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Abstract:
he morphology and density of black carbon (BC) cores in internally mixed BC (In-BC) particles affects their mixing state and absorption enhancement. In this work, we developed a new method to measure the morphology and effective density of 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. The new measurement system can select size-resolved ambient In-BC particles and measure the mobility size and mass of In-BC cores. The morphology and effective density of ambient In-BC cores are then calculated. For In-BC cores in the atmosphere, changes in the dynamic shape factor (χ) and effective density (ρeff) can be characterized as a function of aging process (Dp ⁄ Dc) measured by SP2 and VTDMA. During an intensive field study, the ambient In-BC cores had an average 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 %. With the measured morphology and density, the average shell ⁄ core ratio and absorption enhancement (Eab) from ambient black carbon were estimated to be 2.1–2.7 and 1.6–1.9 for different sizes of In-BC particles at 200–350 nm. When assuming the In-BC cores have a void-free BC sphere with a density of 1.8 g cm−3, the shell ⁄ core ratio and Eab could be overestimated by ∼ 13 and ∼ 17 % respectively. The new approach developed in this work will help improve calculations of mixing state and optical properties of ambient In-BC particles by quantification of changes in morphology and density of ambient In-BC cores during aging process.
