10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The Effect of Chemistry and Particle Total Surface Area on Loss Rate of Highly Oxidized Multifunctional Organic Molecules (HOMs)
IIDA PULLINEN, Jürgen Wildt, Einhard Kleist, Monika Springer, Cheng Wu, Stefanie Andres, Sebastian H. Schmitt, Andreas Wahner, Thomas F. Mentel, University of Eastern Finland
Abstract Number: 1123 Working Group: Aerosol Chemistry
Abstract The loss of Highly Oxidized Multifunctional Organic Molecule (HOM) peroxy radicals on existing particle surface as reported here might have implications on atmospheric chemistry. Particle densities used in the experiments correspond to mass loadings observed in the Troposphere. HOM-RO2 lifetimes in the atmosphere are also likely to be longer than in our experiments, leading to proportionally higher importance of condensational loss onto particles.
HOMs are a group of organic molecules produced by oxidation of VOC, and are generally considered to have low to extremely low vapour pressures, which makes them potentially important in particle formation and growth processes in the atmosphere. Better understanding the formation and loss processes of these molecules can thus lead to a better understanding of particle formation.
HOM formation can be explained by peroxy radical chemistry: a HOM peroxy radical (HOM-RO2) is formed via autoxidation, followed by a termination via classical peroxy radical pathways. The main stable end product pathways are those leading to formation of ketones or alcohols (RO2+RO2’), and hydroperoxides (RO2+HO2).
When studying the formation and loss processes of HOMs in chamber experiments we discovered that there were indications that the particles were participating photochemistry, in that they acted as a condensation sink to both HO2 and HOM-RO2. This was observed during determinations of effective uptake coefficients of HOM on particles. For HOM with extremely low vapour pressures uptake coefficients are 1, i.e. uptake by particles is collision limited. At high particle surface (> 1.0 · 10-3 m2m-3), uptake coefficients for some HOM seemed to be higher than 1, while for other HOM loss rates decreased with increasing particle surface. Our hypothesis is that also peroxy radicals are lost on particles. When in presence of high particle surface, losses of peroxy radicals on particles affect the production rates of termination products. Lower production rates mimic too high uptake coefficients and vice versa higher production rates can mimic decreases of uptake coefficients at higher particle surface.
Examples for this behaviour are the HOM dimers. Lower peroxy radical concentrations due to efficient losses on particles decrease production rates and pretend uptake coefficients higher than collision limited. Another observation indirectly indicates very efficient HO2 losses on organic particles. HOMs preliminary identified as hydroperoxides show the same behaviour as dimers and other HOM preliminary identified as ketones show the opposite trend. We suggest that the HO2 losses lead to lower production rates of hydroperoxides with the result of unrealistically high uptake coefficients. The apparent lowering of uptake coefficients for ketones suggests that the HO2 losses can also lead to an increasing production rate in RO2+RO2 reactions.