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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  16 Jun 2020

16 Jun 2020

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This preprint is currently under review for the journal ACP.

The Production and Hydrolysis of Organic Nitrates from OH Radical Oxidation of β-Ocimene

Ana Cristina Morales1,, Thilina Jayarathne1,a,, Jonathan H. Slade2, Alexander Laskin1,3, and Paul B. Shepson1,3,4 Ana Cristina Morales et al.
  • 1Department of Chemistry, Purdue University, West Lafayette,IN47906, USA
  • 2Department of Chemistry & Biochemistry, University of California San Diego, La Jolla, CA 92093, USA
  • 3Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47906, USA
  • 4School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794, USA
  • anow at: Bristol Myers Squibb, 1 Squibb Drive, New Brunswick, NJ 08901, USA
  • These authors contributed equally to this work.

Abstract. Biogenic volatile organic compounds (BVOCs) emitted by plants represent the largest source of non-methane hydrocarbon emissions on Earth. Photochemical oxidation of BVOCs represents a significant pathway in the production of secondary organic aerosol (SOA), affecting Earth’s radiative balance. Organic nitrates (RONO2), formed from the oxidation of BVOCs in the presence of NOx, represent important aerosol precursors, and affect the oxidative capacity of the atmosphere, in part by sequestering NOx. In the aerosol phase, RONO2 hydrolyze to form nitric acid and numerous water-soluble products, thus contributing to an increase in aerosol mass. However, only a small number of studies have investigated the production of RONO2 from •OH oxidation of terpenes, among those, few have studied their hydrolysis. Here, we report a laboratory study of OH radical-initiated oxidation of β-ocimene, an acyclic, triolefinic monoterpene released during the daytime from vegetation, including forests, agricultural landscapes, and grasslands. We conducted studies of the OH radical oxidation of β-ocimene in the presence of NOx using a 5.5 m3 all-Teflon photochemical reaction chamber, during which we quantified the total (gas- and particle-phase) RONO2 yield and the SOA yields. We sampled the organic nitrates produced and measured their hydrolysis rate constants at different solution pH. The total organic nitrate yield was determined to be 33(±7) %, consistent with the available literature regarding the dependence of organic nitrate production (from RO2 + NO) on carbon number. We found the hydrolysis rate constants to be highly pH-dependent, with a hydrolysis lifetime of 51(±13) min at pH = 4, and 24(±3) min at pH = 2.5, a typical pH for deliquesced aerosols. We also employed high-resolution mass spectrometry for product identification, which is used to infer key mechanisms of gas–particle partitioning. The results indicate that the ocimene SOA yield under relevant aerosol mass loadings in the atmosphere is significantly lower (< 1 %) than reported yields from cyclic terpenes, such as α-pinene, likely due to alkoxy radical decomposition and formation of smaller higher-volatility products. This is also consistent with the observed lower particle-phase organic nitrate yields of β-ocimene, 1.5(±0.5) %, under dry conditions. We observed the expected hydroxy nitrates by chemical ionization mass spectrometry (CIMS), and some secondary production of the di-hydroxy di-nitrates, likely produced by oxidation of the first-generation hydroxy nitrates. Lower RONO2 yields were observed under high relative humidity (RH) conditions, indicating the importance of aerosol-phase RONO2 hydrolysis under ambient RH. This study provides insight into the formation and fate of organic nitrates, ocimene SOA yields, and NOx cycling in forested environments from daytime monoterpenes, which are not currently included in atmospheric models.

Ana Cristina Morales et al.

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Ana Cristina Morales et al.

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Short summary
Organic nitrates formed from the oxidation of biogenic volatile organic compounds impact both ozone and particulate matter as they remove nitrogen oxides, but represent important aerosol precursors. We conducted a series of reaction chamber experiments that quantified the total organic nitrate and secondary organic aerosol yield from the OH radical-initiated oxidation of ocimene, and also measured their hydrolysis lifetimes in the aqueous phase, as a function of pH.
Organic nitrates formed from the oxidation of biogenic volatile organic compounds impact both...