Is Black Carbon an Unimportant Ice‐Nucleating Particle in Mixed‐Phase Clouds?

It has been hypothesized that black carbon (BC) influences mixed‐phase clouds by acting as an ice‐nucleating particle (INP). However, the literature data for ice nucleation by BC immersed in supercooled water are extremely varied, with some studies reporting that BC is very effective at nucleating ice, whereas others report no ice‐nucleating ability. Here we present new experimental results for immersion mode ice nucleation by BC from two contrasting fuels (n‐decane and eugenol). We observe no significant heterogeneous nucleation by either sample. Using a global aerosol model, we quantify the maximum relative importance of BC for ice nucleation when compared with K‐feldspar and marine organic aerosol acting as INP. Based on the upper limit from our laboratory data, we show that BC contributes at least several orders of magnitude less INP than feldspar and marine organic aerosol. Representations of its atmospheric ice‐nucleating ability based on older laboratory data produce unrealistic results when compared against ambient observations of INP. Since BC is a complex material, it cannot be unambiguously ruled out as an important INP species in all locations at all times. Therefore, we use our model to estimate a range of values for the density of active sites that BC particles must have to be relevant for ice nucleation in the atmosphere. The estimated values will guide future work on BC, defining the required sensitivity of future experimental studies. Plain Language Summary The ability of black carbon particles to trigger the formation of ice from liquid cloud droplets is assessed by combining laboratory measurement of its efficiency with a global aerosol model. The new estimates suggest that these particles are not as efficient for ice nucleation when immersed in water droplets as previously thought and therefore not as atmospherically relevant. Key Points Soot particles generated from two different fuels do not nucleate ice above the handling blanks when immersed in water droplets Previous representations of the ice‐nucleating ability of BC particles overestimate its importance in the atmosphere We use a global aerosol model to estimate the required ice‐nucleating efficiency of BC for it to compete with other INP in the atmosphere