Pressure and Temperature Dependence of Methyl ... - ACS Publications

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Pressure and Temperature Dependence of Methyl Nitrate Formation in the CH3O2 + NO Reaction Nadezhda Butkovskaya, Alexandre Kukui,† and Georges Le Bras* Institut de Combustion, Aérothermique, Réactivité et Environnement (ICARE), CNRS-INSIS 1C Av. de la Recherche Scientifique, 45071 Orléans Cedex 2, France ABSTRACT: The branching ratio β = k1b/k1a for the formation of methyl nitrate, CH3ONO2, in the gas-phase CH3O2 + NO reaction, CH3O2 + NO → CH3O + NO2 (1a), CH3O2 + NO → CH3ONO2 (1b), has been determined over the pressure and temperature ranges 50−500 Torr and 223−300 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K, the CH3ONO2 yield has been found to increase linearly with pressure from 0.33 ± 0.16% at 50 Torr to 0.80 ± 0.54% at 500 Torr (errors are 2σ). Decrease of temperature from 300 to 220 K leads to an increase of β by a factor of about 3 in the 100−200 Torr range. These data correspond to a value of β ≈ 1.0 ± 0.7% over the pressure and temperature ranges of the whole troposphere. Atmospheric concentrations of CH3ONO2 roughly estimated using results of this work are in reasonable agreement with those observed in polluted environments and significantly higher compared with measurements in upper troposphere and lower stratosphere.

1. INTRODUCTION This work is a continuation of our studies of small (n ≤ 4) alkyl nitrates formation in the reaction of alkyl peroxy radicals with NO:

CH3O2 + NO reaction. At the same time, an accurate knowledge of methyl nitrate formation yields under the full ranges of atmospheric pressure and temperature is necessary for modeling of atmospheric processes, in particular, ozone formation. This article reports the results of a study of methyl nitrate formation in the CH3O2 + NO reaction,

RO2 + NO → RO + NO2 (a) → RONO2

(b)

CH3O2 + NO → CH3O + NO2 (1a)

where R = CnH2n+1 is the alkyl radical. Such reactions are of interest for atmospheric chemistry because (a) leads to the formation of NO2, which further photolysis results in formation of O3, whereas (b), forming organic nitrate, a reservoir or a sink for both NOx and RO2, leads to a decrease in O3 formation [e.g., ref 1]. In two previous recent studies in our lab, formations of ethyl nitrate in the C2H5O2 + NO reaction,2 and of isopropyl nitrate in the i-C3H7O2 + NO reaction,3 have been investigated over ranges of pressure and temperature of interest for the troposphere. The aim of these studies is to complete the database for alkyl nitrate branching ratios, because most of the earlier studies were carried out for the reactions of alkyl peroxy radicals with n ≥ 5 (see refs 4−8 and references therein). Prior to ours, investigations of small alkyl nitrate formation using their direct detection by a chemical ionization massspectrometer (CIMS) were made by Elrod and co-workers9−11 in a turbulent flow reactor. For ethyl and isopropyl nitrates, the branching ratios of 0.6% were determined at 100 Torr and T = 298 K along with a negative temperature dependence over the 213−298 K range.10,11 Formation of methyl nitrate in the CH3O2 + NO reaction has never been observed. Only an upper limit of β ≤ 3% was determined at T = 298 and 213 K and 100 Torr pressure in the study of Scholtens et al.,9 where CH3ONO2 was monitored as a positive ion from proton transfer reaction (PTR). The authors noted that the observed nitrate signal was close to the detection limit. That study was a single attempt of direct detection of CH3ONO2 from the © 2012 American Chemical Society

→ CH3ONO2

(1b)

using a turbulent flow reactor (TFR) coupled with a CIMS. In our laboratory, different regimes of CIMS analysis have been tested for the identification and quantification of organic nitrate species.2,3 In the present work, methyl nitrate and other products were detected in positive mode using the PTR method. The experimental procedure and detection methods are presented below along with the description of the calibration approach. The branching ratio β = k1b/k1a was determined over the pressure range 50−500 Torr at room temperature and the temperature dependence was determined over the temperature range 300−223 K at pressures of 100, 150, and 200 Torr corresponding to the conditions of the upper troposphere. The data set obtained for β, combined with the recommended value for the rate constant k1, allows estimation of the rate constant for CH3ONO2 formation, k1b, over almost the whole range of tropospheric pressure and temperature. Special Issue: A. R. Ravishankara Festschrift Received: November 8, 2011 Revised: February 13, 2012 Published: February 13, 2012 5972

dx.doi.org/10.1021/jp210710d | J. Phys. Chem. A 2012, 116, 5972−5980

The Journal of Physical Chemistry A

Article

Reaction Schemes. F + CH4 + O2 + NO (Main System). Typical concentrations of the reactants in the reactor were [CH4] ≈ 1 × 1015, [O2] ≈ (1−2) × 1016 and [NO] ≈ (0.5−2) × 1015 molecules cm−3. The initial concentration of F-atoms was in the range [F]0 = (2−20) × 1011 molecules cm−3. The reactions of F-atoms with O2 and NO, competing with the F + CH4 reaction 2 and forming FO2 and FNO, respectively, consumed less than 5% of the atoms emerging from the injector. More serious loss of active species was due to the consumption of methyl radicals in the reaction with NO, competing with their reaction with O2 (3):

2. EXPERIMENTAL SECTION Chemical Reactor. Chemical reactions took place in a turbulent flow reactor (TFR) coupled to a quadrupole massspectrometer with chemical ionization described earlier.13 The scheme of the experimental setup is presented in Figure 1. For

CH3 + NO + M → CH3NO + M

Depending on NO concentration, from 20 to 60% of CH3 radicals were consumed by NO forming nitrosomethane, CH3NO. Under the conditions of our experiments, nitrosomethane is a stable molecule that does not react with other products. The methyl peroxy radical reacted with NO giving the major products CH3O and NO2 and a small fraction of methyl nitrate:

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.

