Interactions between aerosol organic components and liquid water content during haze episodes in Beijing

Abstract. 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 (ALW inorg ) was modelled by ISORROPIA-II, and ALW contributed by organics (ALW org ) was estimated with κ-Kohler theory, where a real-time hygroscopicity parameter of the organics (κ org ) calculated from the real-time organic oxygen-to-carbon (O/C) was used. Particle overall hygroscopicity (κ total ) was computed by weighting component hygroscopicity parameters based on their volume fractions in the mixture. We found that ALW org , which is often neglected in traditional ALW modelling, contributes a significant fraction (18–32 %) to the total ALW in Beijing. The highest ALW org fraction appears in the cleanest days when both the organic fraction and κ org 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 κ org , instead of using a fixed κ org , to calculate ALW org in Beijing. The significant variation of κ org which was calculated from O/C, together with the highly variable organic or inorganic volume fractions, leads to a wide range of κ total (between 0.20 and 0.45), which exerts great impacts on the water uptake. The variation of organic O/C, or derived κ org , 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 κ org , 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 κ org , together with the decreasing particle organic fraction (or increasing particle inorganic fraction), increases κ total , thus further promotes the ability of particles to uptake water.