Pressure and Temperature Dependence of Ethyl Nitrate Formation in

Dec 21, 2009 - Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), CNRS-IPSL, Verrières-le-Buisson, France. Cite this:J. Phys. Chem...
0 downloads 0 Views 617KB Size
Download PDF
956

J. Phys. Chem. A 2010, 114, 956–964

Pressure and Temperature Dependence of Ethyl Nitrate Formation in the C2H5O2 + NO Reaction Nadezhda Butkovskaya,* Alexandre Kukui,† and Georges Le Bras Institut de Combustion, Ae´rothermique, Re´actiVite´ et EnVironnement (ICARE), CNRS-INSIS, 1C AV. de la Recherche Scientifique, 45071 Orle´ans Cedex 2, France ReceiVed: October 19, 2009; ReVised Manuscript ReceiVed: NoVember 30, 2009

The branching ratio β ) k1b/k1a for the formation of ethyl nitrate, C2H5ONO2, in the gas-phase C2H5O2 + NO reaction, C2H5O2 + NO f C2H5O + NO2 (1a), C2H5O2 + NO f C2H5ONO2 (1b), was determined over the pressure and temperature ranges 100-600 Torr and 223-298 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K the C2H5ONO2 yield was found to increase linearly with pressure from about 0.7% at 100 Torr to about 3% at 600 Torr. At each pressure, the branching ratio of C2H5ONO2 formation increases with the decrease of temperature. The following parametrization equation has been derived in the pressure and temperature ranges of the study: β(P,T) (%) ) (3.88 × 10-3 · P (Torr) + 0.365) · (1 + 1500(1/T - 1/298)). The atmospheric implication of the results obtained is briefly discussed, in particular the impact of β on the evolution of ethyl nitrate in urban plumes. 1. Introduction Production of organic nitrates in the reaction of alkyl peroxy radicals with NO is recognized to have a significant role in atmospheric chemistry:1

RO2 + NO f RO + NO2 f RONO2

(a) (b)

where R ) CnH2n+1 is the alkyl radical. Reaction a leads to the formation of the alkoxy radical RO and NO2, with further photolysis of NO2 resulting in formation of O3. In contrast, reaction b, forming organic nitrate, is a reservoir or sink for both NOx (NO, NO2) and RO2, leading to a decrease in O3 formation. Many studies have been devoted to the determination of the nitrate yield in reaction of alkyl peroxy radicals with NO (Aschmann et al.2 and references therein). It was established that formation of alkyl nitrate usually is a minor channel of the RO2 + NO reaction with yields ranging from ∼4% for propyl nitrate (n ) 3) to ∼30% for heavier alkyl nitrates with n g 7, at T ) 298 K and atmospheric pressure.3 Most of the reported studies were carried out using static chamber reactors and gaschromatography for alkyl nitrate detection. Systematic studies were carried out only for alkyl radicals with n g 4, whereas NO reaction with the ubiquitous CH3O2 and C2H5O2 radicals were not fully investigated because of their low nitrate yield and related detection problems. Moreover, most of the studies were carried out at room temperature (298 ( 2 K) and atmospheric pressure. Several investigations were made at other temperatures, such as those of Atkinson et al.4,5 in chambers, Harris and Kerr6 and Cassanelli et al.7 in atmospheric flow reactors, and Elrod and co-workers8,9 in a turbulent flow reactor. The only pressure dependent studies were those of Atkinson and co-workers on the formation of C5-C7 alkyl nitrates.2,4,5 On the basis of the data obtained, Carter and Atkinson derived * Corresponding author. E-mail: [email protected]. Permanent address: Institute of Chemical Physics of the Russian Academy of Sciences, Moscow, Russian Federation. † Laboratoire Atmosphe`res, Milieux, Observations Spatiales (LATMOS), CNRS-IPSL, Verrie`res-le-Buisson, France.

a general expression for the estimation of the branching ratios for the alkyl nitrate forming channels.1,10 However, the applicability of this expression to the formation of nitrates with n e 4 remains an open question. At the same time, an accurate knowledge of organic nitrate formation yields under the full ranges of atmospheric pressure and temperature is needed for modeling of atmospheric processes, in particular, ozone formation. This article reports the results of a study of ethyl nitrate formation in the C2H5O2 + NO reaction,

C2H5O2 + NO f C2H5O + NO2 f C2H5ONO2

(1a) (1b)

using a turbulent flow reactor (TFR) coupled with a chemical ionization mass-spectrometer (CIMS) for detection and quantification of ethyl nitrate. The branching ratio, β ) k1b/k1a, which was experimentally determined as the ethyl nitrate yield, is an important parameter for modeling the atmospheric degradation of many volatile organic compounds, including ethane and propanal, which are precursors of the C2H5O2 radical.11 Up to now, two studies of the branching ratio of reaction 1 have been reported.3,8 In the early chamber study of Atkinson et al.3 using gas-chromatography with flame ionization detection (GC-FID), only an upper limit β e 1.4% was determined at T ) 298 K and atmospheric pressure. In the most recent study, Ranschaert et al.8 directly detected ethyl nitrate formed in a turbulent flow reactor at 100 Torr and T ) 213-299 K by CIMS with a radioactive ion source. C2H5ONO2 was monitored as a positive ion from proton transfer reaction (PTR). A branching ratio of about 0.6% at 298 K and 2% at 213 K was found in agreement with the temperature dependence observed for the reactions of heavier alkyl peroxy radicals.4-7 The authors noted that the observed nitrate signal was close to the detection limit at 100 Torr, which corresponded to the optimum instrument performance. They evaluated the accuracy of their measurements to 50%. In our laboratory, different regimes of CIMS analysis have been tested for the identification and quantification of organic nitrate species. In the present work, ethyl nitrate and other products were detected both in positive mode (PTR) and using

10.1021/jp910003a  2010 American Chemical Society Published on Web 12/21/2009

Ethyl Nitrate Formation

J. Phys. Chem. A, Vol. 114, No. 2, 2010 957

Figure 1. Experimental setup: 1, ion source; 2, ion-molecule reactor (IMR); 3, temperature controller; 4, turbulizer; 5, injector; 6, resistance; 7, liquid nitrogen cooling bath; 8, discharge tube; 9, microwave discharge; 10, sampling cones; 11, temperature sensor; 12, FeII(SO4) filter; 13, liquid nitrogen/ethanol cold bath; 14, NO cylinder.

negative ion chemical ionization (NICI). The experimental and detection methods are presented below along with the description of the calibration procedure used. Using these methods, the branching ratio β ) k1b/k1a could be determined over the pressure range 100-600 Torr at room temperature. Then, the temperature dependences of β were determined over the 300-223 K temperature range at different pressures between 100 and 600 Torr. The data set obtained for β combined with the recommended value for the rate constant k1 allows the estimation of the rate constant for C2H5ONO2 formation, k1b, over the nearly whole range of tropospheric pressure and temperature.

