Surface and boundary layer exchanges of volatile organic compounds, nitrogen oxides and ozone during the GABRIEL campaign

摘要

We present an evaluation of sources, sinks and turbulent transport ofnitrogen oxides, ozone and volatile organic compounds (VOC) in the boundarylayer over French Guyana and Suriname during the October 2005 GABRIELcampaign by simulating observations with a single-column chemistry andclimate model (SCM) along a zonal transect. Simulated concentrations ofO₃ and NO as well as NO₂ photolysis rates over the forest agreewell with observations when a small soil-biogenic NO emission flux wasapplied. This suggests that the photochemical conditions observed duringGABRIEL reflect a pristine tropical low-NO_x regime. The SCM uses acompensation point approach to simulate nocturnal deposition and daytimeemissions of acetone and methanol and produces daytime boundary layer mixingratios in reasonable agreement with observations. The area average isopreneemission flux, inferred from the observed isoprene mixing ratios andboundary layer height, is about half the flux simulated with commonlyapplied emission algorithms. The SCM nevertheless simulates too highisoprene mixing ratios, whereas hydroxyl concentrations are stronglyunderestimated compared to observations, which can at least partly explainthe discrepancy. Furthermore, the model substantially overestimates theisoprene oxidation products methlyl vinyl ketone (MVK) and methacrolein(MACR) partly due to a simulated nocturnal increase due to isopreneoxidation. This increase is most prominent in the residual layer whereas inthe nocturnal inversion layer we simulate a decrease in MVK and MACR mixingratios, assuming efficient removal of MVK and MACR. Entrainment of residuallayer air masses, which are enhanced in MVK and MACR and other isopreneoxidation products, into the growing boundary layer poses an additional sinkfor OH which is thus not available for isoprene oxidation. Based on thesefindings, we suggest pursuing measurements of the tropical residual layerchemistry with a focus on the nocturnal depletion of isoprene and itsoxidation products.