Simulating atmospheric composition over a South-East Asian tropical rainforest: performance of a chemistry box model T. A. M. Pugh1, A. R. MacKenzie1, C. N. Hewitt1, B. Langford1, P. M. Edwards2, K. L. Furneaux2,†, D. E. Heard2, J. R. Hopkins3, C. E. Jones4, A. Karunaharan5, J. Lee3, G. Mills6, P. Misztal7,8, S. Moller4, P. S. Monks5, and L. K. Whalley2 1Lancaster Environment Centre, Lancaster University, Lancaster, UK 2School of Chemistry, University of Leeds, Leeds, UK 3National Centre for Atmospheric Science, University of York, York, UK 4Department of Chemistry, University of York, York, UK 5Department of Chemistry, University of Leicester, Leicester, UK 6School of Environmental Sciences, University of East Anglia, UK 7Centre for Ecology and Hydrology Edinburgh, U.K. 8School of Chemistry, The University of Edinburgh, U.K. †Sadly passed away on 28 July 2009
Abstract. Atmospheric composition and chemistry above tropical rainforests is currently
not well established, particularly for south-east Asia. In order to examine our
understanding of chemical processes in this region, the performance of a box model
of atmospheric boundary layer chemistry is tested against measurements made at the top
of the rainforest canopy near Danum Valley, Malaysian Borneo. Multi-variate
optimisation against ambient concentration measurements was used to estimate
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 the overall modelling
strategy, and suggests that this method may be applied where measured fluxes
are not available, assuming that the local chemistry and mixing are adequately understood.
The largest contributors to the optimisation cost function
at the point of best-fit are OH (29%), NO (22%) and total peroxy radicals (27%).
Several factors affect the modelled VOC chemistry. In particular concentrations
of methacrolein (MACR) and methyl-vinyl ketone (MVK) are substantially overestimated,
and the hydroxyl radical (OH) concentration is substantially underestimated; as
has been seen before in tropical rainforest studies. It is shown that inclusion
of dry deposition of MACR and MVK and wet deposition of species with high Henry's
Law values substantially improves the fit of these oxidised species, whilst also
substantially decreasing the OH sink. Increasing OH production arbitrarily,
through a simple OH recycling mechanism , adversely affects the model fit
for volatile organic compounds (VOCs). Given the constraints on isoprene flux
provided by measurements, a substantial decrease in the rate of 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%, in conjunction
with increased deposition of intermediates and some modest OH recycling, is able to
produce both isoprene and OH concentrations within error of those measured.
Whilst we cannot rule out an important role for missing chemistry, particularly
in areas of higher isoprene flux, 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 physical and micrometeorological
effects than errors in the chemical scheme.
Citation: Pugh, T. A. M., MacKenzie, A. R., Hewitt, C. N., Langford, B., Edwards, P. M., Furneaux, K. L., Heard, D. E., Hopkins, J. R., Jones, C. E., Karunaharan, A., Lee, J., Mills, G., Misztal, P., Moller, S., Monks, P. S., and Whalley, L. K.: Simulating atmospheric composition over a South-East Asian tropical rainforest: performance of a chemistry box model, Atmos. Chem. Phys., 10, 279-298, doi:10.5194/acp-10-279-2010, 2010.