Experimental and Theoretical Investigation of the Reaction NO + OH +

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Article

Theoretical and Experimental Investigation of the Reaction NO + OH + O # HO + NO 2

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Christa Fittschen, Emmanuel Assaf, and Luc Vereecken J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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The Journal of Physical Chemistry

Theoretical and Experimental Investigation of the Reaction NO + OH + O2 → HO2 + NO2

Christa Fittschen1*, Emmanuel Assaf1, Luc Vereecken2*

1

Université Lille, CNRS, UMR 8522 - PC2A - Physicochimie des Processus de Combustion et

de l’Atmosphère, F-59000 Lille, France 2

Institut für Energie und Klimaforschung, Forschungszentrum Jülich GmbH, 52428 Jülich,

Germany Corresponding authors : [email protected]; [email protected]

Revised Version April 19, 2017

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ABSTRACT The possible formation of HO2 and NO2 as products from the reaction of OH and NO under atmospheric conditions, in competition to the formation of HONO, has been investigated experimentally and theoretically. Experiments have been carried out by directly measuring the formation of HO2 radicals using laser photolysis coupled to cw-CRDS. OH radicals have been generated from the reaction of F-atoms with H2O, and absolute HO2 and OH profiles have been recorded at different NO concentrations. The potential energy surface has been calculated and the rate constant has been obtained from RRKM master equation modeling. Both experiment and theory, show that the HO + NO reaction in the presence of O2 bath gas is not a competitive source of HO2 + NO2.


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INTRODUCTION Photolysis of NO2 is the only known pathway of O3 formation in the troposphere, whereby the major source of NO2 is the reaction of HO2 and RO2 radicals with NO. Recent field studies1-3 on the rate of formation of O3 (P(O3)) in polluted environments suggest that the measured P(O3) is higher than the one obtained from modeling including the known NOx chemistry. One of the hypotheses developed by the authors to explain this disagreement1,2 is that the reaction of OH with NO in the presence of O2 could lead to formation of HO2 and NO2, OH + NO + O2 → HO2 + NO2

(R1a)

in competition to the currently accepted sole formation of HONO OH + NO (+M) → HONO (+M)

(R1b)

However, in order to bring into agreement measurement and model, the reaction would need to proceed with an effective bimolecular reaction rate of k1a = (3–15) ×10-11 cm3s-1. The formation of HO2 and NO2 from the reaction of vibrationally excited HONO with O2 has already been mentioned by Staikova et al.4, but has never been investigated experimentally. The reaction is difficult to measure, because the expected reaction product HO2 would be concerted back to OH by fast reaction with NO (R2) (see Table 2) and only low steady state concentrations of HO2 can be expected, depending on [NO] and the ratio of k1a/k1b. Therefore, a selective and sensitive detection of HO2 is needed for investigation of this reaction. The current paper describes a combined experimental and theoretical study to elucidate this reaction path.



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METHODOLOGY Experimental Section The experimental technique has been described earlier5-7. It consists of two continuouswave cavity ring-down spectroscopy (cw-CRDS) absorption paths and a high repetition rate laser induced fluorescence (LIF) techniques coupled simultaneously to a laser photolysis cell. cwCRDS has been used to measure absolute concentration-time profiles of OH and HO2 radicals. OH profiles8 have been obtained at 7028.831 cm-1 while HO2 radicals9 have been quantified at 6638.205 cm-1. Ring-down events have been acquired over 50 to 100 photolysis pulses. The ringdown time τ is then converted to absorbance α using the following equation:

with τ0 and τt the ring-down times in the presence and absence of the absorbing species (respectively before and after the photolysis shot), c the speed of the light, RL the ratio between the cavity length and the absorption path length (respectively 79 cm and the overlap between the photolysis laser beam and the near-IR laser beam 37.7 cm), σ the absorption cross section of the absorbing species. Relative OH profiles were also probed in the (1-0) vibrational band of the A-X electronic transition at around 282 nm using the doubled output of a dye laser (SIRAH) pumped at 532 nm using the doubled output of a Nd:YAG laser (Spectra Physics). The pulse repetition frequency of the dye laser was 10 kHz. The fluorescence is collected perpendicular to the photolysis beam at 310 20 nm after passing through an interference filter and was detected using a photomultiplier


