Measurement of Density and Shape for Single Black Carbon Aerosols in a Heavily Polluted Urban Area

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 (chi) of ambient BC particles with three different aerodynamic diameters (D-a, 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 showed that BC-dominated particles whose BC core mass (similar to 2.2 fg) was almost equal to particle mass (similar to 2.3 fg) were observed in particles with 200 nm D-a. The morphology of these BC-dominated particles was near-spherical (chi approximate to 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 D-a 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.