Abstract
Black carbon (BC) aerosol imposes adverse effects on atmospheric visibility, climate, and health. The particle density and morphology are often needed to investigate the mixing state and aging process of BC particles. A method, combining an aerodynamic aerosol classifier (AAC), a differential mobility analyzer (DMA), a single-particle soot photometer (SP2) and a single particle aerosol mass spectrometer (SPAMS), was developed to determine the density and dynamic shape factor (χ) of ambient BC particles with three different aerodynamic diameters (Da, 200 nm, 350 nm, and 500 nm) in Shanghai, China, a typical urban area. The BC particles were either classified as “BC-dominated particle” which is mainly made of black carbon or “BC-mixed particle” which is a mixture of both BC and non-BC substances. The results show that BC-dominated particles whose BC core mass (~2.2 fg) was almost equal to particle mass (~2.3 fg) were observed in particles with 200 nm Da. The morphology of these BC-dominated particles was near-spherical (χ ≈ 1.02), indicating that they had undergone rapid morphology modification from the initial highly irregular morphology to near-spherical shape. Most BC particles with 350 nm or 500 nm Da were BC-mixed particles. Combining the effective densities (1.62~1.77 g cm-3) and average single particle mass spectra of particle, the ammonium sulfate and ammonium nitrate were found to be the main secondary substances of these BC-mixed particles, indicating that condensation of inorganic species such as nitrates and sulfates could play a significant role in the aging process of fresh BC in Shanghai. Generally, the morphology and density information of single BC particle is crucial to identify the mixing state and aging process of BC aerosols.
Highlights
It has been well acknowledged that black carbon (BC) aerosol exerts significant impacts on air quality, climate and human health (Menon et al, 2002; Anenberg et al, 2012; Bond et al, 2013; Wu et al, 2018)
Given that both in winter sampling time of 2018 and summer sampling time of 2019 the largest peak’s location of the electrical mobility size distribution of these aerodynamic aerosol classifier (AAC)-selected particles was constant at ~135nm (See Figure 3), the setpoints of AAC and differential mobility analyzer (DMA) were respectively set as 200 nm and 135 nm to select the particles in Mode (DDaa =200 nm, DDmm =135 nm), which were further measured by SP2
The density and morphology of ambient BC particles in Shanghai, China were investigated by measuring the BC particles in three typical modes: Mode (DDaa =200 nm, DDmm =135 nm), Mode (DDaa =350 nm, DDmm =260 nm) and Mode (DDaa =500 nm, DDmm =359 nm)
Summary
It has been well acknowledged that black carbon (BC) aerosol exerts significant impacts on air quality, climate and human health (Menon et al, 2002; Anenberg et al, 2012; Bond et al, 2013; Wu et al, 2018). BC aerosol can provide surfaces for many important atmospheric reactions, such as catalyzing SO2 oxidation, which could play a critical role in driving the formation, and aging of regional haze (Zhang et al, 2020). It can affect climate directly by scattering and absorbing solar radiation, and indirectly by influencing cloud formation through acting as cloud condensation nuclei (CCN) (Sun and Ariya, 2006; Li et al, 2008; Li et al, 2011).
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