully automated equipment that monitors continuously.

Following progress in airsheds

The Ministry has developed a reporting framework for air quality in airsheds. This shows the location of airsheds around New Zealand and reports annual summary air quality information provided by regional and unitary authorities. This information will be updated yearly.

The summary information shows:

peak levels of PM₁₀ plotted against the airshed’s Straight or Curved Line Path (if the standard has been breached)
the number of times the PM₁₀ standard is exceeded each calendar year
the most recent estimated PM₁₀ emissions in the airshed

PM₁₀ in the urban air

PM₁₀ comes from the burning of fuels such as coal, wood, oil, petrol and diesel. The main sources in the urban environment are home heating and vehicles, as well as some industrial processes.

The level of PM₁₀ in the air depends on how much pollution is being produced (the emissions). However, other factors like the weather and local geography have a big effect. For example, windy conditions help move pollution away but features such as valleys can cause pollution to linger. Temperature inversions that occur during cold, calm conditions can trap in pollution causing levels to be high. Years where there have been a lot of temperature inversions often have worse air quality. So sometimes pollution can be worse during a particular year because of the weather, rather than because we are polluting more. Conversely, a windy year with few temperature inversions may show fewer pollution episodes regardless of any changes in pollution emitted.

It is important to remember these effects when trying to assess improvements in air quality. Councils take this into account when setting the starting point of their Straight or Curved Line Path.