Characterization of Size-Resolved Hygroscopicity of Black Carbon-Containing Particle in Urban Environment

The hygroscopic properties of BC-containing particles (BCc) are important to determine their wet scavenging, atmospheric lifetime, and interactions with clouds. Such information is still lacking in the real world because of the challenges in isolating BCc from other aerosols to be directly characterized. In this study, the size-resolved chemical components of BCc including the refractory BC core and associated coatings were measured by a soot particle-aerosol mass spectrometer in suburban Nanjing. The size-resolved hygroscopicity parameter of BCc (κ_BCc) was obtained based on this full chemical characterization of BCc. We found increased inorganic fraction and more oxidized organic coatings with thicker coatings, which modified κ_BCc besides the determinant of particle size. The bulk κ_BCc was observed to range from 0.11 to 0.34. The size-resolved κ_BCc consistently showed minima at coated diameter (D_coated) of 100 nm, parametrized as κ(x) = 0.28-0.35 × exp(-0.004 × x), x = D_coated. Under critical supersaturations (SS) of 0.1% and 0.2%, the D₅₀ values of BCc were 200 ± 20 and 135 ± 18 nm, respectively. On average 33 ± 16% and 59 ± 20% of BCc in number could be activated at SS = 0.1% and 0.2%, respectively. These results provide constraints on surface CCN sources for the light-absorbing BC-containing particles.