CH3O2 + NO → CH3O + NO2 (1a) → CH3ONO2

(2)

CH3 + O2 + M → CH3O2 + M

(3)

(1b)

with k1 = 7.8 × 10−12 cm3 molecule−1 s−1. In all the experiments, reaction 1 was completed within less than 0.5 ms, giving NO2 and methyl nitrate final products. Methoxy radical from reaction 1a could further react with NO2:

the pressure dependence study, the reactor was operated at room temperature (298 ± 2 K) and at pressure from 50 to 500 Torr with typical flow velocity of N2 carrier gas of about 18 m/s (Reynolds number Re ≈ 2700 at 50 Torr and Re ≈ 12000 at 500 Torr). Reaction of F-atoms with methane in the presence of O2 was used as the source of methyl peroxy radicals: F + CH 4 → CH3 + HF

(4)

CH3O + NO2 + M → CH3ONO2 + M −11

−1

(5) −1

with k 5 = 1.4 × 10 cm molecule s . The NO 2 concentration consisted of the product from reaction 1a and a trace impurity in NO. Typically, the NO2 background concentration in the presence of NO was (5−7) × 1011 molecules cm−3. Secondary reaction 5 also forms methyl nitrate, complicating the measurements of the nitrate yield from reaction 1. Two competing secondary reactions reduced formation of methyl nitrate in reaction 5:

with k2 = 7 × 10−11 and k3 = 1 × 10−12 cm3 molecule−1 s−1 (unless specified, the rate constants given in the text are the recommended values from ref 14 at 298 K and 200 Torr). Fatoms were generated by dissociation of CF4 (Messer) in a microwave discharge. CF4 was added to the He carrier-gas flowing in a quartz tube concentrically connected to a moveable injector. The distance from the injector tip to the orifice of the entrance into the ion−molecule reactor was usually 40 cm, which corresponded to a residence time in the TFR of about 20 ms. During the low temperature experiments, the carrier gas was cooled by passing through a metal coil immersed into a liquid N2 bath. The temperature in the reactor was controlled by resistive heating of the inlet tube using a CB100 digital controller (RKC Instrument). Methane (AlphaGaz N45) was introduced into the reactor upstream of the tip of the movable injector. 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-liquid N2 cooled trap and a (Fe)IISO4 filter to prevent penetration of NO2 impurity into the reactor. Methanol (Sigma-Aldrich, 99.8%) used for calibration purposes was introduced into the reactor as preprepared 10% mixture in He. Its concentration was calculated from the rate of the gas mixture pressure drop in a calibrated volume. Calibration of NO2 was done using commercial 0.5% NO2/N2 mixture (AlphaGaz).

3

CH3O + O2 → CH2O + HO2

(6)

CH3O + NO + M → CH3ONO + M (7a) → CH2O + HNO (7b)

with k6 = 1.7 × 10−15 and k7 = 2.2 × 10−11 cm3 molecule−1 s−1. As reaction 6 is relatively slow, NO acted as the principal scavenger. The contribution from reaction 5 to the measured CH3ONO2 yield could be estimated as the k5[NO2]/(k6[O2] + k7[NO]) ratio. Corresponding corrections were made using pressure and temperature dependences for k5, k6, and k7 recommended in ref 14. The branching ratio was obtained as the ratio of the measured nitrate concentration to the final concentration of formaldehyde or methyl nitrite, provided that the stoichiometric conversion of the methoxy radicals from reaction 1a into CH2O and CH3ONO occurs by reaction 7. Reaction 7 occurs via two channels: addition channel 7a and hydrogen abstraction channel 7b. According to recent recommendations, under conditions of our experiments the rate constant of the addition channel (7a) is in the falloff regime with k7a = 1.3 × 10−11 cm3 molecule−1 s−1 at P = 50 Torr and k7a = 2.6 × 10−11 cm3 molecule−1 s−1 at P = 500 Torr.14,15 A thorough study of reaction 7 by Caralp et al.16 combining experimental and 5973



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theoretical methods, showed that at P > 50 Torr the rate constant of the abstraction channel (7b) does not depend on pressure, the recommended value being k7b = 2.3 × 10−12 cm3 molecule−1 s−1 15 on the basis of several the studies.16−18 This gives a pressure dependent CH2O yield ranging from α = k7b/ (k7a + k7b) = 17 ± 3% at 50 Torr to α = 8 ± 1% at 500 Torr. The characteristic reaction time for reaction 7 was of the order of 10−4 s, ensuring complete conversion of the methoxy radicals to the final stable products, formaldehyde and methylnitrite. In such a case,

having the same mass as HOCH2NO2, the calibration was carried out in the presence of O2, causing reaction 10: CH2OH + O2 → CH2O + HO2

(10)

with k10 = 9.6 × 10−12 cm3 molecule−1 s−1. We took advantage of the fact that the rate constant of reaction 10 is orders of magnitude higher than that of O2 reaction with the methoxy radical (6) (k6 = 1.7 × 10−15 cm3 molecule−1 s−1): CH3O + O2 → CH2O + HO2

(6)

An oxygen concentration of about 3 × 10 molecules cm−3 removed the CH2OH radicals without noticeable reduction of CH3O concentration. Sensitivity to CH2O was usually calculated from its signal intensity in the F + CH3OH + NO2 system. The formaldehyde concentration was determined as Δ[CH2O] = (1 − γ)·Δ[CH3OH], where Δ[CH3OH] is the methanol consumption in reaction 8. Sensitivity of the mass spectrometer to formaldehyde produced in consecutive reactions 8b and 10 was calculated from its signal intensity ΔIKcal as S(CH2O) = Δ[CH2O]/ΔIKcal = (1 − γ)·Δ[CH3OH]NO2/ΔIKcal, where K is the mass number for formaldehyde detection. To calibrate methyl nitrite, a flow of NO instead of NO2 was introduced into the CH3OH + F system described above. The concentration of NO was about 1 × 1014 molecules cm−3. The methoxy radicals were converted into the nitrite and formaldehyde via reaction 7 discussed above: 15

