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One of the key air quality objectives in the State highway environmental plan is:

A1: Understand the contribution of vehicle traffic to air quality.

The impact of road transport on air quality in New Zealand varies locally. Motor vehicles produce a complex mix of contaminants. It is not feasible to monitor all of these, so we use nitrogen dioxide (NO2) as a general proxy for motor vehicle-related air pollution. This is consistent with the recommendation of the World Health Organization which states that 'nitrogen dioxide levels are generally a reasonable marker of exposure to traffic-related emissions.'

Monthly monitoring, using NO2 passive samplers, is undertaken at around 130 sites across the state highway and local road network. Measurements are made at a variety of potential sensitive locations near to roads, including residences and schools.

Data from these tubes allow air quality associated with vehicle emissions to be established and tracked over time. The primary objectives of the Transport Agency national air quality monitoring network are to:

  1. Report against the air quality performance indicator from the State Highway Environmental Plan,
  2. Identify where there may be an air quality problem on the state highway network, and
  3. Indicate trends and seasonal variations.

The overall aim is to see a decreasing trend in NO2 concentrations. This aligns with the Government’s desired long term objective which seeks to 'reduce transport's negative effects on the local environment and public health' as stated in the Government Policy Statement for Land Tranport Funding(external link).

A web tool has been developed to make summary data for transport-related air quality monitoring readily available.

Below you'll find additional links, documents and presentations relevant to the monitoring network.

Please note, the method for calculating annual and seasonal averages is as follows:

  • at least 75% valid data and one monthly average in each summer and winter to calculate an annual average;
  • at least two valid monthly averages to calculate a seasonal average
    • the summer averaging period is now January to March of the current year
    • the winter averaging period is now July to September of the current year
  • For triplicate sites, these requirements also apply but at least two triplicate samplers must have a minimum of 75% valid data in order to calculate a triplicate average.

Air quality monitoring reports

View annual monitoring reports, site metadata and operating manuals and trend reports:

National air quality (NO2) monitoring network


For further information contact