Enhanced soot particle ice nucleation ability induced by aggregate compaction and densification

Soot particles, acting as ice nucleating particles (INPs), can contribute to cirrus cloud formation which has an important influence on climate. Aviation activities emitting soot particles in the upper troposphere can potentially impact ice nucleation (IN) in cirrus clouds. Pore condensation and freezing (PCF) is an important ice formation pathway for soot particles in the cirrus regime, which requires the soot INP to have specific morphological properties, i.e. mesopore structures. In this study, the morphology and pore size distribution of two kinds of soot samples were modified by a physical agitation method without any chemical modification, by which more compacted soot sample aggregates could be produced compared to the unmodified sample. The IN activities of both fresh and compacted soot particles with different sizes, 60, 100, 200 and 400 nm, were systematically tested by the Horizontal Ice Nucleation Chamber (HINC) under mixed-phase and cirrus clouds relevant temperatures (T). Our results show that soot particles are unable to form ice crystals at T > 235 K (homogeneous nucleation temperature, HNT) but IN was observed for compacted and larger size soot aggregates (> 200 nm) well below homogeneous freezing relative humidity (RHhom) at T < HNT, demonstrating PCF as the dominating mechanism for soot IN. We also observed that mechanically compacted soot particles can reach a higher particle activation fraction (AF) value for the same T and RH condition, compared to the same aggregate size fresh soot particles. The results also reveal a clear size dependence for the IN activity of soot particles with the same agitation degree, showing that compacted soot particles with large sizes (200 and 400 nm) are more active INPs and can convey the single importance of soot aggregate morphology for the IN ability. In order to understand the role of soot aggregate morphology for its IN activity, both fresh and compacted soot samples were characterized systematically using particle mass and size measurements, comparisons from TEM (transmission electron microscopy) images, soot porosity characteristics from argon (Ar) and nitrogen (N2) physisorption measurements, as well as soot-water interaction results from DVS (dynamic vapor sorption) measurements. Considering the soot particle physical properties along with its IN activities, the enhanced IN abilities of compacted soot particles are attributed to decreasing mesopore width and increasing mesopore occurrence probability due to the compaction process.

Analysis of the physical properties, composition and structure of soot particles

A soot particle consisting of crystallites representing several parallel layers of densely packed hexagons with carbon atoms located at the vertices is considered. Such a crystallite structure is similar to a graphite crystal lattice, but less ordered with large distances between the layers, where individual layers can be arbitrarily rotated relative to each other and relative to their common normal. It is shown that the core of a particle with a high salt content of exhaust gases (EG) of a diesel internal combustion engine (DICE) is discharged more, which allows us to distinguish models of the structure of a spherical nanoscale particle.

A Numerical and Experimental Study of Soot Precursor and Primary Particle Size of N-Butylbenzene in Laminar Flame

The current study analyzed the soot precursor of the n-butylbenzene found in diesel and kerosene in laminar flame, and integrated the corresponding poly-aromatic hydrocarbon (PAH) growth mechanism with the popular n-butylbenzene oxidation mechanisms to improve the soot formation prediction of n-butylbenzene. The size of soot precursor was determined by the fringe length in the core of soot particle since the nanostructure of the core of soot particle is similar with that of nascent soot particle formed by soot precursor nucleation. The geometric mean fringe length in core of soot particles was measured to be 0.67 nm approximating to the size of five-ringed PAH (A5). An A5 growth mechanism was added on a popular n-butylbenzene mechanism, and the combined mechanism was further reduced. After validation by the ignition delay time in literature, the combined mechanism was then validated by the primary particle diameter in laboratory and soot volume fraction of n-propylbenzene in literature. The calculated soot precursor concentration and PAH condensation rate of the combined mechanism are smaller than that of the base mechanism. The simulated primary soot particle diameter of proposed combined mechanism agrees well with the measure primary soot particle diameter. Comparing to the simulated soot volume fraction of base n-butylbenzene mechanism, the simulated soot volume fraction of proposed combined n-butylbenzene-A5 mechanism agrees well with the measure soot volume fraction of n-propylbenzene in literature. This study provides certain support for further investigation of soot formation of n-butylbenzene and its relative fuel like diesel and kerosene.

Immersion Freezing Ability of Freshly Emitted Soot with Various Physico-Chemical Characteristics

