Phase behavior and viscosity in biomass burning organic aerosol and climatic impacts

Smoke particles generated by burning biomass consist mainly of organic aerosol termed biomass burning organic aerosol (BBOA). BBOA influences the climate by scattering and absorbing solar radiation or acting as nuclei for cloud formation. The viscosity and the phase behavior (i.e., the number and type of phases present in a particle) are properties of BBOA that are expected to impact several climate-relevant processes but remain highly uncertain. We studied the phase behavior of BBOA using fluorescence microscopy and showed that BBOA particles comprise two organic phases (a hydrophobic and a hydrophilic phase) across a wide range of atmospheric relative humidity (RH). We determined the viscosity of the two phases at room temperature using a photobleaching method and showed that the two phases possess different RH-dependent viscosities. The viscosity of the hydrophobic phase is largely independent of the RH from 0 to 95%. We use the Vogel–Fulcher–Tamman equation to extrapolate our results to colder and warmer temperatures, and based on the extrapolation, the hydrophobic phase is predicted to be glassy (viscosity >10¹² Pa s) for temperatures less than 230 K and RHs below 95%, with possible implications for heterogeneous reaction kinetics and cloud formation in the atmosphere. Using a kinetic multilayer model (KM-GAP), we investigated the effect of two phases on the atmospheric lifetime of brown carbon within BBOA, which is a climate-warming agent. We showed that the presence of two phases can increase the lifetime of brown carbon in the planetary boundary layer and polar regions compared to previous modeling studies. Hence, the presence of two phases can lead to an increase in the predicted warming effect of BBOA on the climate.