Cross-sections and quantum yields for the atmospheric photolysis of the potent greenhouse gas nitrogen trifluoride

Atmospheric Environment 44 (2010) 1186e1191

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Cross-sections and quantum yields for the atmospheric photolysis of the potent greenhouse gas nitrogen trifluoride Terry J. Dillon*, Abraham Horowitz, John N. Crowley Max Planck Institute for Chemistry, Joh.-Joachim-Becher-Weg 27, 55128 Mainz, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 October 2009 Received in revised form 17 December 2009 Accepted 21 December 2009

Although NF3, a trace gas of purely anthropogenic origin with a large global warming potential is accumulating in the Earth's atmosphere, little photochemical data exists from which to calculate its atmospheric removal rate. In this study, photodissociation quantum yields, F1, were derived following 193.3 nm laser photolysis of NF3, and quantitative conversion of the F-atom photoproducts to OH, which was detected by laser induced fluorescence. Values of F1(P, T) ¼ (1.03  0.05) were determined at pressures between 28 and 100 mBar of He or N2 and at either room temperature or 255 K. Absorption cross-sections, s, obtained between 184 and 226 nm were combined with the values of F1(P, T) to confirm a long (z700 year) photolysis lifetime for NF3. No evidence for reaction of OH with NF3 was found, indicating that this process makes little or no contribution to NF3 removal from the atmosphere. These results underpin recent calculations of an NF3 atmospheric lifetime s z 550 years, largely controlled by photolysis in the stratosphere. In the course of this work the rate coefficient k2(298 K) ¼ (1.3  0.2)  1011 cm3 molecule1 s1 was obtained for the reaction F þ H2O. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Photolysis Cross-sections Quantum yields Greenhouse gas NF3

1. Introduction Nitrogen trifluoride, NF3, a gas of purely anthropogenic origin, is widely used in the microelectronics industry for plasma etching silicon wafers. Molina et al. (1995) studied NF3 atmospheric chemistry, and measured absorption cross-sections, s, for NF3 considerably smaller than those reported previously (Makeev et al., 1975). NF3 was not removed by other atmospheric processes (reaction with O3, aqueous uptake etc.), leading Molina et al. (1995) to calculate an atmospheric lifetime s > 700 years, largely controlled by photolysis in the upper-stratosphere. This long lifetime, coupled with strong IR absorption features reported for NF3 (Robson et al., 2006) are indicative of a large Global Warming Potential (GWP). Interest in NF3 has been stimulated by dramatic expansion of its production, prompted by the phasing out of alternatives such as the perfluorocarbons (restricted by the Kyoto protocol) and increased demand for flat-screen devices. More recently Prather and Hsu (2008) incorporated the absorption crosssections of Molina et al. (1995) and data on the O(1D) mediated loss of NF3 (Sorokin et al., 1998), into a 3-D chemistry and transport model to calculate a shorter lifetime of z550 years. Prather and Hsu (2008) noted that NF3, which is not included in the Kyoto

* Corresponding author. Tel.: þ49 6131 305 313; fax: þ49 6131 305 388. E-mail address: [email protected] (T.J. Dillon). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.12.026

protocol, has a larger GWP (on a 100 year time horizon ¼ 16 800), than the perfluorocarbons it replaces. The first atmospheric measurements of NF3 (Weiss et al., 2008), have recently shown that ambient levels (0.454 ppt in 2008) are increasing at about 11% per year, corresponding to release of z16% of the global NF3 production estimate (4 Mg), larger than the 2% estimated by industry (Weiss et al., 2008). Such estimates are however precarious as long as some of the photochemical parameters required to calculate lifetimes are uncertain, missing or are based on assumptions or estimates. In this work laboratory techniques were used to determine cross-sections, s and for the first time photolysis quantum yields, F for NF3. Attention was given to measurements of s in the actinic range 200e210 nm, where literature discrepancies are considerable (factor of z75). The larger portion of the manuscript describes laser-based experiments to determine F for F-atom formation in NF3 photolysis (R1). Since NF3 contains only NeF bonds, the F-atom quantum yield may be equated to the overall quantum yield, relevant for lifetime calculations.

2. Experimental The experiments detailed in this work used Pulsed Laser Photolytic (PLP) initiation of reaction coupled to OH detection by pulsed Laser Induced Fluorescence (LIF). NF3 and H2O concentrations were accurately monitored by VUV absorption spectroscopy.

