Articles | Volume 16, issue 12
Atmos. Chem. Phys., 16, 7725–7741, 2016
Atmos. Chem. Phys., 16, 7725–7741, 2016

Research article 24 Jun 2016

Research article | 24 Jun 2016

Understanding isoprene photooxidation using observations and modeling over a subtropical forest in the southeastern US

Luping Su1, Edward G. Patton2, Jordi Vilà-Guerau de Arellano3, Alex B. Guenther4, Lisa Kaser5, Bin Yuan6,7, Fulizi Xiong8, Paul B. Shepson8,9, Li Zhang10, David O. Miller10, William H. Brune10, Karsten Baumann11, Eric Edgerton11, Andrew Weinheimer5, Pawel K. Misztal12, Jeong-Hoo Park13, Allen H. Goldstein12,14, Kate M. Skog15, Frank N. Keutsch16,17, and John E. Mak1 Luping Su et al.
  • 1School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA
  • 2Mesoscale and Microscale Meteorology Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
  • 3Meteorology and Air Quality Section, Wageningen University and Research Centre, Wageningen, the Netherlands
  • 4Department of Earth System Science, University of California, Irvine, CA, USA
  • 5Atmospheric Chemistry Observations & Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
  • 6Earth System Research Laboratory, Chemical Sciences Division, National Oceanic and Atmospheric Administration, Boulder, CO, USA
  • 7Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 8Department of Chemistry, Purdue University, West Lafayette, IN, USA
  • 9Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN, USA
  • 10Department of Meteorology, Pennsylvania State University, University Park, PA, USA
  • 11Atmospheric Research and Analysis Inc., Cary, NC, USA
  • 12Department of Environmental Science, Policy, & Management, University of California at Berkeley, Berkeley, CA, USA
  • 13Climate and Air Quality Research Department, National Institute of Environmental Research, Incheon, Republic of Korea
  • 14Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA, USA
  • 15Department of Chemistry, University of Wisconsin, Madison, WI, USA
  • 16School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 17Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA

Abstract. The emission, dispersion, and photochemistry of isoprene (C5H8) and related chemical species in the convective boundary layer (CBL) during sunlit daytime were studied over a mixed forest in the southeastern United States by combining ground-based and aircraft observations. Fluxes of isoprene and monoterpenes were quantified at the top of the forest canopy using a high-resolution proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS). Snapshot (∼  2 min sampling duration) vertical profiles of isoprene, methyl vinyl ketone (MVK) + methacrolein (MACR), and monoterpenes were collected from aircraft every hour in the CBL (100–1000 m). Both ground-based and airborne collected volatile organic compound (VOC) data are used to constrain the initial conditions of a mixed-layer chemistry model (MXLCH), which is applied to examine the chemical evolution of the O3–NOx–HOx–VOC system and how it is affected by boundary layer dynamics in the CBL. The chemical loss rate of isoprene (∼  1 h) is similar to the turbulent mixing timescale (0.1–0.5 h), which indicates that isoprene concentrations are equally dependent on both photooxidation and boundary layer dynamics. Analysis of a model-derived concentration budget suggests that diurnal evolution of isoprene inside the CBL is mainly controlled by surface emissions and chemical loss; the diurnal evolution of O3 is dominated by entrainment. The NO to HO2 ratio (NO : HO2) is used as an indicator of anthropogenic impact on the CBL chemical composition and spans a wide range (1–163). The fate of hydroxyl-substituted isoprene peroxyl radical (HOC5H8OO·; ISOPOO) is strongly affected by NO : HO2, shifting from NO-dominant to NO–HO2-balanced conditions from early morning to noontime. This chemical regime change is reflected in the diurnal evolution of isoprene hydroxynitrates (ISOPN) and isoprene hydroxy hydroperoxides (ISOPOOH).

Final-revised paper