Articles | Volume 9, issue 4
Research article
27 Apr 2016
Research article |  | 27 Apr 2016

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

Yuxuan Zhang, Qiang Zhang, Yafang Cheng, Hang Su, Simonas Kecorius, Zhibin Wang, Zhijun Wu, Min Hu, Tong Zhu, Alfred Wiedensohler, and Kebin He

Abstract. 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 (χ) and effective density (ρeff) can be characterized as a function of the aging process (DpDc) measured by SP2 and VTDMA. During an intensive field study, the ambient In-BC cores had an average shape factor χ of  ∼ 1.2 and an average density of  ∼ 1.2 g cm−3, 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 (Eab) 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 Eab 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.

Short summary
We develop a novel method in this work for in situ measurements of the morphology and effective density of ambient In-BC cores using a volatility tandem differential mobility analyzer and a single-particle soot photometer. We find that In-BC cores hardly transform the morphology of BC into a void-free sphere. Taking the morphology and density of ambient In-BC cores into account, our work provides a new insight into the enhancement of light absorption for In-BC particles in the atmosphere.