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Harvard Forest Symposium Abstract 2007

  • Title: Observations of biogenic volatile organic compounds and their oxidation products at Harvard Forest
  • Primary Author: Karena McKinney (Harvard School of Engineering and Applied Sciences)
  • Abstract:

    Simultaneous in situ observations of biogenic volatile organic compounds (BVOCs) and their oxidation products provide information on the rates and mechanisms of BVOC oxidation and the roles of BVOCs in regional ozone production and the formation and growth of secondary organic aerosol (SOA). In this study, Proton Transfer Reaction Mass Spectrometry (PTR-MS) was used to monitor concentrations of common BVOCs and their oxidation products such as isoprene, monoterpenes, methyl vinyl ketone (MVK), and methacrolein (MACR), and to identify prevalent terpene oxidation products and characterize their gas-phase abundances at Harvard Forest. Laboratory studies of authentic samples of known terpene oxidation products were used to characterize the PTR-MS response to these compounds, including identification of parent ion mass-to-charge ratios and fragmentation patterns. During the summer of 2005 and 2006, PTR-MS measurements of isoprene, total monoterpenes, MVK + MACR, acetone, methanol, and a number of other VOCs were recorded. Isoprene concentrations exhibited a very strong diurnal cycle, with peak values above 5 ppbv at midday and minima around 100 pptv at night. In contrast, maximum monoterpene concentrations typically were less than 1 ppbv and occurred in the early evening, with a local minimum at midday. The MVK + MACR diurnal cycle lags that of isoprene, with lower daytime maxima and higher nighttime minima, resulting in an average MVK + MACR / isoprene ratio of ~0.35 during daylight hours and ~1.8 at night. For a total of 26 days between July and September 2005, PTR-MS signals at mass-to-charge ratios corresponding to expected monoterpene oxidation products such as nopinone, pinonaldehyde, alpha-pinene oxide, and others were monitored above the forest canopy. Estimated gas-phase mixing ratios 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 meterorlogical conditions are used to provide information on the mechanisms controlling their production and loss in the gas phase.

  • Research Category: Forest-Atmosphere Exchange