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.\nIn order to examine our understanding of chemical processes in this\nregion, the performance of a box model of atmospheric boundary layer\nchemistry is tested against measurements made at the top of the rainforest\ncanopy near Danum Valley, Malaysian Borneo. Multi-variate optimisation\nagainst ambient 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\nthe overall modelling strategy, and suggests that this method may\nbe applied where measured fluxes are not available. The largest contributors\nto the optimisation cost function at the point of best-fit are OH\n(41%), NO (18%) and total monoterpenes (16%). Several factors affect\nthe modelled VOC chemistry. In particular concentrations of methacrolein\n(MACR) and methyl-vinyl ketone (MVK) are substantially overestimated,\nand the hydroxyl radical [OH] concentration is substantially underestimated;\nas has been seen before in tropical rainforest studies. It is shown\nthat inclusion of dry deposition of MACR and MVK and wet deposition\nof species with high Henry???s Law values substantially improves\nthe fit of these oxidised species, whilst also substantially decreasing\nthe OH sink. Increasing [OH] production arbitrarily, through a simple\nOH recycling mechanism, adversely affects the model fit for volatile\norganic compounds (VOCs). Given the constraints on isoprene flux\nprovided by measurements, a substantial decrease in the rate of reaction\nof VOCs with OH is the only remaining option to explain the measurement/model\ndiscrepancy for OH. A reduction in the isoprene+OH rate constant\nof 50???70% is able to produce both isoprene and OH concentrations\nwithin error of those measured. Whilst we cannot rule out an important\nrole for missing chemistry, particularly in areas of higher isoprene\nflux, this study demonstrates that the inadequacies apparent in box\nand global model studies of tropical VOC chemistry may be more strongly\ninfluenced by representation of detailed physical and micrometeorological\neffects than errors in the chemical scheme.