Droplet activation behaviour of atmospheric black carbon particles in fog as a function of their size and mixing state
2018
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
diameter and hygroscopicity, facilitating droplet activation. This affects BC
radiative forcing through aerosol–cloud interactions (ACIs) and BC life
cycle. To gain insights into these processes, we performed a field campaign
in winter 2015–2016 in a residential area of Zurich which aimed at
establishing relations between the size and mixing state of BC particles and
their 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 in the morning hours of weekdays, 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 very low effective
peak supersaturations (SS peak ) 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
593 nm) 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 SS peak , while BC particles surrounded by
thick coating have very similar activation behaviour to BC-free particles.
Using simplified κ -Kohler 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.
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