Effects of NO x and seed aerosol on highly oxygenated organic molecules (HOM) from cyclohexene ozonolysis

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.