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

Abstract

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.

Biomass-burning derived particles from a wide variety of fuels: Part 1: Properties of primary particles

“…[rBC] measurements, and are ~35%. The derived MACBC values are very similar to those recently reported by Forestieri et al (2018) for fresh BC particles: MACBC,pure = 11.9 m 2 g -1 at 405 nm and 8.8 m 2 g -1 at 532 nm, with an extrapolated value at 781 nm of 5.7 m 2 g -1 . The value at 532 nm is somewhat higher than that suggested by (7.75 m 2 g -1 at 532 nm).…”

Light Absorption by Ambient Black and Brown Carbon and its Dependence on Black Carbon Coating State for Two California, USA, Cities in Winter and Summer

“…We suggest that a reasonable precision-limited uncertainty on the MAC BC,ref values for both campaigns is 10%. It is noteworthy that the derived campaign-specific MAC BC,ref values are in reasonable agreement with laboratory measurements Forestieri et al, 2018) and literature assessments . They are, however, substantially smaller than recently determined from filter-based measurements made across Europe for ambient particles, where the MAC at 637 nm was reported as 10.0 m 2 /g, which would correspond to an MAC at 532 nm of 12 m 2 /g assuming an AAE = 1.…”

Radiative absorption enhancements by black carbon controlled by particle-to-particle heterogeneity in composition

“…Particle composition measurements made using an aerosol mass spectrometer confirmed that the coating material was completely removed and established the relative composition for the mixed coating experiments. The absorption enhancement was determined as the ratio between the coated and uncoated (thermally denuded) particle absorption corrected for losses in the thermodenuder and for the influence of multiply charged particles (42). Further details are available in refs.…”