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

Abstract

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.

Black Carbon Particles Do Not Matter for Immersion Mode Ice Nucleation

“…However, significant ice nucleation in the cirrus regime at T < 235 K is reported in the same studies (Table 2). Studies reporting immersion freezing of soot using large surface areas per droplet from large aggregates or coagulated particles or using large droplet sizes (Brooks et al, 2014; Dymarska et al, 2006; Gorbunov et al, 2001; Popovicheva et al, 2008; Vergara‐Temprado et al, 2018), have been excluded from Table 2. The data from the literature and from this study support the idea that soot particles derived from fossil and hydrocarbon fuel combustion are inactive INPs in the immersion freezing mode relevant to the MPC regime.…”

Deposition of ionic species and black carbon to the Arctic snowpack: combining snow pit observations with modeling

“…Further improvements regarding the modeling of the Arctic snowpack are needed to better address physical properties (e.g., the evolution of the snow density) and post-depositional processes acting upon the vertical distribution of impurities in the snowpack. Although the treatment of impurities was recently implemented into the CRO-CUS snowpack model (Tuzet et al, 2017), the impact of processes modifying the vertical distribution of impurities in the Arctic snowpack like blowing snow, sublimation, and percolation are still not fully considered in most models. The full implementation of post-depositional processes into complex snow models may offer the opportunity to exploit further snowpack and ice core observations for the reconstruction of climate and pollution.…”

“…The recommended method to determine dry deposition relies mostly on the calculation of fluxes based on atmospheric composition and an estimated dry deposition velocity (Vet et al, 2014), which shows a large uncertainty. In the case of BC, the calculated deposition varies considerably across models since it depends on the applied assumptions and parameters concerning the size of the aerosols and the mixing state (Bond et al, 2013).…”

Bounding Global Aerosol Radiative Forcing of Climate Change

“…An increase in cloud ice through an increase in the number of INPs might be expected to increase precipitation rates (Field & Heymsfield, ; Lohmann, ), but the resulting impact on cloud water and amount along with the corresponding radiative effect of anthropogenic aerosols is currently not well constrained, with GCM studies suggesting the net overall effect to be small (Hoose et al, ; Lohmann, ). Models that include explicit treatment of INP sources and cloud microphysics at high resolution suggest a strong link between (and reflected SW radiation) and INP driven by changes in precipitation (Vergara‐Temprado, Holden, et al, ).…”