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 m2 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=πdp/λ, where dp 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.
Highlights
Soot particles, which contain light-absorbing black carbon (BC), are a byproduct of the incomplete combustion of fossil fuels and biomass
Average and median mass absorption coefficients (MACs) values were determined for BC particles from the methane diffusion flame with x > 0.9. (The dimensionless size parameter x = π dp/λ, where dp is particle diameter and λ is wavelength.) There are no systematic differences in the MAC values between nascent and nascent–denuded (Df,m = 2.16 ± 0.1) and coated–denuded (Df,m = 2.64 ± 0.1) soot at any wavelength for this range of x despite some degree of collapse for the thickly coated– denuded particles (Figs. 1 and S2)
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
Summary
Soot particles, which contain light-absorbing black carbon (BC), are a byproduct of the incomplete combustion of fossil fuels and biomass. These particles affect climate directly by absorbing and scattering solar radiation (Bond et al, 2013) and indirectly by acting as cloud condensation nuclei, especially following chemical processing (Lohmann and Feichter, 2005). Soot particles absorb shortwave radiation and have an overall warming effect on climate. The exact magnitude of the climate impacts of BC remains uncertain. One estimate puts top-of-the-atmosphere direct forcing by BC as high as 0.9 W m−2, which is comparable in magnitude to that of CO2 (Ramanathan and Carmichael, 2008). Other more recent assessments yield 0.71 W m−2 with 90 % uncertainty bounds of 0.08 to 1.27 W m−2 (Bond et al, 2013) or 0.61 [+0.16 to +1.40] W m−2 (Wang et al, 2016), while the IPCC suggests a value of 0.40 [+0.05 to +0.80] W m−2 (Boucher et al, 2013)
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