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Universidade de Aveiro (2001)

Origem e composição da matéria orgânica presente nos aerossóis atmosféricos

Alves, Célia dos Anjos

Titre : Origem e composição da matéria orgânica presente nos aerossóis atmosféricos

Auteur : Alves, Célia dos Anjos

Université de soutenance : Universidade de Aveiro

Grade : Doutoramento em Ciências Aplicadas ao Ambiente 2001

Atmospheric aerosols are of great interest for climate studies and important indicators of natural or anthropic pollution. It has become clear in recent studies that organic compounds constitute a significant fraction of the aerosol mass. However, due to analytical difficulties and the complexity of compounds present, data on the organic composition of aerosol particles are still limited. The objective of this Thesis is to contribute, through field work, for a better understanding of the organic composition of atmospheric aerosols, taking simultaneously into consideration the primary contribution of biogenic and anthropogenic sources, and the secondary formation of oxidation products from volatile organic compounds. Atmospheric particulate matter was simultaneously collected at three Portuguese sites during an intensive campaign in August 1996. Two of the sampling sites - Anadia and Tábua - were located in rural areas near or within forests. The third site was in the university campus on the outskirts of the town of Aveiro. A second field experiment took place in an Abies boressi forest in central Greece and in Giesta, a coastal-rural site in the centre of Portugal, during the period of 20 July-12 August 1997. Size segregated aerosols were also obtained in the Greek site by making use of a high volume cascade impactor sampler. The collected aerosols were analysed in relation to black and organic carbon content. The particulate organic matter was extracted with solvents and separated into different fractions by using column chromatography on silica gel. The five organic classes resulting from this preparative technique were analysed by Gas Chromatography and Mass Spectrometry. Concentrations of solvent extractable organic matter are lower in Tábua, which is the remote sampling site. This is probably due to dry deposition and atmospheric dispersion over the forest. Seeing that Aveiro is influenced by urban emissions, anthropogenic particulate organic compounds are present at higher levels in this region than in Anadia and Tábua. The detection of over 1000 organic compounds in atmospheric particles from the Greek forest renders this site the one where a higher number of constituents could be quantified. In general, the identifiable portion of the organic matter exhibit higher concentrations in aerosols from Greece than those acquired for the other regions. The organic material consisted primarily of aliphatic hydrocarbons, acids, alcohols, and ketones, with a predominance of molecular weights > C20, derived from vascular plant waxes. Biomarkers for vegetation sources such as phytosterols and triterpenic compounds were also detected. Microbial components (< C20), petroleum residues and pyrogenic aromatic hydrocarbons were present in the various aerosol extracts. Photochemical products deriving from volatile organic compounds emitted by vegetation or from anthropogenic precursors were identified, representing a small fraction of the total chromatographically resolved organic mass (1-3%). These secondary include alkane derivatives, di- and monocarboxylic acids, nitroaromatics and many terpene photo-oxidation products. Among them we can refer, as examples, to the pinonic and pinic acids, nopinone, pinonaldehyde, pinanone, pinenol, trimethylnorpinanol, campholadehyde, campholenic acid, camphanone, carenol, phellandral, limonene oxide, sabinaketone, geranaldehyde and caryophyllene oxide. Some of these oxygenated compounds have been detected in aerosol samples from smog chamber experiments with terpenes emitted by vegetation and atmospheric oxidants. To our knowledge many of the mentioned products have been identified for the first time in the field. In the apportionment of the various organic fractions to sources, vegetation waxes dominated, contributing to 50-60% of the solvent-extractable material in the aerosols. Petroleum residues and microbial components comprise 20-30% and 25-25% of the organic matter, respectively. Samples showed higher concentrations of material in the submicrometer size range. The mass distribution of compounds present a mode for diameters < 0.49 mm, confirming the secondary origin of aerosol products formed by nucleation/condensation mechanisms from gaseous precursors. Another mode was present for compounds in particles with diameters between 1 and 3 mm as a result of condensation on the surface of the pre-existing particles or of mechanical processes

Mots clés : Química da atmosfera - Teses de doutoramento ; Poluição ambiental ; Aerossóis - Poluição atmosférica ; Compostos orgânicos - Aspectos ambientais


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