T.J. Dillon et al. / Atmospheric Environment 44 (2010) 1186e1191

The apparatus has been detailed previously (Wollenhaupt et al., 2000; Dillon et al., 2006) and is described only briefly below.

2.1. Determination of UV absorption cross-sections, s, for NF3 Absorption cross-sections, s, were determined by measuring the attenuation of UV light by static samples of pure NF3 in an absorption cell of path length l ¼ 30.4 cm, fitted with quartz windows. The absolute pressure of NF3 was monitored by a calibrated 1000 Torr capacitance manometer (MKS). Collimated light from either a low-pressure Hg lamp (l ¼ 184.9 nm) or a D2 lamp (180e260 nm) transmitted the sample and was focussed onto the entrance slit of a 0.5 m monochromator equipped with a grating (300 lines mm1 blazed at 200 nm) and a diode-array detector (Oriel Instaspec2). The volume around the lamp, cell, and monochromator was purged with N2 to minimise absorption by O2 or H2O. Spectra were recorded at an experimental resolution of 0.4 nm, the full-width at half-maximum of the l ¼ 184.9 nm Hg line.

2.2. F-atom quantum yields from NF3 photolysis PLP-LIF experiments to determine F were conducted in a 500 cm3 jacketed photolysis cell. Circulation of a cryogenic fluid through the outer jacket provided temperature control. The cell pressure and the gas flow rate (1e3 L (STP) min1) were regulated to ensure that a fresh gas sample was available for photolysis at each laser pulse (10 Hz). Pulsed light (z20 ns) was provided by an exciplex laser (Lambda Physik) operating at l ¼ 193.3 nm (ArF). The output from a frequency doubled Nd-YAG (Quantel Brilliant B) pumped dye laser (Lambda Physik Scanmate with Rhodamine 6G dye) was used to excite OH via the 282 nm A2Sþ (v ¼ 1) ) X2P (v ¼ 0) transition. Laser induced fluorescence was detected by a photomultiplier tube screened by 309 nm (interference) and BG 26 (glass) filters. F-atom photoproducts formed in (R1) were converted (R2) to OH by reaction with H2O:

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N2 O D hnð193:3 nmÞ / Oð1 DÞ D N2

(R4)

Oð1 DÞ D H2 O / 2OH

(R5)

The arbitrary LIF signals associated with (R1) and (R2) were converted into F1 values (see Section 3.2) by comparison with OH observed from (R4) and (R5). In the majority of experiments, He bath gas at 28 < P/ mbar < 100 was used. Typically, [OH] in the range of (1e10)  1010 molecule cm3 were generated from laser fluences of 8 mJ cm2 in conjunction with (in units of 1013 molecule cm3) [H2O] ¼ 400e1000 and either [NF3] ¼ 10e100 or [N2O] ¼ 1e10. Concentrations of H2O were determined optically at l ¼ 184.9 nm (s184.9nm ¼ 7.08  1020 cm2 molecule1 (Chung et al., 2001)) in a 43.8 cm absorption cell located downstream of the reactor. Other reagent concentrations were determined by manometric methods (estimated accuracy  15%), though larger [NF3] were monitored at 184.9 nm and, using s values from this work, were in good agreement (5%) with manometric determinations. Chemicals: He, N2 and O2 (all Messer 99.999%) were used as supplied. NF3 (ABCR 99.99%), and N2O (Messer 99.5%) were subject to repeated T ¼ 77 K freeze-pump-thaw cycles prior to dilution in He and storage in blackened glass bulbs. An alternative supply of pre-mixed NF3 (Air Liquide 5% in 99.999% N2) was used as supplied. H2O (“milli-Q” de-ionised water) was subject to similar freezepump-thaw cycles, dilution and storage in glass bulbs, or without further purification, was added to the reactor via a bubbler when larger concentrations were required. H2O2 (Fluka 35%) was concentrated by bubbling a small flow of N2 through the sample at P z 14 mbar for several hours prior to use. 3. Results and discussion

NF3 D hnð193:3 nmÞ / F D ðother productsÞ

(R1)

Determinations of absorption cross-sections (s, Section 3.1), photolysis quantum yields (F, Section 3.2) and rate coefficients (k, Section 3.3) from this work are detailed below. Unless stated otherwise, errors quoted throughout are two standard deviations, precision only.

