The effect of local sources on aerosol particle number size distribution, concentrations and fluxes in Helsinki, Finland

Three years of aerosol particle number concentrations (PNCs), size distributions and vertical particle fluxes measured at the semi-urban SMEAR III station in Helsinki, Finland, were studied. The purpose is to study the local emission sources and their effect on particle concentrations and size distributions. By means of cluster analysis, six representative size distributions were identified. Their occurrence together with particle concentrations and fluxes were found to vary significantly with wind direction. Lower particle concentrations and fluxes were measured downwind from vegetated and residential areas compared to directions where the measurement site is downwind from roads passing near the measurement site. For these directions, contributions of the local sources on the measured particle concentrations and size distributions were evident. In particular, size distributions with a mode in the size range 20–40 nm were found to be more affected by local traffic emissions, whereas the mode shifted towards larger sizes when contribution from distant sources was more evident. Using flux footprint functions, mixed vehicle fleet emission factors (EFs) were derived from the particle flux measurements. EFs of 6.03 (±0.19)·10¹⁴ # Veh⁻¹ km⁻¹ and 3.65 (±0.12)·10¹⁴ # Veh⁻¹ km⁻¹ were estimated for cold (October–March) and warm (April–September) periods, respectively. Emission factors increased with decreasing air temperature (T) following a linear relationship EF=−0.20·10¹⁴ # Veh⁻¹ km⁻¹°C⁻¹ T+6.98·10¹⁴ # Veh⁻¹ km⁻¹ (RMSE=3.7·10¹⁴ # Veh⁻¹ km⁻¹).