Effect of Thermodenuding on the Structure of Nascent Flame Soot Aggregates

Bhandari, Janarjan; China, Swarup; Onasch, T. B.; Wolff, Lindsay; Lambe, Andrew T.; Davidovits, P.; Cross, Eben S.; Ahern, Adam; Olfert, Jason S.; Dubey, M. K.; Mazzoleni, Cláudio

doi:10.3390/atmos8090166

Cited by 23 publications

(41 citation statements)

References 76 publications

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“…For the reverse-coating experiments, which gave broader BC per-particle mass distributions compared to forward-coating distributions, we found no dependence of the derived MAC values on the distribution width. The collapse was presumably due to the effect of evaporation or condensation of the coating material and not due to the denuding process alone (Bhandari et al, 2017). The observed D f,m independence of the MAC (x > 0.9) is consistent with Radney et al (2014).…”

Section: Soot Optical Properties From the Methane Diffusion Flame 41supporting

confidence: 61%

Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot

et al. 2018

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Abstract. Optical properties of flame-generated black carbon (BC) containing soot particles were quantified at multiple wavelengths for particles produced using two different flames: a methane diffusion flame and an ethylene premixed flame. Measurements were made for (i) nascent soot particles, (ii) thermally denuded nascent particles, and (iii) particles that were coated and then thermally denuded, leading to the collapse of the initially lacy, fractal-like morphology. The measured mass absorption coefficients (MACs) depended on soot maturity and generation but were similar between flames for similar conditions. For mature soot, here corresponding to particles with volume-equivalent diameters >∼ 160 nm, the MAC and absorption Ångström exponent (AAE) values were independent of particle collapse while the single-scatter albedo increased. The MAC values for these larger particles were also size-independent. The mean MAC value at 532 nm for larger particles was 9.1 ± 1.1 m 2 g −1 , about 17 % higher than that recommended by Bond and Bergstrom (2006), and the AAE was close to unity. Effective, theory-specific complex refractive index (RI) values are derived from the observations with two widely used methods: Lorenz-Mie theory and the Rayleigh-Debye-Gans (RDG) approximation. Mie theory systematically underpredicts the observed absorption cross sections at all wavelengths for larger particles (with x > 0.9) independent of the complex RI used, while RDG provides good agreement. (The dimensionless size parameter x = π d p /λ, where d p is particle diameter and λ is wavelength.) Importantly, this implies that the use of Mie theory within air quality and climate models, as is common, likely leads to underpredictions in the absorption by BC, with the extent of underprediction depending on the assumed BC size distribution and complex RI used. We suggest that it is more appropriate to assume a constant, size-independent (but wavelength-specific) MAC to represent absorption by uncoated BC particles within models.

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Section: Soot Optical Properties From the Methane Diffusion Flame 41supporting

confidence: 61%

Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot

et al. 2018

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“…Temperatures during the coating process were adjusted for each organic material, and kept below the homogenous nucleation point of the coating substance. At temperatures below 200 K, the morphology of the BC aerosol would not change significantly (Bhandari et al, 2017). The phase and the thickness of the coating material was not directly determined in these experiments.…”

Section: Aerosol Generation and Characterizationmentioning

confidence: 91%

Laboratory study of the heterogeneous ice nucleation on black carbon containing aerosol

