1Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180, Brussels, Belgium
2Atmospheric Chemistry Division, National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, Colorado, 80305, USA
3Institute for Atmospheric and Climate Science, ETH Zurich, Universitaetsstr. 16, 8092 Zurich, Switzerland
4LAGEO, Institute for Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
*now at: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA
Abstract. Due to the scarcity of observational constraints and the rapidly changing environment in East and Southeast Asia, isoprene emissions predicted by models are expected to bear substantial uncertainties. The aim of this study is to improve upon the existing bottom-up estimates, and investigate the temporal evolution of the fluxes in Asia over 1979–2012. To this purpose, we calculate the hourly emissions at 0.5° × 0.5° resolution using the MEGAN-MOHYCAN model driven by ECMWF ERA-Interim climatology. This study incorporates (i) changes in land use, including the rapid expansion of oil palms, (ii) meteorological variability according to ERA-Interim, (iii) long-term changes in solar radiation (dimming/brightening) constrained by surface network radiation measurements, and (iv) recent experimental evidence that South Asian tropical forests are much weaker isoprene emitters than previously assumed, and on the other hand, that oil palms hold a strong isoprene emission capacity. These effects lead to a significant lowering (factor of two) in the total isoprene fluxes over the studied domain, and to emission reductions reaching a~factor of 3.5 in Southeast Asia. The bottom-up annual isoprene emissions for 2005 are estimated at 7.0, 4.8, 8.3, 2.9 Tg in China, India, Indonesia and Malaysia, respectively.
Changes in temperature and solar radiation are the major drivers of the interannual variability and trend in the emissions. An annual positive flux trend of 0.2% and 0.52% is found in Asia and China, respectively, through the entire period, related to positive trend in temperature and solar radiation. The impact of oil palm expansion in Indonesia and Malaysia is to enhance the trends over that region, e.g. from 1.17% to 1.5% in 1979–2005 in Malaysia. A negative emission trend is derived in India (−0.4%), owing to the negative trend in solar radiation data associated to the strong dimming effect likely due to increasing aerosol loadings.
The bottom-up emissions are evaluated using top-down isoprene emission estimates derived from inverse modelling constrained by GOME-2/MetOp-A formaldehyde columns through 2007–2012. The satellite-based estimates appear to support our assumptions, and confirm the lower emission rate in tropical forests of Indonesia and Malaysia. Additional flux measurements are clearly needed to better characterize the spatial variability of emission factors. Finally, a decreasing trend in the top-down Chinese emissions inferred after 2007, is in line with the cooling episode recorded in China after that year, thus suggesting that the satellite HCHO columns are able to capture climate-induced changes in emissions.