Abstract
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, 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 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 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 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 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 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, β-ocimene SOA yields, and NOx cycling in forested environments from daytime monoterpenes not currently included in atmospheric models.
Highlights
Biogenic volatile organic compounds (BVOCs) constitute the largest flux (88 %) of all non-methane organic compounds to the atmosphere (Goldstein and Galbally, 2007; Guenther et al, 1995, 2012)
BVOCs participate in chemical reactions with regulated air pollutants, including NO2 and O3, and with radical species such as OH and NO3, which leads to the formation of low-volatility oxygenated compounds that partition into aerosol particles and represents a source of secondary organic aerosol (SOA) (Atkinson and Arey, 2003; Hallquist et al, 2009; Hatakeyama et al, 1991; Isaksen et al, 2009; Lee et al, 2016; Monks et al, 2009; Perring et al, 2013; Pye et al, 2015; Tuazon and Atkinson, 1990)
This study aims to quantify the yields of β-ocimene-derived RONO2 and the hydrolysis rate constants to better understand how organic nitrates may impact air quality and climate
Summary
Biogenic volatile organic compounds (BVOCs) constitute the largest flux (88 %) of all non-methane organic compounds to the atmosphere (Goldstein and Galbally, 2007; Guenther et al, 1995, 2012). By sequestering NOx, RONO2 slow the production of tropospheric O3, but it can act as a source of NOx and O3 downwind upon further oxidation and decomposition (Browne and Cohen, 2012; Pusede et al, 2015). These low-volatility, water-soluble compounds are important precursors to and constituents of SOA due to their efficient partitioning into the condensed phase (Biesenthal et al, 1997; Fry et al, 2009, 2014; Perraud et al, 2012; Rollins et al, 2012). The hydrolysis lifetime of organic nitrates in the aerosol phase (∼ 1–3 h) is much shorter than the lifetime of the aerosol itself to deposition (∼ 1 week), which suggests the contribution of organic nitrates to aerosol mass may have been underestimated from the observational data (Romer et al, 2016)
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