liquid N2 cooled trap and a (Fe)IISO4 filter to prevent penetration of NO2 impurity into the reactor. Acetaldehyde (Fluka, 99.9%) and ethanol (Sigma-Aldrich, 99.8%) used for calibration purposes were introduced into the reactor as preprepared 10% mixtures in He. Their concentrations were calculated from the rate of the gas mixture pressure drop in a calibrated volume. Calibration of NO2 was done using commercial mixture (AlphaGaz, 0.5% in N2) which flow rate was controlled by a CELERITY controller. Reaction Schemes. C2H6 + F + O2 + NO Main System. Typical concentrations of the reactants in the reactor were [C2H6] ≈ 4 × 1014, [O2] ≈ 5 × 1015 and [NO] ≈ 3 × 1015 molecules cm-3. Initial concentration of F-atoms was typically in the range of [F]0 ) (4-10) × 1011 molecules cm-3. When ethane was flowed directly into the reactor, the reactions of F-atoms with O2 and NO, competing with the F + C2H6 reaction (2), consumed about 10% of the atoms emerging from the injector, and the reaction of ethyl radicals with NO, competing with their reaction with O2 (3), consumed about 1% of the radicals. Ethyl peroxy radical reacted with NO giving the major products C2H5O and NO2 and a small fraction of ethyl nitrate:

C2H5O2 + NO f C2H5O + NO2

(1b)

f C2H5ONO2 -12

-1

-1

with k1 ) 8.7 × 10 cm molecule s . In all the experiments, reaction 1 was completed within less than 1 ms, giving NO2 and ethyl nitrate final products. The ethoxy radical from reaction 1a could further react with NO2: 3

C2H5O + NO2 + M f C2H5ONO2 + M

2. Experimental Methods

(1a)

(4)

Chemical Reactor. Chemical reactions took place in the turbulent flow reactor (TFR) coupled with the quadrupole massspectrometer with chemical ionization described earlier.12 The scheme of the experimental setup is presented in Figure 1. For the pressure dependence study, the reactor was operated at room temperature (298 ( 2 K) and at pressure from 100 to 600 Torr with typical flow velocity of N2 carrier gas of about 18 m/s (Reynolds number Re ≈ 4500 at 100 Torr and Re ≈ 12000 at 600 Torr). Reaction of F-atoms with ethane in the presence of O2 was used as the source of ethyl peroxy radicals:

with k4 ) 2.8 × 10-11 cm3 molecule-1 s-1. The NO2 concentration consisted of the product from reaction 1a and the trace impurity in NO. Typically, the NO2 background concentration in the presence of NO was (5-7) × 1011 molecules cm-3. Secondary reaction 4 also forms ethyl nitrate, complicating the measurements of the nitrate yield from reaction 1. Two reactions helped to scavenge C2H5O radicals to minimize formation of ethyl nitrate in reaction 4:

F + C2H6 f C2H5 + HF

C2H5O + NO + M f C2H5ONO + M

C2H5 + O2 + M f C2H5O2 + M

(2) (3)

with k2 ) 1.1 × 10-10 and k3 ) 7.2 × 10-12 cm3 molecule-1 s-1 (unless specified, the rate constants given in the text are the recommended values from ref 13 at 298 K and 200 Torr). F-atoms were generated by a microwave discharge in a 5% F2/ He commercial mixture (AlphaGaz 2) in a quartz tube concentrically connected to the moveable injector. The distance from the injector tip to the orifice of the inlet cone of the ion-molecule reactor was usually 30 cm, which corresponded to the residence time in the TFR of about 16 ms. During the low temperature experiments, the carrier gas was cooled by passing through a metal coil immersed into liquid N2. Reactor temperature was controlled by resistive heating of the inlet tube using a CB100 digital controller (RKC Instrument). Ethane (AlphaGaz N45) was introduced either into the injector or directly into the reactor upstream of the tip of the movable injector. It was found that radical losses were less when ethane was flowed into the reactor. Oxygen (AlphaGaz N45) was added to the main nitrogen flow using a TYLAN flow controller. Nitrogen monoxide (AlphaGaz N20) was introduced through a special line passing successively through an ethanol-

C2H5O + O2 f CH3CHO + HO2 f CH3CHO + HNO

(5) (6a) (6b)

with k5 ) 1.0 × 10-14 and k6 ) 4.4 × 10-11 cm3 molecule-1 s-1. As reaction 5 is relatively slow, NO acted as a principal scavenger. The contribution from reaction 4 to the measured C2H5ONO2 signal intensity depended on the competition between reaction 4 and reaction 6 and was much lower than the 6 × 10-3 primary nitrate yield of reaction 1 obtained previously.8 More accurate calculations using computer simulation showed that at T ) 298 K this secondary contribution did not exceed 8% of the total nitrate yield at P e 200 Torr and 4% at P g 300 Torr. High NO concentration, used to favor the C2H5O scavenging reaction relatively to reaction 4, was also important to reduce additional NO2 production by the reaction of NO with HO2 formed in reaction 5:

HO2 + NO f OH + NO2

(7)

with k7 ) 8.1 × 10-12 cm3 molecule-1 s-1. When the branching ratio was determined as the ratio of the C2H5ONO2 and NO2 concentrations from reaction 1, β ) ∆[C2H5ONO2]/∆[NO2], the contribution from reaction 7 to the measured NO2 concentration was less than 1% under the conditions used. Another reason to

958

J. Phys. Chem. A, Vol. 114, No. 2, 2010

Butkovskaya et al.

make reactions 5 and 7 negligible is the possible formation of HONO in the OH reaction with NO that could interfere with the NICI detection of the C2H5ONO product (vide infra). An alternative way to obtain the branching ratio was to relate the measured nitrate concentration to the concentration of acetaldehyde or ethyl nitrite, provided that the stoichiometric conversion of the ethoxy radicals from reaction 1a into CH3CHO and C2H5ONO occurs by reaction 6. Reaction 6 occurs via two channels with a known branching ratio. The rate constant of the hydrogen abstraction channel (6b), k6b ) 1.1 × 10-11 cm3 molecule-1 s-1, does not depend on pressure, while that of the addition channel (6a) is in its high pressure limit at P g 100 Torr with k6a ) 3.3 × 10-11 cm3 molecule-1 s-1.14-17 This gives for the CH3CHO yield R ) k6b/(k6a + k6b) ) 25 ( 3%. A characteristic reaction time for reaction 6 was of the order of 10-5 s, ensuring complete conversion of the ethoxy radicals to the final stable products. In such a case, β ) R · ∆[C2H5ONO2]/ ∆[CH3CHO] or β ) (1 - R) · ∆[C2H5ONO2]/∆[C2H5ONO], where ∆[CH3CHO] and ∆[C2H5ONO] are the acetaldehyde and ethyl nitrite concentrations from reaction 6. Acetaldehyde and NO2 were calibrated using commercial compounds. Calibrations of ethyl nitrate and ethyl nitrite were done by producing them in situ as described in the next section. Further, a product used for determination of the product concentration from channel 1a will be called a reference product (RP). It has to be noted that a “dark” (discharge off) reaction between molecular fluorine and NO took place in the reactor (k8 ) 1.2 × 10-14 cm3 molecule-1 s-1 18):

F2 + NO f F + FNO

(8)