F D H2 O / OH D HF

(R2)

3.1. Absorption cross-sections (s) for NF3

Whilst other products of (R1), eg. NF2 are thermodynamically unlikely to react with H2O to produce OH, (R2) is both rapid (see Section 3.3 below) and product-specific (IUPAC, 2009). A large excess of H2O was used to ensure complete titration of F to OH, and also ensured that changes in other reagent concentrations had a minimal effect on the LIF detection sensitivity, eg. by fluorescence quenching or other energy transfer processes in OH (A2Sþ). 193.3 nm photolysis of H2O (R3) also produces OH (IUPAC, 2009). H2 O D hnð193:3 nmÞ / OH D H

(R3)

Any LIF signals observed were not therefore attributed solely to (R1) and (R2), but included a component from (R3), which was quantified in experiments where NF3 was absent. The H atoms formed are expected to be long lived in the chemical systems employed as reactions with H2O and N2O (see later) have large barriers and are only rapid at very high temperatures (Baulch et al., 1992, 2005; Arthur et al., 1997). Similarly, at the low radical densities employed, recombination of H and OH (k z 1012 cm3 molecule1 s1 at 100 Torr He, Baulch et al., 2005) is insignificant. Back-to-back “calibration” experiments were conducted in unchanged conditions of P, T and [H2O], in which NF3 was replaced by N2O to generate OH in the well-characterised reactions (R4) and (R5):

Attenuation of UV light by NF3 (see Section 2.1) was used to obtain absorption cross-sections, s via the BeereLambert Law (E1).

I ¼ I0  expf  s[½NF3 g

(E1)

where I and I0 are respectively the transmitted and incident light intensity. The optical path length, l was 30.4 cm. Evidence of contamination by small amounts of NO (up to 1 part in 105 NF3) was found in the NF3 samples used in this work. Similarity in the boiling points of NO and NF3 meant that this contamination was difficult to completely remove by distillation. Larger amounts (104  [NF3]) were found in an alternative supply of dilute 5% NF3 in N2 (not subsequently used in these spectroscopic experiments), which may suggest such contamination to be endemic. As a result it was necessary to subtract a scaled NO spectrum from our data. Briefly, absorption from a dilute (2  104 in N2) sample of NO was recorded using unchanged conditions of cell l, P, monochromator setting and resolution. The amount of NO present was calculated by visually-guided subtraction of the spectra. Whilst the NO impurity level was always small (z1  105 for the spectra used in Fig. 1) and did not significantly influence manometric calculations of [NF3], corrections to the optical data could be large. For example, at the NO feature around l ¼ 215 nm the absorption from NO was a factor of z3 greater than that due to NF3. Fig. 1 displays (as the solid line)

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the 200e210 nm range, important for photolysis in the stratosphere, there is excellent agreement (all within 5%) between the cross-sections derived in this work and the results of Molina et al. (1995) which are displayed as triangles on Fig. 1. 3.2. Photolysis quantum yields (F1) for NF3

Fig. 1. Main panel: UV absorption spectra of NF3 from this work (the solid line) and previous studies: the dashed line depicts data retrieved manually from the graphic published in La Paglia and Duncan (1961); diamonds are from Makeev et al. (1975); the star from Inel (1993); and triangles from Molina et al. (1995). The insert displays a BeereLambert plot of l ¼ 184.9 nm data from this work, from which s184.9nm ¼ (3.52  0.05)  1020 cm2 molecule1 was calculated (displayed as the solid circle datapoint on the main panel).

the resulting spectrum from this work, smoothed to a resolution of 2 nm, together with results from four previously published studies. Discrete values for s are reported in Table 1. Also listed in Table 1 is a value for s184.9nm obtained in experiments using the isolated Hg line (see Dillon et al. (2005) for procedure). These data (see plot of ln(I0/I) vs. [NF3] displayed as an insert to Fig. 1) were not corrected for NO contamination, since l ¼ 184.9 nm lies in a local minimum of the NO spectrum. The result of s184.9nm ¼ (3.52  0.05)  1020 cm2 molecule1 (depicted as the filled circle in Fig. 1) was subsequently used for online monitoring of [NF3] in this work (see Section 2.2). There is reasonable agreement (4%) between this discrete value, and linear interpolations from available spectra of s184.9nm ¼ 3.64  1020 cm2 molecule1 (from the continuous spectrum reported in this work) or 3.44  1020 cm2 molecule1 from Molina et al. (1995). Makeev et al. (1975) reported considerably larger cross-sections, which are represented in Fig. 1 by the diamond datapoints. The reasons for this discrepancy are unclear though it seems unlikely to have resulted from NO contamination, as the interfering spectral lines are rather distinctive even at moderate resolution, and sample purity was checked by mass-spectrometry. Large discrepancies remain (see Fig. 1) in the literature values for s in the VeUV between those of La Paglia and Duncan (1961) and Inel (1993). In

