Black carbon physical properties and mixing state in the European megacity Paris M. Laborde1,*, M. Crippa1, T. Tritscher1,**, Z. Jurányi1,***, P. F. Decarlo1,****, B. Temime-Roussel2, N. Marchand2, S. Eckhardt3, A. Stohl3, U. Baltensperger1, A. S. H. Prévôt1, E. Weingartner1, and M. Gysel1 1Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland 2Aix-Marseille Université, CNRS, LCE FRE 3416, 13331, Marseille, France 3NILU – Norwegian Institute for Air Research, Kjeller, Norway *now at: AerosolConsultingML GmbH, Ennetbaden, Switzerland **now at: TSI GmbH, Particle Instruments, Aachen, Germany ***now at: Institute of Aerosol and Sensor Technology, University of Applied Sciences Northwestern Switzerland, Windisch, Switzerland ****now at: Department of Civil, Architectural, and Environmental Engineering, and Department of Chemistry, Drexel University, Philadelphia, PA, USA
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 DMEV ~ 150 nm. The
BC-containing particles were moderately coated (coating thickness
Δcoat ~ 33 nm on average for rBC cores with
DMEV = 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
(Δcoat ~ 2 nm ± 10 nm), to have
the smallest BC core sizes (maximum of the rBC core mass size distribution
at DMEV ~ 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.
Citation: Laborde, M., Crippa, M., Tritscher, T., Jurányi, Z., Decarlo, P. F., Temime-Roussel, B., Marchand, N., Eckhardt, S., Stohl, A., Baltensperger, U., Prévôt, A. S. H., Weingartner, E., and Gysel, M.: Black carbon physical properties and mixing state in the European megacity Paris, Atmos. Chem. Phys., 13, 5831-5856, doi:10.5194/acp-13-5831-2013, 2013.