Understanding the surface and pore features of soot nanoparticles is important for predicting their behavior in combustion environments and atmospheric processes. Here, we present a novel computational framework combining reactive molecular dynamics simulations with detailed atomistic analysis to characterize the morphology of over 2000 incipient soot particles formed from acetylene pyrolysis at 1350–1800 K. The surface and pore features of these nanoparticles are explored directly using three-dimensional atomic surface mesh for the first time. The nanoparticles are found to have a highly irregular shape, with an average sphericity of 0.57 and a surface fractal dimension (D_S) of approximately 2.22, in excellent agreement with experimental data. The particles exhibit significant internal porosity (Φ≈0.22) dominated by micropores (≤2 nm). Micropores contribute to a very high specific surface area of approximately 2652. Three distinct pore types – tunnels, pockets, and isolated cavities – are identified in the incipient soot primary particles. The internal pore network is found to have a fractal dimension (D_VC) of approximately 2.15. Strong positive correlations between pore volume and surface area (R²≈0.70) are observed. The findings point to a complex and irregular external and internal structures of incipient soot nanoparticles and a complex pore network within them.

Coagulation of aerosols at high concentrations plays a key role in determining the morphology of nanoparticle agglomerates in gas-phase synthesis reactors, as well as in natural and industrial combustion environments, such as wildfires and pool fires. Here, coagulation of aerosol agglomerates at solid volume fractions, f _v = 0.05–0.3 is investigated from the continuum to the free molecular regime by discrete element modelling (DEM). The DEM-derived coagulation rates, gelation times, t _g , radii of gyration and fractal dimensions, D _f , are validated in detail against Monte Carlo, Brownian and Langevin dynamics literature. At the high concentrations elucidated here, aerosols rapidly form a space-spanning, gel-like network of polydisperse, compact agglomerates with D _f ≥ 2. Increasing f _v from 0.05 to 0.3 results in smaller, less polydisperse and more compact agglomerates. The transport regime controls gelation dynamics, as well as the agglomerate gel size distribution. In the continuum and lower transition regimes, agglomerates remain smaller and less polydisperse with t _g ∼ f _v ^-1.95. In contrast, larger and more polydisperse agglomerate gels form in the upper transition and free molecular regimes with t _g ∼ f _v ^-1.2. In this regard, simple power laws are derived here to estimate accurately the collision frequency and gelation time of aerosol agglomerates and assist reactor design for gas-phase nanomaterial synthesis.

Although citrus processing wastewater is rich in bioactive compounds and nutrients, the significant potential of the specific feedstock for valorization still remains largely unexploited. Thus, a citrus processing wastewater-based biorefinery was employed to recover carotenoids and polyphenols, while producing microbially derived bacterial cellulose nanocrystals. The solid fraction of the effluent was applied in solid-liquid extraction using various green- and petrochemical-based solvents, yielding 1.97 ± 0.03 mg of carotenoids per g of dry solid via a mixture of D-limonene/ethanol/acetic acid. A continuous adsorption/desorption system was developed, adsorbing 89.1 ± 0.63% (w/v) of the initial phenolic content. Different solvents were assessed as elution agents maximizing the desorption of the phenolic content (76 ± 7.25%, w/v) using 75% ethanol. The remaining sugar-rich liquid was subsequently utilized for bacterial cellulose manufacture employing Komagataeibacter sucrofermentans DSM 15973. Elevated fermentative production was achieved at pH-value 6.0 using a surface-to-volume ratio of 1.53 cm⁻¹, yielding 4.98 ± 0.28 g L⁻¹ of the biopolymer following 6 d of incubation. The bacterial cellulose formed was processed using sulfuric acid, aiming to hydrolyze the amorphous fraction and produce a biomaterial of higher crystallinity as well as enhanced stability and mechanical properties as compared to the pristine biopolymer. The nanocrystals produced exhibited colloidal stability (−24.73 ± 1.11 mV), enhanced crystallinity (85.2%) and reduced thermal stability (297.1 °C) as compared to bacterial cellulose. The study demonstrated a sustainable biorefinery approach for efficient valorization of citrus processing wastewater towards production of industrially important bioactive compounds and biopolymers.

