Road traffic is well known to emit particles from combustion of fossil fuels (or bio fuels), but it also produce particles due to the friction between tires and roadsurface, and within brakes. In terms of mass the mechanically generated particles are a polution source of the same order of magnitude as the combustion generated particles.

We will measure the emissions of toxic aerosols from the Stockholm traffic originating from the wear between car tires and the road, and the vehicle´s brakes, applying direct micrometeorological methods. Past projects have only determined emissions by indirect methods, and in a way that does not represent the entire complex vehicle fleet. The measurement results in the flow of particles from the surface up into the atmosphere, specified for particle sizes from 0.07-10 micrometers, and their chemical composition (with respect to metals). This information is relevant to the aerosol toxic effects. Based on these measurements we will derive emission factors that allow estimates of aerosol emissions from factors such as traffic activity, fuel consumption or carbon dioxide emissions. Furthermore, the measurements will be used to calibrate models for particulate emissions from tire- roadway and brakes. The resulting emission factors will be tested against local and regional emission databases. Emission factors will be applied in a dispersion model for the Stockholm area and become the basis for assessment of health impacts and risks of these pollutants in the region in close cooperation with the local environmental authorities and other stakeholders. The work will provide policy makers with the necessary tools to optimize their efforts to reduce or limit emissions of airborne particles from traffic with respect to their effects on health.

Median diurnal cycles of super micron aerosol number fluxes based on the eddy covariance method for dry (solid curve) and wet (dashed curve) for March to April. For the other seasons there is no significant difference. The dotted and dotter-dashed curves are the 25 and 75 percentiles. From Vogt et al. (2011a).

Some years ago the FORMAS financed and very successfull project TEA (Traffic Emissions of Aerosols) tought us a lot of the source processes of road traffic particle emissions, and improved the way emission factors for aerosols can be estimated. With BREAD we build on the experience of TEA, but we will employ more advanced micrometeorological and analytical methods.