- In 2012, monitoring of nitrogen dioxide indicated that the WHO long-term guideline was met at 98 percent of monitoring sites (121 out of 124). Those where the WHO guideline was exceeded are close to state highways and busy local roads.
- In 2012, all 15 regional council and unitary authority monitoring sites that can be compared directly to guidelines, met both the WHO long-term guideline and national short-term standard for nitrogen dioxide. Many of these sites are where high concentrations are expected.
Nitrogen dioxide and why it’s important
Nitrogen dioxide is a gas and can be directly emitted into the air. However, it is most often formed when nitric oxide emissions react with other chemicals in the air. Both nitrogen dioxide and nitric oxide (collectively known as nitrogen oxide) are emitted from the combustion of fossil fuels such as coal, gas, diesel, and oil, and from natural sources such as volcanoes. Transport, particularly heavy-duty diesel vehicles, and some industrial activities, are especially important emission sources.
Nitrogen dioxide has been linked to increases in asthma symptoms, and reduced lung development and function in children. Nitrogen dioxide can also reduce the lung’s defences against bacteria, making people more susceptible to infections.
In 2012, monitoring of nitrogen dioxide indicated that the World Health Organization (WHO) long-term (annual) guideline of 40 µg/m3 was met at 98 percent of monitoring sites (121 out of 124). However, each year, between 2010 and 2012, three to six sites are likely to exceed the WHO guideline. These sites are close to busy local roads and state highways in major urban centres.
These findings are based on the New Zealand Transport Agency’s monitoring network which uses a screening method. This method gives a good indication of nitrogen dioxide concentrations at a number of sites; but it does not enable the recorded concentrations to be used to determine whether the WHO long-term guideline is exceeded.
Figures 15–17 show the annual average nitrogen dioxide concentrations for three different location types from 2010 to 2012:
- close to state highways (70 in 2012, 69 in 2011, and 68 in 2010)
- close to busy local roads (34 sites in 2012, 33 in 2011, and 32 in 2010)
- urban background sites (not close to state highways or local roads) (20 sites in 2012, 19 in 2011, and 15 in 2010).
The WHO long-term guideline shows a minimum level of protection against health risks from long-term exposure to nitrogen dioxide.
The concentrations at busy local roads and state highway sites vary, with a few sites likely to exceed the WHO long-term guideline. The urban background sites are well below the WHO guideline.
Concentrations are typically much higher in winter than in summer. This is likely because winter conditions prevent dispersal, rather than an increase in transport emissions or winter-specific emission sources.
This graph shows Annual average nitrogen dioxide concentration – background locations 2010–12. Visit the data files page for the full breakdown of the data.
This graph shows Annual average nitrogen dioxide concentration – busy local roads 2010–12. Visit the data files page for the full breakdown of the data.
This graph shows annual average nitrogen dioxide concentration – state highways 2010–12. Visit the data files page for the full breakdown of the data.
See About the case studies for more information on this case study.
In addition to the New Zealand Transport Agency’s monitoring, three regional councils and one unitary authority also monitor nitrogen dioxide and were doing so before 2010. The monitoring methods used by regional councils and unitary authorities can be compared with the WHO long-term guideline, unlike the New Zealand Transport Agency monitoring, which is indicative only (see figures 15, 16, and 17).
Figure 18 shows the levels of nitrogen dioxide concentrations in Auckland and Wellington. Two types of sites are represented: ‘peak sites’ (where concentrations are expected to be high, such as busy transport sites – Queen Street and Khyber Pass Road in Auckland, and Central Wellington) and ‘background sites’ (where concentrations are expected to be low, such as urban areas away from busy roads – Glen Eden, Auckland and Upper Hutt, Wellington).
At Khyber Pass Road and Central Wellington, concentrations have decreased since monitoring began. In contrast, concentrations at Glen Eden and Upper Hutt have remained relatively constant. In 2012, all 15 monitoring sites of the three regional councils and one unitary authority that monitor nitrogen dioxide met the WHO long-term guideline.
This graph shows annual average nitrogen dioxide concentrations – selected locations, 2002–12. Visit the data files page for the full breakdown of the data.
Nitrogen dioxide monitoring for short-term exposure
In 2012, hourly nitrogen dioxide concentrations were recorded by four councils at 15 monitoring sites. These sites met the one-hour standard for nitrogen dioxide provided in the National Environmental Standards for Air Quality. Most of these monitoring sites are at peak sites. This therefore suggests that concentrations of these pollutants in non-monitored locations would also likely be lower than the national standard.