Development and application of a box model to boundary layer chemistry above a tropical rainforest

Biogenic volatile organic compounds (BVOCs), in particular isoprene and monoter penes, dominate the oxidation capacity of the tropical boundary layer and play an important role in the formation of the radiatively and ecologically significant gas ozone. This work investigates the oxidation of isoprene and monoterpenes in the tropical boundary layer using a box model of atmospheric chemistry, which is adapted for this purpose. The model output is compared with measurements of atmospheric composition and fluxes made over the tropical rainforest during the Oxidant Particle and Photochemical Processes (OP3) campaign in Sabah, Malaysia. The model can replicate 03 and NOx concentrations well during the daytime, but at night, the simplified mixing assumptions of the model compromise the representation of deposition of these species. A simple parametrisation for an effective night-time deposition velocity for ozone can correct this poor representation. Conversely, the ability of the model to simulate the daytime oxidation chemistry of isoprene and the concentration of the oxidant OH is severely compromised. Possible reasons for the poor model performance in this regard are discussed and tested. The most satisfactory solution to improve model fit appears to be a reduction in the rate of reaction of lsoprene/rnonoterpenes with OH, based upon the hypothesis of segregation of these species in the boundary layer. However, a novel method of calculating the segregation intensity from isoprene concentration measurements rules out this hypothesis. The reason for the model-measurement discrepancies of isoprene and OH remains unsolved. Following this detailed assessment of the strengths and limitations of the model, it is used as a predictive tool to investigate the effect of a change in BVOC emission on atmospheric chemistry. Conversion of natural rainforest to oil palm plantation substantially increases isoprene emissions, and will lead to dangerously high boundary layer ozone concentrations if future NOx concentrations are allowed to approach those in Western Europe. Meanwhile, landscape-scale circadian control can modify the magnitude and timing of isoprene emission, significantly affecting boundary layer ozone and oxidant concentrations. Further measurement studies to confirm landscape-scale circadian control of isoprene emissions are required.EThOS - Electronic Theses Online ServiceGBUnited Kingdo