Formation of highly oxygenated organic molecules from chlorine-atom-initiated oxidation of alpha-pinene

Wang, Yonghong; Riva, Matthieu; Xie, Hongbin; Heikkinen, Liine; Schallhart, Simon; Zha, Qiaozhi; Yan, Chao; He, Xu-Cheng; Peräkylä, Otso; Ehn, Mikael

Highly oxygenated organic molecules (HOMs) from atmospheric oxidation of alpha-pinene can irreversibly condense to particles and contribute to secondary organic aerosol (SOA) formation. Recently, the formation of nitryl chloride (inline-formulaClNO2) from heterogeneous reactions, followed by its subsequent photolysis, is suggested to be an important source of chlorine atoms in many parts of the atmosphere. However, the oxidation of monoterpenes such as alpha-pinene by chlorine atoms has received very little attention, and the ability of this reaction to form HOMs is completely unstudied. Here, chamber experiments were conducted with alpha-pinene and chlorine under low- and high-nitrogen-oxide (inline-formulaNOx, inline-formula M3inlinescrollmathml chem normal NO x = chem normal NO + chem normal NO normal 2 85pt13ptsvg-formulamathimgebcfd1c61bf385469f587d6678306295 acp-20-5145-2020-ie00001.svg85pt13ptacp-20-5145-2020-ie00001.png ) conditions. A nitrate-based CI-APi-ToF (chemical ionization–atmospheric pressure interface–time of flight) mass spectrometer was used to measure HOM products. Clear distributions of monomers with 9–10 carbon atoms and dimers with 18–20 carbon atoms were observed under low-inline-formulaNOx conditions. With increased concentration of inline-formulaNOx within the chamber, the formation of dimers was suppressed due to the reactions of peroxy radicals with NO. We estimated the HOM yields from chlorine-initiated oxidation of alpha-pinene under low-inline-formulaNOx conditions to be around 1.8 %, though with a substantial uncertainty range (0.8 %–4 %) due to lack of suitable calibration methods. Corresponding yields at high inline-formulaNOx could not be determined because of concurrent ozonolysis reactions. Our study demonstrates that also the oxidation of alpha-pinene by chlorine atoms and yield low-volatility organic compounds.

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Wang, Yonghong / Riva, Matthieu / Xie, Hongbin / et al: Formation of highly oxygenated organic molecules from chlorine-atom-initiated oxidation of alpha-pinene. 2020. Copernicus Publications.

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