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

摘要

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 (iχ/i) and effective density (iρ/isubeff/sub) can be characterized as a function of the aging process (iD/isubp/sub∕iD/isubc/sub) measured by SP2 and VTDMA. During an intensive field study, the ambient In-BC cores had an average shape factor iχ/i of ?～?1.2 and an average density of ?～?1.2?g?cmsup?3/sup, indicating that ambient In-BC cores have a near-spherical shape with an internal void of ?～?30?%. From the measured morphology and density, the average shell?∕?core ratio and absorption enhancement (iE/isubab/sub) 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?cmsup−3/sup, the shell?∕?core ratio and iE/isubab/sub were overestimated by ?～?13 and ?～?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.