Articles | Volume 16, issue 14
Atmos. Chem. Phys., 16, 9349–9359, 2016
Atmos. Chem. Phys., 16, 9349–9359, 2016

Research article 28 Jul 2016

Research article | 28 Jul 2016

Speciation of OH reactivity above the canopy of an isoprene-dominated forest

J. Kaiser1,a, K. M. Skog1, K. Baumann2, S. B. Bertman3, S. B. Brown4,5, W. H. Brune6, J. D. Crounse7, J. A. de Gouw4,5,8, E. S. Edgerton2, P. A. Feiner6, A. H. Goldstein9,10, A. Koss4,8, P. K. Misztal9, T. B. Nguyen7, K. F. Olson9, J. M. St. Clair7,b,c, A. P. Teng7, S. Toma3, P. O. Wennberg7,11, R. J. Wild4,8, L. Zhang6, and F. N. Keutsch12 J. Kaiser et al.
  • 1Department of Chemistry, University of Wisconsin-Madison, Madison, WI, USA
  • 2Atmospheric Research & Analysis Inc, Cary, NC, USA
  • 3Department of Chemistry, Western Michigan University, Kalamazoo, MI, USA
  • 4Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA
  • 5Department of Chemistry, University of Colorado, Boulder, CO, USA
  • 6Department of Meteorology, Pennsylvania State University, University Park, PA, USA
  • 7Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
  • 8Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
  • 9Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA
  • 10Department of Civil and Environmental Engineering, University of California, Berkeley, CA, USA
  • 11Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA
  • 12School of Engineering and Applied Sciences and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA
  • anow at: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • bnow at: Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA
  • cnow at: Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA

Abstract. Measurements of OH reactivity, the inverse lifetime of the OH radical, can provide a top–down estimate of the total amount of reactive carbon in an air mass. Using a comprehensive measurement suite, we examine the measured and modeled OH reactivity above an isoprene-dominated forest in the southeast United States during the 2013 Southern Oxidant and Aerosol Study (SOAS) field campaign. Measured and modeled species account for the vast majority of average daytime reactivity (80–95 %) and a smaller portion of nighttime and early morning reactivity (68–80 %). The largest contribution to total reactivity consistently comes from primary biogenic emissions, with isoprene contributing ∼  60 % in the afternoon, and ∼  30–40 % at night and monoterpenes contributing ∼  15–25 % at night. By comparing total reactivity to the reactivity stemming from isoprene alone, we find that ∼  20 % of the discrepancy is temporally related to isoprene reactivity, and an additional constant ∼  1 s−1 offset accounts for the remaining portion. The model typically overestimates measured OVOC concentrations, indicating that unmeasured oxidation products are unlikely to influence measured OH reactivity. Instead, we suggest that unmeasured primary emissions may influence the OH reactivity at this site.

Short summary
OH reactivity can be used to assess the amount of reactive carbon in an air mass. “Missing” reactivity is commonly found in forested environments and is attributed to either direct emissions of unmeasured volatile organic compounds or to unmeasured/underpredicted oxidation products. Using a box model and measurements from the 2013 SOAS campaign, we find only small discrepancies in measured and calculated reactivity. Our results suggest the discrepancies stem from unmeasured direct emissions.
Final-revised paper