Hygroscopicity of materials internally mixed with black carbon measured in Tokyo

摘要

Black carbon (BC) aerosols become internallymixed with non-BC compounds (BC coatings) during aging in the atmosphere. In this study, we measured the hygroscopicity parameter κ on a single-particle basis for both BC-coatingmaterials (κ_(BC-coat)) and BC-free particles (κ_(BC-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 particlemass analyzer, then humidified, and then passed to the SP2 for detection of their refractory BC mass content and optical diameter. A quadrupole aerosol mass spectrometer was also employed to interpret the hygroscopicity data. During the observation period, the measured κ_(BC-coat) generally agreed with κ_(BC-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. However, occasionally κ_(BC-coat) was much higher than κ_(BC-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 cloud condensation nuclei activity of BC-containing particles, which strongly influences their wet removal, and optical properties in the ambient air.