Comparative studies of the NOx impacts on the photooxidation mechanisms of isomeric monoterpenes of β-pinene and limonene

Abstract

It is highly challenging to precisely compare the impacts of anthropogenic pollutants on the photooxidation of isomeric volatile organic compounds with respect to molecular compositions and particle number/mass concentrations of secondary organic aerosols (SOAs). In this study, we conducted a series of well-defined indoor chamber experiments to compare the effects of NOx (NO and NO2) on the photooxidation of isomeric monoterpenes of β-pinene and limonene. For the photooxidation of β-pinene with NOx, the increase of the initial concentrations of NO ([NO]0) shows a monotonous suppression of the particle mass concentration, whereas the increase of [NO2]0 shows a monotonous enhancement of the particle mass concentration. For the photooxidation of limonene with NOx, the increase of [NO]0 exhibits a monotonous suppression of the particle mass concentration, whereas the increase of [NO2]0 shows a parabolic trend of the particle mass concentration. Utilizing a newly developed vacuum ultraviolet free electron laser (VUV-FEL), the online threshold photoionization mass spectrometry reveals a series of novel compounds at molecular weight (MW) = 232 and 306 for the β-pinene + NOx system and MW = 187, 261, 280, and 306 for the limonene + NOx system. The molecular structures and formation pathways of these species were inferred, which led to the prediction of the diversity and difference of SOA products (i.e., ester and peroxide accretion products) formed from different monoterpene precursors. To improve the predictions of future air quality, it is recommended that climate models should incorporate the NOx-driven diurnal photooxidation of monoterpenes for SOA formation mechanisms.