Little is known about the the relevance of mechanically produced particles of road traffic from abrasion and resuspension processes in relation to the exhaust pipe particles. In a project which was jointly realized by EMPA and PSI, concentration measurements of particles and nitrogen oxides (NO_x) in ambient air were performed on both sides of busy roads. During meteorological conditions with winds across the street it is possible to determine the contribution of the local traffic from upwind-downwind differences. Alternatively, these contributions can also be obtained from differences between kerbside sites and nearby background sites.

Hourly dilution factors were calculated from the measured concentration differences of nitrogen oxides, the number of vehicles, and published NO_x emission factors. The emission factors for particles were then calculated from the measured concentration differences, assuming that these undergo the same dilution as nitrogen oxides. Two vehicle categories were distinguished: LDV (light duty vehicles < 6 m, i.e. petrol and diesel passenger cars, vans, motor cycles) and HDV (heavy duty vehicles > 6 m, i.e. lorries and coaches).

In order to distinguish between exhaust pipe emissions and emissions from abrasion and resuspension the PM10 and PM1 fractions of the particles were measured separately. PM1 was interpreted as direct exhaust pipe emissions, and PM10 as total fine particle emissions. The difference PM10-PM1 thus represents the emissions from abrasion and resuspension.

Emission factors of PM10 and PM1 for LDV and HDV could be derived for differnt imporant traffic regimes. It was shown, that the emissions from abrasion and resuspension processes are considerable in relation to the exhaust pipe particles. Given the fact, that the emissions from abrasion and resuspension processes represent roughly half or even more of the total primary particle emissions of road traffic, the question of possible impacts of these emissions has to be raised. Though quantitatively highly significant, these contributions have a completely different particle size distribution and chemical composition than exhaust pipe emissions. While the latter consist mainly of fine soot and organic particles (partly known as cancerogenic), abrasion and resuspension predominantly leads to coarse mineral and metallic particles. The current knowledge of the mechanisms, which are responsible for adverse effects of fine particles does not allow a conclusive judgement concerning the relative importance of the emissions stemming from exhaust pipe as compared to abrasion and resuspension. Further studies should attempt to close this significant gap.

Based on the new information from this project, the total primary PM10 particle emissions of road traffic in Switzerland were recalculated. For the year 2000 they were roughly 25% lower than estimated in an earlier publication in 2001. Consequently road traffic contributes approximatly 20% to the total primary PM10 emissions in Switzerland.

Acknowledgment

We gratefully acknowledge the financial support of the Swiss Federal Roads Authority (ASTRA) and the Federal Office for the Environment (FOEN).

Further information
Please direct questions and/or comments about this project:

Robert Gehrig
Tel. ++41 44 823 42 34

Matz Hill
Tel. ++41 44 823 46 68