Abstract
Abstract. We aimed at quantifying biogenic volatile organic compound (BVOC) emissions in June from three Mediterranean species located at the O3HP site (southern France): Quercus pubescens, Acer monspessulanum and C. coggygria (for isoprene only). As Q. pubescens was shown to be the main BVOC emitter with isoprene representing ≈ 99% of the carbon emitted as BVOC, we mainly focused on this species. C. coggygria was found to be a non-isoprene emitter (no other BVOCs were investigated). To fully understand both the canopy effect on Q. pubescens isoprene emissions and the inter-individual variability (tree to tree and within canopy), diurnal variations of isoprene were investigated from nine branches (seven branches located to the top of canopy at ≈ 4 m above ground level (a.g.l.), and two inside the canopy at ≈ 2 m a.g.l.). The Q. pubescens daily mean isoprene emission rate (ERd) fluctuated between 23 and 98 μgC gDM−1 h−1. Q. pubescens daily mean net assimilation (Pn) ranged between 5.4 and 13.8, and 2.8 and 6.4 μmol CO2 m−2 s−1 for sunlit and shaded branches respectively. Both ERd and isoprene emission factors (Is), assessed according to Guenther et al. (1993) algorithm, varied by a factor of 4.3 among the sunlit branches. While sunlit branches ERd was clearly higher than for shaded branches, there was a non-significant variability of Is (59 to 77 μgC gDM−1 h−1). Diurnal variations of isoprene emission rates (ERs) for sunlit branches were also investigated. ERs were detected at dawn 2 h after Pn became positive and were mostly exponentially dependent on Pn. Diurnal variations of ERs were not equally well described throughout the day by temperature (CT) and light (CL) parameters according to G93 algorithm. Temperature had more impact than photosynthetically active radiation (PAR) on the morning emissions increase, and ER was no longer correlated to CL × CT between solar noon (maximum ER) and mid-afternoon, possibly due to thermal stress of the plant. A comparison between measured and calculated emissions using two isoprene algorithms (G93 and MEGAN – Model of Emissions of Gases and Aerosols from Nature) highlighted the importance of isoprene emission factor Is value used, and some weakness in assessing isoprene emissions under Mediterranean climate conditions (drought) with current isoprene models.
Highlights
Isoprene (2-methylbuta-1,3-diene) is the most abundant biogenic volatile organic compound (BVOC) released into the atmosphere with a global annual flux estimation of 400– 660 TgC yr−1 (Guenther et al, 2006)
The annual cumulative precipitation in 2012 in the rain exclusion plot became significantly different since the beginning of May and was around 30 % lower compared to the control plot
Q. pubescens was found to be a strong isoprene emitter, with a daily mean value of isoprene emission rate (ERiso) of 98 μgC g−DM1 h−1 representing, on average, 98.8 % of the carbon emitted by the Qp4 branch
Summary
Isoprene (2-methylbuta-1,3-diene) is the most abundant biogenic volatile organic compound (BVOC) released into the atmosphere with a global annual flux estimation of 400– 660 TgC yr−1 (Guenther et al, 2006). Once in the atmosphere and due to the high quantity emitted, isoprene strongly impacts the atmospheric chemistry. This molecule is going to react quickly with the main oxidant compound (OH), leading to the formation of oxidative highly reactive products in the atmosphere (Atkinson, 2000; Ciccioli et al, 1999; Claeys et al, 2004; Steiner and Goldstein, 2007). NOx being mainly emitted by anthropogenic sources, isoprene emissions occurring close to megacities surrounded by large ecosystem areas (such as the Mediterranean) can significantly contribute to high O3 levels in summer (Curci et al, 2009)
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