Dependence of size‐resolved CCN spectra on the mixing state of nonvolatile cores observed in Tokyo

Abstract

Size‐resolved cloud condensation nuclei (CCN) spectra and volatility tandem differential mobility analyzer (VTDMA) measurements were made simultaneously in Tokyo in wintertime 2007 to characterize CCN activity near the urban center. Ambient particles with mobility diameters of 30–200 nm were investigated at supersaturations (SSs) of 0.32, 0.60, and 0.88%. The size distributions of the nonvolatile cores of size‐selected particles measured by the VTDMA were bimodal; one mode showed relatively small changes (<10%) in peak diameter by volatilization, and the other mode showed significant changes in diameter (>10% in peak diameter). The former mode is referred to as less volatile (LV), and the latter mode is called more volatile (MV). The main component of nonvolatile cores in Tokyo is known to be black carbon (BC). Therefore, it is likely that LV particles correspond to soot particles. The size‐resolved CCN spectra were broader than those of (NH4)2SO4, indicating that the observed particles were not uniformly mixed. In addition, CCN/CN (CN: condensation nuclei) ratios were smaller than unity after the stepwise increase. A CCN‐LV correlation analysis shows that for small (<80 nm) particles, the slopes of the correlations were smaller than unity, although the correlations were significant. This indicates that CCN‐inactive fractions are explained by LV particles and particles coemitted with LV particles (likely primary organic aerosol particles). For larger particles (>100 nm), the CCN‐inactive fractions were close to the LV particle fractions, suggesting that CCN‐inactive particles were composed of fresh soot.