Kinetics and Thermodynamics of Atmospherically Relevant Aqueous Phase Reactions of α-Pinene Oxide

Abstract

Recent work has demonstrated that isoprene-derived epoxide intermediates are responsible for a wide variety of chemical species found in ambient secondary organic aerosol (SOA). Since the second most abundant biogenic hydrocarbon, α-pinene, is also known to form an epoxide intermediate, nuclear magnetic resonance techniques were used to study products, kinetics, and equilibria of the aqueous phase reactions of that epoxide, α-pinene oxide. The present results indicate that α-pinene oxide will react very quickly with aqueous atmospheric particles, even under low acidity conditions. Depending on the acid concentration of the aqueous solutions, a number of new products are observed from the reaction of α-pinene oxide, some of which are expected to partition back to the gas phase. In contrast to some previous results, no long-lived organosulfate or organonitrate species are observed, and no species which retain the α-pinene bicyclic carbon backbone are observed. Rather, the overall product distribution can be explained by various rearrangements of the initial carbocation intermediate formed in the ring opening of α-pinene oxide, all of which can be rationalized by the thermodynamically driven relief of the bicyclic ring strain in the α-pinene carbon backbone.

Aqueous Secondary Organic Aerosol Formation in Ambient Cloud Water Photo-Oxidations

“…Chamber pinonic acid-OH oxidations have resulted in highly functionalized, lowvolatility molecules (Müller et al, 2012) similar to unpublished pinonic acid aqueous photo-oxidations in our laboratory (identified via ESI-ToF-MS). Our observation of dominant carbonyl formation compares favorably to single-precursor pinonic acid photo-oxidations in our photoreactor (Schurman, 2014) and other aqueous cis-pinonic acid (Aljawhary et al, 2016) and α-pinene oxidations (Bleier and Elrod, 2013); however, α-pinene products appear to vary with OH exposures, with dominant carboxylic acid formation and little change in f43 under high-OH conditions (George and Abbatt, 2010), and carbonyl formation in the form of acetone, formaldehyde, formic acid, etc. under lower [OH] (Nozière et al, 1999); Lignell et al (2013) show that direct aqueous PA photolysis is a minor mechanism.…”

Self-limited uptake of α-pinene oxide to acidic aerosol: the effects of liquid–liquid phase separation and implications for the formation of secondary organic aerosol and organosulfates from epoxides

“…Due to structural similarities, we expect the diffusivities of αPO and its reaction products to be similar. We note that, from the results of Bleier and Elrod (2013), the timescale for reaction of αPO in 10 M sulfuric acid will be ∼ 5 min, suggesting a short reacto-diffusive length of ∼ 0.05 cm. Therefore, we conclude that αPO will react near the interface, and analysis of the depth of the colored layer can give an estimate of the diffusivity of the colored products.…”

“…Standard organic synthesis has shown that, at least in bulk sulfuric acid solutions, the hydrolysis products involve opening of the 4-member ring in αPO (Coelho et al, 2012). This reaction mechanism was also evident in the recent work of Bleier and Elrod (2013). This chemistry is not likely to be reversible and retains a double bond that will allow for further reaction, such as oligomerization at high αPO concentrations.…”

“…In addition, while initial OS formation may drive uptake to aerosol, less-substituted OSs or those with nearby electron-withdrawing groups may readily hydrolyze to diol compounds (Hu et al, 2011). A recent study employing low αPO concentrations reacting in bulk sulfuric acid solutions did not show significant formation of organosulfates; rather unsaturated compounds formed almost exclusively following opening of the αPO 4-member ring (Bleier and Elrod, 2013). Laboratory experiments have observed monoterpenederived epoxide uptake to extremely acidic aerosol (pH ∼ 0), but results at acidities above 0 and less than 7 have not been reported.…”

Reaction Kinetics of Trans-Sobrerol and 8-p-Menthen-1,2-diol with Hydroxyl Radical in Aqueous Solution: A Combined Experimental and Theoretical Study

“…α-Pinene is the second most abundant biogenic nonmethane hydrocarbon emitted by vegetation into the atmosphere [8,9], and is known to contribute to formation of SOAs [10]. T rans-sobrerol (Sob) [11] is the significant hydrolysis product of α-pinene oxide which is an epoxide formed by O atom insertion across the endocyclic double bond in α-pinene and has been observed as a product in the gas phase photooxida-tion of α-pinene in simulation chamber experiments [12,13]. Limonene is also an important member in the monoterpene family and has high emission rates from both biogenic sources and household solvents [10].…”