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

Abstract. Aerosol hygroscopicity and refractory black carbon (rBC) properties were characterised during wintertime at a suburban site in Paris, one of the biggest European cities. Hygroscopic growth factor (GF) frequency 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-containing particles in Paris were mainly originating from fresh traffic emissions, whereas biomass burning only gave a minor contribution. The mass size distribution of the rBC cores peaked on average at an rBC core mass equivalent diameter of D_MEV ~ 150 nm. The BC-containing particles were moderately coated (coating thickness Δ_coat ~ 33 nm on average for rBC cores with D_MEV = 180–280 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-containing 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 (D₀) larger than D₀ = 110 nm contained an rBC core. rBC from traffic emissions was further observed to be uncoated within experimental uncertainty (Δ_coat ~ 2 nm ± 10 nm), to have the smallest BC core sizes (maximum of the rBC core mass size distribution at D_MEV ~ 100 nm) and to have the smallest MAC (~ 7.3 m²g⁻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 1.1 ≤ GF ≤ 1.2 (and D₀ = 265 nm) contained a detectable rBC core. The BC-containing particles from biomass burning were found to have a medium coating thickness as well as slightly larger mean rBC 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%), in the more-hygroscopic mode at D₀ = 265 nm, did not contain a detectable rBC core. A significant fraction of the BC-containing particles had a substantial coating with non-refractory aerosol components. MAC values of ~ 8.8 m²g⁻¹ and ~ 8.3 m²g⁻¹ at λ = 880 nm and mass mean rBC core diameters of 150 nm and 200 nm were observed for the aged and continental air mass types, respectively. The reason for the larger rBC core sizes compared to the fresh emissions – transport effects or a different rBC 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-containing particles are non- or slightly-hygroscopic, which makes them poor cloud condensation nuclei. It can therefore be expected that wet removal through nucleation scavenging is inefficient for fresh BC-containing particles in urban plumes. The mixing-state-specific cloud droplet activation behaviour of BC-containing particles including the effects of atmospheric aging processes should be considered in global simulations of atmospheric BC, as the wet removal efficiency remains a major source of uncertainty in its life-cycle.