ALBERT

Description: Aerosol liquid water (ALW) is ubiquitous in ambient aerosol and plays an important role in the formation of both aerosol organics and inorganics. To investigate the interactions between ALW and aerosol organics during haze formation and evolution, ALW was modelled based on long-term measurements of submicron aerosol compositions from different seasons in Beijing. ALW contributed by aerosol inorganics (ALWinorg) was modelled by ISORROPIA-II, and ALW contributed by organics (ALWorg) was estimated with κ-Köhler theory, where a real-time hygroscopicity parameter of the organics (κorg) calculated from the real-time organic oxygen-to-carbon (O/C) was used. Particle overall hygroscopicity (κtotal) was computed by weighting component hygroscopicity parameters based on their volume fractions in the mixture. We found that ALWorg, which is often neglected in traditional ALW modelling, contributes a significant fraction (18–32 %) to the total ALW in Beijing. The highest ALWorg fraction appears in the cleanest days when both the organic fraction and κorg are relatively high. The distinct variation of O/C, from 0.2 to 1.3, indicates the wide variety of organic components. This emphasizes the necessity of using real-time κorg, instead of using a fixed κorg, to calculate ALWorg in Beijing. The significant variation of κorg which was calculated from O/C, together with the highly variable organic or inorganic volume fractions, leads to a wide range of κtotal (between 0.20 and 0.45), which exerts great impacts on the water uptake. The variation of organic O/C, or derived κorg, was found to be influenced by T, ALW, and aerosol mass concentrations. Among which, T and ALW both have promoting effects on O/C. During high-ALW haze episodes, although the organic fraction decreases rapidly, O/C, or derived κorg, increases with the increase in ALW, suggesting the formation of more soluble organics via aqueous/heterogeneous-phase process. A positive feedback loop is thus formed: during high-ALW episodes, the ever-increasing κorg, together with the decreasing particle organic fraction (or increasing particle inorganic fraction), increases κtotal, thus further promotes the ability of particles to uptake water.