This reaction produced F-atoms, complicating the kinetics of the yield measurements. Though the reaction has a low rate constant, its characteristic time was comparable with the residence time in the reactor due to the high NO concentrations used. To avoid complications, the product signal intensities were measured as a difference of signals with open and closed F2 flow, keeping the discharge permanently switched on. Since F2 entirely dissociates in the discharge, such operation eliminated the influence of reaction 8. For comparison, some measurements were carried out with a permanent F2 flow setting discharge on-off, with minimized effect of reaction 8 by working at a shorter residence time of about 7 ms. F + C2H5OH Calibration Reaction. For calibration, ethyl nitrate was produced in the reactor by using the reaction of F-atoms with ethanol as the source of ethoxy radicals:

F + C2H5OH f C2H5O + HF

(9a)

f CH3CHOH + HF

(9b)

f HOCH2CH2 + HF

(9c)

The total rate constant of this fast reaction is k9 ) 1.2 × 10-10 cm3 molecule-1 s-1. This source was recommended by Bogan and Nesbitt,19 who determined the branching fraction of C2H5O formation γ ) k9a/k9 ) 55 ( 4%. When NO2 was introduced into the reactor ([NO2] ≈ 1 × 1013 molecules cm-3), ethyl nitrate was formed via addition reaction 4:

C2H5O + NO2 + M f C2H5ONO2 + M

(4)

Nitrate concentration was determined as ∆[C2H5ONO2] ) γ · ∆[C2H5OH], where ∆[C2H5OH] is the ethanol consumption in reaction (9). Sensitivity of the mass spectrometer to ethyl nitrate produced in consecutive reactions 9a and 4 was calculated from its signal intensity ∆Ical M as SM(C2H5ONO2) ) ∆[C2H5ONO2]/ cal ∆Ical M ) γ · ∆[C2H5OH]/∆IM , where M is the mass number for

nitrate detection. The two radical coproducts from reactions 9b and 9c could also combine with NO2, to form products with the same mass as ethyl nitrate. But, as will be shown below, detection of C2H5ONO2 in negative mode is not affected by other products of reaction (9). Combination reactions of NO2 with CH3CHOH and HOCH2CH2 have not been reported so far, but to avoid a possible interference, the calibration was carried out in the presence of O2, to scavenge the CH3CHOH and HOCH2CH2 radicals:

CH3CHOH + O2 f CH3CHO + HO2

(10)

HOCH2CH2 + O2 + M f HOCH2CH2O2 + M

(11) with k10 ) 1.9 × 10-11 and k11 ) 3.0 × 10-12 cm3 molecule-1 s-1. We took advantage of the fact that the rate constants of reactions 10 and 11 are by orders of magnitude higher that the rate constant of O2 reaction with ethoxy radical (5):

C2H5O + O2 f CH3CHO + HO2

(5)

with k5 ) 1.0 × 10-14 cm3 molecule-1 s-1. A moderate oxygen concentration of about 5 × 1014 molecules cm-3 removed the CH3CHOH and HOCH2CH2 radicals without reducing the C2H5O concentration by more than 1.5%. A possible fate of the HOCH2CH2O2 radical includes its self-reaction and its reactions with HO2 and NO2:

2HOCH2CH2O2 f 2HOCH2CH2O + O2

(12)

f HOCH2CH2OH + HOCH2CHO + O2 (13) HOCH2CH2O2 + HO2 f HOCH2CH2OOH + O2

(14) HOCH2CH2O2 + NO2 + M f HOCH2CH2OONO2 + M (15) with k12 ) 1.1 × 10-12, k13 ) 1.2 × 10-12, and k14 ) 1.5 × 10-11 cm3 molecule-1 s-1. The rate constant of reaction 15 is expected to be similar to that of the CH3CH2O2 + NO2 addition reaction, 4.2 × 10-12 cm3 molecule-1 s-1. The HOCH2CH2O radical rapidly decomposes, giving formaldehyde (molecular weight MW ) 30) and hydroxy methyl radical (k16 ) 5 × 10-5 s-1 20):

HOCH2CH2O + M f CH2O + HOCH2 + M

(16) The formed stable products, CH2O (MW ) 30), HOCH2CH2OH (MW ) 62), HOCH2CHO (MW ) 60), HOCH2CH2OOH (MW ) 78), and HOCH2CH2OONO2 (MW ) 123), do not interfere with the detection of C2H5ONO2 in PTR regime. To calibrate ethyl nitrite, a flow of NO instead of NO2 was introduced into the C2H5OH + F system described above. Concentration of NO was about 5 × 1013 molecules cm-3. The ethoxy radicals were converted into the nitrite and acetaldehyde via reaction 6:

C2H5O + NO + M f C2H5ONO + M f CH3CHO + HNO

(6a) (6b)

Nitrite concentration was determined as ∆[C2H5ONO] ) (1 - R) · γ · ∆[C2H5OH], and its detection sensitivity was calculated as SN(C2H5ONO) ) ∆[C2H5ONO]/∆Ical N ) γ · ∆[C2H5OH]/ ∆INcal, where ∆INcal is the signal intensity of C2H5ONO detected

Ethyl Nitrate Formation

J. Phys. Chem. A, Vol. 114, No. 2, 2010 959

at mass number N. In this case O2 was not added, and the reactions of the accompanying radicals from reactions 9b and 9c were

TABLE 1: Observation of Ions Produced in the IMR from the F + C2H6 + O2 + NO and F + C2H5OH + O2 + NO2/NO Reaction Systemsa Positive Mode (PTR)

CH3CHOH + NO + M f CH3CH(OH)NO + M (17a) f CH3CHO + HNO

(17b)

HOCH2CH2 + NO + M f HOCH2CH2NO + M

(18) with k17 ) 2.4 × 10-11 and k18 ) 2.6 × 10-11 cm3 molecule-1 s-1. The products of reactions 17 and 18 do not interfere with the detection of ethyl nitrite or ethanol using NICI. The next section describes the ionization schemes chosen to detect the C2H5ONO2, C2H5ONO, NO2, CH3CHO, and C2H5OH species. CIMS Detection. Ion-Molecule Reactor and Primary Ions. Gas mixture from the TFR was sampled through a Teflon cone into the ion-molecule reactor (IMR). The pressure of the Ar carrier gas in the IMR was about 1 Torr. The electrons and primary Ar+ ions were generated in the ion source with a heated filament. SF6 was continuously introduced into the IMR downstream of the ion source. The primary SF6- negative ions were produced by attachment of thermalized electrons to SF6. In the PTR regime, a small flow of water vapor was added to the SF6 flow, resulting in formation of the H3O+ ion and its water clusters. A new ionization scheme using primary F- ions was tested in this study for detection of nitrates. In this case NF3 was introduced instead of SF6 to generate F- ions through the dissociative electron attachment reaction:21

NF3 + e- f F- + NF2

(1i)