A typical dataset of OH profiles resulting from the back-to-back photolysis of NF3 or N2O in the presence of water vapour or from the photolysis of pure H2O at 193.3 nm are displayed in Fig. 2. The data in this figure were recorded at P ¼ 100 mbar (He) and T ¼ 298 K and in the presence of [H2O] ¼ 6.4  1015 molecule cm3. The triangles depict OH formed following 193.3 nm photolysis (R3) of H2O only. Larger LIF signals were observed upon addition of either NF3 or N2O to the photolysis mixture. The square datapoints in Fig. 2 depict the LIF signal observed following addition of [NF3] ¼ 8.2  1014 molecule cm3. The larger OH signal is attributed to photolysis of NF3, followed by titration of the F-atoms product with H2O (R2). This experiment was repeated at several different [NF3] on a constant background [H2O], with larger OH signal enhancement observed for larger [NF3]. Fig. 2 also displays OH signals (circle datapoints) recorded with N2O added to the 193.3 nm H2O photolysis mixture instead of NF3. In this case, OH is formed following N2O photolysis at 193.3 nm and subsequent titration of O(1D) to OH (R5), in addition to that direct from (R3). Combining the large concentrations of H2O relative to N2O with the known rate coefficients for reaction of O(1D) with H2O and N2O (IUPAC, 2009) we can show that O(1D) was converted to OH with a yield of 2. Similarly, in the absence of reaction partners other than H2O in this system, F-atoms are converted to OH with a yield of unity. The simple kinetic expression (E2) was found to adequately represent the data from all three experiments and was used in nonlinear least-squares fits to obtain the yield of OH, Sobs:

S ¼ Sobs expðkloss tÞ

(E2)

where S is the time dependent LIF signal and kloss is the first-order rate coefficient for OH loss. As OH is unreactive with respect to NF3,

Table 1 NF3 absorption cross-sections, s (in 1022 cm2 molecule1) measured in this work.

l/nm

s

l/nm

s

l/nm

s

184.0 184.9 186.0 188.0 190.0 192.0 194.0 196.0

422.0 350.0 299.0 206.0 137.0 89.6 57.0 35.6

198.0 200.0 202.0 204.0 206.0 208.0 210.0 212.0

21.5 13.6 8.68 5.36 3.63 2.46 1.68 1.10

214.0 216.0 218.0 220.0 222.0 224.0 226.0

0.76 0.58 0.46 0.35 0.23 0.14 0.15

Fig. 2. LIF signals recorded at 298 K following 193.3 nm photolysis of NF3/N2O/H2O mixtures in 100 mbar of He bath gas: triangles ¼ photolysis of [H2O]; circles ¼ photolysis of N2O in presence of H2O (R4) and (R5); squares ¼ photolysis of NF3 in presence of H2O (R1) and (R2). The data were fit with kinetic expression (E2) to obtain observed LIF yields.

T.J. Dillon et al. / Atmospheric Environment 44 (2010) 1186e1191

N2O and H2O, and because low radical densities were used, the loss of OH (z100 s1) is dominated by slow transport out of the LIF area. This expression is valid as long as OH is not removed on the time scale of its generation. The large concentrations of H2O ensure that this is always the case. Values of Sobs from the NF3, N2O and H2O photolysis systems can be converted to relative quantum yields if the relative number of photons absorbed by NF3, N2O or H2O are known. The number of photons (per cm3) absorbed (Nabs) by each one of these absorbers can be calculated (E3) from their measured concentrations and known absorption cross-sections at 193.3 nm via the modified BeereLambert law:

