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Johannes Gutenberg Universität in Mainz (2004)

Emissions of volatile organic compounds from tropical savanna vegetation and biomass burning

Kleiss, Betina

Titre : Emissions of volatile organic compounds from tropical savanna vegetation and biomass burning

Auteur : Kleiss, Betina

Université de soutenance : Johannes Gutenberg Universität in Mainz

Grade : Doktor der Naturwissenschaften 2004

Résumé partiel
This dissertation focuses on characterizing the emissions of volatile organic compounds (VOCs) from tropical savanna vegetation and biomass fires. The measurements were performed with a proton-transfer-reaction mass spectrometer (PTR-MS), which enabled the online detection of a large number of VOCs. The biogenic emissions of tropical savanna vegetation were studied in a woodland savanna in Venezuela. Two field campaigns were carried out, the first during the wet season in September/October 1999, and the second in March/April 2000 during the dry season. Three of the most important grass and tree species of the Venezuelan savanna were studied, namely the grasses Trachypogon plumosus, Hyparrhenia rufa and Axonopus canescens, and the tree species Byrsonima crassifolia, Curatella americana and Cochlospermum vitifolium. The emission rates and the controlling variables were determined with a dynamic plant enclosure system. Most biogenic emissions showed a diurnal variation, with highest values at noon and early afternoon, and low or no emissions during the night. In general, the emissions increased exponentially with increasing temperature and solar radiation, but correlated better with temperature. The emission rates of VOCs showed high variability caused, primarily, by the natural fluctuations of meteorological conditions during field measurements. The emission data were therefore normalized to a standard temperature of 30°C, and standard emission rates thus determined allowed for interspecific and seasonal comparisons, as well as with data from the literature. The range of average daytime (10:00-16:00) emission rates of total VOCs measured from green (mature and young) grasses was between 510-960 ngC/g/h. Methanol (detected at protonated mass 33) was the primary emission (140-360 ngC/g/h), followed by acetaldehyde (mass 45), butene and butanol (mass 57) and acetone (mass 59) with emission rates between 70-200 ngC/g/h. The emissions of propene (mass 43) and methyl ethyl ketone (MEK, mass 73) were <80 ngC/g/h, and the emission of isoprene and C5-alcohols (mass 69) was between 10-130 ngC/g/h. The normalized average emissions at 30°C (standard emissions) of total VOCs was in the range 200-400 ngC/g/h. The oxygenated species methanol, acetaldehyde, acetone and MEK, represented 70-75% of the total. The remaining 30% consisted of olefins (propene ; butene) and other unidentified species. The total daytime VOC emissions from dry grasses were in the range 120-470 ngC/g/h, and the standard emissions were between 24-44 ngC/g/h. Methanol accounted for about half of the emissions from dry grasses. The emission of VOCs was found to vary by up to a factor of three between plants of the same species (for both, green and dry grasses), and by up to a factor of two between the different species. The annual source of methanol from savanna grasses worldwide estimated in this work was 3 to 4.4 TgC, which could represent up to 12% of the current estimated global emission from terrestrial vegetation. For acetone, acetaldehyde and MEK, the savannas may contribute up to  10%, 4% and 30% of the biogenic source, respectively, but these numbers are very uncertain since particularly the biogenic sources of these species are poorly quantified

Mots clés : Atmosphäre ; VOC <Ökologische Chemie> ; Biomasse ; Verbrennung ; Massenspektrometrie

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