Search

Combustion and air-pollution aerosol particles often contain a substantial mass fraction of carbon. In biomass or residual-fuel smoke, this carbon may span a continuum of molecular sizes and physical properties, from macromolecular black carbon (BC) and tarballs to smaller molecules of organic particulate matter (OM). The quantification of these materials presents a metrological challenge which rapid temperature programmed oxidation instruments (RTPOs) may address. RTPOs collect particles on filters, which are then rapidly heated (> 700 K/min) in ambient or synthetic air while CO2 in the emitted gases is quantified either with (e.g. the "FATCAT" instrument) or without (e.g., "TCA08") first passing gases over an oxidation catalyst. Both RTPOs monitor CO2 concentrations in real time to provide thermograms; both report total carbon mass by reference to baseline-subtracted CO2 thermograms. In this study, we investigated the response of these instruments to a variety of laboratory samples. Integrated carbon mass from the FATCAT and TCA08 agreed with one another, and with reference measurements, for both mature, fully graphitized soot (OC/TC < 0.1) and young, partially formed soot (OC/TC = 0.6 after denuding at 350 °C). However, due to its lack of an oxidation catalyst, the TCA08 CO2 measurements were 10-fold lower than reference measurements for two model OM compounds (dioctyl sebacate and sucrose). Since a previous atmospheric study has observed consistency between a TCA08 and a reference instrument, our data imply a large difference in the evaporation kinetics of these surrogates and atmospheric OM, as expected for highly-viscous (glassy) atmospheric OM. Finally, we discuss the thermograms of nebulized BC surrogates (C60, graphene, graphite), which showed two peaks at lower and higher temperatures, even after denuding at 350 °C. [ABSTRACT FROM AUTHOR]

Black carbon, or soot, is one of the highest contributors to global warming. The International Civil Aviation Organization (ICAO) has adopted regulatory standards for soot from aircraft engines, also referred to as a nonvolatile particulate matter (nvPM), to limit or reduce the harmful impacts of nvPM on the environment. Sustainable aviation fuels (SAF) offer advantages to reduce soot emissions and overall environmental impact but require extensive testing and evaluation before wider adoption. Typical measurements of soot produced by combustion of aviation fuels require full-sized jet engines and large volumes of fuel, which can be prohibitively expensive. This study investigates flame spray pyrolysis (FSP) as a simple bench-top tool for comparison of soot emissions from the combustion of different liquid jet fuels. A sampling assembly is designed for soot collection and analysis. Morphological analysis follows from transmission electron microscopy (TEM) image analysis and mobility (differential mobility analyzer) classification. Morphologies are compared to previous measurements from aircraft turbines. Soot agglomerate size distributions and elemental to total carbon ratios (EC/TC) are measured for three liquid fuels and flame conditions with Reynolds numbers and burner equivalence ratios ranging from 6100 to 9100 and 7 to 13, respectively. Day-to-day variations in the dilution ratio resulted in up to 20% variability in the measured total agglomerate number-based concentration and mobility diameter. Geometric mean primary particle and mobility diameter values are below 21 and 104 nm, respectively, in excellent agreement with those emitted from jet engines and prior work using FSP. EC/TC remains > 0.75 for most flame conditions and fuels and increases with burner equivalence ratio, but values as low as 0.63 are measured from SAF combustion. Copyright © 2024 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Soot aggregates form as open, fractal-like structures, but aged atmospheric particles are often observed to be restructured into more-compact shapes due to internal mixing ("coating"). This compaction has a major effect on the radiative properties of the aggregates, and may also influence their aerosol-cloud interactions, toxicity, and deposition in human lungs. Recent laboratory studies have presented conflicting arguments on whether this compaction occurs during condensation or during the evaporation of coatings. In this three-part study, we combine theory and experiments to explain these conflicting results. First, we review the surface-science literature and identify explicit mechanisms for condensation-compaction as well as evaporation-compaction. We also identify a mechanism for avoiding compaction during condensation, based on heterogeneous nucleation theory and the kinetic barriers to capillary formation. Second, we review the soot-restructuring literature and find clear evidence for both condensation- and evaporation-compaction, with condensation-compaction being the norm. Third, we present new experimental results where the capillary forces due to anthracene coatings were "switched on" or "switched off" by using solid or liquid phases during coating addition and removal. Consequently, we demonstrate condensation-compaction, evaporation-compaction, and no compaction, for the same soot source. Overall, our study indicates that soot particles will typically undergo compaction when internal mixing occurs by the condensation of liquid coatings, while compaction may be avoided when internal mixing occurs through coagulation or the gas-to-particle formation of solid or highly viscous coatings. Copyright © 2022 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The thermal-optical analysis (TOA) of black carbon in particulate matter (PM) collected on filters has been recommended and used for the calibration of mass-concentration instruments. However, filter-based TOA calibrations have substantial practical limitations, requiring high sample flow rates (>10 litres per minute), long sampling times (up to 3 h), and subsequently manual filter processing with long analysis times (>15 min per filter). These limitations are avoided by in situ calibration techniques such as the centrifugal particle mass analyzer (CPMA)–electrometer reference mass system (CERMS). The CERMS is capable of producing and monitoring in situ reference mass concentrations below 1 µg m−3 and in real-time (∼1 Hz). Additional advantages of the CERMS are its improved repeatability (1.1%) over TOA (9–11%), and its measurement of a well-defined quantity: total post-CPMA suspended PM mass. In the present work, we demonstrate closure between these two techniques in terms of PM mass concentration for three different soot generators (viz., the Argonaut miniature inverted soot generator, Jing miniCAST, and National Research Council inverted-flame burner) under carefully controlled conditions. We also demonstrate the sensitivity of the CERMS by characterizing the limits of detection of a commercial laser-induced incandescence instrument and a photoacoustic instrument. Our data support the use of the CERMS with well-characterized PM sources to provide reference mass concentrations for the calibration of instruments measuring PM or black-carbon mass concentration. [ABSTRACT FROM AUTHOR]

