Assessing the Difference in the Effects of NOx on the Photooxidation Mechanisms of Isomeric Compounds of α-Pinene and Δ³-Carene

Elucidating the difference between the photooxidation mechanisms of isomeric volatile organic compounds (VOCs) by anthropogenic pollutants helps to unravel the structural dependence of VOCs on the formation of secondary organic aerosols (SOAs). Herein, the effects of NOx (NO and NO2) on the SOA formation from the photooxidation of isomeric monoterpenes of α-pinene and Δ3-carene were compared by a series of experimental and theoretical studies. For both the α-pinene and the Δ3-carene systems, the increase of NO and NO2 concentration ([NO] and [NO2]) first enhances and then suppresses the particle number concentration but enhances the particle size growth (except for the photooxidation of Δ3-carene with NO). The increase of [NOx] first promotes and then suppresses the SOA yields of α-pinene. In particular, the increase of [NO] suppresses the SOA yields of Δ3-carene, whereas the increase of [NO2] promotes the SOA yields of Δ3-carene. These findings imply that the "rate effect" (a faster oxidation rate leads to a higher SOA yield) of photooxidation for Δ3-carene with NO2 may be more pronounced than that of α-pinene. The number of SOA components with higher O/C in the Δ3-carene system is larger than that in the α-pinene system, which could be rationalized that the bond tension of the three-membered ring structure of Δ3-carene is larger than that of the four-membered ring of α-pinene. The present findings serve as a model for clarifying the effects of NOx on the SOA formation of VOCs with the same chemical formula and stimulate systematic studies on a broad class of isomeric VOCs toward the improvement of SOA models.