Book/Dissertation / PhD Thesis

FZJ-2016-04109

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Photochemistry of Highly Oxidized Multifunctional Organic Molecules: a Chamber Study

Pullinen, L. I. M. (Corresponding author)FZJ*

2017
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-260-3

Please use a persistent id in citations: http://hdl.handle.net/2128/15698  urn:nbn:de:0001-2017121314

Abstract: Highly oxidized multifunctional organic molecules (HOMs) are a newly-found class of compounds that are formed in volatile organic compound (VOC) oxidation. Due to high O:C ratios of the HOMs, they are suggested to participate in atmospheric processes, such as new particle formation (NPF) and particle mass formation. Thus studying HOMs gives important insight into mechanisms of particle formation and growth under different chemistry regimes. OH is the main oxidant during daytime chemistry, however so far the photochemical HOM formation has not been studied in detail. This study focusses on the photochemical HOM production from-pinene, on chemical transformation of HOMs as well as on their loss processes. Autoxidation was found to be a dominant process of photochemical HOM formation. However, comparison of the photochemical HOM patterns from $\alpha$-pinene and its main primary oxidation product pinonaldehyde showed that also secondary OH oxidation is likely to contribute to some extent. In one experiment the oxygen content of the chamber during the experiment was lowered below 1% and the HOM formation was not affected, which indicates that autoxidation must be very fast. OH oxidation of pinonaldehyde, $\beta$-pinene, cyclohexene, benzene, and methyl salicylate led to HOM formation. If at all, these compounds do not react efficiently with ozone, suggesting that photooxidation might be a source of HOMs in general. The effect of photochemistry on HOM formation from $\alpha$-pinene was studied in more detail. The yield of HOMs from $\alpha$-pinene photooxidation was found to depend on [OH] and estimated to be between 1.8 and 7%. Adding NO$_{x}$ led to the formation of organic nitrates as well as to a general increase of HOM formation. The formation of organic nitrates confirmed the assignment of HOMs being peroxy radicals. The general increase of HOM formation observed up to moderate NO$_{x}$ levels was mainly due to OH recycling by HO$_{2}$ + NO reactions leading to increased [OH]. Additionally, the presence of NO$_{x}$ also activated the “alkoxy-peroxy pathway”. Alkoxy radicals formed in reactions of NO with peroxy radicals might undergo internal H-shifts and subsequent O$_{2}$ additions, instead of degrading. This pathway can form peroxy radicals and explain why even at very high [NO$_{x}$] there were still termination products of RO$_{2}$ + RO$_{2}$ reactions observable. High [HO$_{2}$] favoured hydroperoxide formation and diminished formation of other termination products. Altogether, the behaviour of HOMs was compatible to classical models of peroxy radical chemistry. Effective uptake coefficients for HOMs on particles were determined to be in the range of 0.5-0.9 for monomers and unity for dimers. At mass loads above ~ 3 $\mu$g m$^{-3}$ impacts of particles on peroxy radical chemistry became obvious suggesting an impact of particles on photochemistry also under atmospheric conditions.

Note: Universität Köln, Diss., 2016

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Contributing Institute(s):

1. Troposphäre (IEK-8)

Research Program(s):

1. 243 - Tropospheric trace substances and their transformation processes (POF3-243) (POF3-243)
2. HITEC - Helmholtz Interdisciplinary Doctoral Training in Energy and Climate Research (HITEC) (HITEC-20170406) (HITEC-20170406)

Appears in the scientific report 2016

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