Direct Quantification of Droplet Activation of Ambient Black Carbon Under Water Supersaturation

Abstract

Understanding the droplet activation of black carbon (BC) under water supersaturation (SS) conditions is crucial to estimate its lifetime (through activation scavenging) and indirect radiative impacts. However, directly probing the droplet activation of ambient BC is challenging because of the difficulty in discriminating the BC‐containing particles (BCc) from other components. Here, we coupled the measurements of number concentrations between size‐resolved cloud condensation nuclei (CCN) and individual BCc, achieving a direct quantification of droplet activation for ambient BCc. The activation diameters of BCc (D50_BCc), determined as the dry diameter at which 50% of BCc were activated, were 144 ± 21 nm under SS 0.2% corresponding to activated number fraction of total BCc of 60% ± 10%. D50_BCc was determined as a function of the coating hygroscopicity, and the organic content was observed as a limiting factor in controlling BCc activation. The activated BCc number concentration was predicted using the volume‐weighted compositions within particle. The predicted CCN agreed well with the measurements for all particles, while overestimating the activated BCc number concentration by 71%, 27%, and 27% at SS = 0.1%, 0.2%, and 0.3%, respectively. The higher discrepancy under lower SS may result from the higher non‐sphericity of BCc at large sizes. Therefore, only knowing the hygroscopicity distribution of bulk compositions may not be sufficient for estimating the activation of BCc, but considering additional factors such as the non‐sphericity of BCc is necessary to estimate droplet activation.

Vertical distribution of black carbon and its mixing state in urban boundary layer in summer

“…The hygroscopicity of BC-containing particles was calculated based on the volume-weighted Zdanovskii-Stokes-Robinson (ZSR) rule (Stokes and Robinson, 1966) by assuming that the hygroscopicity parameters of BC and coatings were 0 and 0.3, respectively (Pringle et al, 2010). A detailed calculation of the hygroscopicity can be found in (Hu et al, 2021a).…”

“…The present study mainly focused on the vertical distribution of BC mass concentrations (Wang et al, 2018a;Samad et al, 2020;Wu et al, 2021a;Guan et al, 2022). For instance, Lu et al (2019) found that the vertical profile of BC mass concentration could be classified into four types, and the variation in the vertical profile was related to the boundary layer variation and local emissions.…”

“…For instance, Lu et al (2019) found that the vertical profile of BC mass concentration could be classified into four types, and the variation in the vertical profile was related to the boundary layer variation and local emissions. Based on the measured BC vertical concentration, the precise evaluation of BC's radiative property distribution requires additional BC related information, such as size distribution and mass absorption cross section, which is a property that is commonly assumed in previous studies (Wu et al, 2021a). In fact, the optical properties could be significantly influenced by BC's microphysical properties.…”

Constraints on the Role of Laplace Pressure in Multiphase Reactions and Viscosity of Organic Aerosols

“…Note that in-flame nucleation and soot formation results in particles greater than ∼10 nm (Johansson et al, 2018); nanosphere soot and tarballs have diameters of ∼50 and ∼250 nm, respectively (Adachi et al, 2019). Though compaction of soot occurs by capillary forces (Bhandari et al, 2019;D. Hu et al, 2021;Leung et al, 2017;Ma et al, 2013), the pressure in these accumulation-mode particles is also outside the range for which condensed-phase reaction rates are altered (Figure 1, panel a).…”

“…Note that in‐flame nucleation and soot formation results in particles greater than ∼10 nm (Johansson et al., 2018); nanosphere soot and tarballs have diameters of ∼50 and ∼250 nm, respectively (Adachi et al., 2019). Though compaction of soot occurs by capillary forces (Bhandari et al., 2019; D. Hu et al., 2021; Leung et al., 2017; Ma et al., 2013), the pressure in these accumulation‐mode particles is also outside the range for which condensed‐phase reaction rates are altered (Figure 1, panel a). Given that cycloadditions including the Diels‐Alder reaction are the most dramatically accelerated by increased pressure, oligomerization of furanoid compounds during the nucleation and growth of new particles in sunlit biomass burning plumes is likely affected by particle internal pressure.…”