Identification by single‐particle soot photometer of black carbon particles attached to other particles: Laboratory experiments and ground observations in Tokyo

Abstract

Black carbon (BC) aerosols, which are strong contributors to positive radiative forcing, are found in the atmosphere as bare BC or as internal mixtures of BC with non-BC compounds (mixed BC-containing particles). Mixed BC-containing particles can be broadly classified into two morphological types: bare BC on the surface of non-BC particles (attached type) or BC embedded within or coated by non-BC compounds (coated type). For the same amount of mixed non-BC compounds, enhancements of the mass absorption cross section of BC by the coated type are much larger than those of the attached type. Consequently, identification of the two morphological types in mixed BC-containing particles is important for understanding the impact of BC on climate. We develop a new algorithm to classify mixed BC-containing particles into attached and coated types by using the single-particle soot photometer (SP2), based on laboratory experiments generating and detecting mixed BC-containing particles of known morphological types. Our algorithm allows users to choose an optimal set of operational parameters, depending on their purpose and instrumental conditions. We used the algorithm to identify the morphology of mixed BC-containing particles with a BC mass of~8.0 fg in ambient air in Tokyo. The observed number fractions of attached type particles among morphologically identified BC-containing particles were generally less than 0.1 but occasionally reached~0.4 during short events. This finding will motivate further observations, including in known outflow regions, to investigate the regional and global abundance of the attached type among mixed BC-containing particles.

Quantifying black carbon light absorption enhancement with a novel statistical approach

“…A single-particle soot photometer (SP2) study by Sedlacek et al (2012) reported a negative lag time between the scattering and incandescence signals in samples influenced by biomass burning, implying a near-surface location of soot relative to non-absorbing materials. Near-surface mixing of soot has also been observed in Tokyo, but accounted for only 10 % of total mixed soot containing particles (Moteki et al, 2014). Considering the domination of core-shell-type particles in the ambient environment, the core-shell assumption in our optical model is sufficient to approximate reality.…”

On-line Measurement of the Surface Area Concentration of Aerosols in Yokohama, Japan, using the Diffusion Charging Method

“…If these particles are heavily coated with OC, their specific surface area is likely to decrease significantly. Moreover, Moteki et al (2014) showed that the abundance of coated BC particles among total BCcontaining particles can vary considerably with time. In addition, the abundance of coated BC parti cles varies with emission sources (China et al, 2014;China et al, 2013).…”

A New Method to Determine the Number Concentrations of Refractory Black Carbon Ice Nucleating Particles

“…The mixing state of rBC in the atmosphere is often a complex mixture of embedded, partially coated, or bare particles of varying morphologies (Adachi et al 2007;China et al 2013). However, studies have shown the SP2 capable of detecting rBC coated in up to 50 nm of oleic acid or anthracene (Slowik et al 2007) and connected to other solid aerosol particle types (Moteki et al 2014).…”

“…However, the small decrease in N 500 relative to f mixed indicates that the mixed particles were for the most part not removed due to vaporization of their rBC component. Moteki et al (2014) used the SP2 to determine the mixing state of rBC containing particles and were able to distinguish between coated rBC and rBC attached to other solid aerosol species. For this second aerosol type they observed a scattering signal persisting after the rBC component had vaporized, indicating that the scattering component was not completely destroyed due to heating.…”