Global isoprene emissions estimated using MEGAN, ECMWF analyses and a detailed canopy environment model

摘要

The global emissions of isoprene are calculated at 0.5° resolutionfor each year between 1995 and 2006, based on the MEGAN (Model of Emissions of Gases andAerosols from Nature) version 2 model (Guenther et al., 2006) and a detailed multi-layer canopy environment model forthe calculation of leaf temperature and visibleradiation fluxes. The calculation is driven by meteorological fields – air temperature,cloud cover, downward solar irradiance, windspeed, volumetric soil moisture in 4 soil layers –provided by analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF).The estimated annual global isoprene emission ranges between 374 Tg (in 1996) and 449 Tg (in1998 and 2005), for an average of ca. 410 Tg/year over the whole period, i.e. about 30% lessthan the standard MEGAN estimate (Guenther et al., 2006). This difference is due, to a large extent, to theimpact of the soil moisture stress factor, which is found here to decrease the global emissionsby more than 20%. In qualitative agreement with past studies, high annual emissions are foundto be generally associated with El Niño events. The emission inventory is evaluated againstflux measurement campaigns at Harvard forest (Massachussets) and Tapajós in Amazonia,showing that the model can capture quite well the short-term variability ofemissions, but that it fails to reproduce the observed seasonal variation at thetropical rainforest site, with largely overestimated wet season fluxes.The comparison of the HCHO vertical columns calculated by a chemistry and transportmodel (CTM) with HCHO distributions retrieved from space provides useful insights on tropicalisoprene emissions. For example, the relatively low emissions calculated over Western Amazonia(compared to the corresponding estimates in the inventory of Guentheret al., 1995)are validated by the excellent agreement found between the CTM and HCHO data over this region.The parameterized impact of the soil moisture stress on isoprene emissions is found toreduce the model/data bias over Australia, but it leads to underestimated emissions near the endof the dry season over subtropical Africa.