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.)

摘要

Liquid clouds form by condensation of water vapour on aerosol particles inthe atmosphere. Even black carbon (BC) particles, which are known to beslightly hygroscopic, have been shown to readily form cloud droplets once theyhave acquired water-soluble coatings by atmospheric aging processes.Accurately simulating the life cycle of BC in the atmosphere, which stronglydepends on the wet removal following droplet activation, has recently beenidentified as a key element for accurate prediction of the climate forcing ofBC. Here, to assess BC activation in detail, we performed in situ measurementsduring cloud events at the Jungfraujoch high-altitude station in Switzerlandin summer 2010 and 2016. Cloud droplet residual and interstitial(unactivated) particles as well as the total aerosol were selectively sampledusing different inlets, followed by their physical characterization usingscanning mobility particle sizers (SMPSs), multi-angle absorption photometers(MAAPs) and a single-particle soot photometer (SP2). By calculating clouddroplet activated fractions with these measurements, we determined the rolesof various parameters on the droplet activation of BC. The half-risethreshold diameter for droplet activation(Dhalfcloud), i.e. the size above which aerosolparticles formed cloud droplets, was inferred from the aerosol sizedistributions measured behind the different inlets. The effective peaksupersaturation (SSpeak) of a cloud was derived fromDhalfcloud by comparing it to the supersaturationdependence of the threshold diameter for cloud condensation nuclei (CCN)activation measured by a CCN counter (CCNC). In this way, we showed that themass-based scavenged fraction of BC strongly correlates with that of theentire aerosol population because SSpeak modulates the criticalsize for activation of either particle type. A total of 50 % of theBC-containing particles with a BC mass equivalent core diameter of 90 nmwas activated in clouds with SSpeak≈0.21 %,increasing up to ∼80 % activated fraction atSSpeak≈0.50 %. On a single-particle basis, BCactivation at a certain SSpeak is controlled by the BC core sizeand internally mixed coating, which increases overall particle size andhygroscopicity. However, the resulting effect on the population averaged andon the size-integrated BC scavenged fraction by mass is small for tworeasons: first, acquisition of coatings only matters for small cores inclouds with low SSpeak; and, second, variations in BC core sizedistribution and mean coating thickness are limited in the lower freetroposphere in summer. Finally, we tested the ability of a simplified theoretical model, whichcombines the κ-Köhler theory with the Zdanovskii–Stokes–Robinson(ZSR) mixing rule under the assumptions of spherical core–shell particlegeometry and surface tension of pure water, to predict the droplet activationbehaviour of BC-containing particles in real clouds. Predictions of BCactivation constrained with SSpeak and measured BC-containingparticle size and mixing state were compared with direct cloud observations.These predictions achieved closure with the measurements for the particlesize ranges accessible to our instrumentation, that is, BC core diameters andtotal particle diameters of approximately 50 and 180 nm, respectively. Thisclearly indicates that such simplified theoretical models provide asufficient description of BC activation in clouds, as previously shown foractivation occurring in fog at lower supersaturation and also shown inlaboratory experiments under controlled conditions. This further justifiesapplication of such simplified theoretical approaches in regional and globalsimulations of BC activation in clouds, which include aerosol modules thatexplicitly simulate BC-containing particle size and mixing state.