Knowledge of the optical properties of soot black carbon (BC) is required for the prediction of the radiative effects of freshly-emitted and aged BC particles. Here we review BC mass absorption cross section (MAC) and absorption function E(m) measurements, focusing on freshly-emitted BC. First, we review recently reported MACs at 550 nm wavelength as obtained from direct measurements of particulate absorption and mass concentration; we find an average of 8.0 ± 0.7 m2/g from ten measurements, not significantly higher (p > 0.26) than the widely used MAC of 7.5 ± 1.2 m2/g recommended by Bond and Bergstrom [Bond, T. C., and R. W. Bergstrom. 2006. Light absorption by carbonaceous particles: An investigative review. Aerosol Sci. Technol. 40(1):27–67]. Second, we review recently reported E(m), whose retrieval is more complex due to the need to combine measurements with numerical models to estimate the contribution of scattering to extinction. Third, we review recent numerical studies that have aimed to predict the BC MAC using various complex refractive indices (m = n + ik). Most of these studies have used m = 1.95 + 0.79i recommended by Bond and Bergstrom (2006), yet failed to predict a MAC as high as 7.5 or 8.0 m2/g at 550 nm wavelength. Fourth, we summarize a selected range of alternative values of m that has been reported by recent studies and place them in the context of measurements using a contour plot of E(m) on the n–k plane. We show that the widely used m = 1.95 + 0.79i corresponds to an E(m) that is too low to be consistent with the measured MAC values. We conclude that the E(m) of BC in the visible and near infrared should be greater than 0.32, and that the commonly used BC models or the refractive index, or both, are still in need of improvement. [ABSTRACT FROM AUTHOR]