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(Hamamatsu R212). A boxcar integrator (EG&G Model 4121B) integrates the signal from the photomultiplier. A typical signal is composed of 10000 data points (over 1s) separated by 100 µs (i.e. 5000 points before the photolysis pulse to measure any background signal and 5000 points after). Each point is averaged over typically 100 photolysis laser pulses. The signal from the boxcar is then digitized to a computer for data acquisition. OH profiles from LIF and cw-CRDS were found to be identical within the experimental uncertainty. OH radicals were generated through the 248 nm photolysis of XeF2 in the presence of H2O: XeF2 + h

248nm

→ XeF + F → 2 F + Xe

F + H2O → HF + OH

(R1) (R2)

XeF2 has already been used in a previous study10,11 as a clean source of photolytic F-atoms and its handling has been described in detail. Briefly, around 80 mg of XeF2 crystals are introduced into a homemade Teflon bag which is subsequently filled with around 50 L of helium through a calibrated flowmeter. Rapid sublimation of XeF2 leads to a homogeneous mixture at atmospheric pressure. The flow from the bag into the photolysis cell is controlled by a Teflon needle valve, the flow rate is determined by measuring the pressure increase into a known volume. All experiments were carried out at 298 K and at a total pressure of 50 Torr, the photolysis repetition rate was always 0.33 Hz. Two series of experiments have been carried out with total flow rates of 530-550 and 840 cm3 min-1, the difference being due to different flows of XeF2. Typical concentrations of XeF2 were 0.81-0.93 and 1.8×1014 molecules cm-3 for the two flow rate, leading with a photolysis energy of around 100 mJ cm-2 to initial F atom concentrations of around 2-5×1012 molecules cm-3. The H2O concentration was estimated to be 5×1014 molecules cm-3, leading to initial OH5 ACS Paragon Plus Environment


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concentrations of around 2-5×1012 molecules cm-3 in less than 1 ms. Helium, O2 and NO (diluted at 2% in N2) (all Alphagaz) were introduced to the photolysis cell through calibrated flow meters. Two series of measurements were performed for each XeF2 flow: one with varying the NO concentration at a fixed concentration of O2 and one with varying the O2 concentration at a fixed concentration of NO. The NO concentration such as given in Table 1 is the concentration calculated from flow rates and the concentration of the premixed NO gaz. It might slightly be overestimated due to conversion of some NO into NO2 through the termolecular reaction NO + NO + O2 (k = 3.95 x 10-38 cm6 molecule-2 s-1)12 before reaching the reaction volume. However, this reaction is very slow under our conditions. Also, the NO flow has been mixed into the main flow, containing the O2, only just before entering the photolysis cell, thus minimizing their reaction time. Also, the presence of small amounts of NO2 next to NO should not have a strong influence on the results, because its rate constant with OH radicals is similar to the reaction with NO and therefore only a small fraction of OH radicals would react with NO2. It would have an even smaller impact on the HO2 profile, because the rate constant for the reaction of HO2 radicals with NO2 is roughly 50 times slower than the rate constant for the reaction of HO2 with NO. The concentrations of other nitrogen species possibly present in the reaction cell such as N2O3 (from a reaction NO + NO2) or N2O4 will be very low and have not been taken into account in the reaction mechanism. Total flow rates have been kept constant for each series by varying the additional Helium flow. The experimental conditions are summarized in Table 1.


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Table 1: Experimental conditions for all experiments Flow rate

