Heated submicron particle fluxes using an optical particle counter in urban environment

Abstract. From May 2008 to March 2009 aerosol emissions were measured using the eddy covariance method covering the size range 0.25 to 2.5 μm diameter (D_p) from a 105 m tower, in central Stockholm, Sweden. Supporting chemical aerosol data were collected at roof and street level. Results show that the inorganic fraction of sulfate, nitrate, ammonium and sea salt accounts for approximately 15% of the total aerosol mass < 1 μm D_p (PM₁) with water soluble soil contributing 11% and water insoluble soil 47%. Carbonaceous compounds were at the most 27% of PM₁ mass. It was found that heating the air from the tower to 200 °C resulted in the loss of approximately 60% of the aerosol volume at 0.25 μm D_p whereas only 40% of the aerosol volume was removed at 0.6 μm D_p. Further heating to 300 °C caused very little additional losses <0.6 μm D_p. The chemical analysis did not include carbonaceous compounds, but based on the difference between the total mass concentration and the sum of the analyzed non-carbonaceous materials, it can be assumed that the non-volatile particulate material (heated to 300 °C) consists mainly of carbonaceous compounds, including elemental carbon. Furthermore, it was found that the non-volatile particle fraction <0.6 μm D_p correlated (r² = 0.4) with the BC concentration at roof level in the city, supporting the assumption that the non-volatile material consists of carbonaceous compounds. The average diurnal cycles of the BC emissions from road traffic (as inferred from the ratio of the incremental concentrations of nitrogen oxides (NO_x) and BC measured on a densely trafficked street) and the fluxes of non-volatile material at tower level are in close agreement, suggesting a traffic source of BC. We have estimated the emission factors (EFs) for non-volatile particles <0.6 μm D_p to be 2.4 ± 1.4 mg veh⁻¹ km⁻¹ based on either CO₂ fluxes or traffic activity data. Light (LDV) and heavy duty vehicle (HDV) EFs were estimated using multiple linear regression and reveal that for non-volatile particulate matter in the 0.25 to 0.6 μm D_p range, the EF_HDV is approximately twice as high as the EF_LDV, the difference not being statistically significant.