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

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 ( ‡) and effective density ( eff) can be characterized as a function of the aging process ( D pĝ• D c) measured by SP2 and VTDMA. During an intensive field study, the ambient In-BC cores had an average shape factor ‡ of ĝ1/4ĝ€1.2 and an average density of ĝ1/4ĝ€1.2ĝ€gĝ€cmĝ 3, indicating that ambient In-BC cores have a near-spherical shape with an internal void of ĝ1/4ĝ€30ĝ€%. From the measured morphology and density, the average shellĝ€ĝ•ĝ€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 200–350ĝ€nm. When the In-BC cores were assumed to have a void-free BC sphere with a density of 1.8ĝ€gĝ€cm−3, the shellĝ€ĝ•ĝ€core ratio and E ab were overestimated by ĝ1/4ĝ€13 and ĝ1/4ĝ€17ĝ€%, 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.