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

  • Author(s): Su, L
  • Patton, EG
  • De Arellano, JVG
  • Guenther, AB
  • Kaser, L
  • Yuan, B
  • Xiong, F
  • Shepson, PB
  • Zhang, L
  • Miller, DO
  • Brune, WH
  • Baumann, K
  • Edgerton, E
  • Weinheimer, A
  • Misztal, PK
  • Park, JH
  • Goldstein, AH
  • Skog, KM
  • Keutsch, FN
  • Mak, JE
  • et al.
Abstract

The emission, dispersion, and photochemistry of isoprene (C H ) 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)Cmethacrolein (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 O -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 modelderived 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 HO ratio (NO :HO ) 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 q; ISOPOO) is strongly affected by NO:HO2, shifting from NO-dominant to NO-HO -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). 5 8 3 2 2 2

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