Gas phase formation of extremely oxidized pinene reaction products in chamber and ambient air

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Abstract

High molecular weight (300-650 Da) naturally charged negative ions have previously been observed at a boreal forest site in Hyytiälä, Finland. The long-term measurements conducted in this work showed that these ions are observed practically every night between spring and autumn in Hyytiälä. The ambient mass spectral patterns could be reproduced in striking detail during additional measurements of α-pinene (C 10H 16) oxidation at low-OH conditions in the JÃ1/4lich Plant Atmosphere Chamber (JPAC). The ions were identified as clusters of the nitrate ion (NO 3-) and α-pinene oxidation products reaching oxygen to carbon ratios of 0.7-1.3, while retaining most of the initial ten carbon atoms. Attributing the ions to clusters instead of single molecules was based on additional observations of the same extremely oxidized organics in clusters with HSO 4- (Hyytiälä) and C 3F 5O 2- (JPAC). The most abundant products in the ion spectra were identified as C 10H 14O 7, C 10H 14O 9, C 10H 16O 9, and C 10H 14O 11. The mechanism responsible for forming these molecules is still not clear, but the initial reaction is most likely ozone attack at the double bond, as the ions are mainly observed under dark conditions. β-pinene also formed highly oxidized products under the same conditions, but less efficiently, and mainly C 9 compounds which were not observed in Hyytiälä, where β-pinene on average is 4-5 times less abundant than α-pinene. Further, to explain the high O/C together with the relatively high H/C, we propose that geminal diols and/or hydroperoxide groups may be important. We estimate that the night-time concentration of the sum of the neutral extremely oxidized products is on the order of 0.1-1 ppt (∼10 6-10 7 molec cm -3). This is in a similar range as the amount of gaseous H 2SO 4 in Hyytiälä during day-time. As these highly oxidized organics are roughly 3 times heavier, likely with extremely low vapor pressures, their role in the initial steps of new aerosol particle formation and growth may be important and needs to be explored in more detail in the future. © 2012 Author(s).

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Ehn, M., Kleist, E., Junninen, H., Petäjä, T., Lönn, G., Schobesberger, S., … Mentel, T. F. (2012). Gas phase formation of extremely oxidized pinene reaction products in chamber and ambient air. Atmospheric Chemistry and Physics, 12(11), 5113–5127. https://doi.org/10.5194/acp-12-5113-2012

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