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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 25 Sep 2018

Research article | 25 Sep 2018

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

Droplet activation behaviour of atmospheric black carbon particles in fog as a function of their size and mixing state

Ghislain Motos1, Julia Schmale1, Joel Christopher Corbin1,a, Marco Zanatta1,b, 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: Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany

Abstract. Among the variety of particle types present in the atmosphere, black carbon (BC), emitted by combustion processes, is uniquely associated with harmful effects to the human body and substantial radiative forcing of the Earth. Pure BC is known to be non-hygroscopic, but its ability to acquire a coating of hygroscopic organic and inorganic material leads to increased hygroscopicity as well as diameter, facilitating droplet activation. This affects BC radiative forcing through aerosol-cloud interactions (aci) and BC life cycle. To gain insights into these processes, we performed a field campaign in winter 2015/16 in a residential area of Zurich which aimed at distinguishing different particle mixing states regarding hygroscopic properties in the cloud condensation nuclei (CCN)-activated fraction spectrum of urban aerosol and establishing relations between the mixing state of BC and its activation to form droplets in fog. This was achieved by operating a CCN counter (CCNC), a scanning mobility particle sizer (SMPS), a single particle soot photometer (SP2) and an aerosol chemical speciation monitor (ACSM) behind a combination of a total- and an interstitial-aerosol inlet.

Our results indicate that, depending on the time of the day, we sampled both heavily aged internally mixed BC from background air advected to the site and freshly emitted externally mixed BC from local or regional traffic sources. During rush hours in the morning of weekdays, we found clear evidence that the enhanced traffic emissions caused peaks in the number fraction of externally mixed BC particles which do not act as CCN within the CCNC. The mixing state of BC particles was also found to play a key role in their ability to form fog droplets. The very low effective peak supersaturations (SSpeak) occurring in fog (between approximately 0.03 and 0.06% during this campaign) restrict droplet activation to a minor fraction of the aerosol burden (around 0.5 to 1% of total particle number concentration between 20 and 593nm) leading to very selective criteria on diameter and chemical composition. We show that bare BC cores are unable to activate to fog droplets at such low SSpeak, while BC particles surrounded by thick coating have a very similar activation behavior as BC-free particles. The threshold coating thickness required for activation was shown to decrease with increasing BC core size. Using simplified κ-Köhler theory combined with the ZSR mixing rule assuming spherical core-shell particle geometry constrained with single particle measurements of respective volumes, we found good agreement between the predicted and the directly observed size and mixing state resolved droplet activation behaviour of BC-containing particles in fog. This successful closure demonstrates the predictability of their droplet activation in fog with a simplified theoretical model only requiring size and mixing state information, which can also be applied in a consistent manner in model simulations.

Ghislain Motos et al.
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Short summary
Clouds form by condensation of water vapour on aerosol particles. We showed that black carbon, a subset of particles responsible for a climate warming due to their strong light absorption and known to be insoluble in water, were able to form droplets when the humidity of the air is very slightly over 100 %. This is made possible by their acquisition of a "coating" made of hydrophilic material during atmospheric aging. The predictability of this process using theory was successfully tested.
Clouds form by condensation of water vapour on aerosol particles. We showed that black carbon, a...