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Harvard Forest Symposium Abstract 2010

  • Title: Determination of biogenic VOC fluxes and observations of VOC oxidation at Harvard Forest
  • Primary Author: Karena McKinney (Harvard School of Engineering and Applied Sciences)
  • Additional Authors: Alexandra Chang-Graham (Amherst College); Brian Merrigan (Amherst College); Andrew Vasta (Amherst College); jack angiolillo (Amherst College)
  • Abstract:

    Biogenic volatile organic compounds (BVOCs) play a critical role in determining oxidant concentrations and secondary organic aerosol (SOA) production in forested environments. Recent findings have suggested that the emission rates of BVOCs and the range of species emitted could be larger than previously thought. In this study, Proton Transfer Reaction Mass Spectrometry (PTR-MS) was used to obtain fast (<1 Hz) measurements of the predominant BVOC species, including isoprene, monoterpenes, methanol, acetone, and methyl ethyl ketone, above the Harvard Forest canopy. Together with vertical wind data, these measurements are used to determine fluxes of BVOCs out of the forest using the eddy covariance method. Isoprene is the dominant emitted species, with peak emission rates and midday mixing ratios of ~4 mg isoprene m-2 h-1 and ~5 ppbv, respectively. Isoprene emission rates are expected to vary with temperature and radiation (PAR) levels, and compare favorably with standard emission algorithms based on these parameters. Interannual variability in isoprene emission rates is also observed. In contrast to isoprene, maximum monoterpene concentrations typically were less than 1 ppbv and occurred in the early evening, with a local minimum at midday. Monoterpene fluxes are about an order of magnitude smaller than those of isoprene. The amplitude of the diurnal cycle suggests monoterpene emissions at Harvard Forest may exhibit light dependence as well as temperature dependence. Fluxes of oxygenated VOCs, including methanol, acetone, methyl ethyl ketone, and oxygenated terpenes that have rarely been observed previously, have also been determined. Current work involves investigating the dependence of these emission rates on factors such as temperature, radiation levels, meteorological conditions, and season.

    Concentrations of additional VOCs, including methyl vinyl ketone + methacrolein, acetonitrile, aromatics, and terpene oxidation products such as nopinone, pinonaldehyde, alpha-pinene oxide, and others have also been measured. Estimated gas-phase concentrations ranged from near the detection limit (several pptv) to several ppbv. Observations of the diurnal cycles of these compounds and the relationship between their atmospheric concentrations and environmental factors are being used to provide information on the mechanisms controlling their production and loss in the gas phase.

    Solid phase microextraction (SPME), a new but promising technique for sampling volatile organic compounds in air using absorbent polymer fibers, was also used to identify terpene compounds and quantify their concentrations in forest air. SPME samples were collected and analyzed using gas chromatography-mass spectrometry (GC-MS). A number of terpenes, including a-pinene, b-pinene, camphene, 3-carene, and limonene, as well as terpene derivatives, were detected. The results are consistent with the expected compound distribution based on the forest tree inventory and with total monoterpene concentrations measured using PTR-MS.

  • Research Category: Forest-Atmosphere Exchange