Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon

Abstract

Abstract. The presence of clear coatings on atmospheric black carbon (BC) particles is known to enhance the magnitude of light absorption by the BC cores. Based on calculations using core/shell Mie theory, we demonstrate the enhancement of light absorption (EAbs) by atmospheric black carbon (BC) when coated in mildly absorbing material (CBrown) is reduced, relative to the enhancement by non-absorbing coatings (CClear). This reduction, sensitive to CBrown shell thickness and imaginary refractive index (RI), can be up to 50% for 400 nm radiation and 25% averaged across the visible radiation spectrum for reasonable core/shell diameters. The enhanced direct radiative forcing possible due to the enhancement effect of CClear is therefore reduced if the coating is absorbing. Additionally, the need to explicitly treat BC as an internal, as opposed to external, mixture with CBrown is shown to be important to the calculated single scatter albedo only whensub models treat BC as large spherical cores (>50 nm). For smaller BC cores (or fractal agglomerates) consideration of the BC and CBrown as an external mixture leads to relatively small errors in the particle single scatter albedo of <0.03. It is often assumed that observation of an absorption Angstrom exponent (AAE) >1 indicates non-BC absorption. Here, it is shown that BC cores coated in CClearcan reasonably have an AAE of up to 1.6, a result that complicates the attribution of observed light absorption to CBrown within ambient particles. However, an AAE<1.6 does not exclude the possibility of CBrown, rather CBrown cannot be confidently assigned unless AAE>1.6. Comparison of these results to some ambient AAE data shows that large-scale attribution of CBrown is a challenging task using current in-situ measurement methods. We suggest that coincident measurements of particle core and shell sizes along with the AAE may be necessary to distinguish absorbing and non-absorbing OC.

The Absorption Ångström Exponent of black carbon: from numerical aspects

“…Furthermore, Schnaiter et al (2005) also measured the absorption of coated BC particles at 450, 550 and 700 nm (see Fig. 9 of their paper), and from these three wavelengths we do see that coating does decrease BC AAE (from approximately 1.1 to 0.8), thereby undermining the AAE results in Lack and Cappa's (2010) study . 25 When the true BC AAE is in doubt, one can alternatively investigate BC AAE by measuring the absorption of BC particles in the atmosphere, which turns out to be more challenging.…”

“…Here, we define coating thickness as the radius of a coated BC particle minus the radius of the core. As pointed out in section 1, Lack and Cappa (2010) assumed that coating thickness is linearly proportional to the core radius. On the other hand, if we apply aerosol condensation physics, we conclude that small core radii are associated with larger coating thicknesses, since condensation reduces the diameter differences between big and small particles.…”

“…In the aforementioned BC/BrC absorption separation studies 10 (Kirchstetter and Thatcher, 2012;Lu et al, 2015), the AAE for uncoated BC was implicitly assumed to be the same as that for coated BC. Lack and Cappa (2010) used a core-shell Mie code to investigate how BC changes its AAE value with respect to coating. At realistic particle sizes, they showed that the BC AAE increases to 1.4~1.6 after coated.…”

“…When BC particles grow in size by coating, the particle growth is governed by condensation. Theoretically, condensation reduces the diameter spread between big particles and small particles over time (Seinfeld and Pandis, 2016), on the contrary to the assumption by Lack and Cappa (2010). Schnaiter et al (2005) coated BC particles with secondary organic aerosol material in a lab, and found that the coating increases particle sizes, while reduces geometric standard deviation (see 20 Fig.…”

Characteristics of Mass Absorption Efficiency of Elemental Carbon in Urban Chengdu, Southwest China: Implication for the Coating Effects on Aerosol Absorption

“…Generally, the AAE of externally mixed EC is 1.0 for EC-contained particle with < 50 nm diameter (Moosmuller et al 2010), while the AAE of internally mixed EC by the presence of transparent coatings (non-absorbing coating) can reach to 1.7 (Gyawali et al 2009;Lack and Cappa 2010). Moreover, the AAE of internally mixed EC by the presence of non-EC absorbers (e.g., brown carbon, which is mainly emitted from biomass burning) coatings can increase even higher (Corr et al 2012).…”