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The coating of black carbon (BC) with inorganic salts and organic compoundscan enhance the magnitude of light absorption by BC. To elucidate theenhancement of light absorption of aged BC particles and its relation to themixing state and morphology of individual particles, we conductedobservations of particles at an Asian outflow site in Noto Peninsula, Japan,in the spring of 2013. Absorption and scattering coefficients at 405, 532,and 781 nm and mass concentrations/mixing states of refractory BC in PM<sub>2.5</sub>were measured using a three-wavelength photoacoustic soot spectrometer and asingle-particle soot photometer (SP2), respectively, after passage through athermodenuder (TD) maintained at 300 or 400 °C or abypass line maintained at room temperature (25 °C). The averageenhancement factor of BC light absorption due to coating was estimated bycomparing absorption coefficients at 781 nm for particles that with andwithout passing through the TD at 300 °C and was found to be1.22. The largest enhancements (> 1.30) were observed under highabsorption coefficient periods when the air mass was long-range transportedfrom urban areas in China. Aerosol samples were also analyzed using atransmission electron microscope (TEM) equipped with an energy dispersiveX-ray analyzer. The morphological features and mixing states ofsoot-containing particles of four samples collected during the highabsorption events were analyzed by comparing microphotographs before andafter the evaporation of beam-sensitive materials by irradiation with a high-density electron beam. The majority of the soot in all samples was found asmixed particles with sulfate-containing spherules or as clusters of suchspherules. For samples showing high enhancement (> 1.30) of BClight absorption, the TEM showed that the internally mixed soot-containingparticles tended to have a more spherical shape and to be thickly coated.The SP2 measurements also suggested that the proportion of thickly coatedsoot was greater. Thus, the observed enhancement of BC light absorption wasfound to differ according to the mixing states and morphology ofsoot-containing particles. The enhancement of BC light absorption in our in situmeasurements and its relation with individual features of soot-containingparticles will be useful to evaluate direct radiative forcing in thedownwind areas of large emission sources of BC

Light absorption and morphological properties of soot-containing aerosols observed at an East Asian outflow site, Noto Peninsula, Japan