XeF2 /

He /

O2 /

NO /

k’OH / s-1

/ cm3 min-1 1013 cm-3 1017 cm-3 1017 cm-3 1013 cm-3 LIF/CRDS 550

9.3

11.3

4.4

0

38/43

550

9.3

11.3

4.4

4.7

100/102

550

9.3

11.3

4.4

8.4

169/166

530

9.3

11.3

4.4

2.8

68/74

530

8.1

15.7

0.004

4.9

63/80

530

8.1

11.1

4.5

0

45/50

530

8.1

11.1

4.5

4.9

76/72

840

18

7.9

8.3

3.1

77/76

840

18

7.9

8.3

1.9

69/61

840

18

7.9

8.3

0

61/53

840

18

13.3

2.9

3.1

86/64

840

18

16.2

0.003

3.1

45/80

Computational Details The potential energy surface of the HO+NO+O2 reaction was characterized at the M06-2X/augcc-pVTZ level of theory13,14. The relative energies were then improved by single-point calculations at the CCSD(T)/aug-cc-pVxZ (x=D,T,Q) level of theory, allowing extrapolation to the complete basis set limit CCSD(T)/CBS(DTQ) using the aug-Schwartz6 extrapolation scheme introduced by Martin15. The initial OH+NO recombination forming HONO is a barrierless reaction; its rovibrational properties and energy profile along the reaction coordinate were characterized by constrained optimization with O−N bond lengths between 2.0 and 4.4 Å. The H-abstraction reaction TS from HONO by O2 was checked by intrinsic reaction coordinate (IRC)



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calculations for both HONO isomers. All quantum chemical calculations were performed using Gaussian-0916. Based on the rovibrational and energetic properties obtained above, the H-abstraction by O2 molecules from chemically activated HONO molecules was studied by RRKM master equation analysis. The intermediates are described within the rigid rotor harmonic oscillator paradigm, after scaling the wavenumbers by 0.97117,18. The initial association TS of OH + NO was implemented using a single geometry along the reaction path, i.e. no variational or microvariational optimization of the reaction rate was performed for this barrierless channel, though its selection was based on the available constrained optimizations along the pathway to find the best bottleneck. Both HONO intermediates (cis- and trans-HONO) were implemented explicitly in the master equation, connected by the internal-rotation TS obtained from quantum chemical calculations. The energy-specific rate coefficient kabstr(E) for bimolecular H-abstraction from these HONO adducts by O2 was modeled using the methodologies described in Maranzana et al.19 and Green and Robertson20, which assume a canonical energy distribution for the O2 coreactant and the relative motion of the two reactants, but allows incorporating a specific internal energy E for the HONO reactant. This methodology was enhanced by asymmetric Eckart tunneling following Miller21. Energy loss by collisions was modeled using an exponential down model (∆Edown ≈ 200 cm-1 near the nascent HONO energies)22 based on Lennard-Jones collision numbers ( ε(HONO) = 250 K, σ(HONO) = 4.0 Å) using air at 1 atm as a bath gas ([O2] = 20%). All state density calculations and master equation trials were performed using the Multiwell2016 program suite23-25.


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RESULTS AND DISCUSSION Experiments F atoms, generated through the photolysis of XeF2 in presence of H2O8, are converted into OH radicals within less than 1 ms under our experimental conditions. The concentration-time profiles of OH and HO2 radicals have then simultaneously been measured by time-resolved continuous wave Cavity Ring Down Spectroscopy (cw-CRDS) in the presence of varying concentrations of NO or O2. Details of the experimental conditions are summarized in Table 1.

2.5×10 1 2 [NO] = 2.8×10

2.0×10 1 2

13

3.0×10 1 0

5.0×10 1 1

2.0×10 1 0

0

k1a = 1×10-11 cm3s-1

k1a = 1×10-11 cm3s-1

10 1 2

1.0×

0.00

0.01

0.02

0.03

t/s

1.0×10 1 0

5.0×10 1 1 0 0.00

1.0×10 1 2

cm

[NO] = 8.4×1013 cm-3 1.5×10 1 2

4.0×10 1 0 -3

[HO2] / cm -3

[OH] / cm -3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.01

0.02

0.03

0 0.00

t/s

0.01

0.02

t/s

Figure 1: OH (left graph) and HO2 (right graph) for two different NO concentrations at [He] = 11.3×1017 cm-3, [O2] = 4.4×1017 cm-3. For all models: k1b=1.4×10-12. Full lines: k1a=1.2×10-13; dashed lines k1a=2.4×10-13; dashed-dotted lines for k1a=0 (in cm3s-1). Inset of HO2 graph: model with k1a = 1×10-11 cm3s-1 for experimental conditions of blue dots, dashed black line in OH profile.

