The chemical composition and evolution of secondary organic aerosol (SOA) in the atmosphere represents one of the largest uncertainties in our current understanding of air quality. Despite vast research, the toxicological mechanisms relating to adverse human health effects upon exposure to particulate matter are still poorly understood. Particle-bound reactive oxygen species (ROS) may substantially contribute to observed health effects by influencing aerosol oxidative potential (OP). The role of radicals in both the formation and aging of aerosol, as well as their contribution to aerosol OP, remains highly uncertain. The profluorescent spin trap BPEAnit (9,10-bis(phenylethynyl)anthracenenitroxide), previously utilized to study combustion-generated aerosol, has been applied to provide the first estimate of particle-bound radical concentrations in SOA. We demonstrate that SOA from different atmospherically important VOC precursors have different particle-bound radical concentrations, estimated for the ozonolysis of α-pinene (0.020 ± 0.0050 nmol/μg), limonene (0.0059 ± 0.0010 nmol/μg), and β-caryophyllene (0.0025 ± 0.00080 nmol/μg), highlighting the potential importance of OH-initiated formation of particle-bound organic radicals. Additionally, the lifetime of particle-bound radical species in α-pinene SOA was estimated, and a pseudo-first-order rate constant of k = 7.3 ± 1.7 × 10-3 s-1 was derived, implying a radical lifetime on the order of minutes.