skip to main content


Title: Characterization of Nascent Soot Particles from Acetylene Pyrolysis: A Molecular Modeling Perspective
Soot or black carbons are combustion-generated carbonaceous nanoparticles formed during the incomplete combustion of hydrocarbon fuels. The complexity of hydrocarbon systems often makes it difficult to investigate the fundamentals of soot formation experimentally. To address this, this study uses reactive molecular dynamics simulations with reactive force field (ReaxFF) potentials. The current work focuses on the formation and evolution of soot during acetylene pyrolysis. The analysis provides insights into the physicochemical aspects of soot formation and the maturation of incipient soot particles. In this work, we focus on the evolution and interdependence of features such as the number of carbon atoms, number of aromatic rings, mass, C/H ratio, the radius of gyration, atomic fractal dimension, surface area, volume, and density. Based on the physicochemical features, two distinct classes of nascent soot can be observed. These are termed type-1 and type-2 particles. The type-1 particles show significant morphological evolution, while the type-2 particles show chemical restructuring without significantly changing the morphology. Qualitative correlations of various degrees are also observed between some of these morphological features.  more » « less
Award ID(s):
2144290
NSF-PAR ID:
10410208
Author(s) / Creator(s):
; ; ; ;
Date Published:
Journal Name:
13th US National Combustion Meeting
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Wildfires, which have been occurring increasingly in the era of climate change, emit massive amounts of particulate matter (PM) into the atmosphere, strongly affecting air quality and public health. Biomass burning aerosols may contain environmentally persistent free radicals (EPFRs, such as semiquinone radicals) and redox-active compounds that can generate reactive oxygen species (ROS, including ·OH, superoxide and organic radicals) in the aqueous phase. However, there is a lack of data on EPFRs and ROS associated with size-segregated wildfire PM, which limits our understanding of their climate and health impacts. We collected size-segregated ambient PM in Southern California during two wildfire events to measure EPFRs and ROS using electron paramagnetic resonance spectroscopy. EPFRs are likely associated with soot particles as they are predominantly observed in submicron particles (PM 1 , aerodynamic diameter ≤ 1 μm). Upon extraction in water, wildfire PM mainly generates ·OH (28–49%) and carbon-centered radicals (∼50%) with minor contributions from superoxide and oxygen-centered organic radicals (2–15%). Oxidative potential measured with the dithiothreitol assay (OP-DTT) is found to be high in wildfire PM 1 , exhibiting little correlation with the radical forms of ROS ( r 2 ≤ 0.02). These results are in stark contrast with PM collected at highway and urban sites, which generates predominantly ·OH (84–88%) that correlates well with OP-DTT ( r 2 ∼ 0.6). We also found that PM generated by flaming combustion generates more radicals with higher OP-DTT compared to those by smoldering or pyrolysis. 
    more » « less
  2. Abstract. In mid-August through mid-September of 2017 a major wildfire smoke and hazeepisode strongly impacted most of the NW US and SW Canada. During this periodour ground-based site in Missoula, Montana, experienced heavy smoke impactsfor ∼ 500 h (up to 471 µg m−3 hourly averagePM2.5). We measured wildfire trace gases, PM2.5 (particulate matter≤2.5 µm in diameter), and black carbon and submicron aerosolscattering and absorption at 870 and 401 nm. This may be the most extensivereal-time data for these wildfire smoke properties to date. Our range oftrace gas ratios for ΔNH3∕ΔCO and ΔC2H4∕ΔCO confirmed that the smoke from mixed, multiple sourcesvaried in age from ∼ 2–3 h to ∼ 1–2 days. Our study-averageΔCH4∕ΔCO ratio (0.166±0.088) indicated a largecontribution to the regional burden from inefficient smoldering combustion.Our ΔBC∕ΔCO ratio (0.0012±0.0005) for our groundsite was moderately lower than observed in aircraft studies (∼ 0.0015)to date, also consistent with a relatively larger contribution fromsmoldering combustion. Our ΔBC∕ΔPM2.5 ratio (0.0095±0.0003) was consistent with the overwhelmingly non-BC (black carbon),mostly organic nature of the smoke observed in airborne studies of wildfiresmoke to date. Smoldering combustion is usually associated with enhanced PMemissions, but our ΔPM2.5∕ΔCO ratio (0.126±0.002)was about half the ΔPM1.0∕ΔCO measured in freshwildfire smoke from aircraft (∼ 0.266). Assuming PM2.5 isdominated by PM1, this suggests that aerosol evaporation, at least nearthe surface, can often reduce PM loading and its atmospheric/air-qualityimpacts on the timescale of several days. Much of the smoke was emitted latein the day, suggesting that nighttime processing would be important in theearly evolution of smoke. The diurnal trends show brown carbon (BrC),PM2.5, and CO peaking in the early morning and BC peaking in the earlyevening. Over the course of 1 month, the average single scattering albedo forindividual smoke peaks at 870 nm increased from ∼ 0.9 to ∼ 0.96.Bscat401∕Bscat870 was used as a proxy for the size and“photochemical age” of the smoke particles, with this interpretation beingsupported by the simultaneously observed ratios of reactive trace gases toCO. The size and age proxy implied that the Ångström absorptionexponent decreased significantly after about 10 h of daytime smoke aging,consistent with the only airborne measurement of the BrC lifetime in anisolated plume. However, our results clearly show that non-BC absorption canbe important in “typical” regional haze and moderately aged smoke, with BrCostensibly accounting for about half the absorption at 401 nm on average forour entire data set. 