β = α·Δ[CH3ONO2 ]/Δ[CH2O]

or

β = (1 − α) ·Δ[CH3ONO2 ]/Δ[CH3ONO]

where Δ[CH2O] and Δ[CH3ONO] are the formaldehyde and methyl nitrite concentrations from reaction 7. In preliminary experiments at P = 200 Torr it was tested that the change of the distance from the injector tip to the orifice of the intrance into the ion−molecule reactor within 20−50 cm did not lead to any change of the signal intensities of the end products. Also, the dependence of the product intensities (CH2O, CH3ONO, CH3ONO2, CH3NO) on NO concentration was measured and compared with computer simulation to verify the supposed mechanism. Calibrations of formaldehyde, methyl nitrate, and methyl nitrite were done by producing them in situ as described in the next section. F + CH3OH (Calibration Reaction). For calibration, methyl nitrate was produced in the reactor using the fast reaction of Fatoms with methanol as the source of methoxy radicals: F + CH3OH → CH3O + HF

CH3O + NO + M → CH3ONO + M (7a) → CH2O + HNO (7b)

(8a)

The nitrite concentration was determined as Δ[CH3ONO] = (1 − α)·γ·Δ[CH3OH], and its detection sensitivity was calculated as SN(CH3ONO) = Δ[CH3ONO]/ΔINcal = γ·Δ[CH3OH]/ΔINcal, where ΔINcal is the signal intensity of CH3ONO detected at mass number N. In this case O2 was also added to avoid a possible interference from the reactions of the hydroxy methyl radical with NO:

→ CH2OH + HF (8b)

The methoxy radicals yield in this fast reaction was measured in a number of studies19−22 that give an estimated value of γ = k8a/(k8a + k8b) = 58 ± 4%. When NO2 was introduced into the reactor ([NO2] ≈ 3 × 1013 molecules cm−3), methyl nitrate was formed via addition reaction 5a: CH3O + NO2 + M → CH3ONO2 + M

CH2OH + NO + M → CH2O + HNO

(5a)

(11a)

→ HOCH2NO + M (11b)

Abstraction channel 5b CH3O + NO2 → CH2O + HONO

with k11 = 2.5 × 10−11 cm3 molecule−1 s−1. Similarly to reaction 9, the expected major reaction pathway is hydrogen abstraction with formation of formaldehyde (11a), but the addition channel (11b) cannot be totally excluded.23 A flow of oxygen was added to eliminate a possible formation of HOCH2NO adduct with the same mass as methyl nitrite by competitive reaction 10. The next section describes the ionization schemes chosen to detect the NO2, CH3ONO2, CH3ONO, CH2O, and CH3OH molecules. CIMS Detection: Ion−Molecule Reactor and Primary Ions. The 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. The ions formed in the IMR entered the ion-optical zone through the 180 μm orifice in the nickel

(5b)

with a rate constant of about k5b = 2 × 10−13 cm3 molecule−1 s−1 20 was not important in our experiments. The nitrate concentration was determined as Δ[CH3ONO2] = γ·Δ[CH3OH], where Δ[CH3OH] is the methanol consumption in reaction 8. Sensitivity of the mass spectrometer to methyl nitrate produced in consecutive reactions 8a and 5a was calculated from its signal intensity ΔIMcal as S(CH3ONO2) = cal Δ[CH3ONO2]/ ΔIM = γ·Δ[CH3OH]NO2/ΔIMcal, where M is the mass number for nitrate detection. The NO2 reaction with CH2OH radical coproduct from reaction 8b is expected to produce formaldehyde and HONO (reaction 9a); however, combination reaction 9b forming a product with the same mass as methyl nitrate cannot be fully excluded:23 CH2OH + NO2 + M → CH2O + HONO

(9a)

→ HOCH2NO2 + M (9b)

As reaction 9 is rather fast, k9 = 2.3 × 10−11 cm3 molecule−1 s−1,23 to avoid a possible interference with CH3ONO2 detection 5974



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peaks result from the side reactions OH + NO2 producing HNO3 (HNO3·H+ at m/z 64 and HNO3·H+·H2O at m/z 82), and CH2OH + NO2 giving HONO (HONO·H+ at m/z 48 and HONO·H+·H2O at m/z 66). According to Aoki et al.,26 NO2+ is the predominant ion in the PTR detection of C1−C2 alkyl nitrates; however, the observed fragmentation of the protonated nitrates is strongly pressure dependent and in case of CH3ONO2 can give a strong protonated peak m/z 78 under certain conditions.27 In the present study, the peak assignment was confirmed using detection with the deuteron transfer reaction (DTR) when the H2O flow into the IMR was changed to D2O. Figure 2b shows the observed NO 2 + , CH 3 ONO 2 ·D + (m/z 79), and CH3ONO2·D+·D2O (m/z 99) ions from CH3ONO2. The high PA of methyl nitrite, 191 kcal/mol,28 provided sensitive detection of the CH3ONO product as the CH3ONO·H+ ion (m/z 62) and CH3ONO·H+·H2O ion cluster (m/z 80) using PTR and their deutero analogs CH3ONO·D+ (m/z 63) and CH3ONO·D+·D2O (m/z 83) using DTR. NO2 was detected in negative mode as the NO2− ion (m/z 46) formed by electron transfer from SF6−:29