ðNabs Þ ¼ E0 f1  expðs193:3 nm C[Þg=[

(E3)

where E0 is the incident laser light intensity, measured by the joulemeter (typically z 7 mJ cm2 equivalent to 7  1015 photons cm2 at 193.3 nm), C is the concentration of absorber, and l is the photolysis light path length. It should be emphasised that, for relative quantum yield measurements, neither the optical path length nor the incident laser energy (which both remain unchanged in back-to-back measurements) need to be known absolutely or accurately. The absorption cross-section used for N2O was s193.3nm (N2O) ¼ 89.5  1021 cm2 molecule1, obtained by linear interpolation of evaluated literature values (IUPAC, 2009). For NF3 a value of s193.3nm(NF3) ¼ 6.78  1021 cm2 molecule1 was obtained from the spectrum derived in this work. By comparison, a value of 6.67  1021 cm2 molecule1 may be obtained by linear interpolation of the data from Molina et al. (1995). For H2O we used s193.3nm (H2O) ¼ 1.26  1021 cm2 molecule1 from Chung et al. (2001). The choice of the latter value was not entirely straightforward, since there is a large spread (50%) in reported literature values (Keller-Rudek and Moortgat, 2009), and evaluated spectra (Sander et al., 2006; IUPAC, 2009) are only reported for shorter wavelengths. Data from the spectrum of Chung et al. (2001) were chosen both to convert optical (184.9 nm e see Section 2.2) measurements to absolute [H2O], and to calculate the photons absorbed (E3) with the reasonable expectation that any systematic errors in s values would cancel out. Note that at l ¼ 184.9 nm, where more data is available, there is good agreement (2%) between the reported value of Chung et al. (2001), interpolation of the evaluated literature cross-sections (IUPAC, 2009), and perhaps most appropriate for our purposes a discrete value determined from extinction of a l ¼ 184.9 nm Hg line from Creasey et al. (2000). Experiments were repeated at different [NF3] or [N2O] for unchanged conditions of P, T and [H2O]. This way a range of Sobs could be obtained whilst not changing the LIF sensitivity or the background amount of OH from (R3). In the presence of large [H2O], the changes in [NF3] or [N2O] do not affect the LIF sensitivity and Sobs is proportional to the absolute [OH] produced. The amount of OH generated is linearly related to the number of photons absorbed (Nabs) and the quantum yield of dissociation of N2O or NF3 with the constant term due to photolysis (R3) of H2O.

Sobs ¼ afNabs F1 þ S3 g

for NF3 data

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example of such a plot is displayed in Fig. 3. Both the NF3 (squares) and N2O (circles) datasets display the linear relationship between Sobs and Nabs with intercepts (S3) coincident with measured signal from H2O only. Values of F1 may therefore be calculated from the relative gradients. Taking into account the stoichiometry of OH formation in (R5) and (R2) the dataset in Fig. 3 results in F1 ¼ (1.13  0.05) where the uncertainty quoted is propagated from the least-squares errors in weighted fits of the data. The experimental conditions and results from determinations of F1 at various He bath gas pressures and at room temperature and 255 K are listed in Table 2. Within the errors reported (statistical only, two standard deviations propagated from errors on the individual slopes), no systematic deviation from F1 ¼ 1 was observed under any conditions. Additional error (z15%) arises from uncertainty in determining the concentrations of NF3 and N2O. We assume no error associated with the use of a quantum yield of unity for N2O dissociation, F4, (IUPAC, 2009). In some experiments the 193.3 nm photolysis of a constant amount of H2O at different laser fluences was used to build up a plot of Sobs versus Nabs as shown in Fig. 3. In this case, the well-known (unity) quantum yield for H2O dissociation at 193.3 nm (IUPAC, 2009) was used to derive values of F1. The results obtained by this method (see Table 2) are somewhat scattered, due to experimental difficulties in measuring absolute [H2O], but are consistent with F1 ¼ 1 obtained via the N2O photolysis (R4) and (R5) method. 3.3. Determinations of F1, k2 and k7 in N2 bath gas Fig. 4 displays three LIF profiles from preliminary experiments in which OH was produced in the photolysis of different concentrations of NF3 in the presence of H2O (R1) and (R2) at T ¼ 298 K and in P ¼ 40 mbar of N2. A considerably smaller [H2O] ¼ 1.6  1014 molecule cm3 was employed compared to the experiments described above so that OH formation via direct H2O photolysis (R3) was minimised and to allow determinations of F1 at low temperatures. Consequently, conversion of F to OH via (R2) was slower than in the experiments detailed above, and was resolved on the

(E4)

or

Sobs ¼ af2Nabs F2 þ S3 g

for N2 O data

(E5)

where a is a constant of proportionality related to the LIF detection efficiency and is common to both expressions, S3 is the LIF signal associated with photolysis of H2O and the factor of 2 in the N2O expression comes from reaction stoichiometry whereby two OH are produced in (R5). It follows from (E4) to (E5) that plots of Sobs vs. calculated values of Nabs may be used to obtain relative F values. An

Fig. 3. Plot of Sobs (in arbitrary units) vs. per cm number of photons absorbed (Nabs). All experiments were conducted in the presence of 6.4  1015 molecule cm3 H2O at P ¼ 100 mbar (He), T ¼ 298 K. Slopes are (1.7  0.05) for NF3 and (3.0  0.1) for N2O. For H2O the different values of Sobs were obtained by varying the laser fluence, yielding slope (1.9  0.1).

