The pyrolysis of cellulose and lignocellulosic materials occurs through a molten intermediate liquid which thermally degrades to volatile components and produces primary aerosols. The generation of primary aerosols from biomass hinders the production of biofuels by pyrolysis, and intensifies the environmental impact of forest fires and the health implications associated with cigarette smoking. High speed photography is utilized to elucidate the ejection mechanism of aerosol particles from thermally decomposing cellulose at the timescale of milliseconds. Fluid modeling, based on first principles, and experimental measurement of the ejection phenomenon supports the proposed mechanism of reactive boiling ejection. Identification of the ejection mechanism of intermediate cellulose confirms the transportation of nonvolatile material to the gas-phase and provides fundamental understanding for predicting the rate of aerosol generation and the residence time distribution of reacting, molten biopolymers.