The ions formed in the IMR entered the ion-optical zone through the 180 µm orifice in the nickel skimmer with a potential of several volts and acting as a first focusing element. After passing through the quadrupole mass analyzer (EXTREL), the ions were registered in the ion counting regime using a Channeltron multiplier and a MTS-100 preamplifier. Detection in PositiWe Mode. C2H5ONO2 and C2H5ONO were detected using proton transfer from the H3O+ ion and its water clusters. In the absence of reactants, the major positive ions observed in the IMR were H3O+ and its water clusters H3O+ · (H2O)n with n ) 0-3. Using H3O+ · (H2O)n ions, organic products with proton affinity (PA) greater than that of H2O, 165 kcal mol-1, can be detected by proton transfer reactions: +

+

H3O · (H2O)n + M f MH · (H2O)m + (n - m + 1) · H2O

m e n (2i)

It is known that ethanol and acetaldehyde with PA ) 186 and 184 kcal mol-1, respectively, can be monitored by PTR with high sensitivity. Ethyl nitrate has a PA of 178 kcal mol-1 22 and also undergoes reaction 2i, which was used in the study of Ranschaert et al.,8 where it was detected as the MH+ · (H2O)2 ion at m/e 128. However, it was found that protonated alkyl nitrates, though observed, were not the major ions in the PTR spectra of C1-C5 alkyl nitrates.23 The major product ions were fragment ions including NO2+, R+, RO+, and ROH · H+. In the case of ethyl nitrate, the NO2+ (m/e 46), C2H5O+ (m/e 45), C2H5OH · H+ (m/e 47), and MH+ (m/e 92) ions were observed with relative intensities 100:20:18:∼1.5. Unfortunately, in our study, all three ions m/e 45, 46, and 47 were also present in the PTR mass spectrum of acetaldehyde, one of the major reaction products, with approximate relative intensities 100:3:2. Therefore, only C2H5ONO2 · H+ ion (m/e 92) or its water clusters could

species

mass

MH+

MH+ · H2O

MH+ · (H2O)2

H2 O C2H5ONO2 C2H5ONO CH3CHO C2H5OH

18 91 75 44 46

19 92 76 45 47

37 110 94 63 65

55 128 112 81 83

Negative Mode species NO2 C2H5ONO2 C2H5ONO CH3CHO C2H5OH

mass 46 91 75 44 46

ion agent

charge exchange, fragmentation

M-H-

M · F-

46 46, 62 46

90

110

-

SF6 FFFF-

43 65

a

Masses in bold indicate ions used in the measurements of the branching ratio of reaction 1.

be used for detection of the nitrate. The detection was made at m/e 110 corresponding to the C2H5ONO2 · H+ · (H2O) ion cluster, which intensity was somewhat higher than the simple protonated ion MH+. Theoretical investigations have shown that proton is attracted rather by the internal oxygen atom between C and N atoms than by the end O-atoms,22 suggesting that the PA of C2H5ONO is similar to that of C2H5ONO2. This was confirmed in the present study by sensitive detection of the C2H5ONO product as the C2H5ONO · H+ ion (m/e 76) in ethane and ethanol systems. The observed ions are listed in Table 1. Detection in NegatiWe Mode. NO2 from reaction 1a was detected at m/e 46 as the NO2- ion formed by electron transfer from SF6- with the rate constant k3i ) 1.3 × 10-10 cm3 molecule -1 s-1:24

SF6- + NO2 f NO2- + SF6

(3i)

Charge transfer reaction with F- ion,

F- + NO2 f NO2- + F -12

(4i) -1

-1 25

is much slower (k4i e 6 × 10 cm molecule s ) and was inefficient in our experiments. For example, at 200 Torr, the typical sensitivities to NO2 using SF6- and F- reagent ions were 1.2 × 108 and 2.7 × 109 molecules cm-3/cps, respectively. Thus, for reliable monitoring of NO2, reaction 3i was preferred. Ethyl nitrate and ethyl nitrite were detected using a new ionization scheme with F- ions (F--NICI). Thermochemical considerations suggest that the following ion-molecule reactions can occur (R ) C2H5): 3

F- + RONO2 f NO3- + CH3CH2F f [RF · NO3-] F- + RONO2 f NO3- + C2H4 + HF f [R-H · NO3-] + HF

∆Hr ) -42.0 (5ia)

(5ib) ∆Hr ) -46.4 (5ic)

(5id)

960

J. Phys. Chem. A, Vol. 114, No. 2, 2010

F- + RONO2 f NO2- + CH2O + CH3F ∆Hr ) -30.8

Butkovskaya et al.

(5ie)

-1

The given enthalpy of reactions (in kcal mol ) were calculated from the heats of formation of the reactant and product molecules taken from ref 13. The electron affinities (EA) of F, NO2, and NO3 are 3.40, 2.3, and 3.94 eV, respectively.26 In the mass spectra from the C2H5OH + F + NO2 system, the major peaks were at m/e 46 and m/e 62 with relative intensities 100: 50. As well, two peaks at m/e 90 (∼1%) and m/e 110 (∼0.04%) were detected, which were assigned to the [C2H4 · NO3-] and [C2H5F · NO3-] ion complexes, respectively. Though very minor, these peaks are specific for ethyl nitrate, while the NO2- and NO3- ion peaks may be common for all nitrates. As a rule, the NO3- ion forms rather strong complexes with polar molecules. For example, its binding energies with H2O, HBr, and HNO3 ligands are 12.5, 25, and 27 kcal mol-1, respectively.27 Though the binding energy in the [C2H5F · NO3-] complex is not known, it is expected to be of the order of a few kcal mol-1. Evidently, the available reaction energy is much higher, and the complex easily decomposes to NO3- and C2H5F. Elimination of HF helps to stabilize the [R-H · NO3-] complex, explaining its stronger signal in the mass spectrum. To our knowledge, only the rather widely employed CH4-NICI method was tested for alkyl nitrates.28 In this method negative ions were generated by electron attachment with methane as a reagent gas. For ethyl nitrate, a mass spectrum with three peaks at m/e 43, 46, and 62 with relative intensities 20:75:5 was obtained.28 The F--NICI method is preferable for our study, since it gives less NO2and more NO3- ions, which is important because of the presence of nitrite among the major products. Besides, nitrate-selective ions [RF · NO3-] and [R-H · NO3-] were also observable with the F--NICI method, while the CH4-NICI one gave only the [RO-H2-] selective ion at m/e 43 interfering with acetaldehyde detection. Using F--NICI, C2H5ONO can also be detected using the fast single channel reaction:25

F- + C2H5ONO f NO2- + HF + C2H4

(6i)

-

The F -NICI spectra in the presence of CH3CHO and C2H5OH in the TFR, showed that both acetaldehyde and ethanol could be efficiently detected at m/e 43 and m/e 65, respectively. These mass numbers correspond to CH2CHO- and C2H5OH · F- ions, presumably formed in reactions 7i and 8i:

F- + CH3CHO f CH2CHO- + HF F- + C2H5OH f C2H5OH · F-

∆Hr ) -10.3 (7i)

(8i)

