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

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 measurement of submicron aerosol composition in 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-Köhler theory, where the real-time hygroscopicity parameter of the organics (κ_org) was calculated from the real-time organic oxygen-to-carbon ratio (O/C). Overall particle 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 ALW_org fraction is largest on the cleanest days when both the organic fraction and κ_org are relatively high. The large variation in 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 fixed κ_org, to calculate ALW_org in Beijing. The significant variation in κ_org (calculated from O/C), together with highly variable organic or inorganic volume fractions, leads to a wide range of κ_total (between 0.20 and 0.45), which has a great impact on water uptake. The variation in organic O/C, or derived κ_org, was found to be influenced by temperature (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 and derived κ_org increase with the increase in ALW, suggesting the formation of more soluble organics via heterogeneous uptake or aqueous processes. A positive feedback loop is thus formed: during high-ALW episodes, increasing κ_org, together with decreasing particle organic fraction (or increasing particle inorganic fraction), increases κ_total, and thus further promotes the ability of particles to uptake water.