Photooxidation and ozonolysis of α-pinene and limonene mixtures: Mechanisms of secondary organic aerosol formation and cross-dimerization

Elucidating the interplay between different volatile organic compounds (VOCs) is crucial for understanding the formation mechanisms of secondary organic aerosols (SOA). This study systematically investigated the interactions between the bicyclic monoterpene α-pinene and the monocyclic monoterpene limonene under NO₂ photooxidation and dark ozonolysis conditions. The results show that under the NO₂ photooxidation conditions, the increase of [limonene]₀ enhances particle mass concentrations, number concentrations, and particle size; the increase of [α-pinene]₀ results in the increase of particle mass and number concentrations without the increase of particle size. Under the dark ozonolysis conditions, particle mass and number concentrations increase with the increase of [α-pinene]₀ and [limonene]₀; the effect of increasing [α-pinene]₀ on SOA mass concentration is significantly smaller than that of increasing [limonene]₀, which may be attributed to the shift to a smaller particle size with the increase of [α-pinene]₀. As the [limonene]₀/[mixed VOCs]₀ ratio increases from 0 % to 100 %, the SOA yield increases from 54.6 % to 65.2 % under the NO₂ photooxidation conditions and from 19.8 % to 26.2 % under the O₃ conditions. Using threshold photoionization mass spectrometry based on a vacuum ultraviolet free electron laser, a series of new products were detected at molecular weights of 364, 366, 380, 386, 411, 413, and 414, which could be assigned to the cross-dimers, such as organic peroxides, ester dimers, and organic nitrates. Our study underscores the critical role of monocylic monoterpene limonene in the SOA formation and advances our understanding of the SOA formation in the atmosphere.