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 MTS100 preamplifier. CH3ONO2, CH3ONO, CH2O, and CH3OH 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 = 1−3. As a rule, the water concentration was chosen to produce equal signals from H3O+ and H3O+·(H2O) ions at 200 Torr. 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 → MH+· (H2O)m + (n − m + 1) · H2O m≤n

(1i)

In our study, methanol and formaldehyde with PA = 180 and 170 kcal mol−1, respectively, were monitored at mass numbers m/z 33 and 51 (methanol) and m/z 31 (formaldehyde). Methyl nitrate has a PA of 175.5 kcal mol−1 24,25 and is also expected to undergo reaction 1i. In the PTR mass spectrum of the reaction products in the CH3OH + F + O2 + NO2 system (Figure 2a), two series of lines were identified as belonging to CH2O and CH3ONO2 reaction products. Formaldehyde was observed at m/z 31, 49, and 67 corresponding to CH2O·H+, CH2O·H+·H2O, and CH2O·H+·(H2O)2 ions); CH3ONO2 was observed at m/z 46, 78, and 96 corresponding to NO2+, CH3ONO2·H+, and CH3ONO2·H+·H2O ions. Other minor

SF6− + NO2 → NO2− + SF6

(3i)

The calibrated NO2 signal was used to estimate the trace impurity level of NO2 as well as to measure NO2 concentration from reaction 1a.

3. RESULTS AND DISCUSSION Branching Ratio as a Function of Pressure at Room Temperature. The branching ratio of reaction 1 was measured at T = 298 ± 2 K between P = 50 and 500 Torr using PTR detection method and between 53 and 200 Torr using DTR one. As a rule, each experiment preceded by NO2 calibration and measurement of NO2 background intensity in the presence of NO. The CH3ONO2, CH2O, and CH3ONO product signal intensities were measured from the F + CH4 + O2 + NO system under above determined conditions at several NO concentrations. Then, calibration followed by CH3ONO and CH3OH signal intensity measurements from the F + CH3OH + NO + O2 reaction system and CH3ONO2, CH2O, and CH3OH signal intensity measurements from the F + CH3OH + NO2 + O2 reaction system. The final equations to calculate the branching ratio by using different products originating from channel 1a were cal cal β = (αγ /(1 − γ))(ΔIM /ΔI M )(ΔIK /ΔIK )

(using CH2O)

(E1)

cal cal β = (ΔIM /ΔI M )(ΔIN /ΔIN )(Δ[CH3OH]NO2

/Δ[CH3OH]NO )

(using CH3ONO)

(E2)

cal ΔIM

In these equations, ΔIM and are the CH3ONO2 signal intensities from the main and calibration reaction systems, respectively; ΔIN and ΔINcal are the CH3ONO signal intensities from the main and calibration reaction systems, respectively; ΔIK and ΔIKcal are the CH2O signal intensities from the main and calibration reaction systems, respectively; and Δ[CH3OH]NO2 and Δ[CH3OH]NO denote methanol consumption during the nitrate and nitrite calibration, respectively. The methanol consumption was determined as the average from the measurements at both m/z 33 and 51 intense

Figure 2. Product mass spectra from the F + CH3OH + O2 + NO2 system using PTR (a) and DTR (b) detection modes measured at 200 Torr with [F]0 = 1.2 × 1012, [CH3OH] = 5.2 × 1012, [O2] = 2.3 × 1014, [NO2] = 2.4 × 1013 molecules cm−3. 5975



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Table 1. Determination of the Branching Ratio β = k1b/k1a at Different Pressures (T = 298 K) P (Torr)

[CH4] 1015

[O2] 1016

[NO] 1015

[CH3O]a 1011

[CH3O]b 1011

[CH3ONO2] 109

βc (%)

βd (%)

50

0.96

0.97

70

1.0

0.95

100

1.1

1.1

200

1.0

1.1

300

1.1

1.5

2.1 1.1 0.55 2.1 1.1 0.52 2.0 1.0 0.55 2.1 1.0 0.51 2.2 1.1 0.56

6.91 9.79 13.5 9.62 16.6 22.4 10.3 17.1 23.9 11.1 18.9 25.1 8.04 11.9 15.0

PTR 6.42 9.20 12.6 10.6 16.4 22.9 13.1 19.6 24.6 12.6 18.8 23.7 8.45 12.8 13.6

3.33 4.65 6.12 5.77 9.13 12.9 5.71 10.6 14.6 8.35 11.6 15.2 6.71 9.70 10.0

0.41 0.31 0.25 0.42 0.37 0.34 0.46 0.45 0.34 0.62 0.42 0.41 0.70 0.68 0.47

0.43 0.34 0.28 0.36 0.36 0.33 0.37 0.41 0.32 0.53 0.48 0.45 0.68 0.62 0.51

400

1.0

1.7

500

1.2

2.0

2.0 1.0 0.53 2.1 1.0 0.57

2.80 4.70 4.53 2.80 3.07 2.08

2.78 3.55 3.78 2.10 2.57 1.80

0.93 0.65 0.65 0.70 0.74 0.72

0.79 0.63 0.70 0.95 0.91 0.77

0.473 0.521 0.451

0.394 0.375 0.354 0.357 0.546 0.377 0.430 0.367 0.511 0.516 0.452

βe (%)

