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Atmospheric aging promotes internal mixing of black carbon (BC), leading to an enhancement of light absorption and radiative forcing. The relationship between BC mixing state and consequent absorption enhancement was never estimated for BC found in the Arctic region. In the present work, we aim to quantify the absorption enhancement and its impact on radiative forcing as a function of microphysical properties and mixing state of BC observed in situ at the Zeppelin Arctic station (78 degrees N) in the spring of 2012 during the CLIMSLIP (Climate impacts of short-lived pollutants in the polar region) project.

Single-particle soot photometer (SP2) measurements showed a mean mass concentration of refractory black carbon (rBC) of 39 ngm(-3), while the rBC mass size distribution was of lognormal shape, peaking at an rBC mass-equivalent diameter (D-rBC) of around 240 nm. On average, the number fraction of particles containing a BC core with D-rBC > 80 nm was less than 5% in the size range (overall optical particle diameter) from 150 to 500 nm. The BC cores were internally mixed with other particulate matter. The median coating thickness of BC cores with 220 nm < D-rBC < 260 nm was 52 nm, resulting in a core-shell diameter ratio of 1.4, assuming a coated sphere morphology. Combining the aerosol absorption coefficient observed with an Aethalometer and the rBC mass concentration from the SP2, a mass absorption cross section (MAC) of 9.8 m(2) g(-1) was inferred at a wavelength of 550 nm. Consistent with direct observation, a similar MAC value (8.4m(2) g(-1) at 550 nm) was obtained indirectly by using Mie theory and assuming a coated-sphere morphology with the BC mixing state constrained from the SP2 measurements. According to these calculations, the lensing effect is estimated to cause a 54% enhancement of the MAC compared to that of bare BC particles with equal BC core size distribution. Finally, the ARTDECO radiative transfer model was used to estimate the sensitivity of the radiative balance to changes in light absorption by BC as a result of a varying degree of internal mixing at constant total BC mass. The clear-sky noontime aerosol radiative forcing over a surface with an assumed wavelength-dependent albedo of 0.76-0.89 decreased, when ignoring the absorption enhancement, by -0.12 Wm(-2) compared to the base case scenario, which was constrained with mean observed aerosol properties for the Zeppelin site in Arctic spring. The exact magnitude of this forcing difference scales with environmental conditions such as the aerosol optical depth, solar zenith angle and surface albedo. Nevertheless, our investigation suggests that the absorption enhancement due to internal mixing of BC, which is a systematic effect, should be considered for quantifying the aerosol radiative forcing in the Arctic region.