Extensive Soot Compaction by Cloud Processing from Laboratory and Field Observations

Abstract

Soot particles form during combustion of carbonaceous materials and impact climate and air quality. When freshly emitted, they are typically fractal-like aggregates. After atmospheric aging, they can act as cloud condensation nuclei, and water condensation or evaporation restructure them to more compact aggregates, affecting their optical, aerodynamic, and surface properties. Here we survey the morphology of ambient soot particles from various locations and different environmental and aging conditions. We used electron microscopy and show extensive soot compaction after cloud processing. We further performed laboratory experiments to simulate atmospheric cloud processing under controlled conditions. We find that soot particles sampled after evaporating the cloud droplets, are significantly more compact than freshly emitted and interstitial soot, confirming that cloud processing, not just exposure to high humidity, compacts soot. Our findings have implications for how the radiative, surface, and aerodynamic properties, and the fate of soot particles are represented in numerical models.

The Impact of Cloud Processing on the Ice Nucleation Abilities of Soot Particles at Cirrus Temperatures

“…The difference in particle morphology is consistent with the findings of China et al. (), reporting higher roundness for soot particles from ice crystal residuals, as compared to nascent soot aggregates and also with the more recent findings of Bhandari et al (), who report strong compaction of soot particles upon cloud processing. In such soot aggregates, ice nucleation through PCF is more likely to take place due to the increase in available pore water and reduction of interpore spacing (David et al., ) and the higher overall spatial density of pores, enhancing the probability of pores with the right characteristics (size and contact angle) to be available for condensation of water and subsequent homogeneous freezing within the pore.…”

“…In fact, China et al. () and more recently Bhandari et al () have reported strongly compacted soot particles to be long range transported within the free troposphere. Moreover, scavenging of soot particles by cloud droplets was recently found to lead to large, compacted aggregates, that can be released back into the atmosphere in MPC conditions, when the cloud droplets evaporate at the expense of growing ice crystals (Ding et al., ).…”

“…Soot particles have recently been shown to form ice at temperatures relevant for cirrus cloud formation via the PCF mechanism (Mahrt et al., ; Nichman et al., ; Ullrich et al., ), which was found to be strongly dependent on the physicochemical particle properties, namely, hydrophilicity (soot‐water contact angle) as well as the availability of pores of the right size (Mahrt et al., ). Previous studies reported a compaction of fractal soot aggregates following condensation and evaporation cycles (Bhandari et al, ; Chamberlain et al., ; Colbeck et al., ; Huang et al., ; Ma et al., ; Tritscher et al., ; Zuberi et al., ) and upon ice formation on the soot aggregates (China et al., ). Furthermore, Miljevic et al.…”

“…Soot particles have recently been shown to form ice at temperatures relevant for cirrus cloud formation via the PCF mechanism (Mahrt et al, 2018;Nichman et al, 2019;Ullrich et al, 2017), which was found to be strongly dependent on the physicochemical particle properties, namely, hydrophilicity (soot-water contact angle) as well as the availability of pores of the right size (Mahrt et al, 2018). Previous studies reported a compaction of fractal soot aggregates following condensation and evaporation cycles (Bhandari et al, 2019;Chamberlain et al, 1975;Colbeck et al, 1990;Huang et al, 1994;Ma et al, 2013;Tritscher et al, 2011;Zuberi et al, 2005) and upon ice formation on the soot aggregates . Furthermore, Miljevic et al (2012) reported a compaction when they tested the restructuring of different soot types after exposure to water and a variety of organic solvents provided that the liquid can wet the particle and the degree of compaction found to be dependent on the initial fractal extent.…”

Aging induced changes in ice nucleation activity of combustion aerosol as determined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy

“…The increase in pore water available to initiate ice formation via PCF can emanate from both physical and chemical changes of the aerosol and specically pore properties upon aging. 32 The aging in aqueous solutions likely caused a restructuring in the soot particle morphology, similar to the compaction of soot particles observed during cloud processing in other studies, 27,86 that can contribute to an overall increase in the number of mesopores present and hence mesopore volume. At the same time, lowering the soot-water contact angle of the pores present could increase the mesopore volume accessible for water to condense at a lower RH w (compared to unaged soot) and hence PCF to occur even in the absence of any morphological change.…”