Secondary organic aerosol phase behaviour in chamber photo-oxidation of mixed precursors

Yu Wang, Aristeidis Voliotis, Yunqi Shao, Taomou Zong, Xiangxinyue Meng, Mao Du, Dawei Hu, Ying Chen, Zhijun Wu, M. Rami Alfarra, Gordon McFiggans

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The phase behaviour of aerosol particles plays a profound role in atmospheric physicochemical processes, influencing their physical and optical properties and further impacting climate and air quality. However, understanding of aerosol phase behaviour is still incomplete, especially that of multicomponent particles which contain inorganic compounds and secondary organic aerosol (SOA) from mixed volatile organic compound (VOC) precursors. We report measurements conducted in the Manchester Aerosol Chamber (MAC) to investigate the aerosol rebounding tendency, measured as bounce fraction, as a surrogate of particle phase behaviour during SOA formation from photo-oxidation of biogenic (α-pinene, isoprene) and anthropogenic (o-cresol) VOCs and their binary mixtures on deliquescent ammonium sulphate seed. Aerosol phase behaviour is RH and chemical composition dependent. Liquid (bounce fraction, BF < 0.2) at RH > 80 % and non-liquid behaviour (BF > 0.8) at RH < 30 % were observed, with a liquid-to-nonliquid transition with decreasing RH between 30 %~80 %. This RH-dependent phase behaviour (RHBF = 0.2, 0.5, 0.8) increased towards a maximum with increasing organic-inorganic-mass ratio (MRorg/inorg) during SOA formation evolution in all investigated VOC systems. With the use of comparable initial ammonium sulphate seed concentration, the SOA production rate of the VOC systems determines the MRorg/inorg, and consequently the change of the phase behaviour. Although less important than RH and MRorg/inorg, the SOA composition plays a second-order role, with differences in liquid-to-nonliquid transition at moderate MRorg/inorg of ~1 observed between biogenic and anthropogenic-containing VOC systems. The real atmospheric consequences of our results are that any processes changing ambient RH or MRorg/inorg will influence their particle phase behaviour. Where abundant anthropogenic VOCs contribute to SOA, compositional changes of SOA may influence phase behaviour at moderate organic mass fraction (~50 %) compared with purely biogenic SOA. Further studies are needed on more complex and realistic atmospheric mixtures.
Original languageEnglish
Number of pages25
JournalAtmospheric Chemistry and Physics
Early online date29 Jun 2021
Publication statusE-pub ahead of print - 29 Jun 2021


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