Molecular composition of secondary organic aerosol from α-pinene ozonolysis affected by relative humidity

Biogenic volatile organic compounds (BVOC) can contribute a significant fraction to secondary organic aerosols (SOA) through atmospheric oxidation, which plays a critical role in climate change and human health. The ozonolysis of α-pinene, one of the most important BVOCs, is a canonical SOA system. At present, the effects of relative humidity (RH) on SOA composition from α-pinene+O3 reaction are still unclear. In this study, we report the SOA composition on molecular level formed in α-pinene+O3 reaction under various RH. The SOA components were measured by an Extractive ElectroSpray Ionization inlet coupled with a long Time-of-Flight Mass Spectrometer (EESI-TOF-MS). We observed RH-dependent SOA chemical composition, including larger contribution of monomer products with increasing RH, although the total O:C remained largely unchanged. The effect of RH may be attributed to the particle-phase reactions of SOA components. This study highlights the necessity of characterizing SOA composition on molecular level and of considering RH dependence of SOA chemical composition and physicochemical properties in atmospheric models.