Characterization of the Gaseous and Particulate Products of Terpene Oxidation at a New England Forest

Oxidation of biogenic volatile organic compounds, particularly terpenes, is known to result in production of polar, oxygenated organic compounds with low vapor pressures, which can contribute to formation and growth of secondary organic aerosol (SOA). A combination of several analytical techniques has been used to monitor terpene concentrations, identify prevalent terpene oxidation products, and characterize their abundances at Harvard Forest, a rural site in Western Massachusetts. Gas-phase concentrations of monoterpenes and their oxidation products were measured by Proton Transfer Reaction Mass Spectrometry (PTR-MS), while information on semi-volatile oxygenated organic compounds in aerosols was obtained by filter sampling followed by extraction and GC-MS analysis. Laboratory studies of authentic samples of known terpene oxidation products were used to characterize the response of each technique to these compounds, including identification of PTR-MS mass-to-charge ratios and fragmentation patterns, and GC-MS retention times and mass spectra. During the summer and fall of 2005 and 2006, PTR-MS signals at mass-to-charge ratios corresponding to expected monoterpene oxidation products such as nopinone, pinonaldehyde, α-pinene oxide, and others were monitored above the forest canopy. Estimated gas-phase concentrations ranged from near the detection limit (several pptv) to approximately 100 pptv. Observations of the diurnal cycles of these compounds and the relationship between their atmospheric concentrations and factors such as season, temperature, radiation levels, and meteorological conditions provide information on the mechanisms controlling their production and loss. Analysis of aerosol samples obtained in parallel are used to help characterize the volatilities of the oxidation products and their role in aerosol growth.