Abstract
The performance of the novel ammonium chemical ionization time of flight mass spectrometer (NH4+-CI3–TOF) utilizing NH4+ adduct ion chemistry to measure first generation oxidized product molecules (OMs) as well as highly oxidized organic molecules (HOMs) was investigated for the first time. The gas-phase ozonolysis of cyclohexene served as a first test system. Experiments have been carried out in the TROPOS free-jet flow system at close to atmospheric conditions. Product ion signals were simultaneously observed by the NH4+-CI3-TOF and the acetate chemical ionization atmospheric pressure interface time of flight mass spectrometer (acetate-CI-API-TOF). Both instruments are in remarkable good agreement within a factor of two for HOMs. For OMs not containing an OOH group the acetate technique can considerably underestimate OM concentrations by 2–3 orders of magnitude. First steps of cyclohexene ozonolysis generate ten different main products, detected with the ammonium-CI3-TOF, comprising 93% of observed OMs. The remaining 7% are distributed over several minor products that can be attributed to HOMs, predominately to highly oxidized RO2 radicals. Summing up, observed ammonium-CI3-TOF products yield 5.6 × 109 molecules cm−³ in excellent agreement with the amount of reacted cyclohexene of 4.5 × 109 molecules cm−³ for reactant concentrations of [O3] = 2.25 × 1012 molecules cm−³ and [cyclohexene] = 2.0 × 1012 molecules cm−³ and a reaction time of 7.9 s. NH4+ adduct ion chemistry is a promising CIMS technology for achieving carbon-closure due to the unique opportunity for complete detection of the whole product distribution including also peroxy radicals, and consequently, for a much better understanding of oxidation processes.
Highlights
Understanding the degradation mechanisms of hydrocarbons with highest biogenic emission rates, such as isoprene and terpenes (Guenther et al, 2012), is important to predict their contribution to the atmospheric oxidation budget and the importance of their oxidation products for particle nucleation and secondary organic aerosol (SOA) formation (Riipinen et al, 2012)
Our results show a detection sensitivity for RO2 radicals and other organic molecules (OMs) of 28 cps/pptv utilizing the NH4+-CI3-TOF which shows promise to reach carbon-closure
The most intense product peaks from the pure O3 reaction are 104.107 m/z (C5H10O)NH4+, 118.086 m/z (C5H8O2)NH4+, 132.102 m/z (C6H10O2)NH4+, 148.097 m/z (C6H10O3) NH4+ and 150.075 m/z (C5H8O4)NH4+. This is in very good agreement with Aschmann et al (2003), who identified the main ozonolysis products as pentanal (C5H10O), glutaraldehyde (C5H8O2), adipaldehyde (C6H10O2), peracid (C5H8O4), and attributed the signal at (C6H10O3) to the secondary ozonide (SOZ) and/or hydroxy dicarbonyl compounds
Summary
Understanding the degradation mechanisms of hydrocarbons with highest biogenic emission rates, such as isoprene and terpenes (Guenther et al, 2012), is important to predict their contribution to the atmospheric oxidation budget and the importance of their oxidation products for particle nucleation and secondary organic aerosol (SOA) formation (Riipinen et al, 2012). Because of the important role that SOA plays in air quality and global climate, the formation of SOA components has been a major area of research in recent years. Despite these efforts we do not have a complete molecular level understanding how SOA is formed from precursor gases. Ozonolysis experiments of the most abundant monoterpenes, α-pinene and limonene showed that highly oxidized RO2 radicals with up to 12 O atoms and the corresponding closed shell products are formed by a step by step O2 insertion initiated by intramolecular H-shifts of RO2 radicals on a time scale of seconds (Jokinen et al, 2014; Crounse et al, 2013)
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