Simulating atmospheric composition over a South-East Asian tropical rainforest: performance of a chemistry box model
Atmospheric composition and chemistry above tropical rainforests is\ncurrently not well established, particularly for south-east Asia. In\norder to examine our understanding of chemical processes in this region,\nthe performance of a box model of atmospheric boundary layer chemistry\nis tested against measurements made at the top of the rainforest canopy\nnear Danum Valley, Malaysian Borneo. Multivariate optimisation against\nambient concentration measurements was used to estimate average\ncanopy-scale emissions for isoprene, total monoterpenes and nitric\noxide. The excellent agreement between estimated values and measured\nfluxes of isoprene and total monoterpenes provides confidence in the\noverall modelling strategy, and suggests that this method may be applied\nwhere measured fluxes are not available, assuming that the local\nchemistry and mixing are adequately understood. The largest contributors\nto the optimisation cost function at the point of best-fit are OH\n(29%), NO (22%) and total peroxy radicals (27%). Several factors\naffect the modelled VOC chemistry. In particular concentrations of\nmethacrolein (MACR) and methyl-vinyl ketone (MVK) are substantially\noverestimated, and the hydroxyl radical (OH) concentration is\nsubstantially underestimated; as has been seen before in tropical\nrainforest studies. It is shown that inclusion of dry deposition of MACR\nand MVK and wet deposition of species with high Henry's Law values\nsubstantially improves the fit of these oxidised species, whilst also\nsubstantially decreasing the OH sink. Increasing OH production\narbitrarily, through a simple OH recycling mechanism, adversely affects\nthe model fit for volatile organic compounds (VOCs). Given the\nconstraints on isoprene flux provided by measurements, a substantial\ndecrease in the rate of reaction of VOCs with OH is the only remaining\noption to explain the measurement/model discrepancy for OH. A reduction\nin the isoprene+OH rate constant of 50%, in conjunction with increased\ndeposition of intermediates and some modest OH recycling, is able to\nproduce both isoprene and OH concentrations within error of those\nmeasured. Whilst we cannot rule out an important role for missing\nchemistry, particularly in areas of higher isoprene flux, this study\ndemonstrates that the inadequacies apparent in box and global model\nstudies of tropical VOC chemistry may be more strongly influenced by\nrepresentation of detailed physical and micrometeorological effects than\nerrors in the chemical scheme.