The production and hydrolysis of organic nitrates from OH radical oxidation of β -ocimene

摘要

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 (RONO 2 ), formed from the oxidation of BVOCs in the presence of NO x , represent important aerosol precursors and affect the oxidative capacity of the atmosphere, in part by sequestering NO x . In the aerosol phase, RONO 2 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 RONO 2 from OH oxidation of terpenes, and among those, few have studied their hydrolysis. Here, we report a laboratory study of OH-initiated oxidation of β -ocimene, an acyclic, tri-olefinic monoterpene released during the daytime from vegetation, including forests, agricultural landscapes, and grasslands. We conducted studies of the OH oxidation of β -ocimene in the presence of NO x using a 5.5?m 3 all-Teflon photochemical reaction chamber, during which we quantified the total (gas- and particle-phase) RONO 2 yield and the SOA yields. We sampled the organic nitrates produced and measured their hydrolysis rate constants across a range of atmospherically relevant pH. The total organic nitrate yield was determined to be 38( ± 9)?%, consistent with the available literature regarding the dependence of organic nitrate production (from RO 2 + 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 preliminary product identification. The results indicate that the ocimene SOA yield ( ?1?%) under relevant aerosol mass loadings in the atmosphere is significantly lower 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 – i.e., 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 dihydroxy dinitrates, likely produced by oxidation of the first-generation hydroxy nitrates. Lower RONO 2 yields were observed under high relative humidity (RH) conditions, indicating the importance of aerosol-phase RONO 2 hydrolysis under ambient RH. This study provides insight into the formation and fate of organic nitrates, β -ocimene SOA yields, and NO x cycling in forested environments from daytime monoterpenes not currently included in atmospheric models.