Fluxes and concentrations of volatile organic compounds from a South-East Asian tropical rainforest

B Langford,A R Mackenzie,C N Hewitt,B Davison,E Nemitz,T A M Pugh,S F Lim,P K Misztal,C Helfter

doi:10.5194/acp-10-8391-2010

Abstract

Abstract. As part of the OP3 field study of rainforest atmospheric chemistry, above-canopy fluxes of isoprene, monoterpenes and oxygenated volatile organic compounds were made by virtual disjunct eddy covariance from a South-East Asian tropical rainforest in Malaysia. Approximately 500 hours of flux data were collected over 48 days in April–May and June–July 2008. Isoprene was the dominant non-methane hydrocarbon emitted from the forest, accounting for 80% (as carbon) of the measured emission of reactive carbon fluxes. Total monoterpene emissions accounted for 18% of the measured reactive carbon flux. There was no evidence for nocturnal monoterpene emissions and during the day their flux rate was dependent on both light and temperature. The oxygenated compounds, including methanol, acetone and acetaldehyde, contributed less than 2% of the total measured reactive carbon flux. The sum of the VOC fluxes measured represents a 0.4% loss of daytime assimilated carbon by the canopy, but atmospheric chemistry box modelling suggests that most (90%) of this reactive carbon is returned back to the canopy by wet and dry deposition following chemical transformation. The emission rates of isoprene and monoterpenes, normalised to 30 °C and 1000 μmol m−2 s−1 PAR, were 1.6 mg m−2 h−1 and 0.46mg m−2 h−1 respectively, which was 4 and 1.8 times lower respectively than the default value for tropical forests in the widely-used MEGAN model of biogenic VOC emissions. This highlights the need for more direct canopy-scale flux measurements of VOCs from the world's tropical forests.

Highlights

Trees assimilate carbon from the atmosphere through the process of photosynthesis, as a result of which, tropical forests are estimated to sequester up to 1.3 Pg of carbon annually (Lewis et al, 2009)
During the daytime, mixing ratios for each compound were always above the calculated limit of detection, with the exception of methanol and m/z’s 83 and 85, which we tentatively ascribe to hexanal and/or cis3-hexenol, and ethyl vinyl ketone (EVK), respectively
Typical daytime fluxes ranged between 0.2 and 4.4 mg m−2 h−1 (10:00–14:00; 5th and 95th percentiles), which, when normalised to standard conditions (30 ◦C; 1000 μmol m−2 s−1 photosynthetically active radiation (PAR)), gave an average base emission rate of 1.6 mg m−2 h−1. This value was found to be 4.1 times smaller than the default standard emission rate used in the MEGAN model for tropical forests

Summary

Introduction

Trees assimilate carbon from the atmosphere through the process of photosynthesis, as a result of which, tropical forests are estimated to sequester up to 1.3 Pg of carbon annually (Lewis et al, 2009). Some of this assimilated carbon is released back into the atmosphere in the form of reactive volatile organic compounds such as isoprene and monoterpenes (Laothawornkitkul et al, 2009). Emissions of biogenic volatile organic compounds (BVOC) contribute to the global carbon cycle They can influence both atmospheric composition and global climate in several key ways.

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Results

Conclusion