Examples include the large-scale wildfires in the Pacific Northwest of North America in 2017 and the Australian "black summer" fires in 2019 -2020. Smoke is also ubiquitous in the upper troposphere and is mixed into the lowermost stratosphere. Smoke from wildfires consists of mostly organic aerosol, called biomass-burning organic aerosol (BBOA). For example, smoke sampled from wildfires in the western U.S.A. comprised of > 90% BBOA by mass. Wildfires also emit large amounts of volatile organic compounds (VOCs). In the atmosphere, hydroxyl radicals (OH) and ozone (O3) oxidize VOCs, and some of the products of these reactions condense onto existing particles, adding to the BBOA mass. OH and O3 can also react directly with BBOA, forming a more oxidized aerosol. These combined processes result in aged BBOA (Fig. ). In the stratosphere, sulfuric acid (H2SO4) can also condense on to the aged BBOA to form aged BBOA-H2SO4 particles (Fig. ). The sources of H2SO4 in the stratosphere includes the oxidation of sulfur dioxide (SO2) from pyroCb events and volcanic eruptions and the oxidation of carbonyl sulfide (COS) from natural and industrial sources. The conversion of SO2 to H2SO4 occurs on the order of several weeks in the stratosphere, while the conversion of COS to H2SO4 occurs on the order of tens of years. BBOA can linger for many months in the stratosphere and contribute to the depletion of the UVblocking stratospheric ozone layer. Key reactions that may occur on BBOA in the stratosphere and contribute to stratospheric ozone depletion include ClONO2+HCl, ClONO2+H2O, and HOCl+HCl. These reactions on BBOA could delay ozone layer recovery for years to come, increasing ultraviolet radiation exposure at the Earth's surface and raising the risk of skin cancer. Additionally, BBOA introduced into the stratosphere by pyroCb events may influence the formation of polar stratospheric clouds (PSCs), potentially causing further depletion of the stratospheric ozone layer. Two important properties of BBOA are the viscosity and phase state (i.e., liquid, semisolid, or amorphous solid). These properties are closely related: liquids have a viscosity < 10 2 Pa s, semisolids range from 10 2 -10 12 Pa s, and amorphous ("glassy") solids exhibit a viscosity > 10 Pa s. Knowledge of the viscosity and phase state of BBOA particles is needed to predict the rates of multiphase reactions, including the multiphase reactions mentioned above that can contribute to ozone depletion. For example, the rate for a multiphase reaction often depends on the square root of the diffusion coefficient within the aerosol particle. Since diffusion is related to 1/viscosity, this implies that the rate often depends on the square root of 1/viscosity.