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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Nov 2018

Research article | 07 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Cloud droplet activation properties and scavenged fraction of black carbon in liquid-phase clouds at the high-alpine research station Jungfraujoch (3580 m a.s.l.)

Ghislain Motos1, Julia Schmale1, Joel C. Corbin1,a, Robin Modini1, Nadine Karlen1,b, Michele Bertò1, Urs Baltensperger1, and Martin Gysel1 Ghislain Motos et al.
  • 1Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
  • anow at: Measurement Science and Standards, National Research Council Canada, 1200 Montreal Road, Ottawa K1A 0R6, Canada
  • bnow at: Institute for Aerosol and Sensor Technology, University of Applied Sciences (FHNW), Windisch, Switzerland

Abstract. Liquid clouds form by condensation of water vapour on aerosol particles in the atmosphere. Even black carbon (BC) particles, which are known to be little hygroscopic, have been shown to readily form cloud droplets once they have acquired water-soluble coatings by atmospheric aging processes. Accurately simulating the life cycle of BC in the atmosphere, which strongly depends on the wet removal following droplet activation, has recently been identified as a key element for accurate prediction of the climate forcing of BC.

Here, to assess BC activation in detail, we performed in-situ measurements during cloud events at the Jungfraujoch high mountain station in Switzerland in summer 2010 and 2016. Cloud droplet residual and interstitial (unactivated) particles as well as the total aerosol were selectively sampled using different inlets, followed by their physical characterization using scanning mobility particle sizers (SMPSs), multi-angle absorption photometers (MAAPs) and a single particle soot photometer (SP2). By calculating cloud droplet activated fractions with these measurements, we determined the roles of various parameters on the droplet activation of BC. The half-rise threshold diameter for droplet activation (Dhalfcloud), i.e. the size above which aerosol particles formed cloud droplets, was inferred from the aerosol size distributions measured behind the different inlets. The effective peak supersaturation (SSpeak) of a cloud was derived from Dhalfcloud by comparing it to the supersaturation dependence of the threshold diameter for cloud condensation nuclei (CCN) activation measured by a CCN counter (CCNC). In this way we showed that the mass-based scavenged fraction of BC strongly correlates with that of the entire aerosol population because SSpeak modulates the critical size for activation of either particle type. Fifty percent of the BC-containing particles with a BC mass equivalent core diameter of 90nm were activated in clouds with SSpeak0.21%, increasing up to ~80% activated fraction at SSpeak0.5%. On a single particle basis, BC activation at a certain SSpeak is controlled by the BC core size and internally mixed coating which increases overall particle size and hygroscopicity. However, the resulting effect on the population averaged and on the size integrated BC scavenged fraction by mass is small for two reasons: first, acquisition of coatings only matters for small cores in clouds with low SSpeak and, second, variations in BC core size distribution and mean coating thickness are limited in the lower free troposphere in summer.

Ghislain Motos et al.
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Status: final response (author comments only)
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Ghislain Motos et al.
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Short summary
Atmospheric Black carbon (BC) particles are strong light absorbers that contribute to global warming. In-situ cloud measurements performed at a high altitude site showed that cloud supersaturation mainly drives the activation of BC to cloud droplets. It was further shown how BC particle size and mixing state modulate this nucleation scavenging in agreement with simplified theoretical predictions. These findings can inform model simulations towards a better representation of the BC life-cycle.
Atmospheric Black carbon (BC) particles are strong light absorbers that contribute to global...