The higher viscosities of our field samples are most likely due to a combination of factors. It could be partially explained by oxidation, which is expected to lead to more polar compounds, higher intermolecular forces, and therefore higher viscosities in the BBOA. However, Xu et al.'s experiments show that on timescales relevant to our study, oxidation alone could not bring the viscosity of BBOA to 2.5×10 8 Pa s. Such high viscosities could also be due to dilution in the atmosphere prior to sampling. When smoke plumes are diluted, low molecular weight and low polarity compounds can evaporate first, leaving behind larger, more polar, and hence more viscous compounds. Recent experiments by Siemens et al. suggest that this dilution can cause an increase in the viscosity of BBOA by up to 3 orders of magnitude. The differences in viscosities could also be due to differences in burn conditions (i.e. flaming vs smoldering) or fuel type. Most of the studies above burn a single wood or fuel type at a single temperature, while our field samples contained BBOA from fires that would have been burning at different temperatures and a wide range of fuels could have contributed to the samples. Additional studies are needed to explore the effect of atmospheric oxidation, dilution, and burning conditions on BBOA viscosity.