Aerosol and molecular processing in the atmosphere occurs in a complex and variable environment consistiog,,of multiple phases and interfacial regions. To explore the effects of such conditions on the ,reactivity of chemical systems; we employ an environmental simulation chamber to investigate the multiphase photolysis of pyruvic acid, which, OH photoreacts in the troposphere in aqueous particles and in the gas phase. Upon irradiation of nebulized pyruvic acid, acetic acid and carbon 'dioxide are rapidly generated, which is, consistent with previous literature on the bulk phase photolysis reactions. Additionally, we identify a new C-6 product, zyrnoic acid, a species that has not previously been reported from pyruvic acid photolysis under any conditions. Its observation here, and corresponding spectroscopic signatures, indicates it could be formed by heterogenerms reactions at the droplet surface. Prior studies of the aqueous photolysis of pyruvic acid have shown that high-molecular-weight:compounds are formed via radical reactions; however, they are inhibited by the presence of oxygen, leading to doubt as to whether the chemistry would occur in the atmosphere. Identification of dimethyltartaric acid from the photolysis of multiphase pyruvic acid in air confirms radical polymerization chemistry can compete with oxygen reactions to some extent under aerobic conditions. Evidence-of additional polymerization within the particles during, irradiation is suggested by the increasing viscosity and, organic content of the particles. The implications of multiphase specific process are then discussed within the broader scope of atmospheric science.