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https://doi.org/10.5194/acp-2020-694
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-694
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  28 Aug 2020

28 Aug 2020

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This preprint is currently under review for the journal ACP.

Effects of NOx and seed aerosol on highly oxygenated organic molecules (HOM) from cyclohexene ozonolysis

Meri Räty1, Otso Peräkylä1, Matthieu Riva1,2, Lauriane Quéléver1, Olga Garmash1, Matti Rissanen1,3, and Mikael Ehn1 Meri Räty 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
  • 3Aerosol Physics Laboratory, Physics Unit, Tampere University, Tampere, Finland

Abstract. Cyclohexene (C6H10) is commonly used as a proxy for biogenic monoterpenes, when studying their oxidation mechanisms and secondary organic aerosol (SOA) formation. The ozonolysis of cyclohexene has been shown to be effective at producing highly oxygenated organic molecules (HOM), a group of molecules known to be important in the formation of SOA. Here, we provide an in depth look at how, on a molecular level, the HOM formation and fate changed with perturbations from NOx and seed particles.

HOM were produced in a chamber from cyclohexene ozonolysis, and measured with a chemical ionisation mass spectrometer (CIMS) using nitrate (NO3) as reagent ion. As high-resolution CIMS instruments provide mass spectra with numerous ion signals and a wealth of information that can be hard to manage, we employed a primarily statistical approach for the data analysis. To utilise as many individual HOM signals as possible, each compound was assigned a parameter describing the quality of the observed signal. These parameters were then used as weights or to determine the inclusion of a given signal in further analyses.

Under unperturbed ozonolysis conditions, known HOM peaks were observed in the chamber, including C6H8O9 as the largest HOM signal, and C12H20O9 as the largest dimer product. With the addition of nitric oxide (NO) into the chamber, the spectrum changed considerably, as expected. Dimer product signals decreased overall, but an increase in dimers with nitrate functionalities was seen, as a result of NO3 radical oxidation. The response of monomer signals to NO addition varied, and while nitrate-containing monomers increased, non-nitrate signals either increased or decreased, depending on the individual molecules.

The addition of seed aerosol increased the condensation sink, which markedly decreased the signals of all low-volatility compounds. Larger molecules were seen to have a higher affinity for condensation, but a more detailed analysis showed that the uptake was controlled mainly by the number of oxygen atoms in each molecule. All non-nitrate compounds with at least 7 oxygen atoms were observed to condense onto the seed aerosol at close to equal rates. Nitrates required higher mass and higher oxygen content to condense at similar rates as the non-nitrate HOM. A comparison to experiments with alpha-pinene reported earlier, showed quite a similar relationship between elemental composition and volatility, although products from alpha-pinene ozonolysis appeared to require slightly higher oxygen numbers for the same decrease in volatility. In addition, two models developed for predicting volatilities of volatile organic compound (VOC) oxidation products were tested on the ozonolysis products of cyclohexene.

Meri Räty et al.

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Short summary
Cyclohexene resembles certain relatively complex compounds present in the atmosphere that through oxidation produce vapours that take part in aerosol formation. We studied the highly oxygenated organic molecules (HOM) formed in cyclohexene ozonolysis, the relationship between their chemical composition and tendency to condense onto seed aerosol, and the effect of NOx pollutants on their signals. Two existing models were also tested in their ability to predict the volatility of the HOMs.
Cyclohexene resembles certain relatively complex compounds present in the atmosphere that...
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