Initially, the OH decays (see left graph, Figure 1) obtained by cw-CRDS or LIF have been fitted to a mono-exponential decay, and the decay rates (see Table 1) have been plotted as a function of the NO concentration, shown in Figure 2 for all experiments: while the values are a bit scattered, a good agreement can be seen between LIF (green dots in Figure 2) and CRDS (blue 9 ACS Paragon Plus Environment

0.03


dots in Figure 2). A linear regression leads to a rate constant for the loss of OH radicals with NO of k = (1.1±0.3)×10-12 cm3s-1, in good agreement with the recommended12 rate constant for the formation of HONO in the reaction of OH with NO, k1b = 1.17×10-12 cm3s-1 (rate constant calculated for 50 Torr N2: this is an approximation, because the bath gas in our experiment is Helium, but high O2 concentrations as well as the probably efficient quencher XeF2 are present).

200

150

k'OH / s-1

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kOH by cw-CRDS kOH by LIF

100

50

0

0

2.0×10 1 3

4.0×10 1 3 6.0×10 1 3 [NO] / cm-3

8.0×10 1 3

1.0×10

Figure 2: Plot of OH decay rates from all experiments, blue dots from fit to OH decays obtained by cw-CRDS, green dots by LIF

A small HO2 signal was always seen directly following the photolysis pulse (right graph of Figure 1), corresponding to ≈1% of the initial F atoms being converted into HO2, possibly through the reaction of F-atoms with impurities such as CH2O (many experiments of Cl + CH3OH have been carried out in the photolysis cell before these experiments, therefore it is very plausible that some CH2O slowly desorbs from the reactor walls) present in the photolysis cell.


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Typical experiments with 2 different NO concentrations (blue trace: 2.8×1013cm-3, red trace: 8.4×1013cm-3) are presented in Figure 1: the OH concentration time profiles in the left graph, the corresponding, simultaneously measured HO2 profiles in the right graph. The traces have been modeled by a small mechanism summarized in Table 2, containing 10 reactions. The results from three runs are shown in Figure 1 using the rate constants from Table 2, with only the yield of HO2 formation in (R1), i.e. k1a, being varied. The OH decays are barely influenced by the change, and thus the three traces are not distinguishable in Figure 1: the decay is mostly governed by the overall rate constant k1 and not by the ratio of k1a/k1b. The HO2 traces however are very sensitive to a change in the rate constant k1a: the best fit is obtained with an HO2 yield of 8% (full line). Increasing k1a by only a factor of 2 (dashed line) not only overestimates the HO2 concentration, but also changes the shape of the signal: the HO2 decay for the higher NO concentration is much faster than observed in the experiment. Removing entirely (R1a), i.e. considering HONO being the only product of reaction (R1), leads to a fast decay of the initial HO2 due to the fast consumption of the initial HO2 by reaction with NO. The modeled trace in the inset of the HO2 profile corresponds to k1a =1×10-11 cm3s-1, still well below of what would be needed to bring into agreement global atmospheric model and field experiments, even though the direct comparison is approximate due to the difference in pressure and [O2] partial pressure, i.e. collision frequencies between (R1a) and (R1b). It should be noted that the HO2 detection scheme is very selective and traces off the HO2 absorption line do not lead to any absorption signal.



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Table 2: Reaction mechanism used to simulate HO2 and OH profiles. Number

Reaction

k / cm3 s-1

Refs.

1a

OH + NO (+O2) → HO2 + NO2

1.2×10-13

This worka

1b

OH + NO (+M) → HONO (+M)

(1.1-1.7) ×10-12

This workb

2

HO2 + NO → NO2 + OH

8.66×10-12

6

3

OH + HO2 → H2O + O2

1.02×10-10

1

-12

4

OH + HONO → NO2 + H2O

6×10

6

5

OH + NO2 (+M) → HONO2 (+M)