A hierarchical modeling approach is presented for describing soot growth dynamics, encompassing reactive Molecular Dynamics (MD) simulations and a monodisperse particle dynamics model. Reactive MD is employed to investigate nucleation of soot nanoparticles during isothermal acetylene pyrolysis at 1200–1800 K. A “lumped” soot nucleation rate is determined by tracking the rate of formation of soot clusters at various fuel concentrations, following a power law dependency with the initial acetylene concentration. The MD-obtained soot nucleation rate is incorporated in a monodisperse particle dynamics model describing soot formation in laminar premixed methane flames. The soot volume fraction predicted by the monodisperse model with the MD-derived nucleation rate is in good agreement with measurements in a methane nucleation flame (φ = 1.95), showing significant improvement (3 orders of magnitude) compared to a semi-empirical nucleation rate. The MD-derived nucleation rate also performs well in the methane sooting flame (φ = 2.32), yielding soot volume fractions comparable to experimental measurements.

Ultra porous carbonaceous nanoparticles were prepared by judicious oxidation of various commercial carbon blacks (CBs) at high temperatures (1200 °C). X-ray diffraction, N₂ adsorption and microscopy analyses revealed that during such oxidation, O₂ diffuses through and reacts with CB, disordering its crystalline structure. The concurrent external and internal oxidation of CB results in tiny pores that greatly increase the specific surface area, SSA, from 240 up to 2185 ± 199 m²/g. This is about 150–200 % larger than the SSA of CB oxidized at low temperatures (450–550 °C), 50–100 % larger than the SSA of most porous CB commercially available and on par with that of commercial activated carbons (e.g. YP80). The potential of this ultra porous CB generated here for energy storage is showcased using electric double layer capacitors (EDLCs). The gravimetric capacitance of EDLCs using the above high SSA CB as active material is up to 60 % larger than those obtained from EDLCs based on YP80 or Ketjenblack at high scan rates (≥ 100 mV/s) and current densities of 0.02–5 A/g. The superior rate performance of these CBs is attributed to the high concentration of pores with a 2–8 nm radius formed largely by internal oxidation. Such pores cannot be produced at large concentrations by low temperature oxidation of CB that is used widely to enhance CB porosity. Hence, close control of the oxidation dynamics of CB can substantially increase supercapacitor performance.

Due to the growth of urban areas in close proximity to wildlands, wildfires increasingly burn both biomass and man-made materials. The physicochemical properties of emitted particulate matter (PM) from such “wildland-urban interface (WUI)” fires may differ substantially from those of wildland fires and other ambient PM sources. However, the associations between properties and hazards of WUI fire PM have not been studied. Here, we employed a wildfire simulator (WiFS) to reproduce biomass and WUI fires by combusting pinewood and a simplistic WUI fire model (1:1 mixture of pinewood and polyethylene), respectively. WUI fire PM contained high concentrations of the highly toxic and carcinogenic PAH benzo[c]fluorene and significant amounts of highly bioactive alkyl and oxygenated PAHs, which were both absent in biomass fire PM, and had a carcinogenicity potential (benzo[a]pyrene equivalents, BaP_Eq) 20 times higher than biomass fire PM. Additionally, exposure of THP-1 macrophages to WUI fire PM, but not biomass fire PM, caused significant reductions in viability and mitochondrial potential, significantly decreased phagocytosis of 1 μm beads, and substantial dysregulation of gene expression. These findings suggest that WUI fire PM exposure may be more hazardous than wildland fire PM exposure, likely due to differences in their chemical profiles.

