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Abstract
Abstract. We have incorporated a semi-mechanistic isoprene emission module into the JULES land-surface scheme, as a first step towards a modelling tool that can be applied for studies of vegetation – atmospheric chemistry interactions, including chemistry-climate feedbacks. Here, we evaluate the coupled model against local above-canopy isoprene emission flux measurements from six flux tower sites as well as satellite-derived estimates of isoprene emission over tropical South America and east and south Asia. The model simulates diurnal variability well: correlation coefficients are significant (at the 95 % level) for all flux tower sites. The model reproduces day-to-day variability with significant correlations (at the 95 % confidence level) at four of the six flux tower sites. At the UMBS site, a complete set of seasonal observations is available for two years (2000 and 2002). The model reproduces the seasonal pattern of emission during 2002, but does less well in the year 2000. The model overestimates observed emissions at all sites, which is partially because it does not include isoprene loss through the canopy. Comparison with the satellite-derived isoprene-emission estimates suggests that the model simulates the main spatial patterns, seasonal and inter-annual variability over tropical regions. The model yields a global annual isoprene emission of 535 ± 9 TgC yr−1 during the 1990s, 78 % of which from forested areas.
Highlights
Isoprene (C5H8) is quantitatively the most important of the non-methane biogenic volatile organic compounds (BVOCs) emitted into the atmosphere (Pacifico et al, 2009)
We describe the validation of a modified version of the Arneth et al (2007b) scheme that has been implemented in the Joint UK Land Environmental Simulator (JULES; Best et al, 2011; Clark et al, 2011; www.jchmr.org/jules)
The use of a locally measured isoprene emission factors (IEFs) instead of the generic IEF improves the magnitude of simulated emissions at La Verdiere, but it has only a small impact on the magnitude of isoprene emissions at Montmeyan, where locally measured IEF and generic IEF are more similar to each other than at La Verdiere (Table 1)
Summary
Isoprene (C5H8) is quantitatively the most important of the non-methane biogenic volatile organic compounds (BVOCs) emitted into the atmosphere (Pacifico et al, 2009). Terrestrial vegetation is the main source (Guenther et al, 2006), not all plants emit isoprene F. Pacifico et al.: Photosynthesis-based biogenic isoprene emission scheme in JULES. Tropical broadleaf trees are considered to be the main contributors to global isoprene emissions (Guenther et al, 2006). Isoprene is a carboncontaining compound and – after oxidation in the atmosphere – a carbon dioxide (CO2) precursor, so it is a potentially significant term in the global carbon cycle (Guenther, 2002). Isoprene modulates tropospheric ozone (O3) and methane (CH4) concentrations (Hofzumahaus et al, 2009) and is a source of secondary organic aerosol (SOA; Claeys et al, 2004), which affects cloud properties and the surface radiation budget
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