Oxidation of Tree Oil Containing a Complex Mixture of Sesquiterpenes: Keeping Track of the Carbon

Oxidation of biogenic volatile organic compounds in the atmosphere leads to a diverse mixture of products spanning a wide range of functionalities and structures, many of which partition to the particle phase. Sesquiterpenes have large secondary organic aerosol (SOA) yields and are therefore potentially significant contributors to atmospheric OA. However, their low concentration and the low volatility of their oxidation products pose challenges for studies of product yields and oxidation mechanisms. Studies done on terpene oxidation focus on a select few representative terpenes. We performed experiments in a chamber to study oxidation products from a complex mixture of tree oil extract containing a large fraction of sesquiterpenes, with the goal of keeping track of as much of the carbon as possible. Additionally, we compare these results to the oxidation of a single sesquiterpene, β-caryophyllene. Gas phase measurements were made using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS), while the particle phase was monitored with an time-of-flight aerosol mass spectrometer (ToF-AMS). Together, these instruments span the wide range of volatility needed to measure the oxidation products. The high mass resolution of the PTR-ToF-MS allows for chemical formulas to be unambiguously assigned to many of the detected masses, while the high time resolution allows us to track the fast oxidation of low volatility VOCs and their subsequent transition to the particle phase. Additionally, we use selective reagent ionization with NO+ ions to provide sensitive ionization of some larger gas-phase molecules with less fragmentation, as well as some structural information.