The signal intensities at m/e 43 and m/e 65 were linear up to CH3CHO and C2H5OH concentrations of 2 × 1013 molecules cm-3. Detection of acetaldehyde at m/e 43 using F- NICI has been previously reported by Tiernan et al.29 We suggest that ionization occurs via exothermic reaction 7i, since the reaction channel forming the acetyl anion CH3CO- is endothermic by 16 kcal mol-1. Reactions with SF6- are energetically forbidden for both acetaldehyde and ethanol. Table 1 contains mass numbers observed from the chemical systems under study with NF3 or SF6 reagents in the IMR. 3. Results and Discussion Branching Ratio as a Function of Pressure at Room Temperature. In the first series of experiments, the branching ratio of reaction (1) was measured between P ) 100 and 600

Torr at T ) 298 ( 2 K using PTR, F--NICI or combined PTR/ SF6 detection methods. As a rule, each experiment consisted of the following procedures: (1) C2H5ONO2, CH3CHO, and C2H5ONO product signal intensity measurements from the F + C2H6 + O2 + NO system under above determined conditions; (2) CH3CHO calibration under the same conditions; (3) C2H5ONO2 and C2H5OH signal intensity measurements from the F + C2H5OH + NO2 (+O2) reaction system; (4) C2H5OH calibration. When C2H5ONO was used as a reference product (RP), C2H5ONO and C2H5OH signal intensities from the F + C2H5OH + NO reaction system were additionally measured. Reaction time was about 7 or 25 ms for the main reaction (C2H6 system) and about 25 ms for calibration reactions (C2H5OH system). In the PTR/SF6 detection method with NO2 as RP, the first step was preceded by NO2 calibration and measurement of NO2 signal intensity from the main reaction without H2O in the IMR. The signals were monitored at the mass numbers indicated in bold in Table 1. The final equation to calculate the branching ratio by using different detection methods were:

β ) Rγ(∆IM /∆Ical M )(∆[C2H5OH]/[CH3CHO]) (PTR and F--NICI with CH3CHO as RP) β ) γ(∆IM /∆Ical M )(∆[C2H5OH]/∆[NO2]) (PTR/SF6 with NO2 as RP) cal β ) (∆IM /∆Ical M )(∆IN /∆IN ) (∆[C2H5OH]NO2 /∆[C2H5OH]NO)

(F--NICI with C2H5ONO as RP) In these equations, ∆IM and ∆Ical M are the C2H5ONO2 signal intensities from the main and calibration reactions, respectively; ∆IN and ∆Ical N are the C2H5ONO signal intensities from the main and calibration reactions, respectively; ∆[C2H5OH]NO2 and ∆[C2H5OH]NO denote ethanol consumption during the nitrate and nitrite calibration, respectively. The obtained results are summarized in Table 2. Figure 2 presents the branching ratio measured using the different detection methods at different pressures. We see that the normalization of the nitrate product concentration by the three different final products (CH3CHO, C2H5ONO, and NO2) gives consistent results and does not show any systematic deviations. The averaged branching ratio is β ) 0.74 ( 0.10, 1.22 ( 0.11, 1.56 ( 0.13, 1.86 ( 0.26, 2.29 ( 0.21, and 2.68 ( 0.34% at P ) 100, 200, 300, 400, 500, and 600 Torr, respectively. These data give a linear pressure dependence described by eq E1:

β(P) (%) ) (3.88 ( 0.62) × 10-3 · P (Torr) + 0.365 ( 0.020 (E1) Extrapolation to zero pressure gives a nonzero intercept of about 0.4%. For P ) 760 Torr extrapolation gives β ) 3.3%. The dependence obtained is in accord with the Aschmann’s et al. data for the reaction of pentyl peroxy radicals with NO, where a linear 4-fold increase of the pentyl nitrate yield was found over the 51-744 Torr pressure range.2 It is worth noting that in the study of Aschmann et al., similar nonzero intercepts have been observed for both nitrate isomers. As well, we obtained a nonzero intercept in the study of the pressure dependence of HNO3 formation in the HO2 + NO reaction.30 The nature of these intercepts is not clear. It is more probable that they are just a result of extrapolation. Existing theoretical studies explain the increase of the nitrate yield with pressure by collisional stabilization of the formed RONO2 and predict zero yield at zero pressure.31,32 Red lines in Figure 2 represent the branching

Ethyl Nitrate Formation

J. Phys. Chem. A, Vol. 114, No. 2, 2010 961

TABLE 2: Determination of the Branching Ratio β ) k1b/k1a at 298 K P (Torr)

detection referencea S(RP)b 1011 × concc method product (RP) (108) (eq 1a) M nitrate

cal d ∆IM (cps)

∆IMe (cps)

SM(nitrate)b (108) conc (1b) f (109)

100 100 100 100

PTR PTR PTR/SF6 F--NICI F--NICI

CH3CHO CH3CHO NO2 CH3CHO C2H5ONO

0.909 0.752 4.60 0.092 0.156

26.0 12.7 31.6 3.35 4.76

110 110 110 62 62

41.1 ( 2.7 1.64 ( 0.36 41.2 ( 3.5 0.83 ( 0.31 70.7 ( 4.2 3.44 ( 1.12 180 ( 23 180 ( 23

150

PTR

CH3CHO

0.372

21.6

110

144 ( 6

200 200 200

PTR PTR PTR PTR/SF6 F--NICI NF3/SF6 F--NICI F--NICI

CH3CHO CH3CHO CH3CHO NO2 CH3CHO NO2 CH3CHO C2H5ONO

0.383 0.108 0.249 2.38 0.194 1.28 0.215 0.242

9.45 6.40 7.09 8.21 3.04 16.6 3.65 3.66

110 110 110 110 90 90 62 62

101 ( 8 96 ( 4 117 ( 4 117 ( 4 109 ( 2 79.6 ( 9.1

F--NICI F--NICI F--NICI F--NICI

CH3CHO C2H5ONO CH3CHO C2H5ONO

0.262 0.360 0.173 0.247

9.01 8.42 5.15 4.76

62 62 62 62

400 400

PTR F--NICI F--NICI

CH3CHO CH3CHO C2H5ONO

2.05 0.187 0.561

20.3 2.89 3.40

110 62 62

500

F--NICI F--NICI

CH3CHO C2H5ONO

0.396 0.613

5.39 4.79

62 62

214 ( 26 214 ( 26

0.495 0.495

600

F--NICI F--NICI

CH3CHO C2H5ONO

0.448 0.956

3.50 3.85

62 62

91 ( 12 91 ( 12

1.08 1.08

200 200 200 300 300

115 94.8 81.6 0.161 0.161

17.1 7.81 28.1 2.89 2.89

5.28 ( 1.13

25.9

18.0

3.02 ( 0.48 3.72 ( 0.99 2.59 ( 0.41 2.59 ( 0.41 1.42 ( 0.46 6.92 ( 2.06 175 ( 22 175 ( 22

34.1 21.6 32.6 32.6 26.5 28.5 0.286 0.286

10.3 8.02 8.46 8.46 3.75 19.8 5.00 5.00

( ( ( (

0.626 0.626 0.270 0.270

14.8 14.8 7.11 7.11

439 0.463 0.463

46.5 5.55 5.55

249 249 263 263

20 20 24 24

10.1 ( 0.9 1.10 ( 0.33 120 ( 16 120 ( 16

10.6 10.6 9.82 9.82

β (%) 0.72 ( 0.20 0.62 ( 0.26 0.89 ( 0.30 0.86 ( 0.20 0.61 ( 0.13 0.74 ( 0.10g 0.83 ( 0.22 0.83 ( 0.22g 1.09 ( 0.23 1.29 ( 0.39 1.19 ( 0.26 1.03 ( 0.20 1.23 ( 0.45 1.19 ( 0.37 1.37 ( 0.28 1.37 ( 0.25 1.22 ( 0.11g 1.64 ( 0.29 1.76 ( 0.30 1.38 ( 0.24 1.49 ( 0.18 1.56 ( 0.13g 2.04 ( 0.56 1.92 ( 0.46 1.63 ( 0.34 1.86 ( 0.26g 2.10 ( 0.32 2.48 ( 0.26 2.29 ( 0.21g 2.80 ( 0.53 2.55 ( 0.44 2.68 ( 0.34g

a Product used for quantification of channel 1a. b Detection sensitivity (molecules cm-3 cps-1). c Concentration (molecules cm-3) of product from channel 1a. d Nitrate signal intensity at m/e ) M (counts per second) from calibration reaction. e Nitrate signal intensity at m/e ) M (counts per second) from reaction 1b. f Concentration (molecules cm-3) of nitrate from channel 1b. g Average.

Figure 2. Pressure dependence of the branching ratio for ethyl nitrate formation in the C2H5O2 + NO reaction at 298 ( 2 K. Different symbols denote different detection methods. The compound in parentheses denotes a reference product (RP). The red cross is the measurement of Ranschaert et al.8 Red lines represent the empirical function with parametrization of Atkinson,1 Carter and Atkinson,10 and Atkinson et al.4 The black cross indicates extrapolation to atmospheric pressure.

ratio calculated according to the empirical falloff equation, socalled Y-function, developed by Atkinson et al.1,4,5,10 The calculation was done using different parameter sets from the earliest one suggested in 19834 to the most recent given in 1994.1 Although ethyl peroxy radical gives a primary nitrate, the calculated branching ratio was not multiplied by a coefficient of 0.4, as recommended for primary RO2 radicals, because of

the absence of nitrate isomers in this reaction. Figure 2 shows that the more recent parametrization1,10 better agrees with our experimental pressure dependence, although it gives nitrate yield values lower by approximately a factor of 1.4. The mean branching ratio obtained at 100 Torr in the present work, β ) 0.74 ( 0.10%, agrees well with the value of 0.6 ( 0.3% obtained by Ranschaert et al. in a similar study.8 The difference between the experimental conditions of the two studies is that these authors worked with much higher O2 concentration (∼3 × 1018 molecules cm-3) and moderate NO concentration, using O2 as C2H5O scavenger. The rate constant k1b in their study was extracted from the computer simulation of the kinetics of the C2H5ONO2 formation with the estimated uncertainty of 50%. In our study, the experimental error in β consists mainly of the error in the nitrate signal intensity measurements from reaction 1b (see Table 2). The accuracy of the C2H5ONO2 signal intensity measurement by monitoring the NO3- ion (m/e 62) from the reaction with F- was about 10% compared to (20-30)% using PTR or monitoring the C2H4 · NO3- ion (m/e 90) by F--NICI. Additional errors in the determination of the product concentrations are connected with the uncertainties in the channelling of reactions 6 and 9. It is necessary to note that the determination of β ) k1b/k1a, using C2H5ONO as RP, eliminates the errors connected with the uncertainties in the branching ratios R and γ of reactions 6 and 9, respectively. Coefficient R vanishes because ethyl nitrite is formed in the same reaction 6 in both the main and calibration systems, and coefficient γ cancels because calibration of both

962

J. Phys. Chem. A, Vol. 114, No. 2, 2010

Butkovskaya et al.

TABLE 3: Determination of the Temperature Dependence of the Branching Ratio β(T) ) k1b/k1aa P ) 100 Torr T (K)

-3

10 ∆I62/∆I43

β/β298

P ) 200 Torr β (%)

[C2H6] ) 2.6; [O2] ) 38; [NO] ) 21 298 15.3 1 0.74 293 18.2 1.19 0.88 288 18.1 1.18 0.87 283 20.6 1.35 1.00 278 23.1 1.51 1.12 274 25.3 1.65 1.22 270 25.2 1.67 1.23 [C2H6] ) 2.3; [O2] ) 35; [NO] ) 19 300 11.2 1 0.74 271 18.8 1.68 1.24 263 20.8 1.85 1.37 258 22.1 1.97 1.46 252 24.0 2.18 1.61 249 24.9 2.27 1.68 244 25.9 2.35 1.74 238 28.0 2.54 1.88

T (K)

10-3∆I62/∆I43

β/β298

β/β298

β (%)

[C2H6] ) 2.3; [O2] ) 42; [NO] ) 25 299 20.8 1 1.22 241 31.7 1.92 2.34 231 31.0 2.07 2.53 222 40.0 2.46 3.00

T (K)

T (K)

10-3∆I62/∆I43

β/β298

-3

10 ∆I62/∆I43

β/β298

β (%)

[C2H6] ) 3.1; [O2] ) 43; [NO] ) 31 325 20.7 0.74 0.90 322 22.1 0.79 0.96 314 25.8 0.92 1.13 306 26.4 0.94 1.15 299 28.0 1 1.22 293 29.6 1.06 1.29 287 29.0 1.03 1.26 277 34.3 1.23 1.50 273 32.9 1.17 1.43 263 35.5 1.27 1.55 256 42.3 1.51 1.84 253 41.6 1.49 1.81 248 44.4 1.59 1.93

P ) 400 Torr β (%)

[C2H6] ) 3.8; [O2] ) 46; [NO] ) 38 298 98.8 1 1.56 282 131 1.33 1.99 272 133 1.34 2.02 278 114 1.15 1.73 265 145 1.47 2.20 252 176 1.78 2.68 244 177 1.79 2.69 232 228 2.31 3.47 238 222 2.25 3.37 222 248 2.51 3.77 a

10 ∆I62/∆I43

P ) 200 Torr

[C2H6] ) 3.0; [O2] ) 35; [NO] ) 20 298 6.34 1 1.22 283 8.72 1.37 1.67 276 9.28 1.46 1.78 268 9.05 1.43 1.74 262 10.8 1.71 2.08 253 11.3 1.79 2.18 250 11.0 1.74 2.12 243 12.6 1.99 2.43 235 13.3 2.09 2.56

P ) 300 Torr T (K)

-3

P ) 600 Torr β (%)

[C2H6] ) 4.0; [O2] ) 42; [NO] ) 24 298 27.6 1 1.86 285 30.6 1.11 2.07 273 39.1 1.42 2.64 262 43.1 1.56 2.91 252 50.7 1.84 3.42 242 57.2 2.07 3.85 235 61.0 2.21 4.12 226 67.3 2.44 4.54

T (K)

