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Organic Aerosol from the Oxidation of Biogenic Organic Compounds: a Modelling Study

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Organic Aerosol from the Oxidation of Biogenic Organic Compounds: a Modelling Study Universiteit Antwerpen Doctoral Thesis Organic aerosol from the oxidation of biogenic organic compounds: a modelling study Promoters: Author: Dr. Jean-Fran¸cois Muller¨ , Karl Ceulemans Prof. dr. Magda Claeys A thesis submitted in fulfilment of the requirements for the degree of Doctor in de wetenschappen: chemie in the Faculty of Science, Department of Chemistry 13 October 2014 Cover design: Anita Muys (Nieuwe Mediadienst, Universiteit Antwerpen) Cover photograph: \Tall Trees in Schw¨abisch Gm¨und",Matthias Wassermann (http: //www.mawpix.com/) ISBN 9789057284649 D/2014/12.293/26 Summary Aerosols exert a strong influence on climate and air quality. The oxidation of biogenic volatile organic compounds leads to the formation of secondary organic aerosol (SOA), which makes up a large, but poorly quantified fraction of atmospheric aerosols. In this work, SOA formation from α- and β-pinene, two important biogenic species, is invest- igated through modelling. A detailed mechanism describing the complex gas phase chemistry of oxidation products is generated, based on theoretical results and structure activity relationships. SOA form- ation through the partitioning of oxidation products between the gas and aerosol phases is modelled with the help of estimated vapour pressures and activity coefficients. Given the large model uncertainties, validation against smog chamber experiments is essential. Extensive comparisons with available laboratory measurements show that the model is capable of predicting most SOA yields to within a factor of two for both α- and β-pinene. However, for dark ozonolysis experiments, the model largely overestimates the temperat- ure dependence of the yields and largely underestimates their values above 30 ◦C, by up to a factor of ten. This model behaviour is improved when additional pathways towards non-volatile products are incorporated. Discrepancies are found between the observed and modelled SOA composition, as the model cannot explain the formation of several observed SOA tracers, and important model SOA species, such as hydroperoxides, await experimental identification. For β-pinene, uncertainties in key reactions in the newly implemented mechanism are shown to strongly impact ozone and SOA yields. Favouring the ring-opening of the main alkyl radical formed upon β-pinene OH-addition in the model is found to improve both ozone and SOA predictions. Products from bi-peroxy radicals formed in the ozonolysis of β-pinene are found to potentially contribute significantly to SOA. The model has been used to simulate the long-term photooxidation (ageing) of SOA formed in α-pinene chamber experiments. The model is found to overestimate aerosol concentrations upon SOA ageing by a factor of two or more in many cases, while mod- elled O/C ratios of aged SOA are also strongly overestimated. Possible causes for these discrepancies include missing gas phase or aerosol photochemistry, and the uncertain impact of wall losses. iv Summary Finally, a simple parameterisation has been developed, suitable for use in large-scale models, and capable of reproducing SOA concentrations as simulated by the full model in various conditions. Samenvatting v Samenvatting Organisch aerosol gevormd bij de oxidatie van biogene organische stoffen: een modelleringsstudie Aerosolen oefenen een grote invloed uit op het klimaat en op de luchtkwaliteit. De oxida- tie van biogene vluchtige organische stoffen leidt tot de vorming van secundair organisch aerosol (SOA), dat een grote, maar slecht gekwantificeerde fractie van atmosferische aerosolen uitmaakt. In dit werk wordt de aerosolvorming als gevolg van de oxidatie van α- en β-pineen, twee biogene stoffen die in grote hoeveelheden door bomen worden uitgestoten, onderzocht door middel van modellering. Een gedetailleerd mechanisme dat de complexe gasfasechemie van oxidatieproducten be- schrijft, werd gegenereerd, gebaseerd op structuur-activiteitsrelaties en theoretische re- sultaten. SOA-vorming door partitie van oxidatieproducten tussen gas en aerosol wordt gemodelleerd met behulp van geschatte dampdrukken en activiteitsco¨effici¨enten. Ge- zien de grote modelonzekerheden is validatie aan de hand van smogkamerexperimenten essentieel. Vergelijkingen met beschikbare metingen tonen aan dat het model voor α- en β-pineen in de meeste gevallen in staat is om de SOA-concentraties te benaderen tot binnen een factor twee nauwkeurig. Voor donkere ozonolyse overschat het model echter sterk de tem- peratuursafhankelijkheid van de SOA-vorming, met grote onderschattingen boven 30 ◦C tot gevolg. Dit kan verbeterd worden door bijkomende reactiepaden naar niet-volatiele producten toe te voegen. Ook