Traffic Emissions of Aerosols

Sammanfattning: Non-exhaust traffic emissions are unregulated, yet the mass emission of non-exhaust particles may be larger than exhaust emissions. In addition, their emission factors (EF) are more uncertain than exhaust emissions. This thesis presents aerosol flux measurements in Stockholm using the eddy covariance method. Prior to this work, no direct flux measurements using micrometeorological techniques had been performed to study non-exhaust emissions. The challenges of measuring non-exhaust emissions include low particle number concentrations and the complex meteorological influences due to the dependence on road suspension processes. A novel approach applied here is the use of simultaneous flux measurements of CO2 and particles enabling improved parameterizations and understanding of source processes. It was shown that the EF for particles in the size range 0.25 to 2.5µm diameter are only 0.1% of the total particle number EF, indicating that ultrafine particles dominate the number fluxes. Sub- and super-micron particle emissions have been quantified, characterized, and parameterized. Dependence on wind speed, road surface condition and CO2 correlation indicates that the super-micron particles are primarily emitted through wind and vehicle induced turbulence. On the other hand, good correlation between sub-micron particles and CO2 flux indicates that these particles are primary emissions from traffic. The sub-micron particles (0.25 to 0.6 µm Dp) consisted of 60% semi-volatile components and 40% of non-volatile material. For the super-micron particle emissions, clear seasonal characteristics were found with the highest average values during spring. Heavy duty vehicles were found to emit 30 times more than light duty vehicles per km. This indicates that trucks and buses are more efficient at suspending super-micron particles than light duty vehicles. These results are consistent with the hypothesis that the particles originate from abrasion of road surfaces by studded tires.