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.)
Authors: Motos Ghislain , Schmale Julia , Corbin Joel C. , Modini Rob. L. , Karlen Nadine , Bertò Michele , Baltensperger Urs , Gysel-Beer Martin 
Published: March 2019

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
slightly 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-altitude 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. A total of 50 % of the
BC-containing particles with a BC mass equivalent core diameter of 90 nm
was activated in clouds with SSpeak≈0.21 %,
increasing up to ∼80 % activated fraction at
SSpeak≈0.50 %. 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. Finally, we tested the ability of a simplified theoretical model, which
combines the κ-Köhler theory with the Zdanovskii–Stokes–Robinson
(ZSR) mixing rule under the assumptions of spherical core–shell particle
geometry and surface tension of pure water, to predict the droplet activation
behaviour of BC-containing particles in real clouds. Predictions of BC
activation constrained with SSpeak and measured BC-containing
particle size and mixing state were compared with direct cloud observations.
These predictions achieved closure with the measurements for the particle
size ranges accessible to our instrumentation, that is, BC core diameters and
total particle diameters of approximately 50 and 180 nm, respectively. This
clearly indicates that such simplified theoretical models provide a
sufficient description of BC activation in clouds, as previously shown for
activation occurring in fog at lower supersaturation and also shown in
laboratory experiments under controlled conditions. This further justifies
application of such simplified theoretical approaches in regional and global
simulations of BC activation in clouds, which include aerosol modules that
explicitly simulate BC-containing particle size and mixing state.