0.33 ± 0.08

0.36 ± 0.08

0.39 ± 0.11

0.49 ± 0.12

0.61 ± 0.18

53

0.76

2.40

100

1.15

1.07

200

1.31

2.98

1.95 0.98 0.74 0.49 4.05 2.02 1.01 0.48 2.05 1.03 0.52

0.476 0.604 0.630

3.31 4.88 4.26 2.11 2.55 1.97 DTR 1.27 1.68 1.76 1.87 0.546 0.928 1.45 2.10 0.444 0.609 0.630

5.97 8.80 9.64 12.2 3.18 4.16 8.29 11.2 2.53 3.85 4.29

0.73 ± 0.21

0.80 ± 0.27

0.37 ± 0.08

0.43 ± 0.08

0.49 ± 0.12

a

Concentration of the CH3O product determined from the observed CH2O concentration. bConcentration of the CH3O product determined from the observed CH3ONO concentration. cNormalized by CH2O and corrected for contribution from reaction 5. dNormalized by CH3ONO and corrected for contribution from reaction 5. eAverage branching ratio (error is 1σ). All concentrations are in molecules cm−3.

measurements, the largest contribution being due to the weak CH3ONO2 signal from reaction 1b. Because sensitivity drops with an increase of pressure, the error in CH3ONO2 intensity changed from ∼4% at 50 Torr to ∼22% at 500 Torr (2σ). Additional errors in the determination of the product concentrations are connected with the uncertainties in the channeling of reactions 7 and 8, when formaldehyde is used for normalization of nitrate formation. The uncertainties (1σ) in β indicated in Table 1 include the experimental errors in signal intensity measurements, errors in methanol consumption (∼5%), uncertainties in α (∼14%) and γ (∼7%), and uncertainties connected with the secondary reaction 5. As already mentioned in the Experimental Section, secondary reaction 5 can contribute to the methyl nitrate signal, resulting in an overestimation of its yield from the main reaction 1. This contribution depends on the ratio of NO 2 to NO concentrations and was calculated as the k5[NO2]/k7[NO] ratio. The NO2 concentration is the sum of the NO 2 concentration produced in reaction 1a and the background NO2 concentration introduced by the NO flow. The latter was measured at each pressure at several NO concentrations using

methanol lines. It is necessary to note that the determination of β = k1b/k1a, using CH3ONO as the reference product, is free from the errors connected with the uncertainties in the branching ratios α and γ of reactions 7 and 8, respectively. Coefficient α vanishes because methyl nitrite is formed in the same reaction (7) in both the main and calibration systems, and coefficient γ cancels because calibration of both methyl nitrate and methyl nitrite uses reaction 8 as the initial step. On the other hand, determination of the sensitivity to CH2O simultaneously with that to CH3ONO2 in the F + CH3OH + O2 + NO2 system allows us to eliminate the experimental errors connected with the measurements of methanol consumption. The obtained results are summarized in Table 1. We see that the normalization of the nitrate product concentration by the two different final products (CH2O and CH3ONO) gives consistent results and does not show any systematic deviations. The mean branching ratio at 100 Torr obtained in the present work, β = 0.4 ± 0.1%, is lower than that for ethyl nitrate obtained in our previous work, 0.74 ± 0.10%,2 and in a similar study of Ranschaert et al., 0.6 ± 0.3%.10 The experimental error in β consists mainly of the errors in the signal intensity 5976



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regression presented in Figure 3, where both PTR and DTR results are presented. The dependence obtained is in accord with the general trend of the increase of the nitrate yield with the increase of pressure, which is explained by the necessity of collisional deactivation of the excited intermediates leading to nitrate formation.30,31 It is accepted that the reaction proceeds through the formation of the CH3OONO intermediate complex which can further decompose to CH3O and NO2 or isomerize to CH3ONO2. The isomerization occurs via a loose transition state in the exit channel of the decomposition. The calculated value of CH3ONO2 yield in reaction 1 obtained by Barker et al.30 was found to be ∼0.05% in modeling the conditions of the experiments of Scholtens et al.,9 i.e., P = 100 Torr of N2 carrier gas. However, direct comparison with our results is not quite reasonable, because at the existing level of theory the properties of the transition state are not known, and many arbitrary parameters were used in the calculation. Temperature Dependence of the Branching Ratio. The low temperature measurements with PTR detection of CH2O, CH3ONO, and CH3ONO2 at m/z 31, 62, and 78, respectively, were done at 100 and 200 Torr. It is important that all three observed ions corresponded to protonated molecules without water ligands, because a change of temperature could lead to some redistribution of ionizing water clusters in the IMR. The change of the branching ratio with temperature was determined using as reference products both formaldehyde, β(T)/β(298) = (ΔI78/ΔI31)T/(ΔI78/ ΔI31)298, and methyl nitrite, β(T)/β(298) = (ΔI78/ΔI62)T/ (ΔI78/ΔI62)298. The latter gave more accurate results due to the absence of background and better stability of the signals. The above expressions assume that the branching ratio of reaction 7 does not change with temperature. However, all three rate coefficients for reaction 7 are temperature dependent in accordance with the following power functions: k0(7a)(T) = 2.3 × 10−29 × (T/300)−2.8 cm6 molecule−2 s−1, k∞(7a)(T) = 3.8 × 10−11 × (T/300)−0.6 cm3 molecule−1 s−1,14 and k7b = 2.3 × 10−12 × (T/300)−0.7 cm3 molecule−1 s−1.15 As all three coefficients are negative power functions, the branching fraction of formaldehyde formation, α, only slightly changes through the whole temperature range. For example, it decreases by 18% at 100 Torr and only by 13% at 200 Torr when temperature changes from T = 298 K to T = 228 K. Although not large, the corresponding corrections were done to account for this effect. The results of the measurements are presented in Table 2. To estimate the accuracy of the corrections, we used the recommendations of Atkinson et al.,15 giving for all three rate constants k0(7a)(T), k∞(7a)(T), and k7b the uncertainty of Δn = ± 0.5 over the 200−300 K temperature range for a power parameter n in the k(T) expression. At T = 228 K this gives an uncertainty in the rate constant of about 13% (2σ). The correction coefficients for βT/β298 are α(T)/α(298) for the data with formaldehyde as a reference product (a) and {1 − α(T)}/ {1 − α(298)} for the measurements with methyl nitrite as the reference product (b). The corresponding uncertainties are about 18% and 2%, respectively. At higher pressures and temperatures the corrections are smaller. We see from Table 2 that the βT/β298 values obtained using CH2O and CH3ONO agree well and do not show any systematic deviations, indicating the validity of the corrections used. As the resulting β value for each temperature is an average of (a) and (b), in each case we gave an average additional uncertainty due to Arrhenius factors.

