Comparison with previous BBOA viscosity measurements. As shown in Figure , our BBOA field samples have higher viscosities under more humid conditions than several other BBOA samples generated in the laboratory. Kiland et al. used poke-flow to measure the viscosity of SOA made from the oxidation of BBOA-like phenolic compounds, as a proxy for biomass burning SOA. Those samples were highly viscous and cracked at low RH, but their viscosities decreased by an order of magnitude at 20% RH. Schnitzler et al. used poke-flow to measure the viscosity of water-soluble extracts of primary BBOA generated in the lab from smoldering pine wood. The water-soluble BBOA consistently had a viscosity lower than Kiland et al.'s SOA, and far lower than the field samples herein. Gregson et al. used the fluorescence recovery after photobleaching (FRAP) method to measure the diffusion coefficients of fluorescent dyes in BBOA and then converted the results into the viscosity of the hydrophobic phase and the hydrophilic phase of the BBOA. Gregson et al. reported viscosities of about 10 2 Pa s at 30% RH and 293 K for both phases, estimated from the diffusion coefficients measurements. This is far lower than our field BBOA.