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)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 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 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 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 q; 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).

We present trace gas vertical profiles observed by instruments on the NASA DC-8 and at a ground site during the Korea-US air quality study (KORUS) field campaign in May to June 2016. We focus on the region near the Seoul metropolitan area and its surroundings where both anthropogenic and natural emission sources play an important role in local photochemistry. Integrating ground and airborne observations is the major research goal of many atmospheric chemistry field campaigns. Although airborne platforms typically aim to sample from near surface to the free troposphere, it is difficult to fly very close to the surface especially in environments with complex terrain or a populated area. A detailed analysis integrating ground and airborne observations associated with specific concentration footprints indicates that reactive trace gases are quickly oxidized below an altitude of 700 m. The total OH reactivity profile has a rapid decay in the lower part of troposphere from surface to the lowest altitude (700 m) sampled by the NASA DC-8. The decay rate is close to that of very reactive biogenic volatile organic compounds such as monoterpenes. Therefore, we argue that photochemical processes in the bottom of the boundary layer, below the typical altitude of aircraft sampling, should be thoroughly investigated to properly assess ozone and secondary aerosol formation.