Size resolved aerosol and gas fluxes were measured in Stockholm from 1 April 2008 to 15 April 2009 over both urban and green sectors. CO₂ and H₂O fluxes peaked in daytime for all seasons. CO₂ concentrations peaked in winter. Due to vegetation influence the CO₂ fluxes had different diurnal cycles and magnitude in the two sectors. In the urban sector, CO₂ fluxes indicated a net source. The sector dominated by residential areas and green spaces had its highest aerosol fluxes in winter. In spring, super micrometer concentrations for both sectors were significantly higher, as were the urban sector rush hour fluxes. The submicrometer aerosol fluxes had a similar diurnal pattern with daytime maxima for all seasons. This suggests that only the super micrometer aerosol emissions are dependent on season. During spring there was a clear difference in super micrometer fluxes between wet and dry streets. Our direct flux measurements have improved the understanding of the processes behind these aerosol emissions. They support the hypothesis that the spring peak in aerosol emissions are due to road dust, produced during the winter, but not released in large quantities until the roads dry up during spring, and explain why Stockholm has problems meeting the EU directive for aerosol mass (PM₁₀).

Unlike exhaust emissions, non-exhaust traffic emissions are completely unregulated and in addition, there are large uncertainties in the non-exhaust emission factors required to estimate the emissions of these aerosols. This study provides the first published results of direct measurements of size resolved emission factors for particles in the size range 0.25–2.5 μm using a new approach to derive aerosol emission factors based on carbon dioxide (CO₂) emission fluxes. Aerosol fluxes were measured over one year using the eddy covariance method at the top of a 105 m high communication tower in Stockholm, Sweden. Maximum CO₂ and particle fluxes were found when the wind direction coincided with the area of densest traffic within the footprint area. Negative fluxes (uptake of CO₂ and deposition of particles) coincided with periods of sampling from an urban forest area. The fluxes of CO₂ were used to obtain emission factors for particles by assuming that the CO₂ fluxes could be directly related to the amount of fuel burnt by vehicles in the footprint area. The estimated emission factor for the fleet mix in the measurement area was, in number 1.8 × 10¹¹ particle veh⁻¹ km⁻¹ (for 0.25–2.5 μm size range). Assuming spherical particles of density 1600 kg m⁻³ this corresponds to 27.5 mg veh⁻¹ km⁻¹. For particles (0.8–2.5 μm) the emission factors were 5.1 × 10⁹ veh⁻¹ km⁻¹ for number and 11.5 mg veh⁻¹ km⁻¹ for mass. But a wind speed dependence was noted for high wind speeds. Thus, for wind speeds larger than 9 m s⁻¹, as measured in the tower at 105 m (U₁₀₅), the emission factor for particle number and mass was parameterised as: E_f (Number, 0.8–2.5 μm) = (6.1 ± 1.7)10⁹ U₁₀₅ −50 ± 188 and E_f (Mass, 0.8–2.5 μm) = (20 ± 12) U₁₀₅ − 171 ±122.

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 (D_p). Emission velocities (v_e) were calculated for the same size range and were found to be well correlated with friction velocity (u_*) and CO₂ fluxes (F_CO2). These variables were used to parameterize the emission velocity aswhere v_e and u_* are given in [m s⁻¹], D_p in [μm], and F_CO2 in [mmol m⁻²s⁻¹].

The parameterization reproduces the average diurnal cycle from the observations well for particles sizes up to 0.6 μm D_p. 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_CO2, which represents the traffic source through its fossil fuel consumption and the related CO₂ emissions.