Simulating atmospheric composition over a South-East Asian tropical rainforest
Thomas Pugh1, Robert MacKenzie1, Nicholas Hewitt1, Ben Langford1
1 Lancaster Environment Centre, Lancaster University
O 4.2 in Forest biogeochemistry of reactive trace gases
07.10.2009, 10:05-10:30, Kutschenhaus
Atmospheric composition above tropical rainforests is currently quite poorly defined, particularly for south-east Asia. A box model of atmospheric boundary layer chemistry is compared to measurements made in and over the rainforest at Danum Valley, Malaysian Borneo. Multi-variate optimisation against ambient concentration measurements was used to estimate 24-hour average canopy-scale emissions for isoprene, total monoterpenes and nitric oxide. The excellent agreement between estimated values and measured fluxes of isoprene and total monoterpenes provides confidence in validity of this method, and indicates that it may be applied where measured fluxes are not available. The model performs robustly in representing NOx and O3 concentrations. However several problems affect the modelled VOC chemistry. In particular concentrations of methacrolein (MACR )and methyl-vinyl ketone (MVK ) are greatly overestimated and the hydroxyl radical [OH] is substantially underestimated. It is shown that dry deposition is able to account for the MACR/MVK overestimation. However, increasing [OH] production is not found to be a satisfactory solution to [OH] underestimation, due to negative effects on the model fit for volatile organic compounds (VOCs). Given the constraints on isoprene flux provided by measurements, a substantial decrease in the rate constant for the reaction of VOCs with OH is the only remaining option to explain the measurement/model discrepancy for OH. A reduction in the isoprene + OH rate constant of 50% is able to produce both isoprene and OH concentrations within error of those measured. It is also demonstrated that nighttime isoprene loss cannot be fully explained by the chemistry, leading to the suggestion that inadequate representation of mixing processes within the PBL may explain the discrepancy. This study demonstrates that the inadequacies apparent in box and global model studies of tropical VOC chemistry may be more strongly influenced by representation of detailed micrometeorological effects than errors in the chemical scheme.
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