Hygroscopicity of Materials Internally Mixed with Black Carbon Measured in Tokyo

Black carbon (BC) aerosols become internally mixed with non-BC compounds (BC coatings) during transport in the atmosphere. The hygroscopicity of BC coatings can strongly influence the cloud condensation nuclei (CCN) activity and optical properties of BC-containing particles. In this study, we measured the hygroscopicity parameter k for both BC-coating materials (kBC-coat) and BC-free particles (kBC-free) in the urban atmosphere of Tokyo, using a single particle soot photometer (SP2). In our measurement system, dry ambient particles were first mass-selected by an aerosol particle mass analyzer, then humidified, and then passed to the SP2 laser for detection. There the refractory BC mass content and optical diameter of individual humidified particles were measured by the SP2. During the observation period, the measured kBC-coat generally agreed with kBC-free to within ±25%, and was usually in typical range for inorganic and organic aerosols. These results indicate that BC-coating materials and BC-free particles in Tokyo usually had similar chemical compositions, internal mixtures of inorganic and organic compounds, even in a source region. Occasionally kBC-coat was much higher than kBC-free values, when the mass concentrations of BC and organic aerosols were poorly correlated. This indicates external mixing of BC-containing and BC-free particles from different sources. These findings improve our understanding of the CCN activity of BC-containing particles, which strongly influences their wet removal, and optical properties in the ambient air.