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. Among the variety of particle types present in the atmosphere,black carbon (BC), emitted by combustion processes, is uniquely associatedwith harmful effects to the human body and substantial radiative forcing ofthe Earth. Pure BC is known to be non-hygroscopic, but its ability to acquirea coating of hygroscopic organic and inorganic material leads to increaseddiameter and hygroscopicity, facilitating droplet activation. This affects BCradiative forcing through aerosol–cloud interactions (ACIs) and BC lifecycle. To gain insights into these processes, we performed a field campaignin winter 2015–2016 in a residential area of Zurich which aimed atestablishing relations between the size and mixing state of BC particles andtheir activation to form droplets in fog. This was achieved by operating aCCN counter (CCNC), a scanning mobility particle sizer (SMPS), asingle-particle soot photometer (SP2) and an aerosol chemical speciationmonitor (ACSM) behind a combination of a total- and an interstitial-aerosolinlet. Our results indicate that in the morning hours of weekdays, the enhancedtraffic emissions caused peaks in the number fraction of externally mixed BCparticles, which do not act as CCN within the CCNC. The very low effectivepeak supersaturations (SSpeak) occurring in fog (betweenapproximately 0.03 % and 0.06 % during this campaign) restrictdroplet activation to a minor fraction of the aerosol burden (around0.5 % to 1 % of total particle number concentration between 20 and593 nm) leading to very selective criteria on diameter and chemicalcomposition. We show that bare BC cores are unable to activate to fogdroplets at such low SSpeak, while BC particles surrounded bythick coating have very similar activation behaviour to BC-free particles.Using simplified κ-Köhler theory combined with the ZSR mixingrule assuming spherical core–shell particle geometry constrained with single-particle measurements of respective volumes, we found good agreement betweenthe predicted and the directly observed size- and mixing-state-resolveddroplet activation behaviour of BC-containing particles in fog. Thissuccessful closure demonstrates the predictability of their dropletactivation in fog with a simplified theoretical model only requiring size andmixing state information, which can also be applied in a consistent manner inmodel simulations.

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