17 May, 2017
May 17 2017 03:00 PM - 04:00 PM
Gas-phase oxidation of certain volatile organic compounds (VOC) leads to highly-oxidized and very low volatile material that is capable of acting as a direct source of atmospheric secondary organic aerosol (SOA) [1, 2]. Contrary to previous perception of slow oxidation by consecutive reactions in the atmospheric gas mixture, the generation of highly-oxidized multifunctional compounds (HOM) can be tremendously fast. HOM form by pseudo-one-step reactions, even in a sub-second time-scale, by a process called autoxidation . Autoxidation proceeds by sequential peroxy radical (RO2) hydrogen-shift isomerization reactions and subsequent O2 addition steps, until a termination occurs by unimolecular dissociation or by bimolecular reaction [2-4].
Endocyclic alkenes are cyclic compounds with one or more unsaturated bonds within an organic ring structure (see Figure 1). They constitute a large fraction of biogenically produced terpenes (i.e. compounds that are multiples of C5H8 structural units) and are emitted to the atmosphere in vast quantities . Cyclohexene is a common structural part of many abundant biogenic monoterpenes such as α-pinene and limonene. Its symmetric and relatively simple structure has made it a common surrogate species for tracking detailed oxidation mechanisms and for studying formation of secondary organic material. Previously, we have investigated the autoxidation sequence of cyclohexene leading to gas-phase products with an O/C ratio up to 1.5 , and applied the rules generated to understand the autoxidation mechanisms of methylated cyclohexenes . Since then, the relevant autoxidation reaction steps have been updated to include the latest findings on, for example, ultrafast hydrogen shifts scrambling the peroxy radical distribution , and by further unimolecular dissociation processes. Currently we are investigating the influence of autoxidation perturbation by addition of common atmospheric g as-phase co-reactants NO, NO2, SO2 and HO2, as well as RO2 derived from other smaller VOCs. These reagents are seen to interfere with the oxidation pathways, mainly terminating autoxidation prematurely and forming products which are indicative of the type of perturbing reaction.
To study the autoxidation phenomena, we perform flow reactor investigations of ozonolysis initiated oxidation under ambient conditions utilizing chemical ionization mass spectrometry (CIMS) with nitrate (NO3-) and iodide (I-) reagent ions to detect products with varying levels of oxidation. By careful selection of reacting gas molecules and by perturbing the reaction mixture with certain gas-phase co-reagents (e.g. NO, NO2, SO2 and HO2, as well as RO2) it is possible to infer mechanistic insight into the important reaction steps that transform a completely volatile precursor molecule into a very low volatile molecule that is able to act as a substrate for atmospheric particle formation.
Matti Rissanen completed his undergraduate studies in Chemistry at the University of Helsinki with honors, where he later also received his PhD in Physical Chemistry with the highest possible grade, Laudatur. Since then he has been a Post-Doctoral Researcher at the Division of Atmospheric Sciences at the Physics Department, also at the Univeristy of Helsinki, with research focus on gas-phase reactions creating low volatile material that serves as a substrate for atmospheric new particle formation. Recently he was appointed in a position of Finnish Academy Post-Doctoral Researcher. Rissanen has also remained active with the previous post in Chemistry Department and is still frequently publishing papers on chemical kinetics mostly relevant to low temperature combustion and atmospheric chemistry.
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