the negative ion mode. For example, the NO2 impurity concentration in NO was equal to 5.3 × 1011 molecules cm−3 for [NO] = 4.2 × 1015 molecules cm−3 ([NO2]imp/[NO] = 1.3 × 10−4) and to 1.9 × 1011 molecules cm−3 at [NO] = 2.1 × 1014 molecules cm−3 ([NO2]imp/[NO] = 9.0 × 10−4). We see from Table 1 that the concentration of NO2 impurity, though less than the NO2 concentration produced from reaction 1a, must be taken into account, especially at the higher pressures. The total contribution from reaction 5, k5([CH3O]+[NO2]imp)/ k7[NO], was calculated by assuming that NO2 produced in reaction 1a is equal to the concentration of the CH3O product determined either from the observed CH2O or CH3ONO concentration (Table 1). The nitrate yield measurements were done at several NO concentrations, when the [NO2]tot/[NO] ratio changes substantially. The correction for secondary reaction 5 was more important at low pressures (up to 50%), when it was possible to obtain higher generation of F-atoms, and, concequently, NO2 product concentration. At P ≥ 400 Torr, when the total NO2 concentration was relatively low, the correction was about 12%. At the lowest pressure of 53 Torr for DTR detection, it was possible to measure the nitrate signal intensity using CH3ONO2·D+·D2O ions (m/z 99) with sensitivity 2 orders of magnitude less than that using CH3ONO2·D+ ions (m/z 79). This measurement allowed us to verify the absence of any background contributions to the nitrate signal. The average CH3ONO2 yield obtained in this manner for four NO concentrations used in the experiment was 0.39 ± 0.09% compared to 0.37 ± 0.06% obtained using detection at m/z 79.

Figure 3. Pressure dependence of the branching ratio for methyl nitrate formation in the CH3O2 + NO reaction at 298 ± 2 K. The cross indicates extrapolation to atmospheric pressure.

Figure 3 presents the average branching ratio measured at different pressures. The plot of the nitrate yield against pressure data gives a linear pressure dependence described by eq E3: β(P) (%) = (1.03 ± 0.04) × 10−3· P(Torr) + 0.29 ± 0.01 (E3)

Extrapolation to zero pressure gives a nonzero intercept of about 0.3%. For P = 760 Torr extrapolation gives β = 1.07 ± 0.14%. The uncertainty of the extrapolation (2σ) was calculated from the standard deviation of the nonweighted linear 5977



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Table 2. Temperature Dependence of the Branching Ratio β(T) = k1b/k1a P = 100 Torr (PTR) T (K)

βT/β298a

βT/β298b

298 293 290 283 279 273 263 253 246 242 233

1 1.09 1.09 1.23 1.22 1.35 1.69 2.13 2.15 2.66 2.75

1 1.11 1.07 1.18 1.19 1.27 1.48 1.78 1.89 2.21 2.51

P = 150 Torr (DTR) β (%)

T (K)

βT/β298a

βT/β298b

± ± ± ± ± ± ± ± ± ± ±

300 292 291 282 273 264 251 242 233 228 225

1 1.08 1.12 1.17 1.32 1.68 1.93 2.14 2.19 2.99 2.82

1 1.09 1.16 1.24 1.42 1.81 2.10 2.48 2.44 2.80 2.91

0.39 0.42 0.43 0.47 0.47 0.51 0.62 0.76 0.79 0.95 1.03

0.11 0.15 0.16 0.18 0.18 0.19 0.21 0.29 0.30 0.35 0.37

P = 200 Torr (PTR) β (%)

T (K)

βT/β298a

βT/β298b

± ± ± ± ± ± ± ± ± ± ±

298 288 285 273 261 255 248 241 235 233 223

1 1.17 1.23 1.51 1.35 1.48 1.85 2.16 2.54 2.85 3.84

1 1.07 1.17 1.40 1.50 1.55 2.03 2.35 2.84 2.70 3.86

0.41 0.44 0.46 0.48 0.54 0.69 0.79 0.88 0.90 1.22 1.15

0.11 0.15 0.15 0.16 0.19 0.22 0.27 0.31 0.31 0.40 0.41

β (%) 0.49 0.53 0.58 0.69 0.74 0.77 1.00 1.16 1.21 1.33 1.61

± ± ± ± ± ± ± ± ± ± ±

0.12 0.18 0.20 0.23 0.25 0.26 0.31 0.38 0.41 0.42 0.52

a

Relative change of the CH3ONO2 yield using CH2O as a reference product. bRelative change of the CH3ONO2 yield using CH3ONO as a reference product.