zijn er belangrijke afwijkingen tussen de geobserveerde en gemodelleerde samenstelling van het aerosol. Het model blijkt enerzijds niet in staat te zijn om de vorming van verschillende gekende SOA-tracers te verklaren, en anderzijds zijn veelvoorkomende stoffen in het gemodelleerde aerosol, zoals hydroperoxides, nog niet experimenteel geverifieerd. Voor β-pineen werd aangetoond dat onzekerheden voor enkele cruciale reacties in het nieuwe reactiemechanisme een sterke impact hebben op de vorming van ozon en aerosol in het model. De ringopening van het voornaamste alkylradicaal in de OH-additie van β-pineen blijkt belangrijk te zijn om een redelijke overeenkomst tussen gemodelleerd en geobserveerd ozon en aerosol te bekomen. Biradicalen gevormd tijdens de ozonolyse van β-pineen dragen mogelijk ook sterk bij tot het aerosol. Het model werd gebruikt voor de simulatie van de langdurige foto-oxidatie (veroudering) van aerosol van α-pineen in smogkamers. We stelden vast dat het model de aerosolcon- centraties in dat geval met een factor twee of meer overschat, terwijl de verhouding O/C vi Samenvatting eveneens sterk overschat wordt. Mogelijke oorzaken van deze afwijkingen zijn onder an- dere ontbrekende gas- en aerosolfasechemie, en de onzekere impact van verliezen naar de kamerwanden. Tot slot werd een simpele parametrisering ontwikkeld voor gebruik in grootschalige at- mosferische modellen. Deze is in staat om de aerosolvorming gesimuleerd met het volle- dige aerosolmodel te reproduceren. Acknowledgements I would like to thank everyone who has provided help, advice or support, or shown interest during my work on this dissertation. First, I want to express my gratitude towards my promoter Dr. Jean-Fran¸coisM¨uller,for giving me the opportunity to enter into scientific research, and this in an engaging field. Working with him and within the Tropospheric Chemistry Group always happened in a creative and pleasant atmosphere. I also would like to thank him for making my work towards a doctoral degree possible. His guidance and help as advisor throughout my research at the Belgian Institute for Space Aeronomy (BIRA-IASB) and the writing of this dissertation has been invaluable. I have especially appreciated his invariably positive attitude towards research, combined with a great amount of common sense, friendliness, and patience. I am also very grateful towards my promoter Prof. dr. Magda Claeys, for having accepted the promotership of my doctoral dissertation at the University of Antwerp, and for her continuous guidance throughout the process. Her research and that of her group also provided an important perspective from the observational and experimental side of aerosol science. I would especially like to thank Prof. dr. Annemie Bogaerts (University of Antwerp), Prof. dr. Piet Van Espen (University of Antwerp), Prof. dr. Bernard Aumont (Universit´e Paris-Est Cr´eteilVal de Marne), Prof. dr. Jean-Fran¸coisDoussin (Universit´eParis-Est Cr´eteilVal de Marne), Dr. Jean-Fran¸coisM¨uller(BIRA-IASB), and Prof. dr. Magda Claeys, for their willingness to accept the jury function for this dissertation, and for their interest in my work. My work has been made possible by financial support from the Belgian Science Policy Office (BELSPO), for which I want to express my gratitude. Thanks go to the other members of the Tropospheric Chemistry Group at BIRA-IASB. I would like to give special thanks to Steven Compernolle, with whom I have had the pleasure of collaborating closely during this entire research. In particular, his work on vapour pressure and activity coefficient estimation forms an essential part of the aerosol formation model which I have applied, and he has provided numerous other contribu- tions. Special thanks go to Jenny Stavrakou and Maite Bauwens for many stimulating discussions and support. And I gratefully acknowledge Manuel Capouet, who started up the development of the BOREAM model, and introduced me to it upon my arrival at BIRA. vii viii Acknowledgements I have had the chance to enjoy numerous enlightening exchanges with other scientists, many within the IBOOT and BIOSOA projects. I am indebted to Prof. dr. Jozef Peeters, Dr. Luc Vereecken and Dr. Thanh Lam Nguyen, whose theoretical calculations form the basis of the chemical mechanisms used in this work, and who provided further insights at numerous occasions. Thanks to Dr. Ariane Kahnt for discussions which provided very useful information on smog chamber experiments, and I would also like to thank her former Leipzig colleagues Dr. Anke Mutzel, Dr. Yoshi Iinuma and Dr. Olaf B¨ogefor making available experimental data on β-pinene oxidation. I am grateful towards other former colleagues, such as Yves Christophe, Simon Chab- rillat, Edith Botek, Sergey Skachko, Cindy Senten, and many others, for the good time I had while sharing an office with them, or during the many lively lunch hour
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