Abstract
Abstract. We present an evaluation of sources, sinks and turbulent transport of nitrogen oxides, ozone and volatile organic compounds (VOC) in the boundary layer over French Guyana and Suriname during the October 2005 GABRIEL campaign by simulating observations with a single-column chemistry and climate model (SCM) along a zonal transect. Simulated concentrations of O3 and NO as well as NO2 photolysis rates over the forest agree well with observations when a small soil-biogenic NO emission flux was applied. This suggests that the photochemical conditions observed during GABRIEL reflect a pristine tropical low-NOx regime. The SCM uses a compensation point approach to simulate nocturnal deposition and daytime emissions of acetone and methanol and produces daytime boundary layer mixing ratios in reasonable agreement with observations. The area average isoprene emission flux, inferred from the observed isoprene mixing ratios and boundary layer height, is about half the flux simulated with commonly applied emission algorithms. The SCM nevertheless simulates too high isoprene mixing ratios, whereas hydroxyl concentrations are strongly underestimated compared to observations, which can at least partly explain the discrepancy. Furthermore, the model substantially overestimates the isoprene oxidation products methlyl vinyl ketone (MVK) and methacrolein (MACR) partly due to a simulated nocturnal increase due to isoprene oxidation. This increase is most prominent in the residual layer whereas in the nocturnal inversion layer we simulate a decrease in MVK and MACR mixing ratios, assuming efficient removal of MVK and MACR. Entrainment of residual layer air masses, which are enhanced in MVK and MACR and other isoprene oxidation products, into the growing boundary layer poses an additional sink for OH which is thus not available for isoprene oxidation. Based on these findings, we suggest pursuing measurements of the tropical residual layer chemistry with a focus on the nocturnal depletion of isoprene and its oxidation products.
Highlights
In October 2005 an intensive field campaign including ground-based and airborne measurements has been conducted over the Atlantic Ocean and French Guyana and Suriname to study the atmospheric oxidizing capacity and role of atmosphere-biosphere exchanges in this region (Lelieveld et al, 2008)
The simulation continues for one more day (4 Oc- tropical deforestation on the atmospheric chemistry applying tober) to include a continental day-night-day transition the “big-leaf” approach (Ganzeveld and Lelieveld, 2004), we to study the significance of the nocturnal tropical forest ex- apply for this evaluation the detailed multi-layer model for change regime for daytime atmospheric chemistry and ex- reactive trace gas exchanges
An advantage of interpreting these compounds is that, due to their relatively long chemical lifetimes, chemical transformations do not play a significant role at the timescale of boundary layer (BL) turbulent transport but a major limiting aspect of this interpretation is the lack of knowledge about their biogenic sources and sinks
Summary
In October 2005 an intensive field campaign including ground-based and airborne measurements has been conducted over the Atlantic Ocean and French Guyana and Suriname to study the atmospheric oxidizing capacity and role of atmosphere-biosphere exchanges in this region (Lelieveld et al, 2008). Examples include the two LBAEUSTACH intensive field campaigns (Andreae et al, 2002) at a primary rainforest and a pasture site in the state of Rondonia in southwestern Brazil conducting eddy correlation, gradient and enclosure measurements to study the exchanges of VOCs, nitrogen oxides (NOx=NO+NO2) and ozone Such direct flux measurements are not available for the GABRIEL campaign; through the combined use of observations and models of surface and boundary layer exchanges, empirical estimates can be made and key processes studied.
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