The immersion freezing ability of soot particles has in previous studies been reported in the range of low/insignificant to very high. The aims of this study were to: (i) perform detailed physico-chemical characterisation of freshly produced soot particles with very different properties, (ii) investigate the immersion freezing ability of the same particles, and (iii) investigate the potential links between physico-chemical particle properties and ice-activity. A miniCAST soot generator was used to produce eight different soot samples representing a wide range of physico-chemical properties. A continuous flow diffusion chamber was used to study each sample online in immersion mode over the temperature (T) range from −41 to −32 °C, at a supersaturation of about 10% with respect to liquid water. All samples exhibited low to no heterogeneous immersion freezing. The most active sample reached ice-activated fractions (AF) of 10−3 and 10−4 at temperatures of 1.7 and 1.9 K , respectively, above the homogeneous freezing temperature. The samples were characterized online with respect to a wide range of physico-chemical properties including effective particle density, optical properties, particle surface oxidation and soot maturity. We did observe indications of increasing immersion freezing ice-activity with increasing effective particle density and increasing particulate PAH fraction. Hence, those properties, or other properties co-varying with those, could potentially enhance the immersion freezing ice-activity of the studied soot particle types. However, we found no significant correlation between the physico-chemical properties and the observed ice-nucleating ability when the particle ensemble was extended to include previously published results including more ice-active biomass combustion soot particles. We conclude that it does not appear possible in general and in any straightforward way to link observed soot particle physico-chemical properties to the ice-nucleating ability using the online instrumentation included in this study. Furthermore, our observations support that freshly produced soot particles with a wide range of physico-chemical properties have low to insignificant immersion freezing ice-nucleating ability.

Exploring Nanomechanical Properties of Soot Particle Layers by Atomic Force Microscopy Nanoindentation

In this work, an experimental investigation of the nanomechanical properties of flame-formed carbonaceous particle layers has been performed for the first time by means of Atomic Force Microscopy (AFM). To this aim, carbon nanoparticles with different properties and nanostructures were produced in ethylene/air laminar premixed flames at different residence times. Particles were collected on mica substrates by means of a thermophoretic sampling system and then analyzed by AFM. An experimental procedure based on the combination between semi-contact AFM topography imaging, contact AFM topography imaging and AFM force spectroscopy has been implemented. More specifically, a preliminary topological characterization of the samples was first performed operating AFM in semi-contact mode and then tip-sample interaction forces were measured in contact spectroscopy mode. Finally, semi-contact mode was used to image the indented surface of the samples and to retrieve the projected area of indents. The hardness of investigated samples was obtained from the force–distance curves measured in spectroscopy mode and the images of intends acquired in semi-contact mode. Moreover, the Young’s modulus was measured by fitting the linear part of the retraction force curves using a model based on the Hertz theory. The extreme force sensitivity of this technique (down to nNewton) in addition to the small size of the probe makes it extremely suitable for performing investigation of mechanical properties of materials at the nanoscale. The experimental procedure was successfully tested on reference materials characterized by different plastic behavior, e.g., polyethylene naphthalate and highly oriented pyrolytic graphite. Both hardness and Young’s modulus values obtained from AFM measurements for different soot particle films were discussed.

Ice nucleation activities of soot particles internally mixed with sulphuric acid at cirrus cloud conditions

Soot particles are important candidates for ice nucleating particles (INPs) in cirrus cloud formation which is known to exert a warming effect on climate. Bare soot particles, generally hydrophobic and fractal, mainly exist near emission sources. Coated or internally mixed soot particles are more abundant in the atmosphere and have a higher probability to impact 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 study, two samples, a propane (C3H8) flame soot and a commercial carbon black were coated with varying wt % of sulphuric acid (H2SO4). The ratio of coating material mass to the mass of bare soot particle 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 size and mass distribution of size selected soot particles with/without H2SO4 coating were measured by a scanning mobility particle sizer (SMPS) and a centrifugal particle mass analyser (CPMA) running in parallel. The mixing state and morphology of soot particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, the evidence for the presence of H2SO4 on coated soot particle surface is shown by Energy Dispersive X-ray spectroscopy (EDX). 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 (PCF). 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.

Numerical Investigation on the Effect of the Oxymethylene Ether-3 (OME3) Blending Ratio in Premixed Sooting Ethylene Flames

Synthetic fuels, especially oxygenated fuels, which can be used as blending components, make it possible to modify the emission properties of conventional fossil fuels. Among oxygenated fuels, one promising candidate is oxymethylene ether-3 (OME3). In this work, the sooting propensity of ethylene (C2H4) blended with OME3 is numerically investigated on a series of laminar burner-stabilized premixed flames with increasing amounts of OME3, from pure ethylene to pure OME3. The numerical analysis is performed using the Conditional Quadrature Method of Moments combined with a detailed physico-chemical soot model. Two different equivalence ratios corresponding to a lightly and a highly sooting flame condition have been investigated. The study examines how different blending ratios of the two fuels affect soot particle formation and a correlation between OME3 blending ratio and corresponding soot reduction is established. The soot precursor species in the gas-phase are analyzed along with the soot volume fraction of small nanoparticles and large aggregates. Furthermore, the influence of the OME3 blending on the particle size distribution is studied applying the entropy maximization concept. The effect of increasing amounts of OME3 is found to be different for soot nanoparticles and larger aggregates. While OME3 blending significantly reduces the amount of larger aggregates, only large amounts of OME3, close to pure OME3, lead to a considerable suppression of nanoparticles formed throughout the flame. A linear correlation is identified between the OME3 content in the fuel and the reduction in the soot volume fraction of larger aggregates, while smaller blending ratios may lead to an increased number of nanoparticles for some positions in the flame for the richer flame condition.