2.65×10-12

6c

6

OH + H2O2 → HO2 + H2O

1.7×10-12

6

7a

2 OH → H2O + O

1.48×10-12

6

7b

2 OH (+M) → H2O2 (+M)

4×10-13

6c

8

2 HO2 → H2O2 + O2

1.64×10-12

6

9

OH → diffusion

20

10

HO2 → diffusion

7

a

No clear variation of the profiles with the O2 concentration has been observed and thus the rate constant was treated as second order b All experiments have been carried out at 50 Torr Helium / O2. Rate constant k1b has been varied in the given range to best fit the experimental traces and is in good agreement with the expected literature values (see text) c For simplicity, rate constants have been taken at 50 Torr N2 instead of a mixture of He, O2, XeF2: these reactions are very minor in the present system, and small variations do not have any impact on the OH and HO2 profiles

Figure 3 presents the OH and HO2 profiles from experiments using higher XeF2 flows, constant NO (3.1×1013 cm-3) and varying O2 concentrations (0.02 to 8×1017 cm-3). No changes are seen on the HO2 profiles which are rather noisy due to the low HO2 concentrations, but also due to short averaging times caused by limited availability of XeF2. The OH concentration seems to decay somewhat slower at the highest O2 concentration, in line with a slight increase in the HO2 yield in (R1a) with increasing O2 and subsequent increased recycling of OH through (R2). However, 12 ACS Paragon Plus Environment

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from the small number of experiments no dependence of the rate constant of (R1a) on the O2 concentration can be deduced.

5.0 ×10 1 0 6.0×10 1 2

[O2] = 2.7×1014 cm-3

4.0 ×10 1 0

[HO2] / cm -3

[OH] / cm -3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[O2] = 2.8×1017 cm-3 10 1 2

4.0×

[O2] = 8.3×10

17

-3

cm

3.0 ×10 1 0 2.0 ×10 1 0

10 1 2

2.0×

1.0 ×10 1 0 0 0.00

0.01

0.02

0.03

0.04

0.05

0 0.00

t/s

0.01

0.02

0.03

0.04

t/s

Figure 3: OH (left) and HO2 (right) concentration time profiles for three different O2 initial concentrations at 298 K. [XeF2] = 1.8×1014 cm-3, [NO] = 3.1×1013 cm-3

While it cannot be completely excluded that the observed HO2 signals originate from unknown reactions of impurities, it can be safely concluded that the yield of HO2 formation in the reaction of OH with NO in the presence of O2 is very low at the most and does not play any role in atmospheric chemistry.

Theory The potential energy surface for the OH + NO + O2 reaction system is shown in Figure 4, showing the barrierless association TSadd of OH and NO forming cis- and trans-HONO.


0.05


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Figure 4: Potential energy surface (PES) for the OH + NO reaction in the presence of O2, calculated at the CCSD(T)/CBS(DTQ)//M06-2X/aug-cc-pVTZ level of theory. Molecular models (left to right) show the barrierless association of OH + NO at 2.6 Å separation, TSabstr_cis, and TSabstr_trans.

At the nascent energies, these two conformers are in relative steady-state by fast internal rotation through TSiso. H-abstraction by O2 proceeds through two transition states TSabstr_cis and TSabstr_trans, which are -9.0 and -5.3 kcal mol-1, respectively, below the energy of the separated reactants HO + NO + O2. The pre-reactive complex energy well between HONO and O2 is very shallow, < 1 kcal mol-1, and plays no role of importance. The termolecular mechanism is reminiscent of other important atmospheric NO oxidation reactions, in particular the NO + NO + O2 reaction forming 2 NO2. Like the title reaction, this 14 ACS Paragon Plus Environment

