The effects of morphology, mobility size and SOA material coating on the ice nucleation activity of black carbon in the cirrus regime

Abstract

Abstract. There is evidence that black carbon (BC) particles may affect cirrus formation and hence global climate by acting as potential ice nucleating particles (INPs) in the troposphere. Nevertheless, the ice nucleation (IN) ability of bare BC and BC coated with secondary organic aerosol (SOA) material remains uncertain. We have systematically examined the IN ability of 100–400 nm size-selected BC particles with different morphologies and different SOA coatings representative of anthropogenic (toluene and n-dodecane) and biogenic (β-caryophyllene) sources in the cirrus regime (−46 to −38 °C). Several aerosolized BC proxies were selected to represent different particle morphologies and oxidation levels. Atmospheric aging was further replicated with exposure of SOA-coated BC to OH. The results demonstrate that the 400 nm hydrophobic BC types nucleate ice only at or near the homogeneous freezing threshold (−42 to −46 ˚C). Deposition IN, as opposed to purely homogeneous freezing, was observed to occur for some BC types between 100–200 nm within the investigated temperature range. More fractal BC particles did not consistently act as superior deposition INPs over more spherical ones. SOA coating generated by oxidizing β-caryophyllene with O3 did not seem to affect BC IN ability. However, SOA coatings generated from OH oxidation of various organic species did exhibit higher IN onset supersaturation ratio with respect to ice (SSi) compared with bare BC particles, with toluene SOA coating showing an increase of SSi by 0.1–0.15 while still below the homogeneous threshold. n-dodecane and β-caryophyllene-derived SOA only froze in the homogeneous regime. We attribute the inhibition of IN ability to the filling of the pores on the BC surface by the SOA material coating. OH exposure levels of all SOA coating experiments, from an equivalent atmospheric 10 days to 90 days, did not render significant differences in IN potential. Our study suggests that BC particles with large sizes and/or oxidized surfaces generally exhibit better IN ability, and that the organic coating materials can inhibit ice formation.

Soot-PCF: Pore condensation and freezing framework for soot aggregates

“…Mahrt et al (2020b), who found the ice nucleation ability of processed, compacted soot particles to be significantly higher compared to unprocessed, more fractal soot. This is further supported by recent result of Zhang et al (2020), who investigated different laboratory analogues of soot and found the largest ice nucleation activity to be associated with the most spherical, i.e. compacted soot type.…”

“…Soot-PCF requires pore space available for water to condense. Hydrophobic coatings can fill the pores and make them unavailable for PCF, as has been demonstrated recently for soot coated with different types of secondary organic material (Zhang et al, 2020). Similarly, ice nucleation by soot generated from propane flames was found to shift to higher ice supersaturation and eventually vanish with increasing organic carbon content (Crawford et al, 2011;Mahrt et al, 2018;Möhler et al, 2005b).…”

“…Similarly, ice nucleation by soot generated from propane flames was found to shift to higher ice supersaturation and eventually vanish with increasing organic carbon content (Crawford et al, 2011;Mahrt et al, 2018;Möhler et al, 2005b). More hydrophilic coatings such as strongly oxidized secondary organic material (Zhang et al, 2020) and sulfuric acid (Möhler et al, 2005b(Möhler et al, , 2005a had a less prohibiting effect on the ice nucleation activity but nevertheless increased freezing onsets to higher relative humidity. At the same time, sulfuric acid aging can oxidize and/or remove organic material residing within the soot pores and thus enhance their susceptibility to take up water by capillary condensation and soot-PCF (Mahrt et al, 2020a).…”