Polycyclic aromatic compounds (PACs) are organic pollutants associated with incomplete combustion processes and connected to severe health effects. Their connection to micro-nanoplastics (MNPs) emitted as particulate matter from incineration of plastics remains poorly explored. In this study, we detected 65 Polycyclic Aromatic Hydrocarbon (PAH), oxygenated (OPAH) and nitrated (NPAH) PAC species in MNPs emitted during the incineration of three widely used plastic materials, high-density polyethylene (HDPE), polypropylene (PP) and polyvinyl chloride (PVC). MNPs were generated using the incineration exposure generation system (INEXS) and their PAC profile was offline analyzed using gas chromatography-mass spectrometry. Notably, one of the most abundant species in all three plastics was Benzanthrone, an OPAH previously associated with traffic and biomass burning emissions. MNPs emitted by PVC incineration contained emerging highly toxic PAC species such as benzo(c)fluorene, dibenzopyrenes, and 6-nitrochrysene. Our findings highlight the importance of monitoring beyond the legacy 16 EPA members which accounted only by 12%, 47%, and 41% of the associated carcinogenic potency (expressed as BaPeq), for PVC, PP and HDPE respectively. These results raise concerns for potential health implications and underscore the urgent need for further research on this new environmental challenge, MNP pollution and its association with toxic persistent pollutants.

Wildfire particulate matter from Canadian forest fires significantly impacted the air quality in the northeastern United States during the summer of 2023. Here, we used real-time and time-integrated instrumentation to characterize the physicochemical properties and radiative effects of wildfire particulate matter reaching the metropolitan areas of New Jersey/ New York during this extreme incident. The radiative forcing of −352.4 W/m² derived here based on the measured optical properties of wildfire particulate matter explains, to some extent, the ground level temperature reduction of about 3 °C observed in New Jersey/ New York City during this incident. Such negative radiative forcing in densely populated megacities may limit natural ventilation, increase the residence time of wildfire particulate matter and background air pollutants, exacerbating public health risks. This study highlights the importance of radiative effects from wildfire particulate matter in densely populated areas and their potential implications for climate, air quality and public health.

The toxicity, climate impact, as well as the physical and chemical properties of ultra-fine soot particles emitted from combustion systems are strongly dependent on their size and morphology. Research attention has been paid in the last three decades to developing more accurate and capable methods to model soot particle coagulation in the presence of inception, surface growth, and oxidation, to predict particle size distribution as well as the detailed aggregate morphology of soot. While soot particle concentrations in hydrocarbon flames are primarily governed by soot kinetics, the morphology of soot particles is controlled by both soot kinetics and particle dynamics. Flame-generated soot particles are fractal aggregates formed by polydisperse and nearly spherical primary particles with a certain degree of overlapping. The properties of fractal aggregates, nanoparticle coagulation, and soot formation chemistry all play important roles in soot formation. This article reviews all these aspects but the focus is on recent progress in macro- and meso-scale modeling of soot particle aggregation in laminar sooting flames to avoid the complexities of turbulence. The reviewed macro-scale methods based on the population balance equation include the commonly used sectional methods and methods of moments. The main features of three recently developed state-of-the-art meso-scale methods, namely the event-driven Discrete Element Method, Monte Carlo Aggregation Code, and detailed stochastic population balance model are reviewed. To highlight the complexities of modeling the particle size distribution and detailed particle morphology without and with surface growth, numerical simulations of three test cases were conducted using the event-driven Discrete Element Method, the Monte Carlo Aggregation Code, and the two macro-scale methods. A detailed analysis of the results was presented to understand how different treatments of particle coagulation and surface growth in the two meso-scale methods affect the predicted particle size and morphology. The remaining challenges in modeling detailed soot particle morphology are outlined.

