Critical condensed mass for activation of black carbon as cloud condensation nuclei in Tokyo

In the global modeling of black carbon (BC) particles, BC is categorized into hydrophobic (cloud condensation nuclei (CCN)‐inactive) and hydrophilic (CCN‐active) groups to represent wet removal process. However, the criteria separating these two types of BC have not been quantified. We investigate the dependence of CCN activity of atmospheric BC particles on the mass of condensed compounds (Δm) in Tokyo in April 2007. In order to measure Δm, we developed a volatility tandem aerosol particle mass analyzer (VTAPM), which consisted of two aerosol particle mass analyzers (APMs) connected in tandem and a heater placed between them. The heater temperature was set at 400°C for the quantitative extraction of BC. Δm was quantified as the difference in the particle masses measured by the two APMs. CCN were measured simultaneously with Δm. We found that the critical values of Δm required for the CCN activation of BC particles were 0.18 fg (supersaturation (S) = 0.9%) and 0.07 fg (S = 1.3%) for 100‐nm particles. The chemical compositions of condensed compounds were inferred by changing the heater temperature and by employing Köhler theory. It is concluded that organic compounds constituted the dominant mass fraction of the condensed compounds and that (NH4)2SO4 was a minor component. The present results give a measure of the critical parameter separating the two types (hydrophobic and hydrophilic) of BC.