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Table 2 Experimental conditions and results for determinations of NF3 quantum yields, F1. T/K

P/mBar

[NF3]a

[H2O]a

[N2O]a

F1 b

255 296 298 298 298 298b

100 (He) 28 (He) 99 (He) 100 (He) 100 (He) 40 (N2)

32e97 38e95 27e135 82e270 27e137 270e1100

368 903 320 903 291 16

3.5e11.7 0e3.5 3.0e7.0 5e10 3.0e7.0 n/a

0.92 1.03 0.90 1.13 0.96 1.01

F1c      

0.04 0.03 0.08 0.05 0.04 0.18d

n/a 0.99  0.55 n/a 0.92  0.07 0.86  0.23

notes. a ¼ in units of 1013 molecule cm3. b ¼ calculated relative to OH from N2O photolysis (R4) and (R5). c ¼ calculated relative to OH from H2O photolysis (R3). d N2 experiment used [H2O2] z (13e31)  1013 molecule cm3 to “calibrate” the OH LIF.

experimental time scale. Data obtained at three different NF3 concentrations are displayed in Fig. 4. Kinetic information was extracted from such profiles via the appropriate two-exponential kinetic expression (E6):

S ¼ A  fexpðBtÞ  expðCtÞg

(E6)

where the parameter A is related to the initially formed [F] in (R1), B is the first-order rate coefficient for OH removal, and C is the firstorder rate coefficient for OH formation (equivalent to F-atom destruction). Calculation of the number of photons absorbed (E2), and analysis of the A-parameters was used to build up a plot of Sobs versus Nabs, equivalent to that displayed in Fig. 3. Efficient quenching of O(1D) by N2 meant that, in this bath gas, reference system (R4) and (R5) was no longer suitable. Small H2O concentrations were used, which were difficult to measure accurately and gave rise to small LIF signals (eg. stars in Fig. 4). As a result H2O photolysis (R3) was also an unsuitable reference OH source. For these reasons, back-to-back experiments (not displayed) were conducted using photolysis (R6) of [H2O2] z 1  1014 molecule cm3 as a calibration reaction. H2 O2 D hnð193:3 nmÞ / OH D ðother productsÞ

(R6)

Using literature values for F6 z 1.7 (Gerlach-Meyer et al., 1987; Vaghjiani et al., 1992), a value of F1 ¼ (1.01  0.18) was derived, in agreement with the unity quantum yields obtained in He bath gas. The large error associated with this value derives from uncertainties in [H2O2] measurements. Use of H2O2 at low temperatures was not possible and this experimental approach was not pursued; only one value for F1 in N2 is reported in Table 2. Following corrections for the prompt OH generated (R3) in the laser pulse, analysis of the parameters obtained from expression (E6) allowed the reaction between OH and NF3 (R7) to be investigated. OH D NF3 / ðproductsÞ

(R7)

Specifically, under pseudo first-order conditions of [NF3] >> [OH] the parameter B may be expanded to describe all removal processes for OH, ie. B [ k7 ½NF3  D kloss where kloss accounts for other OH loss processes, mainly transport out of the LIF area. It follows that a plot of B vs. [NF3] has a gradient equivalent to k7 and intercept kloss. A weighted least-squares fit to the data yields k7(298 K) ¼ (3  6)1016 cm3 molecule1 s1, ie. no significant correlation of B with [NF3]. There are no previous determinations of k7 available in the literature. Using enthalpies of formation of reactants and products from Chase (1998) and assuming that the products are HOF þ NF2, (R7) is found to be endothermic (37 kJ mol1), which is consistent with the small rate coefficient at room temperature. C parameters obtained from (E6) contain information on all the processes that affect the rate of OH formation in (R2). Since this OH formation rate is equal and opposite to the rate of F-atom destruction this may be written: C [ k2 ½H2 O D kloss where, given the high reactivity of F-atoms, the term kloss accounting for transport processes is negligible. The main panel on Fig. 5 is a plot of C vs. [H2O] derived from eight experiments where photolysis (R1) of [NF3] ¼ 5  1015 molecule cm3 in P ¼ 40 mbar (N2) at different [H2O] was used to generate OH. A weighted linear fit to the triangles yields a value of k2(298 K) ¼ (1.3  0.1)  1011 cm3 molecule1 s1. All but the largest [H2O] were too small to measure optically at l ¼ 184.9 nm, and were therefore determined from manometric measurements. An extra  20% in k2 was therefore introduced to yield a more realistic k2(298 K) ¼ (1.3  0.3)  1011 cm3 molecule1 s1 which accounts for both statistical and systematic uncertainties. This result is good agreement with the evaluated literature, which lists k2(298 K) ¼ (1.4  0.35)  1011 cm3 molecule1 s1 (IUPAC, 2009).

