ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-4207-2010 Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon Lack D. A. 1 2 Cappa C. D. 3 NOAA Earth System Research Laboratory, Chemical Sciences Division, 325 Broadway, Boulder, CO 80304, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, 216 UCB, Boulder, CO 80309, USA Department of Civil and Environmental Engineering, University of California, Davis, California 95616, USA 06 05 2010 10 9 4207 4220 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/4207/2010/acp-10-4207-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/4207/2010/acp-10-4207-2010.pdf

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><sub>Abs</sub>) by atmospheric black carbon (<i>BC</i>) when it is coated in mildly absorbing material (<i>C</i><sub>Brown</sub>) is reduced relative to the enhancement induced by non-absorbing coatings (<i>C</i><sub>Clear</sub>). This reduction, sensitive to both the <i>C</i><sub>Brown</sub> 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><sub>Clear</sub> 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><sub>Brown</sub> 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><sub>Brown</sub> 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><sub>Clear</sub> 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><sub>Brown</sub> within ambient particles. However, an <i>AAE</i><1.6 does not exclude the possibility of <i>C</i><sub>Brown</sub>; rather <i>C</i><sub>Brown</sub> 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><sub>Brown</sub> 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.

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