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. Aerosol liquid water (ALW) is ubiquitous in ambient aerosol andplays an important role in the formation of both aerosol organics andinorganics. To investigate the interactions between ALW and aerosol organicsduring haze formation and evolution, ALW was modelled based on long-termmeasurement of submicron aerosol composition in different seasons inBeijing. ALW contributed by aerosol inorganics (ALWinorg) was modelledby ISORROPIA II, and ALW contributed by organics (ALWorg) was estimatedwith κ-Köhler theory, where the real-time hygroscopicity parameterof the organics (κorg) was calculated from the real-timeorganic oxygen-to-carbon ratio (O∕C). Overall particle hygroscopicity(κtotal) was computed by weighting component hygroscopicityparameters based on their volume fractions in the mixture. We found thatALWorg, which is often neglected in traditional ALW modelling,contributes a significant fraction (18 %–32 %) to the total ALW in Beijing.The ALWorg fraction is largest on the cleanest days when both theorganic fraction and κorg are relatively high. The largevariation in O∕C, from 0.2 to 1.3, indicates the wide variety of organiccomponents. This emphasizes the necessity of using real-time κorg, instead of fixed κorg, to calculate ALWorg inBeijing. The significant variation in κorg (calculated fromO∕C), together with highly variable organic or inorganic volume fractions,leads to a wide range of κtotal (between 0.20 and 0.45), whichhas a great impact on water uptake. The variation in organic O∕C, or derivedκorg, was found to be influenced by temperature (T), ALW, andaerosol mass concentrations, among which T and ALW both have promotingeffects on O∕C. During high-ALW haze episodes, although the organic fractiondecreases rapidly, O∕C and derived κorg increase with theincrease in ALW, suggesting the formation of more soluble organics viaheterogeneous uptake or aqueous processes. A positive feedback loop is thusformed: during high-ALW episodes, increasing κorg, togetherwith decreasing particle organic fraction (or increasing particle inorganicfraction), increases κtotal, and thus further promotes the abilityof particles to uptake water.

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