Laboratory studies of ice nucleation onto bare and internally mixed soot–sulfuric acid particles

Abstract

Abstract. Soot particles are potential candidates for ice-nucleating particles in cirrus cloud formation, which is known to exert a net-warming effect on climate. Bare soot particles, generally hydrophobic and fractal ones, mainly exist near emission sources. Coated or internally mixed soot particles are more abundant in the atmosphere and have a higher probability of impacting cloud formation and climate. However, the ice nucleation ability of coated soot particles is not as well understood as that of freshly produced soot particles. In this laboratory study, two samples, a propane flame soot and a commercial carbon black, were used as atmospheric soot surrogates and coated with varying wt % of sulfuric acid (H2SO4). The ratio of coating material mass to the mass of bare soot particles was controlled and progressively increased from less than 5 wt % to over 100 wt %. Both bare and coated soot particle ice nucleation activities were investigated with a continuous-flow diffusion chamber operated at mixed-phase and cirrus cloud conditions. The mobility diameter and mass distribution of size-selected soot particles with/without H2SO4 coating were measured by a scanning mobility particle sizer and a centrifugal particle mass analyser running in parallel. The mixing state and morphology of soot particles were characterized by scanning electron microscopy and transmission electron microscopy. In addition, the evidence of the presence of H2SO4 on a coated soot particle surface is shown by energy-dispersive X-ray spectroscopy. Our study demonstrates that H2SO4 coatings suppress the ice nucleation activity of soot particles to varying degrees depending on the coating thickness, but in a non-linear fashion. Thin coatings causing pore filling in the soot aggregate inhibits pore condensation and freezing. Thick coatings promote particle ice activation via droplet homogeneous freezing. Overall, our findings reveal that H2SO4 coatings will suppress soot particle ice nucleation abilities in the cirrus cloud regime, having implications for the fate of soot particles with respect to cloud formation in the upper troposphere.

The dependence of soot particle ice nucleation ability on its volatile content

“…In addition, commercial carbon black FW200 and Printex90 (Orion Engineered Carbons GmbH, OEC, Frankfurt, Main, Germany) with different graphitization levels were used. Dry carbon black powder was sufficiently stirred for two weeks and then aerosolised by a dry dispersion setup introduced in Gao et al 13 Filtered and organic free air (20.9% O 2 ) source was used to generate untreated or air denuded carbon black aerosols, termed FW200, PR90, and FW200-air, PR90-air respectively. Pure N 2 source was deployed in the dry dispersion setup to establish an O 2 free thermal denuding atmosphere, by which generated soot samples are termed FW200-N 2 and PR90-N 2 .…”

“…11,12 Porosity characteristics and surface wettability are viewed as crucial and decisive properties for soot particle IN via PCF. [13][14][15][16] PCF favours both mesopore (2 nm < 𝐷𝐷 p (pore diameter) < 50 nm) 17 structures and a low soot-water contact angle to firstly induce supercooled water condensation in the pore. [18][19][20] An ice crystal can form via homogeneous freezing of supercooled pore water when the temperature (T) is lower than homogeneous nucleation temperature (HNT = 235 K) or by immersion freezing at T > 235 K if active sites are present within the pore and if the pore size is large enough to host an ice germ at the nucleation temperature.…”

Impacts of Cloud‐Processing on Ice Nucleation of Soot Particles Internally Mixed With Sulfate and Organics

“…However, small white dots on the mBrown‐Thin soot‐aggregate can be observed compared to an uncoated soot‐aggregate (mBrown) in Figure 4a, which is different from the thin coating case for mBlack‐Thin (Figure 3c). These could result from a chemical reaction of the organics with H 2 SO 4 coating material, for example, oxidation, which may produce volatile organics and H 2 O (Gao, Zhou, et al., 2022; Mahrt, Alpert, et al., 2020) or light‐molecular‐weight organics. The volatile reaction products can evaporate upon heating under the electron beam during TEM measurements, which explains the white dots on mBrown‐Thin (Figure 4c) presenting a lower mass contrast due to the decrease in mass and thickness caused by the chemical reaction.…”

“…Gao, Zhou, et al. (2022) also noted that the H 2 SO 4 coating effect on soot IN depends on the coating thickness. Thus, the combined aging of cloud‐processing and organic/H 2 SO 4 coating leads to a more complicated task to constrain soot IN.…”

“…(2016) did not observe pre‐activation for ice‐cloud‐processed graphite soot particles with a low organic content. It is also reported that the mixing state of soot particles with different organic (Crawford et al., 2011; Knopf & Koop, 2006; Möhler et al., 2005; Nichman et al., 2019; C. Zhang et al., 2020; Zobrist et al., 2006) or H 2 SO 4 coatings (DeMott et al., 1999; Friedman et al., 2011; Gao, Zhou, et al., 2022) can alter the IN activity of soot particles variably and nonlinearly. Organic particles containing different single species, for example, raffinose (Wagner et al., 2012) or alpha‐pinene (Wagner et al., 2017), already show diverse changes in IN after cloud‐processing.…”