The relationship between 0.25–2.5 μm aerosol and CO₂ emissions over a city

Abstract. 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.