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