Hygroscopic mixing state of urban aerosol derived from size-resolved cloud condensation nuclei measurements during the MEGAPOLI campaign in Paris 1Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
08 Jul 2013
2CNRS-GAME/Météo France, 42 av. Gaspard Coriolis, 31057 Toulouse, France
3Scripps Institution of Oceanography, 9300 Gilman Dr., La Jolla CA 92093, USA
*now at: Institute of Aerosol and Sensor Technology, University of Applied Sciences Northwestern Switzerland, Klosterzelgstrasse 2, 5210 Windisch, Switzerland
**now at: TSI GmbH, Particle Instruments, Neuköllner Strasse 4, 52068 Aachen, Germany
***now at: Ecotech PtyLtd, Knoxfield, Australia and AerosolConsultingML GmbH, Ennetbaden, Switzerland
Received: 03 Dec 2012 – Published in Atmos. Chem. Phys. Discuss.: 21 Jan 2013Abstract. Ambient aerosols are a complex mixture of particles with different physical
and chemical properties and consequently distinct hygroscopic behaviour. The
hygroscopicity of a particle determines its water uptake at subsaturated
relative humidity (RH) and its ability to form a cloud droplet at
supersaturated RH. These processes influence Earth's climate and the
atmospheric lifetime of the particles.
Revised: 23 May 2013 – Accepted: 03 Jun 2013 – Published: 08 Jul 2013
Cloud condensation nuclei (CCN) number size distributions (i.e. CCN number
concentrations as a function of dry particle diameter) were measured close to
Paris during the MEGAPOLI campaign in January–February 2010, covering 10
different supersaturations (SS = 0.1–1.0%). The time-resolved
hygroscopic mixing state with respect to CCN activation was also derived from
these measurements. Simultaneously, a hygroscopicity tandem differential
mobility analyser (HTDMA) was used to measure the hygroscopic growth factor
(ratio of wet to dry mobility diameter) distributions at
RH = 90%. The aerosol was highly externally mixed and its
mixing state showed significant temporal variability. The average particle
hygroscopicity was relatively low at subsaturation (RH = 90%; mean hygroscopicity parameter κ = 0.12–0.27) and increased with
increasing dry diameter in the range 35–265 nm. The mean κ
value, derived from the CCN measurements at supersaturation, ranged from 0.08
to 0.24 at SS = 1.0–0.1%.
Two types of mixing-state resolved hygroscopicity closure studies were
performed, comparing the water uptake ability measured below and above
saturation. In the first type the CCN counter was connected in series with the HTDMA and and
closure was achieved over the whole range of probed dry diameters, growth
factors and supersaturations using the κ-parametrization for the water
activity and assuming surface tension of pure water in the Köhler theory.
In the second closure type we compared hygroscopicity distributions derived
from parallel monodisperse CCN measurements and HTDMA measurements. Very good
agreement was found at all supersaturations, which shows that monodisperse CCN
measurements are a reliable alternative to determine the hygroscopic mixing
state of ambient aerosols.
Citation: Jurányi, Z., Tritscher, T., Gysel, M., Laborde, M., Gomes, L., Roberts, G., Baltensperger, U., and Weingartner, E.: Hygroscopic mixing state of urban aerosol derived from size-resolved cloud condensation nuclei measurements during the MEGAPOLI campaign in Paris, Atmos. Chem. Phys., 13, 6431-6446, doi:10.5194/acp-13-6431-2013, 2013.