Abstract
Abstract. The coating of black carbon (BC) with inorganic salts and organic compounds can enhance the magnitude of light absorption by BC. To elucidate the enhancement of light absorption of aged BC particles and its relation to the mixing state and morphology of individual particles, we conducted observations 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 PM2.5 were measured using a three-wavelength photoacoustic soot spectrometer and a single-particle soot photometer (SP2), respectively, after passage through a thermodenuder (TD) maintained at 300 or 400 °C or a bypass line maintained at room temperature (25 °C). The average enhancement factor of BC light absorption due to coating was estimated by comparing absorption coefficients at 781 nm for particles that with and without passing through the TD at 300 °C and was found to be 1.22. The largest enhancements (> 1.30) were observed under high absorption coefficient periods when the air mass was long-range transported from urban areas in China. Aerosol samples were also analyzed using a transmission electron microscope (TEM) equipped with an energy dispersive X-ray analyzer. The morphological features and mixing states of soot-containing particles of four samples collected during the high absorption events were analyzed by comparing microphotographs before and after the evaporation of beam-sensitive materials by irradiation with a high-density electron beam. The majority of the soot in all samples was found as mixed particles with sulfate-containing spherules or as clusters of such spherules. For samples showing high enhancement (> 1.30) of BC light absorption, the TEM showed that the internally mixed soot-containing particles tended to have a more spherical shape and to be thickly coated. The SP2 measurements also suggested that the proportion of thickly coated soot was greater. Thus, the observed enhancement of BC light absorption was found to differ according to the mixing states and morphology of soot-containing particles. The enhancement of BC light absorption in our in situ measurements and its relation with individual features of soot-containing particles will be useful to evaluate direct radiative forcing in the downwind areas of large emission sources of BC.
Highlights
Black carbon (BC) is contained in particles emitted from fossil fuel combustion and biomass/biofuel burning
In the estimation of direct radiative forcing by BC particles, understanding how to treat the mixing state and optical properties of the BC and other materials is important for reducing this uncertainty (Ma et al, 2012)
The enhancement factor, Eabs (λ), at 405, 532, and 781 nm was determined by comparing the light absorption of aerosol particles with and without passing through a TD maintained at 300 or 400 ◦C
Summary
Black carbon (BC) is contained in particles emitted from fossil fuel combustion and biomass/biofuel burning. For Mexico City, Adachi et al (2010) obtained three-dimensional shapes of soot-containing particles embedded in organic matter and sulfate using electron tomography with a transmission electron microscope (TEM), and calculated the optical properties using a discrete dipole approximation They reported that the core–shell model overestimated light absorption by ∼ 30 % compared to the model for irregularly shaped soot-containing particles. The reported Eabs values are from nondetectable to 1.43 in Toronto (Canada) (Knox et al, 2009; Chan et al, 2011; Healy et al, 2015), 1.38 in Boulder (USA) (Lack et al, 2012), 1.06 in California (USA) (Cappa et al, 2012), 1.10 (August) and non-detectable (January) in Nagoya (Japan) (Nakayama et al, 2014), and 1.4 at Detling (UK) (Liu et al, 2015) In these studies, Eabs was estimated as the ratio between absorption of ambient particles and particles heated in a TD to remove non-BC materials. Specific attention was given to the relations between the enhancement of light absorption and the coating condition of individual soot-containing particles based on the TEM analysis
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