The atmospheric oxidation of biogenic volatile organic compounds (BVOCs), which have annual emission rates (∼ 1100 Tg yr-1 total) roughly 1 order of magnitude larger than anthropogenic non-methane VOCs (Guenther et al., 1995), has a significant impact on air quality and climate. The production of secondary organic aerosol (SOA) from BVOC oxidation products influences the radiative balance of the planet by directly interacting with both solar and terrestrial radiation, as well as indirectly through their role as cloud condensation nuclei (e.g., Ramanathan et al., 2001). Overall, the production of SOA from BVOCs has a cooling effect on global climate, estimated to have a combined radiative forcing as large as -1.5 W m-2 (Scott et al., 2014). Additionally, the inhalation of SOA has a significant impact on the human respiratory system and atmospheric aerosol concentrations are positively correlated with lung cancer and mortality rates (Raaschou-Nielsen et al., 2013). Despite the importance of SOA, the chemical mechanisms that explain the composition of aerosol particles and their chemical processes are still highly uncertain.

The gas phase oxidation of BVOCs also governs tropospheric ozone concentrations by controlling its precursor, NOx (NO + NO2). In the atmosphere, the most common atmospheric oxidant, the OH radical, can either abstract a hydrogen from or add to a BVOC, if it contains an olefinic functionality (e.g., α-pinene), to create a peroxy radical from the rapid addition of molecular oxygen to the organic radical (Fig. 1). In high NOx environments, such as areas within atmospheric transport of combustion emissions, nitric oxide can either add to the peroxy radical to form an organic nitrate (RONO2) or it can be oxidized to create an alkoxy radical and NO2, which can readily photolyze to produce ozone (Fig. 1). The ratio of RONO2 production to NO2 production is referred to as the organic nitrate branching ratio. Since ozone is a greenhouse gas (IPCC, 2007), damages plants and crops (Fiscus et al., 2005), and is a lung irritant (EPA, 2011), the formation and fate of organic nitrates have implications for both climate and environmental health.

With respect to SOA production, among the most important BVOC-derived organic nitrates are products of α-pinene oxidation, due to their relatively low volatility and the high annual global emission rate of α-pinene (∼ 66 Tg yr-1; Guenther et al., 2012). α-Pinene-derived organic nitrates (APNs) can comprise a significant fraction of SOA mass (Xu et al., 2015; Rollins et al., 2010). At elevated relative humidity, when aerosol particles have increased liquid water content, organic nitrates can hydrolyze in the particle phase to eliminate the RONO2 functionality (Liu et al., 2012; Rindelaub et al., 2015; Boyd et al., 2015; Bean and Hildebrandt Ruiz, 2016), leaving the nitrate ion within the particle. However, the products, mechanisms, and kinetics of the APN hydrolysis reactions are still unknown, negatively impacting our understanding of aerosol phase chemistry and the fate of atmospheric NOx.

The conversion of the organic nitrate functionality to a nonvolatile, largely unreactive nitrate ion via a substitution or elimination mechanism would lead to an effective sink of atmospheric NOx and reduce the potential for NOx / O3 transport. Recent results from Romer et al. (2016) indicate that the lifetime of atmospheric boundary layer NOx could be as low as ∼ 2 h, using an assumed short hydrolysis lifetime (Romer et al., 2016). The hydrolysis mechanism could also potentially impact SOA formation and cloud condensation nuclei activity. Thus, the hydrolysis of organic nitrates, and the associated uncertainty, has a significant impact on our understanding of how BVOC–NOx interactions affect climate, air quality, and health.

Despite much study of organic nitrate hydrolysis, the rates and mechanisms at low pH, which is relevant to both ambient (e.g., Guo et al., 2015) and laboratory conditions (Rindelaub et al., 2016), are still very uncertain. While SN2 mechanisms are believed to be more prevalent at high pH (Baker and Easty, 1950; Boschan et al., 1955), recent studies suggest that unimolecular mechanisms are responsible for the fate of organic nitrates under aqueous acidic conditions due to the polar protic solvent system, water's weak nucleophilicity, and the relatively large observed hydrolysis rates for tertiary organic nitrates (Rindelaub et al., 2015). While the reaction is likely acid-catalyzed, the catalysis mechanism is uncertain since both specific and general catalyzed mechanisms have been proposed (Darer et al., 2011; Jacobs et al., 2014).