Here, the light absorption of brown carbon (BrC) emitted by wood combustion and denuded from volatile organic carbon (VOC) at 300 °C is elucidated using a recently developed thermal decomposition platform coupled with a suite of real-time aerosol instrumentation and time-integrated sampling systems. The BrC particle size distribution, morphology and optical properties are closely controlled by increasing the combusted wood mass from 50 to 600 mg to emulate those measured for “real world” wildfire particulate matter (PM) emissions. Size-fractionation of such wildfire-like BrC reveals that the PM_0.1–2.5 fraction contains high molecular weight, carcinogenic polycyclic aromatic hydrocarbons (PAHs) and absorbs up to five times more light compared to the PM_0.1 fraction. Thus, increasing the combusted wood mass from 50 to 600 mg increases the PM_0.1–2.5 concentration by a factor of about eight and enhances the overall BrC mass absorption cross-section, MAC, up to a factor of two at a wavelength of 405 nm. Condensation of VOC on BrC reduces its MAC up to 40 %. Still, the particle size seems to largely determine the BrC light absorption, as large VOC-rich particles absorb more light compared to small VOC-lean ones. The size-resolved BrC MAC measured here can be interfaced with climate models to estimate the climate impact of wildfire PM emissions.

Soot inception by acetylene pyrolysis at 1350-1800 K is investigated using reactive molecular dynamics. The composition and chemical structure of soot precursor molecules formed during inception are elucidated. During soot inception, increasing the process temperature leads to faster depletion of C₂H₂ molecules and faster formation of C₂H₃, C₂H₄, C₂H₆, CH₄, and C₂ with the concurrent appearance of H₂ molecules. Small molecules consisting of 1-5 C atoms (C₁-C₅) are formed due to reactive collisions and grow further to larger hydrocarbon compounds consisting of 6-10 C atoms. At initial stages of inception, prior to the formation of incipient soot, three-member rings are formed, which are associated with the formation of compounds with fewer than 10 C atoms. Once incipient soot is formed, the number of C₁-C₁₀ compounds and the number of three-member rings drop, while the number of five- and six-member rings increases, indicating that the formation of larger rings is associated with the growth of soot clusters. The chemical structure of soot precursor molecules obtained by bond order analysis reveals that molecules with up to 10 C atoms are either linear or branched aliphatic compounds or may contain three-member rings fused with aliphatic components. Molecules with more than 10 C atoms often exhibit structures composed of five- or six-member C rings, decorated by aliphatic components. The identification of molecular precursors contributing to soot inception provides crucial insights into soot formation mechanisms, pinpointing potential pathways of soot formation during combustion.

Stratospheric aerosol injection could mitigate harmful effects of global warming, but could have undesirable side effects, such as warming the stratosphere and depleting the ozone layer. We explore the potential benefits of solid alumina and calcite particles as alternatives to sulfate aerosols by using an experimentally informed aerosol-chemistry-climate model. Compared to sulfur dioxide, injection of solids reduces stratospheric warming by up to 70% and diffuse radiation by up to 40%, highlighting their potential benefits. Achieving −1 W m⁻² of radiative forcing would likely result in very small ozone changes, but sizable uncertainties remain. These arise from poorly understood heterogeneous chemical and microphysical processes, which, under less likely assumptions, could lead to larger global ozone column changes between −14% and +4%. Our work provides recommendations for improving the understanding of stratospheric aerosol injection using materials other than sulfur dioxide, and underscores the need for kinetic laboratory studies.

Chemical nucleation of carbonaceous nanoparticles is investigated during pyrolysis of n-heptane at high temperature (2200–2600 K) by reactive molecular dynamics (MD) simulations. The MD-derived n-heptane consumption rate is in agreement with kinetic modeling, validating the present work at high temperature and high fuel concentration conditions. The critical nucleus size is quantified by the free formation energy at 2200–2600 K for n-heptane concentrations ranging from 3 × 10²⁰−9 × 10²⁰ #/cm³. Increasing temperature leads to smaller critical size, starting from 59 ± 7 carbon atoms at 2200 K that decreases down to 33 ± 3 carbon atoms at 2600 K, while the fuel concentration hardly affects the critical nucleus size. The onset time of nucleation decreases exponentially with temperature, consistent with previous shock tube pyrolysis experiments. The nucleation rate is obtained by the rate of formation of critical and supercritical hydrocarbon molecules. An Arrhenius-type relationship between the nucleation rate and the process temperature is proposed, exhibiting a first-order dependency to the initial fuel concentration. The number density of carbon nuclei derived by this nucleation rate is four to five orders of magnitude higher than that obtained by kinetic models for soot nucleation by reactive polyaromatic hydrocarbon (PAH) dimerization. The present MD-derived nucleation rate provides a computationally efficient pathway to model carbonaceous nanoparticle formation dynamics without relying on individual chemical reaction rate constants or on computationally expensive PAH-based models.

