Traffic aerosol emission velocity derived from direct flux measurements over urban Stockholm, Sweden

Size-resolved aerosol vertical number fluxes were measured using the eddy covariance method, 105 meters above the ground over the city of Stockholm, Sweden, between 1st April 2008 and 15th April 2009. The size range of the measurements cover particles from 0.25 to 2.5 μm diameter (Dp). Emission velocities (ve) were calculated for the same size range and were found to be well correlated with friction velocity ( u ∗ ) and CO2 fluxes ( F CO 2 ). These variables were used to parameterize the emission velocity as v e = ( 129.14 ( 0.01 D p 2 + 0.06 D p + 0.008 ) + 1.22 ) u ∗ F CO 2 where ve and u ∗ are given in [m s−1], Dp in [μm], and F CO 2 in [mmol m−2s−1]. rameterization reproduces the average diurnal cycle from the observations well for particles sizes up to 0.6 μm Dp. For larger particles the parameterization tends to over predict the emission velocity. These larger particles are not believed to be produced by combustion and therefore not well represented by F CO 2 , which represents the traffic source through its fossil fuel consumption and the related CO2 emissions.