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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 absorptionenhancement was never estimated for BC found in theArctic region. In the present work, weaim to quantify the absorption enhancement and its impact on radiativeforcing as a function of microphysical properties and mixing state of BCobserved in situ at the Zeppelin Arctic station (78∘ N) in thespring of 2012 during the CLIMSLIP (Climate impacts of short-lived pollutantsin the polar region) project. Single-particle soot photometer (SP2) measurements showed a mean massconcentration of refractory black carbon (rBC) of 39 ng m−3, while therBC mass size distribution was of lognormal shape, peaking at an rBC mass-equivalent diameter (DrBC) of around 240 nm. On average, thenumber fraction of particles containing a BC core withDrBC&gt;80 nm was less than 5 % in the size range (overalloptical particle diameter) from 150 to 500 nm. The BC cores were internallymixed with other particulate matter. The median coating thickness of BC coreswith 220 nm &lt; DrBC&lt; 260 nm was 52 nm, resultingin a core–shell diameter ratio of 1.4, assuming a coated sphere morphology.Combining the aerosol absorption coefficient observed with an Aethalometerand the rBC mass concentration from the SP2, a mass absorption cross section(MAC) of 9.8 m2 g−1 was inferred at a wavelength of 550 nm.Consistent with direct observation, a similar MAC value (8.4 m2 g−1at 550 nm) was obtained indirectly by using Mie theory and assuming acoated-sphere morphology with the BC mixing state constrained from the SP2measurements. According to these calculations, the lensing effect isestimated to cause a 54 % enhancement of the MAC compared to that of bareBC particles with equal BC core size distribution. Finally, the ARTDECOradiative transfer model was used to estimate the sensitivity of theradiative balance to changes in light absorption by BC as a result of a varyingdegree of internal mixing at constant total BC mass. The clear-sky noontimeaerosol radiative forcing over a surface with an assumed wavelength-dependentalbedo of 0.76–0.89 decreased, when ignoring the absorption enhancement, by−0.12 W m−2 compared to the base case scenario, which was constrainedwith mean observed aerosol properties for the Zeppelin site in Arctic spring.The exact magnitude of this forcing difference scales with environmentalconditions such as the aerosol optical depth, solar zenith angle and surfacealbedo. Nevertheless, our investigation suggests that the absorptionenhancement due to internal mixing of BC, which is a systematic effect,should be considered for quantifying the aerosol radiative forcing in theArctic region.

Effects of mixing state on optical and radiative properties of black carbon in the European Arctic