Journal article

Simulating atmospheric composition over a South-East Asian tropical rainforest: performance of a chemistry box model

Pugh T, MacKenzie A, Hewitt C, Langford B, Edwards P, Furneaux K, Heard D, Hopkins J, Jones C, Karunaharan A, Lee J, Mills G, Misztal P, Moller S, Monks P, Whalley L ...see all

Atmospheric Chemistry and Physics, vol. 10, issue 1 (2010) pp. 279-298

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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. Multivariate 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.

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  • T A M Pugh

  • A R MacKenzie

  • C N Hewitt

  • B Langford

  • P M Edwards

  • K L Furneaux

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