10-3∆I62/∆I43

β/β298

β (%)

[C2H6] ) 6.3; [O2] ) 42; [NO] ) 24 323 39.2 0.87 2.32 308 44.2 0.98 2.62 299 36.9 1 2.68 282 45.9 1.24 3.33 269 50.7 1.37 3.68 253 61.5 1.67 4.47 243 71.2 1.96 5.24 233 91.0 2.46 6.60 223 99.5 2.69 7.22

Concentrations at room temperature are in the units of 1014 molecules cm-3.

ethyl nitrate and ethyl nitrite uses reaction 9 as the initial step. The uncertainties in β, indicated in Table 2, include the experimental errors in signal intensity measurements, errors in ethanol and acetaldehyde calibration (∼5%), and uncertainties in R (12%) and γ (8%). The standard deviations (2σ) for the intensity of the weak C2H5ONO and C2H5ONO2 signals are also given in Table 2. Temperature Dependence of the Branching Ratio. The low temperature measurements were done using the F- · NICI ionization mode with detection of CH3CHO and C2H5ONO2 at m/e 43 and 62, respectively. In preliminary measurements with a constant flow of CH3CHO into the TFR and with the F + C2H5OH + O2 + NO2 reaction system, the intensity of the CH3CHO and C2H5ONO2 signals followed the change of concentrations, in agreement with the equation of state of an ideal gas when the temperature in the TFR was lowered to 223 K at a constant pressure and a constant carrier gas flow. The branching ratio was determined as β ) ∆[C2H5ONO2]/ ∆[CH3CHO] over the whole temperature range. We assumed that the branching ratio of the reaction of ethoxy radicals with NO (6) did not change with temperature. This assumption was based on the experimental data of Arden et al.33 and Baker and Shaw,34 who obtained a constant ratio k6b/k6a ) 0.3 for the 368-408 K range. A very weak positive temperature dependence of the overall rate constant k6 observed over the 286-389 K temperature range in the study of Fittschen et al.15 confirms this assumption. The results of the measurements at 100 (2 runs), 200 (3 runs), 300, 400, and 600 Torr (one run for each) are collected in Table 3. A strong negative temperature effect was found at each pressure.

Figure 3. Temperature dependence of the relative branching ratio for C2H5ONO2 formation, β(T)/β(298 K), at different pressures. Red triangles denote the data of Ranschaert et al.8 at 100 Torr pressure.

Figure 3 represents the normalized branching ratio β(T)/ β(298) for different pressures against reverse temperature. The highest relative change of β was observed at 100 Torr. At this pressure our result agrees well with the temperature dependence obtained by Ranschaert et al.8 At P g 200 Torr, the temperature dependence of the β(T)/β(298) ratio practically does not depend on pressure. In Figure 4, the measured change of the β(T)/β(298) ratio with temperature is compared with the Atkinson’s equation at P ) 100, 200, and 600 Torr. Normalization at T ) 298 K allows us to better reveal the temperature component of the Y-function. The parametriza-

Ethyl Nitrate Formation

J. Phys. Chem. A, Vol. 114, No. 2, 2010 963

β(P,T) (%) ) (3.88 × 10-3 · P (Torr) + 0.365) · (1 + 1500(1/T - 1/298)) (E2) 4. Atmospheric Implication

Figure 4. Comparison of the measured temperature dependence (symbols) with the empirical function at different pressures (red lines) (see caption to Figure 2). The black line is the linear fit to experimental points.

Figure 5. Temperature dependence of the branching ratio β ) k1b/k1a at different pressures. The cross indicates extrapolation to P ) 760 Torr at 298 K. The dotted arrow shows the change of β from the atmospheric conditions near the Earth’s surface to the upper troposphere.

tion of Carter and Atkinson10 gives rather good approximation to the experimental dependences over the 323-243 K range, while the earlier parametrization4 strongly overestimates temperature dependence, especially at T < 250 K. Figure 5 presents a plot of the temperature dependences of the branching ratio at different pressures. It was obtained by multiplying the temperature dependences in Figure 3 by the average pressure coefficients given in Table 2. The picture shows straight lines with the slopes increasing with pressure. It can be assumed with a good approximation that the slope is proportional to pressure. Then, the overall pressure and temperature dependence over the 100 e P e 760 Torr and 215 e T e 300 K ranges can be expressed by the empirical expression:

The data obtained allow an estimate of the rate constant k1b(T,P) over the whole range of tropospheric conditions. The dashed arrow in Figure 5 indicates the change of the branching ratio β(T,P) ) k1b/k1a from the Earth’s surface (β(760 Torr, 298 K) ) 3.3%) to the upper troposphere (β(150 Torr, 215 K) ) 2.8%). We see that β only slightly changes throughout the troposphere, being approximately β ≈ 3%. To derive k1b(T,P), it is necessary to take into account that the overall rate constant for the reaction C2H5O2 + NO, k1 ) k1a + k1b, is invariant with pressure35 but exhibits a significant negative temperature dependence according to the recommended expression k1(T) ) 2.6 × 10-12 exp(365/T) cm3 molecule-1 s-1.13 k1b can be expressed as k1b(T,P) ) 0.01k1(T) β(T,P)/(1 + 0.01β(T,P)) ≈ 0.01k1(T) β(T,P), where β(T,P) is given by eq E2. Then, near the Earth’s surface, k1b ≈ 2.8 × 10-13 cm3 molecule-1 s-1 and in the upper troposphere k1b ≈ 3.9 × 10-13 cm3 molecule-1 s-1. The present value of k1b can be used in chemistry-transport models (CTM) to calculate the impact of reaction 1b on the atmospheric composition through the whole troposphere, in particular the impact on concentrations of species involved in the VOC/NOx/ozone chemistry. This impact may not be negligible considering the rather long lifetime of ethyl nitrate as a NOx reservoir (the lifetime of ethyl nitrate is of the order of one month and is mainly controlled by its photolysis36). The integration of the present results in global tropospheric CTM models should also contribute to assess the importance of the photochemical production of ethyl nitrate from reaction 1b, compared to the natural oceanic source. This may change the total emission rate of oceanic ethyl nitrate that has been recently derived by Neu et al.37 from matching observed and modeled ethyl nitrate tropospheric concentrations, apparently not taking into account the photochemical production of ethyl nitrate in their model. The present value of β at pressure and temperature near the Earth’s surface can also be used to better describe the evolution of ethyl nitrate in urban plumes. The evolution of the alkyl nitrates, as rather long-lived NOx reservoirs, can have a significant impact on regional ozone formation. Such evolution has been first investigated by Bertman et al.,38 who compared measured alkyl nitrate/alkane ratios to calculated ones using a simple sequential reaction model, which assumed a single alkane precursor for each alkyl nitrate. This method was shown to be self-consistent for the secondary C4 and C5 nitrates but the model substantially underestimated ethyl nitrate concentrations. Ranschaert et al.8 have estimated that a value of β ∼ 0.17 was needed to reconcile the measured and modeled ethyl nitrate/ ethane concentration ratios of Bertman et al. This value is much higher than the value β ) 0.014,4 considered in the model of Bertman et al. Our present value, β ) 0.033, although approximately 2 times higher, is still much lower than the value inferred from the atmospheric measurements of the ethyl nitrate/ ethane ratio. Such a difference between the inferred and actual values of β values suggests the existence of other sources of ethyl nitrate than the OH-initiated oxidation of ethane (see ref 38 and references therein). More recently, Sommariva et al.,11 came to the same conclusion by comparing measured concentration ratios ethyl nitrate/ethane with the modeled ones using the master chemical mechanism (e.g., see ref 39). Their model underestimates the measured ethyl nitrate/ethane ratio by a factor