To better understand the organic nitrate hydrolysis mechanism and kinetics under acidic conditions and the corresponding impact on atmospheric processes, hydrolysis reactions were performed focusing on the fate of APNs. The hydrolysis rate constants, specific mechanism, and main product were determined for a laboratory-synthesized APN. The hydrolysis of simple alkyl nitrates, isopropyl nitrate (IPN) and isobutyl nitrate (IBN), were also studied to gain insight into the mechanisms of primary and secondary substituted species and to enable computational chemistry studies of the mechanism and energetics. The results from this study help improve our understanding of organic nitrate chemistry, the fate of atmospherically relevant organic nitrates relating to climate and health, and can help explain important mechanisms that impact aerosol phase chemistry.

In all experiments, the addition of an organic nitrate standard to aqueous solution resulted in hydrolysis of the organic nitrate functionality, with first order loss rates that increased with solution acidity (Figs. 2, 3). For the tertiary APN, hydrolysis rate constants ranged from 3.2 × 10-5 s-1 at neutral pH (6.9) to 2.0 × 10-3 s-1 at low pH (0.25). The hydrolysis rate constants for the secondary isopropyl nitrate and the primary isobutyl nitrate displayed nearly identical kinetics, and had rate constants smaller by more than 2 orders of magnitude relative to the APN, ranging from 1.23 × 10-7 s-1 at neutral pH (6.9) to 1.1 × 10-5 s-1 at low pH (0.25), when data from both experiments were averaged together.

The corresponding hydrolysis lifetimes for the organic nitrates studied are shown in Table 2. APN had a condensed phase hydrolysis lifetime of 8.3 min at low pH, and a lifetime of 8.8 h at neutral pH. Both of these hydrolysis lifetimes are much shorter than the lifetime of a typical atmospheric aerosol particle. The average hydrolysis lifetimes of isopropyl nitrate and isobutyl nitrate were much larger than those for APN, ranging from approximately 1 day at low pH to greater than 8 months at neutral conditions.

The pH dependence of the observed rate constants indicates that the hydrolysis of organic nitrates at low pH is a specific acid-catalyzed mechanism. In specific acid-catalyzed mechanisms, the transfer of the H+ ion from the acid to the reactant is reversible and occurs before the rate determining step, consistent with a unimolecular mechanism. The observed specific acid-catalyzed mechanism is in contrast to Jacobs et al. (2014), who report a general acid-catalyzed mechanism for the hydrolysis of β-hydroxy organic nitrates. It is important to note, however, that Jacobs et al. (2014) did not observe an increase in hydrolysis rates with increasing buffer concentration, a result that is a defining characteristic of specific acid catalysis.

Previous studies indicate that organic nitrate hydrolysis rates increase with alkyl substitution (Darer et al., 2011; Hu et al., 2011). However, in this study, essentially identical kinetics were observed for the primary isobutyl nitrate and secondary isopropyl nitrate. This similarity can be explained through inspection of the unimolecular hydrolysis mechanism. A proposed reaction mechanism for the acid-catalyzed hydrolysis of IBN is shown in Fig. 4, where rearrangement to a relatively stable tert-butyl carbocation drives the rate of the reaction. A similar observation concerning the relatively large hydrolysis rate constant of a primary organic nitrate was recently reported by Jacobs et al. (2014), who concluded that the resonance stabilization of a primary carbocation increased the rate of a nucleophilic substitution reaction. Since carbocation stability drives the rate of the unimolecular hydrolysis reaction, and rearrangements were not possible for the structures studied in Darer et al. (2011) and Hu et al. (2011), the observed hydrolysis rates in the previous studies followed a trend based on the degree of alkyl substitution, as opposed to this work and Jacobs et al. (2014).

The hydrolysis lifetimes measured for the APN are similar to those previously reported for tertiary hydroxyl nitrates. While not as short as the 0.019 h reported by Hu et al. (2011), the measured hydrolysis lifetimes of the APN are consistent with the data presented in Fig. 3 of Darer et al. (2011), which we estimate to display a lifetime of approximately 1 h at pH = 0.8. However, a true comparison of the measured hydrolysis rates is not possible, as solution pH values were not reported in either Darer et al. (2011) or Hu et al. (2011). Nonetheless, one possible explanation for the shorter lifetimes observed in previous work compared to this study may be related to the highly acidic hydrolysis conditions used in previous experiments, which employed the use of strong acids (i.e., acids with negative pKa values) ranging up to 2 M (Darer et al., 2011; Hu et al., 2011). If the organic nitrates studied in both Darer et al. (2011) and Hu et al. (2011) also proceeded via specific acid catalysis, a very low pH environment would likely have led to larger hydrolysis rates than those observed in this study.

