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The Waka Kotahi screening model for estimating air quality near roadways combines the contribution of the road together with the background air quality to arrive at a cumulative concentration.

The model is designed to provide a conservative (worst-case) assessment of air quality risk from a single road for two key transport-related air pollutants – particulate matter (PM10) and nitrogen dioxide (NO2). For more complex situations, specialist advice may be required. The initial version of the model used qualitative indicators of the likely background. The latest version has been upgraded with detailed estimates of background air quality to provide more robust calculations of the cumulative air quality.

The road contribution to PM10 concentrations is calculated using emission factors, which take into consideration the assessment year, the average speed, the amount of traffic and the proportion of heavy vehicles. For NO2 concentrations, the road contribution is based on a general dispersion algorithm, which is only dependent on the amount of traffic. Background air quality data (excluding nearby roads) are available for every location in New Zealand (by census area unit). The background air quality data is provided on our website as an Excel spreadsheet.

Background air quality data

Further information on the key assumptions and limitations of the screening model is available in the users’ notes and other documentation below: