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Soot particles generated by a liquid-fueled miniCAST burner, supplied with diesel B7, was characterized for the first time over a total of 34 operating points covering an overall equivalence ratio (ϕ) range of 0.103–1.645. To characterize the gas and particulate phase emissions, electron microscopy images, mobility-equivalent size distributions and mass concentration of the soot aggregates were recorded, and optical extinction/absorption coefficients of the exhaust were measured. The burner produces soot aggregates with geometric mean diameter size in the range 45-105 nm with primary particle size in the range 18-39 nm. The Ångström absorption exponent (α) was evaluated in the range 400-1000 nm and was found to increase with ϕ and vary in the range 1.05–2.25. The interposition of a catalytic stripper in the sampling line was found to (i) flatten the shape of the size distribution of aggregates, (ii) oxidize most of the gas phase thus impacting optical extinction coefficients particularly below 400 nm and (iii) decrease α. The soot volume fraction (fv) was determined by three independent methods: optical absorption, mass deposit and mobility size distribution combined with morphology data. fv evaluated from size mobility data accounting for morphological aspects agreed within 13% with fv measured optically and within 25% with fv evaluated from mass concentration measurements. The precise methodology developed to characterize engine-like soot particles produced by a liquid-fueled miniCAST can now be transposed to study other regular, renewable, and surrogate liquid fuels to investigate their physical and optical properties before considering their large-scale use. Copyright © 2023 American Association for Aerosol Research

International audience; The transformation process in sooting flames leading to the formation of nanoparticles in the condensed phase from molecular precursors in the gas phase (soot nucleation) is not yet fully understood [1]. Information on the physical chemical properties of the nascent particles is difficult to accessexperimentally but also essential for the validation of the hypotheses on soot nucleation currently being examined by the scientific community. In this work, we characterize the molecular precursors and the soot particles formed in a laminar diffusion methane flame by combining several in situ and ex situ diagnostics (fluorescence, incandescence, secondary ion mass spectrometry and Raman spectroscopy) with the goal of inferring the chemical composition and the structure of the nascent particles. The presented work highlights that several of the investigated physical chemical properties suddenly change within a narrow flame region that acquires the character of a discontinuity point correlated to the soot nucleation process. Notably, the obtained data shows evidences of the transition from flat to curved molecular structures, and the progressive apparition of finite size, sandwiched graphene like layers. Furthermore, the fraction of atomic hydrogen features a local maximum immediately upstream the detection of nascent soot particles consistent with the increasing in flame rate of C-C covalent bonds formation postulated by the major hypotheses explaining the nucleation of soot particles.References[1] J.W. Martin, M. Salamanca, M. Kraft, Soot inception: Carbonaceous nanoparticle formation in flames, Progress in Energy and Combustion Science. 88 (2022) 100956.https://doi.org/10.1016/j.pecs.2021.100956.

International audience; Soot formation in combustion is a major research problem because of the negative impact of the carbonaceous particles in the exhausts on human health and the environment. However, state of the art knowledge on the chemical species and reaction pathways leading to the formation of soot particles is not yet complete. In particular, many questions are still open on the transformation of gaseous molecular precursors into condensed phase soot particles (soot nucleation). One of the hypotheses proposed in the literature, the nucleation driven by the dimerization of small polycyclic aromatic hydrocarbons (PAHs), has been recently used in models to successfully reproduce the soot volume fraction profiles measured in laboratory flames. However, experimental evidences on the existence of dimers in the flame environment are still lacking. In this work, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and high-resolution Atomic Force Microscopy (AFM) are coupled with the aim of characterizing the chemistry of the building blocks involved in the formation of nascent soot particles.

International audience

Particulate emissions from on-road motor vehicles are the focus of intensive current research due to the impact of the ambient particulate matter (PM) levels on climate and human health. Constant improvement in engine technology has led to significant decrease in the number and mass of emitted PM, but particular concern is raised nowadays by the ultrafine particles. In this context, there is a critical lack of certification procedures for the measurement of the smallest-size ( 23 nm) particulate matter emissions. To support the engine development process as well as future certification procedures, a measurement technology for sub-23 nm particles must be designed. The development of a reliable measurement procedure entails understanding the formation and evolution of particles from the engine to the tailpipe via multiple analytical techniques and theoretical simulations. We present here extensive experimental characterization of ultrafine particles emitted by a gasoline direct injection single-cylinder engine as particle generator. The particles were sampled using a cascade impactor which allows size-separation into 13 different size bins. Chemical characterization of the collected size-selected particles was performed using mass spectrometry, which gives access to detailed molecular information on chemical classes of critical interest such as organosulphates, oxygenated hydrocarbons, nitrogenated hydrocarbons, metals, or polycyclic aromatic hydrocarbons. Additionally, the morphology of the emitted particles was probed with atomic force (AFM) and scanning electron microscopy (SEM). Tip-Enhanced Raman Spectroscopy (TERS) was applied for the first time to sub-10 nm combustion-generated particles to gather information on their nanostructure. The extensive database built from these multiple experimental characterizations has been used as input of a theoretical approach to simulate and validate engine out-emissions. These studies were performed in the framework of the H2020 PEMS4Nano project which aims to the development of a robust, reliable and reproducible measurement technology for particles down to 10 nm for both chassis dyno and real driving emissions (RDE).