To further support the unimolecular reaction mechanism of organic nitrate hydrolysis, theoretical enthalpy and free energy profiles of the proposed isobutyl nitrate reaction mechanism are presented in Fig. 5a and b, respectively. Based on extensive benchmarks, thermochemical calculations in the gas phase at the ωB97X-V/aug-cc-pVTZ level of theory are accurate up to ∼ 3.6 kcal mol-1 (Mardirossian and Head-Gordon, 2014). According to recent literature, the ωB97X-V/aug-cc-pVTZ level of theory offers an excellent balance between computational cost and accuracy (Chan and Radom, 2011, 2012). Therefore, the main source of potential inaccuracies in our calculations is the use of a PCM implicit solvent, which is known to provide a less rigorous description of charged species (Takano and Houk, 2005). However, the uncertainty due to using PCM is not quantifiable without calculations involving an explicit solvent model, the pursuit of which is beyond the scope of the present study.

According to the DFT calculations, the isobutyl ion (IB+) corresponds to a saddle point of the energy profile, thus, it is considered a metastable reaction intermediate rather than a stable species. Due to the instability of the primary isobutyl carbocation (IB+), it is likely that a 1,2-hydride shift occurs in concert with bond cleavage of the nitrate group to create the tertiary carbocation intermediate (TB+). In addition, geometry optimizations and frequency calculations indicated that protonation of isobutyl nitrate (IBN) occurs on the terminal oxygen of the nitrate rather than the oxygen of the nitrate ester, as shown in Fig. 4, because the latter produces a metastable species.

Comparing the enthalpy and free energy results, it is observed that both the zero-point vibrational energy and entropic contributions play important roles in determining the most probable products.

Without those contributions, a barrier to reach the isobutyl ion is significantly higher and the overall reaction would be much slower. The entropic contribution also impacts the probability of producing isobutylene via an elimination mechanism, among other products. In any case, computations suggest that the energetically favored product is the nucleophilic substitution product, tert-butyl alcohol (TBA). The difference in calculated free energy of the TB+ intermediate and the final products is likely within the uncertainty of using an implicit solvation model for charged species.

The much larger observed hydrolysis rate constants for APN, as well as that for a secondary β-hydroxynitrate from the Jacobs et al. (2014) study, compared to the IPN/IBN systems is related to carbocation stability in the unimolecular mechanism. Due to its relative size, the α-pinene-derived carbocation will have greater charge stabilization. Further reaction of the tertiary carbocation intermediate readily occurs, as indicated from the identified product pinol using GC–MS (see Fig. 6), further confirming the unimolecular nature of the organic nitrate hydrolysis under acidic conditions. While theoretical calculations were not conducted for this system, the experimental data and supporting theoretical calculations of IBN hydrolysis indicate that the unimolecular mechanism is favored for organic nitrates in acidic environments. Once the α-pinene-derived tertiary carbocation is formed, the reaction can either proceed via unimolecular substitution (SN1) or an elimination (E1) mechanism (Fig. 6; Bleier and Elrod, 2013). In this study, the reaction proceeded via an SN1 pathway to form pinol by intramolecular attack from the secondary hydroxyl group to create a protonated pinol compound. The abstraction of the proton by water completed the acid-catalyzed reaction to create the final pinol product and H3O+.

Pinol has also been observed as a product of α-pinene oxide hydrolysis (Bleier and Elrod, 2013), which is expected to proceed via the same tertiary carbocation intermediate as the APN hydrolysis. In contrast to this study, Bleier and Elrod (2013) also observed the formation of sobrerol and carveol, the alternative SN1 and E1 products, respectively, derived from the tertiary carbocation. While it is peculiar that neither sobrerol nor carveol were detected in this study, the differentiation can at least partially be explained by the greater thermodynamic stability of pinol in comparison to both sobrerol and carveol (Bleier and Elrod, 2013).

As discussed above, particle phase and cloud water hydrolysis is an important reaction concerning the atmospheric fate of organic nitrates. The conversion of the RONO2 functional group within the aerosol phase to the nitrate ion, which has negligible vapor pressure and will exist within particles depending on pH, will reduce the potential of further reactions re-releasing gas phase NOx to the atmosphere, such as through organic nitrate photolysis and/or oxidation.

