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

Abstract

The presence of clear coatings on atmospheric black carbon (<i>BC</i>) particles is known to enhance the magnitude of light absorption by the <i>BC</i> cores. Based on calculations using core/shell Mie theory, we demonstrate that the enhancement of light absorption (<i>E</i>_Abs) by atmospheric black carbon (<i>BC</i>) when it is coated in mildly absorbing material (<i>C</i>_Brown) is reduced relative to the enhancement induced by non-absorbing coatings (<i>C</i>_Clear). This reduction, sensitive to both the <i>C</i>_Brown coating thickness and imaginary refractive index (<i>RI</i>), 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 <i>C</i>_Clear is therefore reduced if the coating is absorbing. Additionally, the need to explicitly treat <i>BC</i> as an internal, as opposed to external, mixture with <i>C</i>_Brown is shown to be important to the calculated single scatter albedo only when models treat <i>BC</i> as large spherical cores (&gt;50 nm). For smaller <i>BC</i> cores (or fractal agglomerates) consideration of the <i>BC</i> and <i>C</i>_Brown as an external mixture leads to relatively small errors in the particle single scatter albedo of &lt;0.03. It has often been assumed that observation of an absorption Angström exponent (<i>AAE</i>)>1 indicates absorption by a non-<i>BC</i> aerosol. Here, it is shown that <i>BC</i> cores coated in <i>C</i>_Clear can reasonably have an <i>AAE</i> of up to 1.6, a result that complicates the attribution of observed light absorption to <i>C</i>_Brown within ambient particles. However, an <i>AAE</i><1.6 does not exclude the possibility of <i>C</i>_Brown; rather <i>C</i>_Brown cannot be confidently assigned unless <i>AAE</i>>1.6. Comparison of these model results to various ambient <i>AAE</i> measurements demonstrates that large-scale attribution of <i>C</i>_Brown is a challenging task using current in-situ measurement methods. We suggest that coincident measurements of particle core and shell sizes along with the <i>AAE</i> may be necessary to distinguish absorbing and non-absorbing OC.