Recent studies have suggested that injection of solid particles such as alumina and calcite particles for stratospheric aerosol injection (SAI) instead of sulfur-based injections could reduce some of the adverse side effects of SAI such as ozone depletion and stratospheric heating. Here, we present a version of the global aerosol-chemistry-climate model SOCOL-AERv2 and the Earth system model (ESM) SOCOLv4 which incorporate a solid-particle microphysics scheme for assessment of SAI of solid particles. Microphysical interactions of the solid particle with the stratospheric sulfur cycle were interactively coupled to the heterogeneous chemistry scheme and the radiative transfer code (RTC) for the first time within an ESM. Therefore, the model allows simulation of heterogeneous chemistry at the particle surface as well as feedbacks between microphysics, chemistry, radiation and climate. We show that sulfur-based SAI results in a doubling of the stratospheric aerosol burden compared to the same mass injection rate of calcite and alumina particles with a radius of 240 nm. Most of the sulfuric acid aerosol mass resulting from SO₂ injection does not need to be lifted to the stratosphere but is formed after in situ oxidation and subsequent water uptake in the stratosphere. Therefore, to achieve the same radiative forcing, larger injection rates are needed for calcite and alumina particle injection than for sulfur-based SAI. The stratospheric sulfur cycle would be significantly perturbed, with a reduction in stratospheric sulfuric acid burden by 53 %, when injecting 5 Mtyr^-1 (megatons per year) of alumina or calcite particles of 240 nm radius. We show that alumina particles will acquire a sulfuric acid coating equivalent to about 10 nm thickness if the sulfuric acid is equally distributed over the whole available particle surface area in the lower stratosphere. However, due to the steep contact angle of sulfuric acid on alumina particles, the sulfuric acid coating would likely not cover the entire alumina surface, which would result in available surface for heterogeneous reactions other than the ones on sulfuric acid. When applying realistic uptake coefficients of 1.0, 10^-5 and 10^-4 for H₂SO₄, HCl and HNO₃, respectively, the same scenario with injections of calcite particles results in 94 % of the particle mass remaining in the form of CaCO₃. This likely keeps the optical properties of the calcite particles intact but could significantly alter the heterogeneous reactions occurring on the particle surfaces. The major process uncertainties of solid-particle SAI are (1) the solid-particle microphysics in the injection plume and degree of agglomeration of solid particles on the sub-ESM grid scale, (2) the scattering properties of the resulting agglomerates, (3) heterogeneous chemistry on the particle surface, and (4) aerosol-cloud interactions. These uncertainties can only be addressed with extensive, coordinated experimental and modelling research efforts. The model presented in this work offers a useful tool for sensitivity studies and incorporating new experimental results on SAI of solid particles.

Here, commercial carbon black (CB) grades are characterized in detail to determine the link between their physicochemical properties and solar steam generation performance. The CB nanoparticles used here have surface mean primary particle diameters of 11–406 nm resulting in specific surface areas of 8–300 m²/g. Thermogravimetric analysis, dynamic light scattering, Raman spectroscopy, and x-ray diffraction reveal that fine CB nanoparticles form large agglomerates, have a more disordered nanostructure and larger organic carbon content than coarse CB grades. Most importantly, UV–vis spectroscopy and Mie theory show that increasing the particle size from 14 to 406 nm reduces the light absorption of CB dispersed in water up to 86%. So, the water evaporation flux of suspensions containing 11–14 nm CB nanoparticles is up to 25% larger than that obtained for suspensions of 406 nm particles. Thus, good control of particle size is essential to optimize the solar steam generation enabled by CB.