Insights into atmospheric oxidation

by Ditte Mogensen

Institution: University of Helsinki
Department: Department of Physics, Division of Atmospheric Sciences
Year: 2015
Keywords: physics
Record ID: 1138229
Full text PDF: http://hdl.handle.net/10138/154171


Forests emit biogenic volatile organic compounds (BVOCs) that, together with e.g. sulfuric acid, can operate as aerosol precursor compounds when oxidised. Aerosol particles affect both air visibility, human health and the Earth s radiative budget, thus making the emission inputs and oxidation mechanisms of VOCs absolutely crucial to understand. This thesis discusses the life cycle of compounds in the atmosphere. Specifically, we studied the representations of emission of BVOCs, the atmosphere s oxidation ability along with the sources and sinks of sulfuric acid. The main tool to achieve this was numerical modelling, often compared to field observations. Additionally, we performed computational chemistry simulations in order to calculate transitions in sulfuric acid. The main findings of this thesis can be summarised into the following: (1) Biological understanding of VOC emission processes needs to be enhanced in order to predict VOC concentrations with a high precision. (2) The unexplained fraction of the total OH reactivity in the boreal forest is larger than the known fraction and known secondary organic oxidation products of primary emitted terpenes cannot explain the missing reactivity. (3) OH is the main oxidation agent of organic compounds in the boreal atmosphere. (4) Criegee Intermediates, produced from unsaturated hydrocarbons, can oxidise SO2 effectively in order to provide as an essential source of sulfuric acid in areas with high VOC concentrations. (5) Two-photon electronic excitation did not turn out to be a significant sink of gaseous sulfuric acid in the stratosphere. This thesis closes a large part of the sulfuric acid concentration gap in VOC rich environments. Further, this thesis raises awareness of the fact that we still do not fully comprehend the mechanisms leading to BVOC emissions nor the organic atmospheric chemistry in the boreal forest. Finally, this work encourage to study alternative BVOC emission sources as well as alternative atmospheric oxidants. Skogar avger utsläpp av flyktiga organiska molekyler, så-kallade VOC. Oxidationsprodukterna av dessa molekyler kan, tillsammans med bl.a. svavelsyra, både skapa nya aerosolpartiklar i atmosfären, samt öka storleken av existerande partiklar. Aerosolpartiklarna i atmosfären påverkar både jordens klimatsystem, sikten och människors hälsa. Det är därför helt avgörande att förstå VOC-molekylernas källor och oxidationsmekanismer. Denna avhandling diskuterar och studerar molekylers livscykel i atmosfären. De specifika studieobjekten var 1) representationen av utsläppen av VOCs i atmosfärkemimodeller, 2) atmosfärens oxidationsförmåga, och 3) källorna och sänkor för svavelsyra i luften. Det viktigaste verktyget i studierna var numeriska simuleringar, som ofta jämfördes med fältobservationer. Dessutom utförde vi beräkningskemiska simuleringar för att beräkna elektroniska transitioner i svavelsyramolekylen. De viktigaste resultaten av denna avhandling är: (1) Biologisk förståelse av utsläppsprocesserna för VOC bör stärkas för att kunna…