Biomass burning is one of the most significant sources of organic aerosol in the atmosphere. Liquid-liquid phase separation (LLPS) has been observed in BBOA, leading to core-shell particles with the hydrophobic phase on the outside of the hydrophilic phase, potentially impacting the evolution of light-absorbing components through multiphase processes. Here, we investigate how multiphase processing differs between the water-soluble (i.e., hydrophilic) and insoluble (i.e., hydrophobic) phases of BBOA. BBOA generated from the sapwood of eastern red cedar was used to prepare thin films, which were placed in a coated-wall flow tube to monitor loss of ozone from the gas phase due to reactive uptake, a direct measure of reactivity. The effects of relative humidity (RH) and ultraviolet (UV) irradiation, in a photoreactor, were determined. Experimental timeseries were used to inform simulations using multilayer kinetics modelling. Among non-irradiated thin films, the uptake coefficient, i.e., the fraction of reactive collisions, was greatest for the water-soluble phase at high RH (75%), associated with a modelled diffusion coefficient, D_BrC, of about 3 x 10^-9 cm² s^-1, and least for the same phase at low RH (0%), associated with a D_BrC of 1 x 10^-10 cm² s^-1. The uptake coefficient for the water-insoluble phase fell between these two, regardless of RH, and the associated D_BrC increased only slightly from 8 x 10^-10 cm² s^-1 at 0% RH to 9 x 10^-10 cm² s^-1 at 75% RH. These results indicate that, upon LLPS, a shell of water-insoluble species could shield the core of water-soluble species at moderate to high RH, in agreement with recent measurements of diffusivity. The uptake coefficients of both phases at 0% RH decreased significantly after UV irradiation, consistent with a transition from viscous liquid to solid, which was supported by qualitative microscopy observations. These results indicate that UV irradiation of both phases can further shield light-absorbing components from multiphase processing. Finally, the model parameters were used to estimate the lifetimes of primary BrC components with respect to multiphase ozonolysis in the atmosphere. In the future, the effects of LLPS and UV irradiation may need to be incorporated into chemical transport models for accurate representations of the atmospheric lifetime of BrC from biomass burning.

Manabu Shiraiwa

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