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reaction proceeds through a short-lived intermediate species, ONOO26,27, formed from two coreactants NO + O2. ONOO then intercepted by the third reactant, NO forming several N2O4 isomers28-32 which ultimately decompose to 2 NO2. There are, however, some key differences. For the 2 NO + O2 reaction, the intermediate is formed from the excess reactant O2, and its equilibrium concentration is low but high enough to be intercepted by a low-concentration coreactant. For OH + NO + O2, in contrast, two low-concentration reactants combine to form a highly transient intermediate, i.e. energized HONO†, which survives only a limited number of collisions with the bath gas with a lifetime of ns, and as such can only be intercepted by coreactants that make up a high fraction of the bath gas itself, in casu O2. A minor mechanistic difference is also that the ONOO + NO reaction proceeds through low-barrier N2O4 adduct formation prior to 2 NO2 formation, whereas the HONO + O2 reaction involves a high-barrier direct H-abstraction leading directly to bimolecular products; while this has little influence on the termolecular capturing mechanism as such, it strongly affects the co-reactant population fraction that contributes effectively to product formation. The high-pressure rate coefficient for HONO + O2, where both reactants have thermal energy content, is very low, k(298K) ~ 10-40 cm3 molecule-1 s-1 at our chosen level of theory (including tunneling through an asymmetric Eckart barrier), due to the high endothermicity of 30 kcal mol-1 for this H-abstraction. The pseudo-first order energy-specific rate coefficient kabstr(E) at the HONO nascent energy (E ~48 kcal mol-1) for 0.2 bar O2 remains very low, of the order of 300 to 600 s-1, despite enhancement by tunneling. The main reason is the entropic disadvantage of the H-abstraction transition state, where the degrees of freedom for relative translation and rotation of the HONO and O2 moieties are converted into vibrational modes with a much smaller state density. While the abstraction TS are submerged relative to the free reactants, the limited excess 15 ACS Paragon Plus Environment


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energy afforded by this submergence remains limited to less than 10 kcal mol-1, insufficient to counter the entropic disadvantage by an energetic advantage. Even accounting for several orders of magnitude of uncertainty (e.g. multi-dimensional tunneling corrections, uncertainty on the relative energies), it is clear that these H-abstraction rates are not competitive against collisional energy loss (collision number ~ 3.4 × 10-10 cm3 molecule-1 s-1, i.e. a pseudo-first order rate coefficient ~ 1010 s-1), or at higher energies even against the redissociation of energized HONO† to OH + NO, which does not face a similar entropic disadvantage. For the obtained H-abstraction rates, there appear to be no reaction conditions where the OH + NO reaction in the presence of O2 can yield a sizable yield of HO2 + NO2. Changing the bath gas to 100% O2 can only increase the pseudo-first order rate coefficients for abstraction by a factor of 5. Lowering the pressure will slow down the collisional energy loss but will equally reduce the O2 co-reactant concentration, such that the ratio of collisional energy loss rate versus bimolecular H-abstraction is essentially unchanged. Even using the less efficient collider He as a bath gas appears insufficient to allow H-abstraction to compete against collisional stabilisation. Our theoretical analysis thus shows that the HO + NO reaction in the presence of O2 bath gas might be a small source of HO2 + NO2, but cannot play a major role in the chemistry of the atmosphere.

CONCLUSION The possible formation of HO2 + NO2 in the reaction of OH + NO in the presence of O2 has been investigated by experimental and theoretical methods. Experiments have been carried out by laser photolysis coupled to a selective and sensitive detection of HO2 by cw-CRDS. The reaction has been initiated by pulsed photolysis of XeF2 in the presence of H2O. Different concentrations of NO and O2 have been added, and time resolved OH and HO2 profiles have been measured. 16 ACS Paragon Plus Environment

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From these experiments it cannot be excluded that HO2 + NO2 are formed in a small yield next to the main reaction product HONO. The potential energy surface for the OH + NO reaction in the presence of O2 has been calculated at the CCSD(T)/CBS(DTQ)//M06-2X/aug-cc-pVTZ level of theory, the rate constant has been obtained from RRKM master equation modeling. From the obtained H-abstraction rates there appear to be no reaction conditions where the OH + NO reaction in the presence of O2 can yield a sizable yield of HO2 + NO2. Therefore, from the combined experimental and theoretical investigations it can be concluded that the reaction of OH with NO might be at the most a small source of HO2 + NO2, but cannot play a major role in the chemistry of the atmosphere.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Rovibrational and energetic data obtained from quantum chemical calculations (PDF)

AUTHOR INFORMATION Corresponding Authors Email: [email protected]; [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEGDEMENT This project was supported by the French ANR agency under contract No. ANR-11-LabEx0005-01 CaPPA (Chemical and Physical Properties of the Atmosphere).



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Experimental and Theoretical Investigation of the Reaction NO + OH +

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