Biogenic sources dominate annual emissions of volatile organic compounds (VOCs) to the atmosphere. A large fraction of these are monoterpenes, which react with OH radicals, NO 3 radicals, or O 3 to form oxidized products, some of which partition to particles as secondary organic aerosol (SOA). Here, we compare the results of studies of the reaction of NO 3 radicals, a nighttime oxidant, with five monoterpenes: Δ-3-carene, β-pinene, α-pinene, limonene, and ocimene. Whereas all of these monoterpenes have the molecular formula C 10 H 16 , they differ by having 1, 2, or 3 C�C double bonds and 0, 1, or 2 rings. Experiments were conducted in an environmental chamber under conditions in which RO 2 • + RO 2 • reactions were dominant, and gas-and particle-phase products were analyzed using mass spectrometry, gas and liquid chromatography, infrared spectroscopy, and derivatization-spectrophotometric methods. Gas-phase products were first-generation compounds with 2−4 functional groups, whereas SOA products were mostly acetal and hemiacetal dimers formed by particle-phase accretion reactions. The large contribution of dimers formed from hydroxycarbonyl nitrate and hydroxynitrate monomers indicates that they might be used as atmospheric tracers for NO 3 radicalinitiated reactions of monoterpenes. Conversely, gas-phase formation of ROOR dimers was negligible. Functional group analysis of SOA indicated ∼1 nitrate, ∼0.2−0.7 carbonyl groups, and ∼0−0.4 hydroxyl, carboxyl, ester, and peroxide groups per C 10 product for all the monoterpenes. SOA mass yields were 56, 89, 48, 78, and 69% for Δ-3-carene, β-pinene, α-pinene, limonene, and ocimene, which combined with functional group analysis gives lower-limit estimates of organic nitrate yields of 34, 56, 35, 50, and 40%. Results were used to develop reaction mechanisms to explain the formation of gas-and particle-phase products and provide improved understanding of the role of molecular structure in VOC oxidation and particle-phase accretion reactions.

The oxidation of volatile organic compounds (VOCs), which are emitted to the atmosphere from natural and anthropogenic sources, leads to the formation of ozone and secondary organic aerosol (SOA) particles that impact air quality and climate. In the study reported here, we investigated the products of the reactions of five biogenic monoterpenes with OH radicals (an important daytime oxidant) under conditions that mimic the chemistry that occurs in polluted air, and developed mechanisms to explain their formation. Experiments were conducted in an environmental chamber, and information on the identity of gas-phase molecular products was obtained using online mass spectrometry, while liquid chromatography and two methods of functional group analysis were used to characterize the SOA composition. The gas-phase products of the reactions were similar to those formed in our previous studies of the reactions of these monoterpenes with NO 3 radicals (an important nighttime oxidant), in that they all contained various combinations of nitrate, carbonyl, hydroxyl, ester, and ether groups. But in spite of this, less SOA was formed in OH/NO x reactions and it was composed of monomers, while SOA formed in NO 3 radical reactions consisted of acetal and hemiacetal oligomers formed by particle-phase accretion reactions. In addition, it appeared that some monomers underwent particle-phase hydrolysis, whereas oligomers did not. These differences are due primarily to the arrangement of hydroxyl, carbonyl, nitrate, and ether groups in the monomers, which can in turn be explained by differences in OH and NO 3 radical reaction mechanisms. The results provide insight into the impact of VOC structure on the amount and composition of SOA formed by atmospheric oxidation, which influence important aerosol properties such as volatility and hygroscopicity.

Organic aerosol in the atmosphere has an impact on climate, visibility, and human health. Oxidation of biogenic volatile organic compounds forms secondary organic aerosols by lowering the volatility of the product molecules and thus enhancing partitioning to the particle phase. The NO 3 -initiated oxidation of Δ-3-carene was studied because it connects the interaction between biogenic and anthropogenic emissions to form aerosols. This work characterized the first-generation gas-phase products of the NO 3initiated oxidation of Δ-3-carene in a 7.4 m 3 Teflon FEP chamber using a Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus) and a high-resolution time-of-flight chemicalionization mass spectrometer (CIMS) using iodide adducts. The mass spectra not only show the presence of most of the expected products, including dicarbonyl, hydroxy nitrate, carbonyl nitrate, hydroxy dicarbonyl, and dicarbonyl nitrate but also show significant fragmentation of the parent ions in the Vocus through the loss of water and/or nitric acid and other neutral fragments. Parent and fragment ions were grouped together, taking advantage of gas-wall interactions in Teflon tubing, which separate the molecules by their inlet delay time. After grouping product ions, the Vocus signals were used to determine the gas-phase product yields for the NO 3 + Δ-3-carene reaction. A comparison between the Vocus and iodide CIMS data allowed the sensitivity of the iodide CIMS to be investigated. Understanding the mechanism of the oxidation of Δ-3-carene by NO 3 radicals allows for a better understanding of the sources of organic nitrate in the atmosphere and can improve the interpretation of field data and the representation of this chemistry in models.