| dcterms.abstract | Biogenic volatile organic compounds (BVOC) represent an important source of reactive chemical species emitted into the troposphere. BVOC have significant impact on global tropospheric chemistry and global radiation budget through ozone production and secondary organic aerosol formation, respectively. Isoprene constitutes the major BVOC species emitted from terrestrial vegetation. Large uncertainties exist in the chemical mechanisms of isoprene photooxidation as well as its turbulent distribution and evolution inside the planetary boundary layer (PBL). One intensive field observation campaign, the Southeast Atmosphere Study (SAS) campaign was carried out during during June 1 - July 15, 2013 in Alabama. The campaign included comprehensive observations of BVOCs and other trace gases (e.g., O3, NOx, and HOx) from airborne and ground-based platforms. During the campaign, vertical profiles of BVOCs were quantified with airborne sampling and subsequent measurements by using a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF-MS). Ground-based eddy covariance (EC) was used to measure BVOC fluxes on a tower above the forest canopy. A mixed-layer chemistry model was used to study how different processes (entrainment, boundary layer dynamics, surface emission, deposition, chemical production and loss) control the evolution of trace gases inside the PBL. A companion laboratory chamber study, the Focused Isoprene eXperiment at the California Institute of Technology (FIXCIT), was carried out during January 2014. A series of isoprene photooxidation experiments were carried out under controlled conditions. The results from the SAS campaign show that the chemical loss rate of isoprene (~1 h) is similar to the turbulent mixing time scale (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 PBL 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 PBL 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 condition from early morning to noontime. This chemical regime change is reflected in the diurnal evolution of isoprene hydroxynitrates (ISOPN) and isoprene hydroxy hydroperoxides (ISOPOOH). | |
