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. Atmospheric black carbon (BC) particles are a concern due to their impact onair quality and climate. Their net climate effect is, however, stilluncertain. This uncertainty is partly related to the contribution of coatedBC particles to the global cloud condensation nuclei (CCN) budgets. In this study, laboratorymeasurements were performed to investigate CCNactivity of BC (REGAL 400R pigment black) particles, in pure state or coated throughevaporating and subsequent condensation of glutaric acid, levoglucosan (bothwater-soluble organics) or oleic acid (an organic compound with lowsolubility). A combination of soot particle aerosol mass spectrometer(SP-AMS) measurements and size distribution measurements with a scanningmobility particle sizer (SMPS) showed that the studied BC particles werenearly spherical agglomerates with a fractal dimension of 2.79 and that theywere coated evenly by the organic species. The CCN activity of BC particlesincreased after coating with all the studied compounds and was governed bythe fraction of organic material. The CCN activation of the BC particlescoated by glutaric acid and levoglucosan were in good agreement with thetheoretical calculations using the shell-and-core model, which is based on acombination of the CCN activities of the pure compounds. The oleic acidcoating enhanced the CCN activity of the BC particles, even though the pureoleic acid particles were CCN inactive. The surprising effect of oleic acidmight be related to the arrangement of the oleic acid molecules on thesurface of the BC cores or other surface phenomena facilitating watercondensation onto the coated particles. Our results show that presenttheories have potential for accurately predicting the CCN activity ofatmospheric BC coated with organic species, given that the identities andamounts of the coating species are known. Furthermore, our results suggestthat even relatively thin soluble coatings (around 2 nm for the compoundsstudied here) are enough to make the insoluble BC particles CCN active attypical atmospheric supersaturations and thus be efficiently taken up bycloud droplets. This highlights the need for an accurate description of thecomposition of atmospheric particles containing BC to unravel their netimpact on climate.

Cloud droplet activation of black carbon particles coated with organic compounds of varying solubility