ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-15425-2016The acid-catalyzed hydrolysis of an α-pinene-derived organic
nitrate: kinetics, products, reaction mechanisms, and
atmospheric impactRindelaubJoel D.jrindela@purdue.eduBorcaCarlos H.HostetlerMatthew A.SladeJonathan H.LiptonMark A.SlipchenkoLyudmila V.ShepsonPaul B.pshepson@purdue.eduDepartment of Chemistry, Purdue University, West Lafayette, IN 47907, USAJoel D. Rindelaub (jrindela@purdue.edu) and Paul B. Shepson
(pshepson@purdue.edu)13December20161623154251543211August201612August201610November201617November2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/15425/2016/acp-16-15425-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/15425/2016/acp-16-15425-2016.pdf
The production of atmospheric organic nitrates (RONO2) has a large
impact on air quality and climate due to their contribution to secondary
organic aerosol and influence on tropospheric ozone concentrations. Since
organic nitrates control the fate of gas phase NOx (NO + NO2),
a byproduct of anthropogenic combustion processes, their atmospheric
production and reactivity is of great interest. While the atmospheric
reactivity of many relevant organic nitrates is still uncertain, one
significant reactive pathway, condensed phase hydrolysis, has recently been
identified as a potential sink for organic nitrate species. The partitioning
of gas phase organic nitrates to aerosol particles and subsequent hydrolysis
likely removes the oxidized nitrogen from further atmospheric processing, due
to large organic nitrate uptake to aerosols and proposed hydrolysis
lifetimes, which may impact long-range transport of NOx, a tropospheric
ozone precursor. Despite the atmospheric importance, the hydrolysis rates and
reaction mechanisms for atmospherically derived organic nitrates are almost
completely unknown, including those derived from α-pinene, a biogenic
volatile organic compound (BVOC) that is one of the most significant
precursors to biogenic secondary organic aerosol (BSOA). To better understand
the chemistry that governs the fate of particle phase organic nitrates, the
hydrolysis mechanism and rate constants were elucidated for several organic
nitrates, including an α-pinene-derived organic nitrate (APN). A
positive trend in hydrolysis rate constants was observed with increasing
solution acidity for all organic nitrates studied, with the tertiary APN
lifetime ranging from 8.3 min at acidic pH (0.25) to 8.8 h at neutral pH
(6.9). Since ambient fine aerosol pH values are observed to be acidic, the
reported lifetimes, which are much shorter than that of atmospheric fine
aerosol, provide important insight into the fate of particle phase organic
nitrates. Along with rate constant data, product identification confirms that
a unimolecular specific acid-catalyzed mechanism is responsible for organic
nitrate hydrolysis under acidic conditions. The free energies and enthalpies
of the isobutyl nitrate hydrolysis intermediates and products were calculated
using a hybrid density functional (ωB97X-V) to support the proposed
mechanisms. These findings provide valuable information regarding the organic nitrate hydrolysis mechanism and its
contribution to the fate of atmospheric NOx, aerosol phase processing,
and BSOA composition.
Introduction
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.
The formation of atmospheric organic nitrates and ozone from the gas
phase oxidation of α-pinene, initiated by the OH radical.
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.
ExperimentMaterials and methods
Organic nitrate hydrolysis reactions were studied for isopropyl nitrate
(Sigma Aldrich, > 99 %), isobutyl nitrate (Sigma Aldrich,
> 97 %), and a hydroxyl organic nitrate derived from α-pinene by injecting 10 µL of a given standard into a 100 mL
buffered solution that was continuously mixed. The α-pinene-derived
nitrate, shown in Fig. 1, was synthesized based on Pinto et al. (2007).
Briefly, α-pinene oxide was added to a 1.0 M solution of
Bi(NO3)3⚫ 5H2O in dichloromethane (DCM), and stirred
for 1 h under N2 before purification using flash chromatography with a
20 % ethyl acetate in hexane solvent system. Product identification and
purity were assessed using 1H and 13C nuclear magnetic resonance
spectroscopy (NMR) (see Sect. S1 in the Supplement). Aliquots of 5 mL were
taken at varying time points from the reaction mixture and extracted with
5 mL of tetrachloroethylene (C2Cl4) before analysis using Fourier
transform infrared spectroscopy (FTIR) for organic nitrate quantification.
