Effect of temperature on the formation of highly oxygenated organic molecules (HOMs) from alpha-pinene ozonolysis

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Highly oxygenated organic molecules (HOMs) are important contributors to secondary organic aerosol (SOA) and new-particle formation (NPF) in the boreal atmosphere. This newly discovered class of molecules is efficiently formed from atmospheric oxidation of biogenic volatile organic compounds (VOCs), such as monoterpenes, through a process called autoxidation. This process, in which peroxy-radical intermediates isomerize to allow addition of molecular oxygen, is expected to be highly temperature-dependent. Here, we studied the dynamics of HOM formation during <span classCombining double low line"inline-formula">α</span>-pinene ozonolysis experiments performed at three different temperatures, 20, 0 and <span classCombining double low line"inline-formula">-15</span>&thinsp;<span classCombining double low line"inline-formula">ĝ</span>C, in the Aarhus University Research on Aerosol (AURA) chamber. We found that the HOM formation, under our experimental conditions (50&thinsp;ppb <span classCombining double low line"inline-formula">α</span>-pinene and 100&thinsp;ppb ozone), decreased considerably at lower temperature, with molar yields dropping by around a factor of 50 when experiments were performed at 0&thinsp;<span classCombining double low line"inline-formula">ĝ</span>C, compared to 20&thinsp;<span classCombining double low line"inline-formula">ĝ</span>C. At <span classCombining double low line"inline-formula">-15</span>&thinsp;<span classCombining double low line"inline-formula">ĝ</span>C, the HOM signals were already close to the detection limit of the nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer used for measuring gas-phase HOMs. Surprisingly, comparing spectra measured at 0 and 20&thinsp;<span classCombining double low line"inline-formula">ĝ</span>C, ratios between HOMs of different oxidation levels, e.g., the typical HOM products <span classCombining double low line"inline-formula">C10H14O7</span>, <span classCombining double low line"inline-formula">C10H14O9</span>, and <span classCombining double low line"inline-formula">C10H14O11</span>, changed considerably less than the total HOM yields. More oxidized species have undergone more isomerization steps; yet, at lower temperature, they did not decrease more than the less oxidized species. One possible explanation is that the primary rate-limiting steps forming these HOMs occur before the products become oxygenated enough to be detected by our CI-APi-TOF (i.e., typically seven or more oxygen atoms). The strong temperature dependence of HOM formation was observed under temperatures highly relevant to the boreal forest, but the exact magnitude of this effect in the atmosphere will be much more complex: the fate of peroxy radicals is a competition between autoxidation (influenced by temperature and VOC type) and bimolecular termination pathways (influenced mainly by concentration of reaction partners). While the temperature influence is likely smaller in the boreal atmosphere than in our chamber, both the magnitude and complexity of this effect clearly deserve more consideration in future studies in order to estimate the ultimate role of HOMs on SOA and NPF under different atmospheric conditions.

OriginalsprogEngelsk
TidsskriftAtmospheric Chemistry and Physics
Vol/bind19
Nummer11
Sider (fra-til)7609-7625
Antal sider17
ISSN1680-7316
DOI
StatusUdgivet - 7 jun. 2019

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