3.4. Conclusions and atmospheric implications

Fig. 4. Time dependent LIF signals recorded following photolysis of different concentrations of NF3 in the presence of H2O (1.6  1014 molecule cm3) at a pressure of 40 mbar (N2). LIF signals resulting from H2O photolysis only (the stars) were used to correct data recorded with NF3 present prior to fitting with expression (E6).

The absorption cross-sections of NF3 obtained in this study are in good agreement (z6% larger) with those reported by Molina et al. (1995) but disagree (orders of magnitude lower) with those of Makeev et al. (1975) in the wavelength region between 200 and 210 nm, which is important for dissociation of NF3 in the stratosphere. NF3 photolysis was investigated at 193.3 nm to derive a quantum yield of unity for formation of F-atoms at this wavelength. Although most data were obtained using He bath gas, the lack of a pressure dependence, and agreement with the limited N2 dataset suggests that the results can be applied to air.

T.J. Dillon et al. / Atmospheric Environment 44 (2010) 1186e1191

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Altogether these results validate the most recent calculations from Prather and Hsu (2008) of a lifetime of z550 years for NF3. Acknowledgements The authors thank Katrin Dulitz for technical assistance in the measurement of NF3 cross-sections, and Dr Mark Blitz for helpful discussions. References

Fig. 5. Main panel shows plot of OH formation rate parameter C vs [H2O]. The gradient is the rate coefficient for F þ H2O, k2(298 K) ¼ (1.3  0.1)  1011 cm3 molecule1 s1. The insert displays a plot of B vs. [NF3] to yield a value of k7(298 K) ¼ (3  6)  1016 cm3 molecule1 s1.

The thermodynamically accessible photolysis channels at 193.3 nm are:

NF3 þ hv/F þ NF2

R1a; DH ¼ 241 kJ mol1 ; l < 497nm

NF3 þ hv/F2 þ NF


R1b; DH ¼ 359 kJ mol1 ; l < 333 nm

NF3 þ hv/2F þ NF





R1c; DH ¼ 514 kJ mol1 ; l < 232 nm

Our data are thus consistent with F-atom production via channels (R1a) and (R1c). Should solely (R1a) be operative, the quantum yield of F-atoms formation would equivalent to the overall quantum yield of dissociation, i.e. unity. Channel (R1c) could be a concerted elimination of two F-atoms, or formation of highly excited NF2 radicals, which subsequently dissociate thermally to NF and F. In both cases, a measured unity yield of F-atoms implies that the overall dissociation quantum yield was just 0.5. The fact that there was no variation of quantum yield with pressure is consistent with an overall quantum yield of unity from the least energetic channel and whilst a possible contribution from (R1c) cannot be dismissed, we conclude that (R1a) is likely to dominate. As the spectrum of NF3 is continuous out to the longest wavelengths measured in this and previous work (z240 nm) this unity quantum yield should apply to the entire spectrum. No evidence was found for reaction OH and NF3. The upper limit derived at room temperature, k7 < 1  1015 cm3 molecule1 s1, corresponds to an atmospheric lifetime of the order of z20 years, similar to that of methane. It should be noted however, that as (R7) is significantly endothermic for the simple abstraction reaction (to HOF þ F products) the average rate coefficient throughout the troposphere and especially in the stratosphere would be much smaller and the lifetime corresponding larger. Whilst concluding that reaction with OH will not play an important role in NF3 degradation in the atmosphere, we recognise that more sensitive experiments are required to better constrain this parameter.

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