Direct Measurement of Black Carbon's Effective Density by Using a Dma-Cpma-Sp2 System and Implications for Black Carbon's Shape Variations

Abstract

Effective density (ρeff) is a key parameter of black carbon-containing (BCc) particles, relating to their morphology, deposition, and optical properties. In this study, a tandem system was established and used to investigate the ρeff of ambient BCc particles. The results show that the ρeff distribution of ambient BCc particles exhibited a bimodal pattern with a left peak location of 0.69 g/cm3 and a right peak location of 1.45 g/cm3. The average ρeff of BCc particles was 1.38 g/cm3 during the entire observation period. The ρeff of BCc particles showed a clear diurnal pattern with a relatively stable distribution at night and large variation during daytime. The average ρeff of BCc particles increased during the daytime due to the increase in BCc particles in the right mode. The ρeff of BCc particles also depended on BCc particle sizes and pollution levels.The ρeff value was demonstrated to be a good indicator for BCc particle morphology. BCc particles became more regular with increasing ρeff and nearly spherical when ρeff > 1.4 g/cm3. This morphological change was related to the coating thickness variation. The mass ratio (MR) of the coatings to the BC core was found to increase from 0.2 to 7 as ρeff increased from 0.3 to 1.4 g/cm3 and fluctuated between 8 and 10 in a ρeff range of 1.4-2.0 g/cm3. More coating led to BCc particle morphological reconstruction. It was also found that less coating was needed to reconstruct BCc particles under high relative humidity conditions. The chemical composition results showed that the traffic-related BCc particles had lower ρeff and that the BCc particles from solid fuel burning had a higher ρeff. This study will be helpful for understanding BCc particle microphysical properties and health risk evaluation.

Keywords: Black carbon, effective density, shape, mixing state