The reported hydrolysis lifetimes for atmospherically relevant hydroxyl nitrates are generally shorter than those reported for oxidation reactions, which range from 2.5–28 h for gas phase OH radical oxidation and from 0.7–28 h for gas phase O3 oxidation (Lee et al., 2014). Gas phase photolysis lifetimes have been reported from 0.50–1.0 h for carbonyl nitrates (Müller et al., 2014; Xiong et al., 2016) to ∼ 107 h for monofunctional organic nitrates (Higgins et al., 2014). In comparison, previously reported tertiary organic nitrate hydrolysis lifetimes have been measured as short as 0.019 h (Hu et al., 2011) and as short as 0.14 h under acidic conditions in this study. This indicates that the partitioning of organic nitrates to the particle phase is likely a significant sink for atmospheric NOx that may critically diminish the potential for long-range transport of NOx / O3 in the form of organic nitrates. It is important to note that organic nitrates formed from other oxidation processes, such as nitrate radical addition, may produce a larger degree of primary and secondary species that may be more resistant to hydrolysis than the tertiary organic nitrates formed during photooxidation of substituted alkenes (Nah et al., 2016).

To our knowledge, there are currently no reported photolysis or oxidation rate constant measurements for α-pinene-derived organic nitrates. However, using the EPI Suite available at the Environmental Protection Agency website (http://www.epa.gov/opptintr/exposure/pubs/episuite.htm), gas phase atmospheric lifetimes of APN were calculated to be 1.3 and 0.64 h for OH radical and O3-induced oxidation, respectively. Due to the lack of carbonyl functionality, it is expected that the APN photolysis rate is negligibly small. With recently reported aerosol pH value estimates ranging from pH 0.5 to 3.0 in the southeastern United States (Guo et al., 2015), the corresponding ambient hydrolysis lifetimes of the APN would be on the order of a half hour, which would indicate that particle phase hydrolysis is the principal atmospheric sink for α-pinene-derived organic nitrate compounds, in comparison to removal via photolysis and oxidation pathways. Hydrolysis in chamber experiments may be even faster as aerosol pH has recently been directly measured as low as pH -0.68 for laboratory-generated particles (Rindelaub et al., 2016).

Due to the likely large degree of aerosol phase hydrolysis, current ambient measurements of particle phase organic nitrate concentrations may be underestimating the atmospheric production of organic nitrates. Indeed, this chemistry can represent a dominant fate of NOx in forested boundary layers and, at low aerosol pH, protonation of the resultant NO3- can represent a dominant source of atmospheric HNO3 (Romer et al., 2016).

The formation of a relatively high vapor pressure product, pinol, from α-pinene-derived nitrate hydrolysis may lead to a reduction in aerosol mass by the partitioning of products back into the gas phase, lowering particle mass concentrations. For instance, the calculated vapor pressure of pinol is estimated to be 3 orders of magnitude greater than the original organic nitrate, based on calculations using the EPI Suite (see above). It is important to note that APN hydrolysis products can have olefinic functionality, such as the case with pinol, and may react further in the particle phase, especially under acidic conditions where sulfonation and/or oligomerization can occur. Photo-induced chemistry occurring to aerosol phase products may result in oxidation at the double bond (Bateman et al., 2011). Pinol has also been identified as a hydrolysis product of another α-pinene oxidation product, α-pinene oxide (Bleier and Elrod, 2013), thus, pinol may be an important tracer for the hydrolysis of α-pinene-derived species. Both the gas and particle phase fate of pinol warrant further study.

The identification of organic nitrate hydrolysis products is important not only to our understanding of the atmosphere but also to our chemical understanding of the organic nitrate hydrolysis mechanism. Research regarding RONO2 hydrolysis under acidic conditions has been limited and suggests that nucleophilic substitution is the dominant reaction pathway. This study shows that through the unimolecular mechanism, elimination and intramolecular rearrangement/attack are also likely reactive pathways that should be considered when identifying aerosol phase chemical processes and potential tracers of atmospherically relevant compounds.

A specific acid-catalyzed hydrolysis mechanism was determined for a tertiary α-pinene-derived organic nitrate, which has implications for atmospheric air quality and climate. This finding, along with supporting theoretical calculations of the isobutyl nitrate hydrolysis mechanism, helps broaden our chemical understanding of the hydrolysis mechanism of organic nitrates. The hydrolysis rates observed for the organic nitrates studied increased with solution acidity, and the large rates observed for the α-pinene-derived organic nitrate further emphasizes the likelihood of particle phase hydrolysis being a sink for organic nitrates and, transitively, atmospheric NOx. It also highlights the importance of ambient aerosol pH measurements. The hydrolysis of organic nitrates within the particle phase will lead to a decreased effective lifetime for NOx and, thus, decreased ozone transport. However, the observed organic hydrolysis product, pinol, is relatively volatile and may partition back to the gas phase, decreasing organic aerosol mass. In addition to the investigation of α-pinene-derived organic nitrate photolysis and oxidation, future work is needed to assess how the loss of particle phase organic nitrates impacts aerosol hygroscopicity.