4. ATMOSPHERIC IMPLICATIONS The pressure dependence presented in Figure 3 allows extrapolation to atmospheric pressure, giving β = 1.07 near the Earth’s surface. The temperature dependence obtained gives for the altitude of about 12 km (T ≈ 220 K, P ≈ 150 Torr) β = 1.2% (Figure 4). Thus, a generic value of β = k1b/k1a ≈ 1.0 ± 0.7% can be used to estimate the rate constant k1b(T,P) over the whole range of tropospheric conditions from the surface to the upper troposphere (error is 2σ). To derive k1b(T,P), it is necessary to take into account that the overall rate constant for the reaction CH3O2 + NO, k1 = k1a + k1b, is pressure independent32 and exhibits a slight negative temperature dependence according to the recommended expression k1(T) = 3.0 × 10−12 exp(285/T) cm3 molecule−1 s−1.15 Thus, 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)% cm3 molecule−1 s−1. The present data are available to be entered into chemistrytransport models 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 methyl nitrate as a NOx reservoir (the lifetime of methyl nitrate is of the order of 1 week to 1 month and is mainly controlled by its photolysis33). The present data can also be used to compare calculated and measured methyl nitrate concentrations in the atmosphere. Methyl nitrate concentrations have been measured in different environments: polluted troposphere, remote troposphere from the surface to the upper troposphere, lower stratosphere. In urban environments and pollution plumes median measured concentrations are often of the order of 10−20 pptv [e.g., refs 34−36]. In these field studies, the authors mentioned that such values were too high to be explained by the photochemical source consisting of the OH oxidation of methane that produces CH3ONO2 through reaction 1b in the high NOx regime. In their estimation of the methyl nitrate production they considered very low values of the branching ratio β in the absence of data. Considering our measured value of β ≈ 0.01 at the Earth's surface, the production of CH3ONO2 can be estimated to be around 10 pptv after 1 day, taking k(OH + CH4) = 6.3 × 10−15 cm3 molecule−1 s−1, a concentration of [OH] = 1 × 106 molecules cm−3 (daytime average), and 2 ppm of methane, typical of polluted environments. Our estimation is

The temperature dependence was also determined at P = 150 Torr using DTR detection of CH2O, CH3ONO, and CH3ONO2 at m/z 52, 63, and 79, respectively (Table 2). At room temperature of 300 K, the branching ratio determined at this pressure using CH2O as the reference product was β = 0.41%. The results for both PTR and DTR low-temperature measurements are shown in Figure 4 representing β against

Figure 4. Temperature dependence of the branching ratio for CH3ONO2 formation at different pressures.

reverse temperature. A similar negative temperature effect was found at all pressures with about a 3-fold increase of β when the temperature changes from T = 298 K to T = 223 K. Such temperature behavior is typical for nitrate formation yield in the RO2 + NO reaction, as was observed for ethyl and propyl peroxy radicals2,3,10,11 as well as for heavier alkyl peroxy radicals.6−8 Theoretical studies predict a sharp increase of nitrate yield with the decrease of temperature.30,31 According to Zhang et al.,31 the temperature dependence is determined by the energy difference between the transition state for decomposition to CH3O and NO and transition state for isomerization to CH3ONO2. The decomposition dominates at the initial reactant energy, whereas isomerization is important at low energies typical for the thermal decomposition of CH3OONO. 5978



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(2) Butkovskaya, N.; Kukui, A.; Le Bras, G. J. Phys. Chem. A 2010, 114, 956−964. (3) Butkovskaya, N.; Kukui, A.; Le Bras, G. Z. Phys. Chem. 2010, 224, 1025−1038. (4) Aschmann, S. M.; Long, W. D.; Atkinson, R. J. Phys. Chem. A 2006, 110, 6617−6622. (5) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L; Winer, A. M.; Pitts, J. N. Jr. J. Phys. Chem. A 1982, 86, 4563−4569. (6) Atkinson, R.; Carter, W. P. L; Winer, A. M. J. Phys. Chem. 1983, 87, 2012−2018. (7) Atkinson, R.; Aschmann, S. M.; Winer, A. M. J. Atmos. Chem. 1987, 5, 91−102. (8) Cassanelli, P.; Fox, D. J.; Cox, R. A. Phys. Chem. Chem. Phys. 2007, 9, 4332−4337. (9) Scholtens, K. W.; Messer, B. M.; Cappa, C. D.; Elrod, M. J. Phys. Chem. A 1999, 103, 4378−4384. (10) Ranschaert, D. L.; Schneider, N. J.; Elrod, M. J. J. Phys. Chem. A 2000, 104, 5758−5765. (11) Chow, J. M.; Miller, A. M.; Elrod, M. J. J. Phys. Chem. A 2003, 107, 3040−3047. (12) 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− 5786. (13) Kukui, A.; Borissenko, D.; Laverdet, G.; Le Bras, G. J. Phys. Chem. A 2003, 107, 5732−5742. (14) 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. (15) 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, 3625−4055. (16) Caralp, F.; Rayez, M.-Th; Forst, W.; Gomez, N.; Delcroix, B.; Fittschen, C.; Devolder, P. J. Chem. Soc., Faraday Trans. 1998, 94, 3321−3330. (17) Daële, V.; Laverdet, G.; Le Bras, G.; Poulet, G. J. Phys. Chem. 1995, 99, 1470−1477. (18) Frost, M. J.; Smith, I. W. M. J. Chem. Soc., Faraday Trans. 1990, 86, 1757. (19) Durant, J. L. Jr. J. Phys. Chem. 1991, 95, 10701−10704. (20) McCaulley, J. A.; Anderson, S. M.; Jeffries, J. B.; Kaufman, F. Chem. Phys. Lett. 1985, 115, 180−186. (21) Pagsberg, P.; Munk, J.; Anastasi, C.; Simpson, V. J. J. Phys. Chem. 1989, 93, 5162−5165. (22) Dobé, S.; Berces, T.; Temps, F.; Wagner, H.Gg.; Ziemer, H. 25th Symp. Int. Combust. Proc. 1994, 25, 775−781. (23) Pagsberg, P.; Munk, J.; Anastasi, C.; Simpson, V. J. J. Phys. Chem. 1989, 93, 5162. (24) Sunderlin, L. S.; Squires, R. R. Chem. Phys. Lett. 1993, 212, 307− 311. (25) Attinà, M.; Cacace, F.; Yanez, M. J. Am. Chem. Soc. 1987, 109, 5092−5097. (26) Aoki, N.; Inomata, S.; Tanimoto, H. Int. J. Mass Spectrom. 2007, 263, 12−21. (27) Inomata, S.; Tanimoto, H.; Aoki, N. J. Mass Spectrom. Soc. Jpn 2008, 56, 181−187. (28) Hunter, E. P.; Lias, S. G. J. Phys. Chem. Ref. Data 1998, 27, 413. (29) Huey, L. G.; Hanson, D. R.; Howard, C. J. J. Phys. Chem. 1995, 99, 5001−5008. (30) Barker, J. R.; Lohr, L. L.; Shroll, R. M.; Reading, S. J. Phys. Chem. A 2003, 107, 7434. (31) Zhang, J.; Dransfield, T.; Donahue, N. M. J. Phys. Chem. A 2004, 108, 9082−9095. (32) Bacak, A.; Bardwell, M. W.; Raventos, M. T.; Percival, C. J.; Sanchez-Reyna, G.; Shallcross, D. E. J. Phys. Chem. A 2004, 108, 10681−10687.

