Multiphase Processing of the Water-Soluble and Insoluble Phases of Biomass Burning Organic Aerosol.

Biomass burning is one of the most significant sources of organic aerosol in the atmosphere. Biomass burning organic aerosol (BBOA) has been observed to undergo liquid-liquid phase separation (LLPS) to give core-shell morphology with the hydrophobic phase encapsulating the hydrophilic phase, potentially impacting the evolution of light-absorbing components, i.e., brown carbon (BrC), through multiphase processes. Here, we demonstrate how multiphase processing differs between the water-soluble (i.e., hydrophilic) and insoluble (i.e., hydrophobic) phases of BBOA in terms of reactive uptake of ozone in a coated-wall flow tube. Effects of relative humidity (RH) and ultraviolet (UV) irradiation were investigated. Experimental timeseries were used to inform simulations using multilayer kinetic modeling. Among non-irradiated thin films, the uptake coefficient was greatest for the water-soluble phase at 75% RH (3 × 10^-5, corresponding to a diffusion coefficient of BrC, D _BrC, of 3 × 10^-9 cm² s^-1) and least for the same phase at 0% RH (1 × 10^-5, corresponding to D _BrC of 1 × 10^-10 cm² s^-1). The uptake coefficient for the water-insoluble phase fell between these two (about 1.5 × 10^-5), regardless of RH, and the corresponding D _BrC increased only slightly (8 × 10^-10 cm² s^-1 at 0% RH to 9 × 10^-10 cm² s^-1 at 75% RH). The uptake coefficients of both phases at 0% RH decreased significantly after UV irradiation, consistent with a transition from viscous liquid to solid and supported by qualitative microscopy observations. Modeling multiphase ozone oxidation of primary BrC components in the atmosphere demonstrated, first, that LLPS may extend the lifetime of water-soluble BBOA encapsulated by water-insoluble species by a factor of 1.5 at moderate to high RH and, also, that UV irradiation may extend the lifetime of both phases by more than a factor of 2.5.