    more » « less
  3. Isoprene has a strong effect on the oxidative capacity of the troposphere due to its abundance. Under low-NOx conditions, isoprene oxidizes to form isoprene-derived epoxydiols (IEPOX), contributing significantly to secondary organic aerosol (SOA) through heterogeneous reactions. In particular, organosulfates (OSs) can form from acid-driven reactive uptake of IEPOX onto preexisting particles followed by nucleophilic addition of inorganic sulfate, and they are an important component of SOA mass, primarily in submicron particles with long atmospheric lifetimes. Fundamental understanding of SOA and OS evolution in particles, including the formation of new compounds by oxidation as well as corresponding viscosity changes, is limited, particularly across relative humidity (RH) conditions above and below the deliquescence of typical sulfate aerosol particles. In a 2-m3 indoor chamber held at various RH values (30 – 80%), SOA was generated from reactive uptake of gas-phase IEPOX onto acidic ammonium sulfate aerosols (pH = 0.5 – 2.5) and then aged in an oxidation flow reactor (OFR) for 0 – 24 days of equivalent atmospheric ·OH exposure. We investigated the extent of inorganic sulfate conversion to organosulfate, formation of oligomers, single-particle physicochemical properties, such as viscosity and phase state, and oxidation kinetics. Chemical composition of particle-phase species, as well as aerosol morphological changes, are analyzed as a function of RH, oxidant exposure times, and particle acidity to better understand SOA and OS formation and destruction mechanisms in the ambient atmosphere. 
    more » « less
  4. We report the influence of side chain hydrolysis on the evolution of nanoscale structure in thin films fabricated by the reactive layer-by-layer (LbL) assembly of branched poly(ethylenimine) (PEI) and poly(2-vinyl-4,4-dimethylazlactone) (PVDMA). LbL assembly of PEI and PVDMA generally leads to the linear growth of thin, smooth films. However, assembly using PVDMA containing controlled degrees of side chain hydrolysis leads to the growth of thicker films that exhibit substantial nanoscale roughness, porosity, and have resulting physicochemical behaviors (e.g., superhydrophobicity) that are similar to those of some thicker PEI/PVDMA coatings reported in past studies. Our results reveal that the degree of PVDMA partial hydrolysis (or carboxylic acid group content) influences the extent to which complex film features develop, suggesting that ion-pairing interactions between hydrolyzed side chains and amines in PEI promote the evolution of bulk and surface morphology. Additional experiments demonstrate that these features likely arise from polymer/polymer interactions at the surfaces of the films during assembly, and not from the formation and deposition of solution-phase polymer aggregates. When combined, our results suggest that nanoporous structures and rough features observed in past studies likely arise, at least in part, from some degree of adventitious side chain hydrolysis in the PVDMA used for film fabrication. Our results provide useful insight into molecular-level features that govern the growth and structures of these reactive materials, and provide a framework to promote nanoscale morphology reliably and reproducibly. The principles and tools reported here should prove useful for further tuning the porosities and tailoring the physicochemical behaviors of these reactive coatings in ways that are important in applied contexts. 
    more » « less
  5. We perform spatially resolved measurements of light scattering of soot in atmospheric pressure counterflow diffusion flames to complement previously reported data on soot pyrometry, temperature and gaseous species up to three-ring polycyclic aromatic hydrocarbons (PAHs). We compare two flames: a baseline ethylene flame and a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of pre-vaporized toluene. The goal is twofold: directly adding an aromatic fuel to bypass the formation of the first aromatic ring, widely regarded as the main bottleneck to soot formation from aliphatic fuels, and assessing the impact of a common component of surrogates of transportation fuels on soot formation. The composition of the fuel and oxidizer streams are adjusted to ensure invariance of the temperature-time history, thereby decoupling the chemical effects of the fuel substitution from other factors. The doping approach enables the comparison of very similar flames with respect to combustion products, radicals and critical precursors to aromatic formation (C2–C5 species), in addition to the temperature-time history. Doping with toluene boosts the aromatic content and soot volume fraction relative to the baseline ethylene flame, but, surprisingly, the soot number density and nucleation rate are affected modestly. As a result, the observed difference in volume fraction in the toluene-doped flame is reflective of larger initial particles at the onset of soot nucleation. The nucleation rate when soot first appears near the flame is of the same order as the dimerization rate of single-ring aromatics, in contrast with the expectation that the dimerization of larger PAHs initiates the process. Even though in and of itself nucleation contributes modestly to the overall soot loading, nucleation conditions the overall soot loading by affecting the size of the initial particle, which ultimately affects subsequent growth. 
    more » « less