Black carbon physical properties and mixing state in the European megacity Paris

Abstract

Abstract. Aerosol hygroscopicity and black carbon (BC) properties were characterised during wintertime in Paris, one of the biggest European megacities. Hygroscopic growth factor (GF) distributions, characterised by distinct modes of more-hygroscopic background aerosol and non- or slightly hygroscopic aerosol of local (or regional) origin, revealed an increase of the relative contribution of the local sources compared to the background aerosol with decreasing particle size. BC particles in Paris were mainly originating from fresh traffic emissions, whereas biomass burning was only a minor contribution. The mass size distribution of the BC cores peaked on average at a BC core mass equivalent diameter of DMEV≈150 nm. The BC particles were moderately coated (Δcoat≈30 nm on average for BC cores with DMEV =160–260 nm) and an average mass absorption coefficient (MAC) of ~8.6 m2 g−1 at the wavelength λ = 880 nm was observed. Different time periods were selected to investigate the properties of BC particles as a function of source and air mass type. The traffic emissions were found to be non-hygroscopic (GF ≈ 1.0), and essentially all particles with a dry mobility diameter larger than D0 = 110 nm contained a BC core. BC from traffic emissions was further characterised by literally no coating (Δcoat ≈2 nm), the smallest maximum of the BC core mass size distribution (DMEV≈100 nm) and the smallest MAC (~7.3 m2 g−1 at λ = 880 nm). The biomass burning aerosol was slightly more-hygroscopic than the traffic emissions (with a distinct slightly hygroscopic mode peaking at GF≈1.1–1.2). Furthermore, only a minor fraction (⩽10%) of the slightly hygroscopic particles with GF⩾1.1 (and D0 = 265 nm) contained a detectable BC core. The BC particles from biomass burning were found to have a medium coating thickness as well as slightly larger mean BC core sizes and MAC values compared to traffic emissions. The aerosol observed under the influence of aged air masses and air masses from Eastern Continental Europe was dominated by a more-hygroscopic mode peaking at GF≈1.6. Most particles (95%) with a D0 = 265 nm, in this mode, did not contain a detectable BC core. A significant fraction of the BC particles had a substantial coating with non-refractory aerosol components. MAC values of ~8.8 m2g−1 and ~8.3 m2 g−1 at λ = 880 nm and mass mean BC core diameters of 150 nm and 200 nm were observed for the aged and continental air mass types, respectively. The reason for the larger BC core sizes compared to the fresh emissions – transport effects or a different BC source – remains unclear. The dominant fraction of the BC-containing particles was found to have no or very little coating with non-refractory matter. The lack of coatings is consistent with the observation that the BC particles are non- or slightly hygroscopic, which makes them poor cloud condensation nuclei.