Measurement Report: Small effect of regional sources on black carbon properties and concentrations in Southern Sweden background air

Abstract. Soot, or black carbon (BC), aerosol is a major climate forcer with severe health effects. The impacts depend strongly on particle number concentration, size and mixing state. This work reports on two field campaigns at nearby urban and rural sites, 65 km apart, in southern Sweden during late summer 2018. BC was measured using a single particle soot photometer (SP2) and Aethalometers (AE33). Differences in BC concentrations between the sites are driven primarily by local traffic emissions. Equivalent and refractory BC mass concentrations at the urban site were on average a factor 2.2 and 2.5 higher than at the rural site. Peaks in rush hour BC mass concentrations at the urban site were up to a factor ~4 higher than the background levels. The number fraction of particles containing a soot core was significantly higher in the city. BC particles at the urban site were on average smaller by mass and had less coating owing to fresh traffic emissions. The organic components of the fresh plumes were similar in mass spectral signature to “hydrocarbon-like organic aerosol” (HOA), commonly associated with traffic. Despite the intense local traffic (~30 000 vehicles passing per day), PM1, including organic aerosol, was dominated by aged continental air masses even at the curbside site. The fraction of thickly coated particles at the urban site was highly correlated with the mass concentrations of all measured chemical species of PM1, consistent with aged, internally mixed aerosol. Trajectory analysis for the whole year showed that air masses arriving at the rural site from eastern Europe contained approximately double the amount of BC compared to air masses from western Europe. Furthermore, BC from the largest region emissions in the Malmö/Copenhagen urban area transported to the rural site is discernable above background levels only when precipitation events are excluded.