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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Jul 2019

Research article | 04 Jul 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Experimental investigation into the volatilities of highly oxygenated organic molecules (HOM)

Otso Peräkylä1, Matthieu Riva1,2, Liine Heikkinen1, Lauriane Quéléver1, Pontus Roldin3, and Mikael Ehn1 Otso Peräkylä et al.
  • 1Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Finland
  • 2Univ Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, F-69626, Villeurbanne, France
  • 3Division of Nuclear Physics, Lund University, P.O. Box 118, 22100 Lund, Sweden

Abstract. Secondary organic aerosol (SOA) forms a major part of the tropospheric submicron aerosol. Still, the exact formation mechanisms of SOA have remained elusive. Recently, a newly discovered group of oxidation products of volatile organic compounds (VOC), highly oxygenated organic molecules (HOM), have been proposed to be responsible for a large fraction of SOA formation. To assess the potential of HOM to form SOA, and to even take part in new particle formation, knowledge of their exact volatilities would be essential. However, due to their exotic, and partially unknown, structures, estimating their volatility is challenging. In this study, we performed a set of continuous flow chamber experiments, supported by box modelling, to study the volatilities of HOM, along with some less oxygenated compounds, formed in the ozonolysis of alpha-pinene, an abundant VOC emitted by the boreal forests. Along with gaseous precursors, we periodically injected inorganic seed aerosol into the chamber to vary the condensation sink (CS) of low volatility vapours. We monitored the decrease of oxidation products in the gas phase in response to increasing CS, and were able to relate the responses to the volatilities of the compounds. We found that HOM monomers are mainly of low volatility, with a small fraction being semi-volatile. HOM dimers were all of at least low volatility, but probably of extremely low volatility: however, our method is not directly able to distinguish between the two. We were able to explain the volatility of the oxidation products in terms of their carbon, hydrogen, oxygen and nitrogen numbers. We found that increasing levels of oxygenation correspond to lower volatilities, as expected, but that the decrease is less steep than would be expected based on many existing models for volatility, such as SIMPOL. The hydrogen number of a compound also explained its volatility, independently of the carbon number, with higher hydrogen numbers corresponding to lower volatilities. This can be explained in terms of the functional groups making up a molecule: high hydrogen numbers are associated with e.g. hyrdoxy groups, which lower the volatility more than e.g. carbonyls, that are associated with a lower hydrogen number. The method presented should be applicable to systems other than α-pinene ozonolysis, and with different organic loadings in order to study different volatility ranges.

Otso Peräkylä et al.
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Otso Peräkylä et al.
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Short summary
Highly oxygenated molecules (HOM) have recently been suggested to form a large fraction of secondary organic aerosol. However, due to their exotic structures, the volatilities of HOM are not well known. We experimentally investigated the volatility of HOM formed in the ozonolysis of the monoterpene alpha-pinene. We found that the volatilities of HOM broadly fall within the range of previous estimates. Further, the volatility of HOM could be well explained using their molecular formulae.
Highly oxygenated molecules (HOM) have recently been suggested to form a large fraction of...