not inconsistent with measured field concentrations of CH 3ONO 2 although the photochemical production of CH3ONO2 through reaction 1b can be higher than estimated here without accounting for additional sources of CH3O2 such as decomposition of large alkoxy radicals in polluted environments [e.g., ref 12]. In the remote troposphere the measured concentrations of methyl nitrate generally do not exceed 5 ppt except for the boundary layer and upper troposphere over the tropical and Southern oceans, where elevated concentrations of methyl nitrate, up to 40−50 ppt, have been observed.37 In these remote regions the predominant source of CH3ONO2 is considered to be oceanic emissions originating from aqueousphase photochemistry.38 This mechanism is supported by the observation of supersaturation of alkyl nitrates in tropical surface waters,39 as well as by comparison of the CH3ONO2 observed and modeled using 3D chemistry-transport models with accordingly adjusted oceanic sources.40,37 Estimation of the CH3ONO2 concentrations using the branching ratio of β = 0.01 derived in this work without accounting for the oceanic sources results in CH3ONO2 concentrations ranging from 60 to 30 ppt for the altitudes from the oceanic surface to the upper troposphere. The steady-state calculations of the median methyl nitrate concentrations have been performed using median averaged values from the data set from the DC8 flights during the PEM-B campaign.41 These concentration levels are about 2−5 times higher than observed (even without accounting for the nitrate emission sources). However, these calculated concentration levels become similar to the observed ones if we consider β = 0.003, the lower limit of our uncertainty domain. At the same time, a significant reduction of the oceanic emission sources is required for getting an agreement of modeled and observed CH3ONO2 concentrations if the branching ratio derived in this work is used. More profound disagreement, at least a factor of 30, should be noted between β found in this work, 0.3% lower limit, and that estimated from the only known to us CH3 ONO 2 measurements in the lower stratosphere, about 0.01%, where the influence of the oceanic emission could be neglected.42 The reason for these disagreements with field measurements is not clear, although it is possible that using a simplified steady-state approach for the CH3ONO2 estimation may be not adequate and that, for an accurate accounting for the long lifetime of methyl nitrate, a detailed chemistry-transport model should be used for comparison with measurements.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), CNRS-IPSL, Verrières-le-Buisson, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study has been performed in the frame of the PRIMEQUAL programme of the French Ministry of Ecology, Energy, Sustainable Development and Sea.

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REFERENCES

(1) Atkinson,R. Phys. Chem. Ref. Data 1994, Monograph No. 2, 1 5979



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(33) Talukdar, R. K.; Burkholder, J. B.; Hunter, M.; Gilles, M. K.; Roberts, J. M.; Ravishankara, A. R. J. Chem. Soc., Faraday Trans. 1997, 93, 2797−2805. (34) Worton, D. R.; Reeves, C. E.; Penkett, S. A.; Sturges, W. T.; Slemr, J.; Oram, D. E.; Bandy, B. J; Bloss, W. J.; Carslaw, N.; Davey, J.; Emmerson, K. M.; Gravestock, T. J.; Hamilton, J. F.; Heard, D. E.; Hopkins, J. R.; Hulse, A.; Ingram, T.; Jacob, M. J.; Lee, J. D.; Leigh, R. J.; Lewis, A. C.; Monks, P. S.; Smith, S. C. Atmos. Environ. 2010, 44, 773−785. (35) Simpson, I. J.; Wang, T.; Guo, H.; Kwok, Y. H.; Flocke, F.; Atlas, E.; Meinardi, S.; Rowland, F. S.; Blake, D. R. Atmos. Environ. 2006, 40, 1619−1632. (36) Flocke, F.; Volz-Thomas, A.; Buers, H. J.; Pätz, W.; Garthe, Hans-Jürgen; Kley, D. J. Geophys. Res. 1998, 103, 5729−5746. (37) Blake, N. J.; Blake, D. R.; Swanson, A. L.; Atlas, E.; Flocke, F. J. Geophys. Res. 2003, 108, 8242. (38) Dahl, E. E.; Saltzman, de Bruyn, W. Geophys. Res. Lett. 2003, 30, 1271. (39) Dahl, E. E.; Yvon-Lewis, S. A.; Saltzman, E. S. Geophys. Res. Lett. 2005, 32, L20817. (40) Neu, J. L.; Lawler, M. J.; Prather, M. J.; Saltzman, E. S. Geophys. Res. Lett. 2008, 35, L13814. (41) GTE Data Archive, PEM-B, DC8 flights, 5 min merged data, ftp://ftp-gte.larc.nasa.gov/pub/TRACEP/MERGES/DC8_LARC/5_ MINUTE/. (42) Flocke, F.; Atlas, E.; Madronich, S.; Schauffler, S. M.; Aikin, K.; Margitan, J. J.; Bui, T. P. Geophys. Res. Lett. 1998, 11, 1891−1894.

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