Laboratory experiments are conducted to investigate aging of size-classified black carbon(BC)particles from OH-initiated oxidation of m-xylene.The variations in the particle size,mass,effective density,morphology,optical properties,hygroscopicity,and activation as cloud condensation nuclei(CCN)are simultaneously measured by a suite of aerosol instruments,when BC particles produced from incomplete combustion of propane are exposed to the oxidation products of the OH-m-xylene reactions.