Gas chromatograph–mass spectrometer (GC–MS) data, which was used for
product identification (see Sect. S2), was only available for data collected
at the reaction endpoints. The FTIR analysis was achieved by integrating the
∼ 1640 cm-1 asymmetric-NO2 stretch unique to organic
nitrates (Nielsen et al., 1995). The reaction solutions used were buffered at
10 mM with either a sulfate, acetate, or phosphate buffer system. Hydrolysis
reactions were studied at pH values 0.25, 1.0, 1.7, 2.5, 4.0, and 6.9.
The thermochemical calculations of a set of reactants, intermediates, and
products involved in the proposed reaction pathways of isobutyl nitrate were
explored using density functional theory (DFT; Hohenberg and Kohn, 1964; Kohn
and Sham, 1965). The set included water (H2O), hydronium ion
(H3O+), nitric acid (HNO3), isobutyl nitrate (IBN), protonated
isobutyl nitrate (IBHN+), isobutyl ion (IB+), tert-butyl
ion (TB+), isobutylene (2MP), tert-butyl alcohol (TBA), and
isobutyl alcohol (IBA); see Table 1. The reactions are assumed to run in an
acidic environment, such that the hydronium ion is prevalent. First, a
systematic, torsional conformational search was performed on the structure of
each molecule of the set, excepting water and hydronium ions. This procedure
was performed in HyperChem (Hyperchem™
Professional 7.51, Hypercube, Inc.)
with the optimized potentials for liquid simulations (OPLSs) force field
(Jorgensen and Tirado-Rives, 1988; Pranata et al., 1991). A maximum of eight
simultaneous variations were allowed, with angles changing every step by a
maximum range of 180∘ at intervals of 15∘. Similar structures
were filtered, with an acceptance criterion set to 5 kcal mol-1 above
the lowest energy conformer. All the following calculations were carried out
using the computational chemistry package Q-Chem 4.3 (Shao et al., 2015).
Second, the lowest energy conformer was optimized employing the long-range
corrected hybrid density functional ωB97X-V (Mardirossian and
Head-Gordon, 2014), with the aug-cc-pVTZ basis set (Kendall et al., 1992),
and polarizable continuum model (PCM) of implicit aqueous solvent (Truong and
Stefanovich, 1995; Barone and Cossie, 1998; Cossi et al., 2003). A
high-accuracy grid was employed, as well as extremely tight convergence
criteria. Third, frequency calculations were executed on the optimized
structures to verify the convergence of the geometry optimizations, and also
to determine if the molecule was a stable species or a reaction intermediate.
These were run using the same setup described above, plus the inclusion of a
thermochemical analysis upon completion of frequency calculations.
Results
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 hydrolysis rate constants (s-1) for isopropyl nitrate (IPN;
red) and isobutyl nitrate (IBN; blue) as a function of solution pH.
The hydrolysis rate constants (s-1) for the α-pinene-derived nitrate as a function of solution pH. The error bars
correspond to 1 standard deviation of replicate measurements.
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 hydrolysis lifetimes of isopropyl nitrate (IPN), isobutyl
nitrate (IBN), and the α-pinene-derived nitrate (APN) at varying pH.
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 proposed unimolecular mechanism for isobutyl nitrate (IBN)
demonstrating the specific acid catalysis and 1,2-hydride shift
rearrangement.
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.
The calculated relative free energies of the intermediates and
products of the proposed acid-catalyzed isobutyl nitrate hydrolysis
mechanism. See Table 1 for calculation stoichiometry. The reaction is not
likely to proceed through the IB+ intermediate due to the instability of
the carbocation.
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+.
The proposed acid-catalyzed hydrolysis mechanism of an α-pinene-derived nitrate.
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).
Discussion
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.
Conclusions
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.
The Supplement related to this article is available online at doi:10.5194/acp-16-15425-2016-supplement.
Acknowledgements
This research was supported in part through computational resources provided
by Information Technology at Purdue University. Paul B. Shepson and
Lyudmila V. Slipchenko acknowledge support from the National Science
Foundation (grants AGS-1228496 and CHE-1465154, respectively). Edited by: M. Ammann Reviewed by: two
anonymous referees
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