Comprehensive Analysis of Products and the Development of a Quantitative Mechanism for the OH Radical-Initiated Oxidation of 1-Alkenes in the Presence of NO_x

Abstract

The reactions of 1-tetradecene and 1-pentadecene, the C14 and C15 linear 1-alkenes, with OH radicals in the presence of NO x were investigated in a series of environmental chamber experiments. Particle-phase β-hydroxynitrates, dihydroxynitrates, dihydroxycarbonyls, and 1,4-hydroxynitrates and gas-phase aldehydes were sampled and then identified and quantified using a suite of offline analytical techniques that included derivatization, gas and liquid chromatography, and multiple types of mass spectrometry. Measured molar yields of products formed by OH radical addition to the CC double bond, including β-hydroxynitrates, dihydroxynitrates, dihydroxycarbonyls (which have not been previously directly quantified with high accuracy), and aldehydes were 0.125 ± 0.01, 0.048 ± 0.005, 0.240 ± 0.04, and 0.268 ± 0.03 (0.264 ± 0.02 and 0.271 ± 0.04 for the formaldehyde and tridecanal/tetradecanal co-products of β-hydroxyalkoxy radical decomposition), respectively. These values give a total molar yield of 0.681 ± 0.05, which agrees very well with the results of kinetics measurements that indicate that the fraction of reaction that occurs by OH radical addition is 0.70. The yields were used to calculate branching ratios for all OH radical addition pathways, including a value of 0.18 for the formation of dihydroxynitrates from the reaction of dihydroxyperoxy radicals with NO and values of 0.47 and 0.53 for β-hydroxyalkoxy radical decomposition and isomerization. The results were used with literature data on the yields of aldehydes measured for similar reactions of smaller alkenes, a model for the effect of carbon number on branching ratios for organic nitrate formation, and a mechanism for H atom abstraction derived from studies of linear alkanes to achieve a complete, quantitative gas-phase reaction mechanism for 1-alkenes. The results should also be useful for constructing mechanisms for more complex reactions of volatile organic compounds.

Products and Mechanisms of Secondary Organic Aerosol Formation from the NO3 Radical-Initiated Oxidation of Cyclic and Acyclic Monoterpenes

“…These values fall within the range of values measured previously for 2-ethylhexyl nitrate and multifunctional organic nitrates. 29,32,52,53 This range is expected, since the presence of multiple functional groups, especially adjacent to the chromophore, affects molar absorptivity. 29 We also know from previous work that during carbonyl analysis a portion of the acetals will be hydrolyzed back to the carbonylcontaining monomers because of the acidic conditions.…”

Gas- and Particle-Phase Products and Their Mechanisms of Formation from the Reaction of Δ-3-Carene with NO3 Radicals

“…The final molar absorptivity of 2660 M −1 cm −1 is roughly midway between 2230 and 2900 M −1 cm −1 , and all three values fall within the range we have measured previously for 2ethylhexyl nitrate and multifunctional organic nitrates. 26,29,47,49 This range is not surprising, since small changes in structure have been shown to affect the molar absorptivity of all types of multifunctional compounds. 26 The final value obtained for the number of nitrate groups per C 10 SOA molecule is then 0.94, close to 1, as expected, with 0.71, 0.15, 0.11, 0.16, 0.13, and 7.80 carbonyl, hydroxyl, carboxyl, ester, peroxide, and methylene groups.…”

Chemistry of Secondary Organic Aerosol Formation from Reactions of Monoterpenes with OH Radicals in the Presence of NOx

“…Reactions with NO will form hydroxynitrates (HN) or hydroxyalkoxy radicals. Our previous measurements of the branching ratios of reactions of hydroxyperoxy radicals with NO indicate that the yields of hydroxynitrates will be ∼20%, with the other ∼80% forming hydroxyalkoxy radicals. , Although RO 2 · radicals formed in our experiments (and in polluted atmospheres) will also react with NO 2 radicals to form peroxynitrates, ROONO 2 , because these compounds decompose reversibly on a time scale of a few seconds, the RO 2 · radicals will eventually react with NO.…”

“…Nitrate groups were quantified from the absorbance measured at 210 nm, where nitrates have a natural absorbance that is orders of magnitude larger than that of any of the other functional groups . The molar absorptivity of nitrate groups can vary significantly, however, depending on the molecular structure. ,,, To account for this, for Δ-3-carene, β-pinene, α-pinene, and limonene we used an iterative process that is described elsewhere , and begins with a molar absorptivity of a 2-ethylhexyl nitrate standard (3030 M –1 cm –1 ). The final calculated molar absorptivities were 2480, 2575, 2554, and 2640 M –1 cm –1 , respectively.…”

“…Accretion reactions occurred to a much lesser extent between products of the reaction of an acyclic monoterpene with NO 3 radicals, 17 and barely at all in SOA formed from reactions of simple acyclic alkenes with OH/NO x . 18,19 This difference can be explained by the presence of a nitrate group adjacent to the carbonyl group in the ring-opened products of the reactions of cyclic monoterpenes with NO 3 radicals, which activates the carbonyl group toward these particle-phase reactions. 20 Other important differences were that during ring opening of cyclic monoterpenes via C−C bond cleavage the carbon backbone remained intact, whereas for the acyclic monoterpene and simple alkenes this led to more volatile fragmentation products.…”