Light Absorption by Carbonaceous Particles: An Investigative Review

Abstract

The optical properties of the light-absorbing, carbonaceous substance often called "soot," "black carbon," or "carbon black" have been the subject of some debate. These properties are necessary to model how aerosols affect climate, and our review is targeted specifically for that application. We recommend the term light-absorbing carbon to avoid conflict with operationally based definitions. Absorptive properties depend on molecular form, particularly the size of sp 2 -bonded clusters. Freshly-generated particles should be represented as aggregates, and their absorption is like that of particles small relative to the wavelength. Previous compendia have yielded a wide range of values for both refractive indices and absorption cross section. The absorptive properties of light-absorbing carbon are not as variable as is commonly believed. Our tabulation suggests a mass-normalized absorption cross section of 7.5 ± 1.2 m 2 /g at 550 nm for uncoated particles. We recommend a narrow range of refractive indices for strongly-absorbing carbon particles, of which the highest is 1.

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

“…Lower MAC values measured in the wet season 2016 could be associated with less coated BC compared to more aged particles in the dry season, which could have thicker coatings. Nevertheless, both values are much higher than the 6.6 m² g -1 suggested by Bond and Bergstrom (2006), especially considering that mineral dust and BrC do not strongly absorb at this wavelength and would therefore have little influence on the apparent MAC. However, they are in agreement with a modelled absorption enhancement of 1.6 calculated for open biomass burning in Brazil .…”

“…Characteristic BC core size distributions measured by the SP2 during the wet and dry season were used in the polydisperse BC-only model to retrieve extinction efficiency and single scattering albedo. The refractive indices used were 1.95 -0.79i for BC (Bond and Bergstrom, 2006) and 1.55 -0.001i for the coating material (Liu et al, 2015). The latter value was only used for the internally mixed BC case.…”

“…The MAC of BC varies between 4 and 11 m² g -1 at 550 nm (Bond and Bergstrom, 2006), having an average of 6.5 m² g -1 at 637 nm for fresh soot. In case of condensation of non-BC material on the BC particles, the MAC can be enhanced due to the well-known 'lensing effect' (Fuller et al, 1999).…”

“…The wavelength dependence is described by the absorption Ångström exponent (å abs ) (Ångström, 1929), which can vary from low values (å abs = 1.0 ± 0.1, weak spectral dependence), usually associated to fossil fuel emitted BC (Bond and Bergstrom, 2006), up to high values (å abs = 6-7, strong spectral dependence) for organic-rich aerosol, e.g., humic-like substances (HULIS) (Hoffer et al, 2006). Measurements at an Amazonian forest site during the dry season resulted in å abs average values below 1.0 for absorption coefficients lower than 15 Mm -1 at 450 nm (Rizzo et al, 2011).…”

Comparison of elemental and black carbon measurements during normal and heavy haze periods: implications for research

“…BC cannot be measured directly but must be inferred from the particle light absorption (b ap ) through a conversion factor that is used to translate b ap to the BC mass concentration (Chow et al 2009;Biswas et al 2003). According to the literature, both EC and BC are considered to assume a graphitic structure and are therefore thermally refractory and light-absorbing in nature (Bond and Bergstrom 2006;Salako et al 2012;Conny et al 2003;Turpin et al 1990). It seems that both EC and BC describe nearly the same fraction of carbonaceous aerosols; however, according to traditional practice, their values are dependent on specified determination methods which focus on different properties, e.g., thermal stability or light absorption (Birch 1998;Chow et al 1993;Hansen et al 1984).…”