Biomass burning events emit a large amount of volatile organic compounds (VOCs), which can be oxidized in the atmosphere, leading to secondary organic aerosol (SOA) formation by gas-toparticle partitioning of the oxidized products. Phenolic compounds such as catechol, guaiacol, and syringol (Fig. ) are major types of VOCs from biomass burning events that lead to the formation of biomass burning SOA (BB-SOA) in the atmosphere. BB-SOA and other particulate matter emitted from biomass burning play an important role in climate and air quality. They alter climate directly by absorbing and scattering radiation, as well as indirectly by acting as nuclei for clouds. BB-SOA also impacts air quality, since they penetrate into the respiratory system, causing adverse health outcomes. Furthermore, the atmospheric load of BB-SOA is expected to increase in the future due to the growing frequency and size of forest fires as a result of climate change. The viscosity of BB-SOA is needed to predict important atmospheric processes related to climate and air quality. For example, the long-range transport of pollutants depends on SOA viscosity. In addition, predictions of gas-particle partitioning (and hence, mass and size distributions) depend on the viscosity of SOA. Similarly, heterogeneous chemical reactions and photochemical reactions in SOA depend on the mobility and uptake of reactants and products, which can be inhibited at high viscosity. Furthermore, the heterogeneous ice nucleation potential of SOA particles may depend on whether it is in a glassy phase state (η > 10 12 Pa s). In addition to the gaseous components that can lead to SOA formation, biomass burning also directly emits organic aerosols into the atmosphere, which are referred to as biomass burning primary organic aerosol (BB-POA). The viscosity of BB-POA from the pyrolysis of pine wood has been studied previously and was found to depend strongly on relative humidity (RH) and temperature. Similarly, the diffusion rates of organic molecules in BB-POA, which are inversely related to particle viscosity, were also found to depend on RH. In contrast to BB-POA, much less is known about the viscosity of BB-SOA. Only one study has reported a viscosity for BB-SOA: catechol + O3 at 0% RH and room temperature was found to have a viscosity (η) greater than 3 × 10 8 Pa s. With only one reported measurement, considerable uncertainty still exists concerning the viscosity of BB-SOA under relevant atmospheric conditions.