The degree to which road traffic pollutes urban air varies hugely, depending how much traffic is nearby. We’ve been mapping traffic pollution in detail across all New Zealand’s towns and cities over the last decade. In doing so we’ve been refining the methods to improve accuracy, and gaining insights into urban and transport design in the process.
From sampling to modelling
Nitrogen dioxide (NO2) arises from combustion, but on city streets traffic exhaust is the overwhelmingly dominant source. A map of NO2 levels is effectively a map of all traffic exhaust pollutants.
We are big fans of passive “diffusion” sampling for nitrogen dioxide (NO2). These plastic tubes (internally coated in a chemical that reacts with NO2) produce a single average concentration measurement over a period of a week to a month. But their simplicity and few-dollars cost means they can be deployed almost anywhere and in large numbers. They are widely used around the world, and their moderate accuracy can be compensated for through smart sampling design.
Over the years we think we are coming close to perfecting that smart sampling design.
Over the past few years we have worked with the NZ Transport Agency to map NO2 from road transport across 8 cities. Using 40 to 60 sampling sites sampled at least three times we have created maps of NO2 at 5 metre resolution, using our “Traffic Impact Model”. This work resulted in an improved sampling design for the ongoing monitoring of long-term trends for all of NZ’s major towns and cities.
Insights on urban design
Our work has indicated how he most polluted locations are not always where the heaviest traffic is – the sheltering created by buildings also has a huge localized impact. This makes downtown areas and infill development particularly sensitive.
Our method has also been used to infer the impact of non-road sources on air quality, or locations with atypical emission patterns (e.g. excess of trucks) such as airports and sea ports, rail yards, etc.
Applications and services
Our approach is empirical, meaning it is based on observations in the real-world rather than entirely theoretical computer modelling. It can be applied to whole cities, to development areas, or to investigate specific urban and transport design features.
We also use these insights to advise on approaches for monitoring of long-term trends, especially where change (in emissions or land-use) is anticipated.
We are also able to convert our maps of NO2 into other traffic pollutants, such as black carbon – a climate pollutant and a risk factor for cancer.
