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This research was commissioned under the 2021 Government Policy Statement on Land Transport. This note details part 2 of a multi-part project investigating the non-exhaust emissions (NEEs) from road transport that are discharged to air and water.

Part 1 (a literature review and gap analysis) made recommendations to improve understanding of how the choice of emission factors used in models affects their outputs. Part 2 addressed this by:

1. determining how existing source-apportionment data could be analysed to help ground-truth current NEE factors
2. undertaking a sensitivity analysis to determine the size of the impact that NEE factors have in air-quality modelling
3. assessing and describing the need for further monitoring.

This work showed a discrepancy between the contribution that NEEs make to roadside PM10 levels when:

• predicted using the Vehicle Emissions Prediction Model (VEPM) 6.3
• resolved from source-apportionment studies.

On average, source-apportionment studies showed an average contribution around 1 μg m^-3 greater than the contribution predicted by the VEPM. The most plausible explanation for this difference was that the ‘missing’ particulate matter (not predicted by the VEPM) came predominantly from dusts being resuspended from the road surface due to tyre contact or traffic-induced turbulence.

This resuspension is not accounted for within the VEPM. In addition, it is unlikely that the accuracy of the brake and tyre emission factors currently being used in the VEPM can be assessed using the existing New Zealand data.

Recommendations for further work coming out of part 2 include:

• introducing a crustal matter emission factor into the VEPM
• changing the method used to derive motor-vehicle-derived coarse particulate matter in the Health and Air Pollution in New Zealand Study
• continuing long-term local authority time series
• conducting various experimental and observational studies to build available NEE data.

Authors:

Elizabeth Somervell, NIWA, Auckland
Perry Davy, GNS Science, Wellington
Ian Longley, NIWA, Auckland
Gustavo Olivares, NIWA, Auckland
Daniel Morrish, NIWA, Auckland<
Guy Coulson, NIWA, Nelson