Biogenic VOC emissions from rainforest and oil palm plantations in South East Asia (contribution to OP3 and ACES projects)

Abstract

For the first time in South-East Asia earlier this year canopy-scale measurements of volatile organic compound (VOC) emission from a selectively logged rainforest and from an oil palm plantation were conducted using PTR-MS combined with virtual disjunct eddy covariance. This was done in the multi-team collaborative effort of the OP3 and ACES projects. Both rainforests and oil palm plantations transfer large amounts of reactive organic carbon to the lower atmosphere, resulting inter alia in formation of tropospheric ozone, secondary organic aerosols (SOA) and peroxy radicals. The impact of biogenic VOCs on the depletion of OH radicals is still not fully recognised. Recent studies over Amazon forests proposed that peroxy radical recycling was responsible for sustaining the atmospheric oxidation capacity (Lelieveld et al., 2008). Isoprene and monoterpenes are regarded as the most important biogenic VOCs, whose fluxes were suggested to be significantly different from rainforest than from oil palms. Although the first lab studies predicted potentially high isoprene emissions from oil palms (Wilkinson et al., 2006), measurements reported here have proven this at the ecosystem scale. Overall at the rainforest, the maximum fluxes of isoprene were observed at about 13:00 with an average of 2.5 mg m-2 h-1. Maximum isoprene emissions from oil palms were recorded earlier in the day, at 11:00, with a mean value of 13 mg m-2 h-1. Initial flux results for total monoterpenes indicate that their mass emission ratio with respect to isoprene was about 1:9 at the rainforest and 1:18 at the oil palm plantation. The results are presented with reference to temperature, photosynthetic radiation and meteorological drivers as well as in comparison with CO2 and H2O fluxes. Although the earlier data for isoprene flux from the Amazon forest during the dry season presented by Karl et al. (2007) seemed higher than our fluxes from the Borneo forest, the emission potentials of the vegetation may be similar after accounting for different temperatures. However, the highest fluxes from oil palms, experiencing steeper exponential growth with temperature than rainforest species, indicate that the basal emission rates of oil palms must be much higher than those of the forest species. The consequences for atmospheric chemistry of land use change from rainforest to oil palm plantation can therefore be quite dramatic. Global models predicting atmospheric changes and bottom-up estimates from the tropics must be constrained by direct measurements such as presented here, taking separate account of these major contributions from oil palm plantations and tropical rainforests