Nichman¹,

Wolf²,

Davidovits³

et al. 2018

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Black carbon (BC) particles are generated in the incomplete combustion of fossil fuels, biomass, and 15 biofuels. These airborne particles affect air quality, human health, and climate. At present, the climate effects of BC particles are not well understood. Their role in cloud formation is obscured by their chemical and physical variability, and by the internal mixing states of these particles with other compounds. The current study focuses on laboratory measurements of the effectiveness of BC-containing aerosol in the formation of ice crystals in cirrus clouds. Ice nucleation in field studies is often difficult to interpret. Nonetheless, most field studies seem to suggest that BC 20 particles are not efficient ice nuclei (IN). On the other hand, laboratory measurements show that in some cases, BC particles can be highly active IN. By working with well-characterized BC-containing particles, our aim is to systematically establish the factors that govern the IN activity of BC.We examine ice nucleation on BC-containing particles under cirrus cloud conditions, commonly understood to be deposition mode ice nucleation. We study a series of well-characterized commercial carbon black particles with 25 varying morphologies and surface chemistries, as well as ethylene flame-generated combustion soot. The carbon black particles used in this study are proxies for atmospherically relevant BC aerosols. These samples were characterized by electron microscopy, mass spectrometry, and optical scattering measurements. Ice nucleation activity was systematically examined in the temperature range from 217 -235 K, using a SPectrometer for Ice Nuclei (SPIN) instrument, which is a continuous flow diffusion chamber coupled with instrumentation to measure light scattering 30 and polarization. To study the effect of coatings on IN, the BC-containing particles were coated with organic acids found in the atmosphere, namely, stearic acid, cis-pinonic acid, and oxalic acid. The results show significant variations in ice nucleation activity as a function of size, morphology and surface chemistry of the BC-containing particles. The measured IN activity dependence on temperature and the physicochemical properties of the BC-containing particles are consistent with an ice nucleation mechanism of pore 35 condensation followed by freezing. Coatings and surface oxidation modify the initial ice nucleation ability of BCcontaining aerosol. Depending on the BC material and the coating, both inhibition and enhancement in IN activity Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-915 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 17 September 2018 c Author(s) 2018. CC BY 4.0 License.were observed. Our measurements at low temperatures complement published data, and highlight the capability of some BC particles to nucleate ice under low supersaturation conditions. These results are expected to help refine theories relating to soot IN activation in the atmosphere.

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“…A study of fractal-like agglomerates produced by a diesel engine similarly measured no significant change to the fractal dimension after removing volatile coatings with a thermodenuder (450 • C; Skillas et al, 1998). Additionally, soot agglomerates emitted by ethylene and methane flames did not exhibit restructuring after being heated to 250-270 • C (Bhandari et al, 2017). If the fractal-like particles underwent restructuring due to heating, one would expect that the fractal dimension (D f ) would increase due to partial collapse of the structure.…”

Section: Determination Of Biomass Burning Aerosolmentioning

confidence: 99%

Investigating biomass burning aerosol morphology using a laser imaging nephelometer

et al. 2018

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Abstract. Particle morphology is an important parameter affecting aerosol optical properties that are relevant to climate and air quality, yet it is poorly constrained due to sparse in situ measurements. Biomass burning is a large source of aerosol that generates particles with different morphologies. Quantifying the optical contributions of non-spherical aerosol populations is critical for accurate radiative transfer models, and for correctly interpreting remote sensing data. We deployed a laser imaging nephelometer at the Missoula Fire Sciences Laboratory to sample biomass burning aerosol from controlled fires during the FIREX intensive laboratory study. The laser imaging nephelometer measures the unpolarized scattering phase function of an aerosol ensemble using diode lasers at 375 and 405 nm. Scattered light from the bulk aerosol in the instrument is imaged onto a chargecoupled device (CCD) using a wide-angle field-of-view lens, which allows for measurements at 4-175 • scattering angle with ∼ 0.5 • angular resolution. Along with a suite of other instruments, the laser imaging nephelometer sampled fresh smoke emissions both directly and after removal of volatile components with a thermodenuder at 250 • C. The total integrated aerosol scattering signal agreed with both a cavity ring-down photoacoustic spectrometer system and a traditional integrating nephelometer within instrumental uncertainties. We compare the measured scattering phase functions at 405 nm to theoretical models for spherical (Mie) and fractal (Rayleigh-Debye-Gans) particle morphologies based on the size distribution reported by an optical particle counter.Results from representative fires demonstrate that particle morphology can vary dramatically for different fuel types. In some cases, the measured phase function cannot be described using Mie theory. This study demonstrates the capabilities of the laser imaging nephelometer instrument to provide realtime, in situ information about dominant particle morphology, which is vital for understanding remote sensing data and accurately describing the aerosol population in radiative transfer calculations.

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Effect of Thermodenuding on the Structure of Nascent Flame Soot Aggregates

Cited by 23 publications

References 76 publications

Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot

Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot

Laboratory study of the heterogeneous ice nucleation on black carbon containing aerosol

Investigating biomass burning aerosol morphology using a laser imaging nephelometer

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