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Harvard Forest Symposium Abstract 2014

  • Title: Determination of biogenic volatile organic compound fluxes from Harvard Forest using PTR-TOF-MS
  • Primary Author: Karena McKinney (Harvard School of Engineering and Applied Sciences)
  • Additional Authors: Ying Liu (Amherst College); J. William Munger (Harvard University)
  • Abstract:

    Forest emissions of biogenic volatile organic compounds (BVOCs) are the largest source of reactive non-methane hydrocarbons to the atmosphere. Quantification of both emission and deposition fluxes of typical BVOCs and their oxidation products is essential to further understanding their roles in tropospheric chemistry, especially their contributions to the formation of ozone and secondary organic aerosol, yet studies suggest that the understanding of the nature and quantity of emitted compounds remains incomplete. Recent findings have indicated the presence of reactive BVOCs within and above forest canopies that have not been quantified previously and have suggested that deposition to the forest canopy can be a major sink of BVOCs. New measurements of BVOC emissions from and concentrations above Harvard Forest have been made from June 8 to September 30, 2012 using Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-TOF-MS). PTR-TOF-MS represents an advance over previous quadrupole-based PTR-MS measurements in that it captures a full, high-resolution (m/delta m ~ 4000) mass spectrum on every scan, resulting in positive identification of molecular formulas. In addition, scans are recorded at high time resolution (5 Hz), allowing true (non-disjunct) eddy covariance fluxes to be determined for each mass-to-charge ratio. Concentration and flux measurements were made simultaneously using a high-sensitivity quadrupole PTR-MS, and results from the two techniques are compared. Measured concentrations of most species agree to within 5%. As in past seasons, isoprene is the major BVOC emitted at Harvard Forest, reaching average midday mixing ratios of ca. 4 ppbv, and its emissions are closely tied to local temperature and light levels. Diurnal and seasonal patterns in emissions of isoprene, monoterpenes, methanol, and acetone are reported and compared with past measurements at the site. In addition, eddy covariance fluxes are calculated for all mass peaks to assess emissions of previously unidentified BVOCs from Harvard Forest.

  • Research Category: Forest-Atmosphere Exchange