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

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–NO_x–HO_x–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).