Atmospheric organics are subject to synergistic oxidation by different oxidants, yet the mechanisms of such processes are poorly understood. Here, using direct measurements and kinetic modeling, we probe the nocturnal synergistic-oxidation mechanism of α-pinene by O₃ and NO₃ radicals and in particular the fate of peroxy radical intermediates of different origins, which will deepen our understanding of the monoterpene oxidation chemistry and its contribution to atmospheric particle formation.