Ambient black carbon (BC) aerosols were analyzed with a hygroscopicity tandem differential mobility analyzer (HTDMA) in-line with a single particle soot photometer (SP2) to investigate the temporal variation of the hygroscopicity and its correlation with the changes of mixing state of BC aerosols. A single particle aerosol mass spectrometer (SPAMS) was used in parallel to provide chemical information at single particle level. 120nm, 240nm and 360nm dry diameter BC particles were humidified at RH=90%. After humidification, particles with growth factors of 1.0, 1.2, 1.4 were selected to represent the BC particles with different hygroscopicities (hydrophobic, moderate hydrophilic, hydrophilic). Our observations include: (1) During nighttime, number fractions of BC particles in hydrophilic modes were much higher than during daytime. (2) For the same hydrophilic mode, BC core size in nighttime tended to be larger than in daytime, with a thinner coating thickness. The single particle mass spectrometry revealed that nighttime condensations of second inorganic species, especially the formation of nitrate on BC cores resulted an increased hygroscopicity. Less strong-water-absorbing materials (thinner coating thickness) could take the same amount of water compared with the secondary organic compounds generated by photo-chemical reactions during daytime. To our knowledge, this is the first measurement of the temporal variations of the hygroscopic growth of BC containing particles in ambient environment. Our findings could improve the understanding of the ageing processe, cloud condensation nuclei activity, wet removal and optical property of BC- particles in the ambient air.

Temporal variation of the hygroscopicity of black carbon aerosols during a summer episode in Shanghai

256th National Meeting (2018)

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