Effects of Volatile Components on Mixing State and Size Distribution of Individual Black Carbon Aerosols

Abstract

In this study, the effects of different volatile components on the mixing state and size distribution of ambient individual black carbon (BC) aerosols were measured using thermo-denudation with single-particle soot photometry and single-particle aerosol mass spectrometry from September 9 to September 27, 2018, in Shanghai, China. The volatility, chemical composition, size distribution, and core-shell structure of BC aerosol were also analysed. To understand the influence of heating temperature on the BC core-shell structure, the mixing state of ageing particles at room temperature (25°C) and the mixing state of residual material heated to 150°C and 300°C were analysed. BC was internally mixed with secondary components, such as ammonium nitrate, ammonium sulphate, a small amount of sodium nitrate, potassium nitrate, sodium sulphate and potassium sulphate, and other nonvolatile components. At 300°C, the secondary components with low volatility escaped, resulting in a thinner BC coating. However, several low-volatile non-BC substances remained, namely organic species, potassium chloride, potassium nitrate, and other nonvolatile substances. The extremely strong photochemical reaction may have produced large amounts of secondary organic carbon on the BC surface in the afternoon, with the high temperature volatilising the whole particle. However, fresh BC particles emitted from traffic during morning rush hour had ultralow volatility, with relatively fewer low-volatile components on the surface.