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.
Supplementary material to "A Systematic Re-evaluation of Methods for Quantification of Bulk Particle-phase Organic Nitrates Using Real-time Aerosol Mass Spectrometry"
