Measuring the morphology and density of internally mixed black carbon with SP2 and VTDMA: new insight into the absorption enhancement of black carbon in the atmosphere

The morphology and density of black carbon (BC) cores in internally mixed BC (In-BC) particles affect their mixing state and absorption enhancement. In this work, we developed a new method to measure the morphology and effective density of the BC cores of ambient In-BC particles using a single-particle soot photometer (SP2) and a volatility tandem differential mobility analyzer (VTDMA) during the CAREBeijing-2013 campaign from 8 to 27 July 2013 at Xianghe Observatory. This new measurement system can select size-resolved ambient In-BC particles and measure the mobility diameter and mass of the In-BC cores. The morphology and effective density of the ambient In-BC cores are then calculated. For the In-BC cores in the atmosphere, changes in their dynamic shape factor (chi) and effective density (rho(eff)) can be characterized as a function of the aging process (D-p/D-c) measured by SP2 and VTDMA. During an intensive field study, the ambient In-BC cores had an average shape factor chi of similar to 1.2 and an average density of similar to 1.2 g cm(-3), indicating that ambient In-BC cores have a near-spherical shape with an internal void of similar to 30 %. From the measured morphology and density, the average shell / core ratio and absorption enhancement (E-ab) of ambient BC were estimated to be 2.1-2.7 and 1.6-1.9, respectively, for In-BC particles with sizes of 200-350 nm. When the In-BC cores were assumed to have a void-free BC sphere with a density of 1.8 g cm(-3), the shell / core ratio and E-ab were overestimated by similar to 13 and similar to 17 %, respectively. The new approach developed in this work improves the calculations of the mixing state and optical properties of ambient In-BC particles by quantifying the changes in the morphology and density of ambient In-BC cores during aging