Laser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex-situ analysis by helium-ion microscopy (HIM) of particles sampled thermophoretically and (2) online size distribution analysis of microprobe-sampled particles using a 1nm-SMPS. The size distributions of the incipient soot particles, found in the nucleation flame and in the early soot region of the Φ = 1.95 flame, derived from time-resolved LII signals are in good agreement with HIM and 1nm-SMPS measurements and are in the range of 2–4 nm. The thermal and optical properties of incipient soot were found to be not radically different from those of mature soot commonly used in LII modeling. This explains the ability of incipient soot particles to produce continuous thermal emissions in the visible spectrum. This study demonstrates that LII is a promising in-situ optical particle sizing technique that is capable of detecting incipient soot as small as about 2.5 nm and potentially 2 nm and resolving small changes in soot sizes below 10 nm.

International audience; Numerous field observations and modeling studies show a significant role of aircraft black carbon (soot) acting as ice nuclei in contrail and cirrus formation leading to radiative forcing and climate changes in the upper troposphere/lower stratosphere. Ship emission particles (soot + impurities) can also act as nuclei for the formation of cloud droplets and affect by that means the life cycle and radiative properties of marine stratus clouds at the top of the marine boundary layer. Lastly, in harbor and airport areas these emissions are considered as dangerous pollutants with respect to human health.Even if serious models have been developed to estimate the impact of soot aerosols upon the atmosphere and the health, field and laboratory measurements are still missing with regard to their physico-chemical properties as well as their ice nucleation properties. We present here the sampling, characterization and estimations of environmental effects of soot particles collected at the outlet of an aircraft engine combustor and emitted at the end-of-pipe of marine and river ships. Using several techniques (TEM, SEM, FTIR, EDX, neutron diffusion, …) we determine common and specific features of carbonaceous particles with respect to their morphology, size, microstructure, chemical composition depending on the fuel used and on combustion conditions. Water uptake measurements quantify the hygroscopic level of these particles acting as cloud and ice nuclei in the atmosphere.

Due to the exponential increase in aircraft traffic in recent decades, the role of soot particles emitted by aircraft engines on the radiative forcing needs to be addressed, and especially their interaction with clouds has to be better understood and quantified. In this work, we investigate the hygroscopic properties of fresh and aged soot sampled on line in a kerosene flame. The activated fraction (Fa) of size selected soot is measured by means of a variable supersaturation condensation nucleus counter at several heights above the burner (HAB), thereby probing soot particles with different residence times in the flame, i.e., different degrees of maturity. In order to simulate atmospheric aging, the activity of soot as cloud condensation nuclei is measured as a function of ozone exposure. We show that fresh soot is hydrophobic (Fa∼0), while Fa increases when soot is exposed to ozone. The measurements depend on the HAB at which soot particles are sampled showing that activation of soot particles is related to their chemical composition. This study brings new results on the link between atmospheric aging of soot and its hygroscopic properties, which is of great interest for understanding the role of soot in the cloud formation. Copyright © 2018 American Association for Aerosol Research

Due to the exponential increase in aircraft traffic in recent decades, the role of soot particles emitted by aircraft engines on the radiative forcing needs to be addressed, and especially their interaction with clouds has to be better understood and quantified. In this work, we investigate the hygroscopic properties of fresh and aged soot sampled on line in a kerosene flame. The activated fraction (Fa) of size selected soot is measured by means of a variable supersaturation condensation nucleus counter at several heights above the burner (HAB), thereby probing soot particles with different residence times in the flame, i.e., different degrees of maturity. In order to simulate atmospheric aging, the activity of soot as cloud condensation nuclei is measured as a function of ozone exposure. We show that fresh soot is hydrophobic (Fa∼0), while Fa increases when soot is exposed to ozone. The measurements depend on the HAB at which soot particles are sampled showing that activation of soot particles is related to their chemical composition. This study brings new results on the link between atmospheric aging of soot and its hygroscopic properties, which is of great interest for understanding the role of soot in the cloud formation. Copyright © 2018 American Association for Aerosol Research