964

J. Phys. Chem. A, Vol. 114, No. 2, 2010

up to 4. However, this factor should probably be significantly reduced by using our present value of β ) 0.033, instead of β ) 0.009 considered in the version of the MCM model used. This will reduce the contribution of the precursors of ethyl nitrate other than ethane considered by Sommariva et al., to explain their measurements. These additional precursors include propanal and various oxy radicals, RO, which are sources of C2H5O2 radicals through photolysis and OH reaction (propanal) and decomposition (RO), similarly to ethane through its OH reaction. Acknowledgment. The study has been performed in the frame of the PRIMEQUAL programme of the French Ministry of Ecology, Energy, Sustainable Development and Sea. References and Notes (1) Atkinson, R. Phys. Chem. Ref. Data 1994, 1, Monograph No. 2. (2) Aschmann, S. M.; Long, W. D.; Atkinson, R. J. Phys. Chem. A 2006, 110, 6617. (3) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Phys. Chem. 1982, 86, 4563. (4) Atkinson, R.; Carter, W. P. L.; Winer, A. M. J. Phys. Chem. 1983, 87, 2012. (5) Atkinson, R.; Aschmann, S. M.; Winer, A. M. J. Atmos. Chem. 1987, 5, 91. (6) Harris, S. J.; Kerr, J. A. Int. J. Chem. Kinet. 1989, 21, 207. (7) Cassanelli, P.; Fox, D. J.; Cox, R. A. Phys. Chem. Chem. Phys. 2007, 9, 4332. (8) Ranschaert, D. L.; Schneider, N. J.; Elrod, M. J. J. Phys. Chem. A 2000, 104, 5758. (9) Chow, J. M.; Miller, A. M.; Elrod, M. J. J. Phys. Chem. A 2003, 107, 3040. (10) Carter, W. P. L.; Atkinson, R. J. Atmos. Chem. 1989, 8, 165. (11) Sommariva, R.; Trainer, M.; de Gouw, J. A.; Roberts, J. M.; Warneke, C.; Atlas, E.; Flocke, F.; Goldan, P. D.; Kuster, W. C.; Swanson, A. L.; Fehsenfeld, F. C. Atmos. EnViron. 2008, 42, 5771. (12) Kukui, A.; Borissenko, D.; Laverdet, G.; Le Bras, G. J. Phys. Chem. A 2003, 107, 5732. (13) Sander S. P.; Friedl, R. R.; Golden, D. M.; Kurylo, M. J.; Moortgat, G. K.; Keller-Rudek, H.; Wine, P. H.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J.; Finlayson-Pitts, B. J.; Huie, R. E.; Orkin, V. L. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies; JPL Publication 06-2, Evaluation Number 15; JPL: Pasadena, CA, 2006. (14) Dae¨le, V.; Ray, A.; Vassalli, I.; Poulet, G.; Le Bras, G. Int. J. Chem. Kinet. 1995, 27, 1121.

Butkovskaya et al. (15) Fittschen, C.; Frenzel, A.; Imrik, K.; Devolder, P. Int. J. Chem. Kinet. 1999, 31, 860. (16) Frost, M. J.; Smith, I. W. M. J. Chem. Soc., Faraday Trans. 1990, 86, 1757. (17) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Atmos. Chem. Phys. 2006, 6, 3625. (18) Turnipseed, A. A.; Birks, J. W. J. Phys. Chem. 1991, 95, 6569. (19) Bogan, D. J.; Nesbitt, F. L. J. Phys. Chem. 1994, 98, 1151. (20) Kukui, A.; Le Bras, G. Phys. Chem. Chem. Phys. 2001, 3, 175. (21) Miller, T. M.; Friedman, J. F.; Stevens-Miller, A. E.; Paulson, J. F. Int. J. Mass Spectrom. Ion Processes 1995, 149, 111. (22) Aschi, M.; Cacace, F.; de Petris, G.; Pepi, F. J. Phys. Chem. 1996, 100, 16522. (23) Aoki, N.; Inomata, S.; Tanimoto, H. Int. J. Mass Spectrom. 2007, 263, 12. (24) Huey, L. G.; Hanson, D. R.; Howard, C. J. J. Phys. Chem. 1995, 99, 5001. (25) Ikezoe, I.; Matsuoka, S.; Takebe, M.; Viggiano, A. Gas phase ionmolecule reaction rate constants through 1986; Maruzen Co.: Tokyo, 1987. (26) NIST Web Book of Chemistry; Standard Reference Database 69; NIST: Gaithersburg, MD. (27) Davidson, J. A.; Fehsenfeld, F. C.; Howard, C. J. Int. J. Chem. Kinet. 1977, 9, 17. (28) Sato, K.; Tanimoto, H.; Imamura, T. Chem. Lett. 2005, 34, 1200. (29) Tiernan, T. O.; Chang, C.; Cheng, C. C. EnViron. Health Perspect. 1980, 36, 47. (30) Butkovskaya, N.; Kukui, A.; Le Bras, G. J. Phys. Chem. A 2007, 111, 9047. (31) Barker, J. R.; Lohr, L. L.; Shroll, R. M.; Reading, S. J. Phys. Chem. A 2003, 107, 7434. (32) Zhang, J.; Dransfield, T.; Donahue, N. M. J. Phys. Chem. A 2004, 108, 9082. (33) Arden, E. A.; Phillips, L.; Shaw, R. J. Chem. Soc. 1964, 5126. (34) Baker, G.; Shaw, R. J. Chem. Soc. 1965, 6965. (35) Bardwell, M. W.; Bacak, A.; Raventos, M. T.; Percival, C. J.; Sanchez-Reyna, G.; Shallcross, D. E. Int. J. Chem. Kinet. 2004, 37, 253. (36) Talukdar, R. K.; Burkholder, J. B.; Hunter, M.; Gilles, M. K.; Roberts, J. M.; Ravishankara, A. R. J. Chem. Soc., Faraday Trans. 1997, 93, 2797. (37) Neu, J. L.; Lawler, M. J.; Prather, M. J.; Saltzman, E. S. Geophys. Res. Lett. 2008, 35, L13814. (38) Bertman, S. B.; Roberts, J. M.; Parrish, D. D.; Buhr, M. P.; Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C.; Montzka, S. A.; Westberg, H. J. Geophys. Res. 1995, 100, 22805. (39) Jenkin, M. E.; Saunders, S. M.; Wagner, V.; Pilling, M. J. Atmos. Chem. Phys. 2003, 3, 181.

JP910003A