Logwood and pellet stoves are popular heating sources around the world. The particulate matter emitted from such stoves contains organic particulate matter (OM), soot, and ash, each of which may have significant effects on climate and health. In this study, the primary OM (POM) emitted from a wood stove and a pellet stove operated according to standard Swiss testing protocols were characterized using aerosol mass spectrometry. The POM mass spectra were found to be highly reproducible, and contained CO+as the dominant ion. Because the POM emitted by such stoves is typically enhanced by the condensation of gaseous organics following atmospheric aging, the secondary OM (SOM) formation potential of these stoves was simulated using the Micro Smog Chamber (MSC) designed by Keller and Burtscher in 2012. In general, OM emission factors from MSC-aged aerosols were comparable to lower-time-resolution results from the literature, although the MSC exposed aerosols to much higher concentrations of oxidants and therefore produced OM that was more oxidized than expected for atmospheric samples. In addition, the logwood-stove particles remained highly aspherical even after oxidation, indicating that mixing with an external aerosol is required for these particles to become spherical. The one exception to this observation occurred when the wood failed to ignite and appeared to generate tar-ball OM particles. [ABSTRACT FROM PUBLISHER]

This study reports the first of a kind data on aircraft engine non-volatile particulate matter (nvPM) number- and mass-based emissions using standardized systems. Two compliant sampling and measurement systems operated by Missouri University of Science and Technology (Missouri S&T) and Empa were evaluated during the Aviation - Particle Regulatory Instrumentation Demonstration Experiment (A-PRIDE) 4 campaign at the SR Technics facilities in Zürich, Switzerland, in November 2012. The Missouri S&T and Empa systems were compared during a series of dedicated engine tests using a CFM56-5B4/2P engine source, and maintenance engine testing using CFM56-7B24/3 and PW4168A engine sources at a range of engine operating conditions. These two compliant systems were found to agree within 6% of each other in terms of nvPM number-based emissions, and within 15% for nvPM mass-based emissions. For the three engine sources studied, at several engine power conditions the mass instruments approached their limit of detection, resulting in high measurement uncertainties. Ancillary instrumentation was used to determine PM size distributions, chemical composition, and effective density from mass-mobility experiments. Particle geometric mean mobility diameter ranged 20–45 nm, and geometric standard deviation varied from 1.55 to 1.9 for the three engine types studied. The fraction of PM organic content measured in the emissions from the CFM56-5B4/2P engine was ∼4% while the size-dependent particle effective density was parameterized with a mass-mobility exponent of 2.57 and a pre-factor of 0.606. Results of this study will contribute to the development of the new nvPM emissions certification standard and emissions inventories from commercial aviation operations. [ABSTRACT FROM PUBLISHER]

The soot particle aerosol mass spectrometer (SP-AMS) instrument combines continuous wave laser vaporization with electron ionization aerosol mass spectrometry to characterize airborne, refractory black carbon (rBC) particles. The laser selectively vaporizes absorbing rBC-containing particles, allowing the SP-AMS to provide direct chemical information on the refractory and non-refractory chemical components, providing the potential to fingerprint various rBC particle types. In this study, SP-AMS mass spectra were measured for 12 types of rBC particles produced by industrial and combustion processes to explore differences in the carbon cluster (Cn+) mass spectra. TheCn+mass spectra were classified into three categories based on their ion distributions, which varied with rBC particle type. The carbon ion distributions were investigated as a function of laser power, electron ionization (on/off), and ion charge (positive or negative). Results indicate that the dominant positive ion-formation mechanism is likely the vaporization of small, neutral carbon clusters followed by electron ionization (C1+to C5+). Significant ion signal from larger carbon cluster ions (and their fragment ions in the small carbon cluster range), including mid carbon (C6+to C29+) and fullerene (greater than C30+) ions, were observed in soot produced under incomplete combustion conditions, including biomass burning, as well as in fullerene-enriched materials. Fullerene ions were also observed at high laser power with electron ionization turned off, formed via an additional ionization mechanism. We expect this SP-AMS technique to find application in the identification of the source and atmospheric history of airborne ambient rBC particles. Copyright 2015 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]