Environ. Sci. Technol. 2005, 39, 1632-1640
Atmospheric Chemistry of Hydrazoic Acid (HN3): UV Absorption Spectrum, HO• Reaction Rate, and Reactions of the •N3 Radical JOHN J. ORLANDO* AND GEOFFREY S. TYNDALL Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado 80305 ERIC A. BETTERTON Department of Atmospheric Sciences, University of Arizona, P.O. Box 210081, Tucson, Arizona 85721-0081 JOE LOWRY AND STEVE T. STEGALL National Enforcement Investigations Center, U.S. E.P.A., Denver Federal Center, P.O. Box 25227, Denver, Colorado 80225
Processes related to the tropospheric lifetime and fate of hydrazoic acid, HN3, have been studied. The ultraviolet absorption spectrum of HN3 is shown to possess a maximum near 262 nm with a tail extending to at least 360 nm. The photolysis quantum yield for HN3 is shown to be ≈1 at 351 nm. Using the measured spectrum and assuming unity quantum yield throughout the actinic region, a diurnally averaged photolysis lifetime near the earth’s surface of 2-3 days is estimated. Using a relative rate method, the rate coefficient for reaction of HO• with HN3 was found to be (3.9 ( 0.8) × 10-12 cm3 molecule-1 s-1, substantially larger than the only previous measurement. The atmospheric HN3 lifetime with respect to HO• oxidation is thus about 2-3 days, assuming a diurnally averaged [HO•] of 106 molecule cm-3. Reactions of •N3, the product of the reaction of HO• with HN3, were studied in an environmental chamber using an FTIR spectrometer for end-product analysis. The •N3 radical reacts efficiently with NO, producing N2O with 100% yield. Reaction of •N3 with NO2 appears to generate both NO and N2O, although the rate coefficient for this reaction is slower than that for reaction with NO. No evidence for reaction of •N3 with CO was observed, in contrast to previous literature data. Reaction of •N3 with O2 was found to be extremely slow, k < 6 × 10-20 cm3 molecule-1 s-1, although this upper limit does not necessarily rule out its occurrence in the atmosphere. Finally, the rate coefficient for reaction of Cl• with HN3 was measured using a relative rate method, k ) (1.0 ( 0.2) × 10-12 cm3 molecule-1 s-1.
Introduction Over the past decade, demand for sodium azide (NaN3), the principal active ingredient in automobile air bag inflators, has rapidly risen to exceed 5 million kg per year (1). This has Corresponding author phone: (303)497-1486; fax: (303)497-1411; e-mail:
[email protected]. 1632
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greatly increased the potential for accidental environmental releases and for human exposure to this highly toxic material. Aqueous sodium azide is readily hydrolyzed to yield hydrazoic acid, HN3 (pKa 4.7), a volatile substance that partitions strongly to the gas phase (KH ) 12 M atm-1) under atmospheric conditions (2). For example, even at concentrations as low as 6.5 ppm (m/v) NaN3 in the aqueous phase (pH 6.5) the gas-phase concentration reaches the threshold limit value of 0.11 ppmv (as hydrazoic acid gas) so there is interest in understanding the fate of atmospheric hydrazoic acid. The problem of significant azide releases to the environment is not a hypothetical one. For example, the town of Mona, UT was evacuated in 1996 afer a tanker truck hauling 80 55-gallon drums of NaN3 overturned (1). The problem of azide disposal will remain for decades, given the many millions of kilograms of NaN3 that is currently being carried by the nation’s automobile fleet (1). Although the tropospheric fate of HN3 has not been the subject of systematic study, sufficient data are available to indicate that reaction with OH and photolysis are likely tropospheric removal processes. Hack and Jordan (3) studied the reaction of HO• with HN3 via the flash photolysis of H2O2/ HN3/He mixtures, with HO• detection via pulsed LIF and reported a value for k1 of 1.3 × 10-12 cm3 molecule-1 s-1.
HO• + HN3 f products
(1)
This would imply a tropospheric lifetime for HN3 of about 10 days (for a diurnally averaged [HO•] ) 106 molecule cm-3). Reaction of O(1D) with HN3 appears to occur at essentially the gas-kinetic rate, k ) (3.2 ( 1.0) × 10-10 cm3 molecule-1 s-1 (4), while values reported for the rate coefficient for its reaction with Cl-atoms lie in the range (9-15) × 10-13 cm3 molecule-1 s-1 (5-8). Given the relatively large value of k1 and the higher abundance of OH compared to Cl and O(1D), these latter two processes are not likely to be of any atmospheric importance. The UV spectroscopy and photochemistry of HN3 has been studied in considerable detail (e.g., refs 9-34), although key data for assessment of the importance of tropospheric photolysis have yet to be obtained. McDonald et al. (18) have reported absorption cross sections for HN3 throughout the vacuum and near UV (100-325 nm). Vacuum UV measurements by Okabe (115-210 nm) (19) and a single wavelength determination at 193 nm (24, 25) indicate that these McDonald et al. (18) data may be low by about 20%. Although the McDonald et al. data do not extend beyond 325 nm and indeed may be systematically low, they do indicate that tropospheric photolysis of HN3 could be significant (photolysis lifetime ≈2-3 days). Numerous photochemistry and photodissociation dynamics studies of HN3 have been conducted at wavelengths ranging from 308 nm into the vacuum UV (e.g., refs 13-17, 19-34). The major photolysis products appear to be NH and N2 at all wavelengths studied (13, 14, 20-27, 29, 30, 32-34), although a minor process to form H and N3 has also been observed at 193, 248, and 266 nm (28, 31-33):
HN3 + hν f HN• + N2
(2a)
f H• + •N3
(2b)
The HN• photoproduct is formed exclusively in the excited a1∆ electronic state at long wavelength (λ g 248 nm) (25), in keeping with spin conservation rules, although other electronic states (A,b,c) have been detected at 193 nm and below 10.1021/es048178z CCC: $30.25
2005 American Chemical Society Published on Web 01/22/2005
(19, 21, 22, 25). A near-unity (φ ≈ 0.8) quantum yield for HN• production near 290 nm has been reported (13), and unit quantum yields are generally assumed at longer wavelengths. Absolute quantum yields for H-atom formation have been made at three wavelengths (28, 32): φ193 ) 0.14; φ248 ) 0.20; and φ266 ) 0.04. No H-atom quantum yield data are available at longer wavelengths, although H-atom production occurs to at least 280 nm (33), and the energy threshold for this process is near 325 nm. Photolysis experiments at higher [HN3] and at higher total pressures have also been carried out. Kodama (18), for example, photolyzed mixtures of HN3 (6.6 × 104 ppmv) in ≈0 to 0.8 atm of Xe buffer gas at 313 nm. Chain reactions were observed (ΦN2 ) 4.85; λ ) 313 nm) which were thought to involve the reaction of the HN• photoproduct with additional HN3, an unlikely path in the atmosphere. In earlier studies of a similar nature, Beckman and Dickinson (11, 12) reported ΦHN3 ) 3.6 (λ ) 190 or 254 nm). Reaction with OH and photolysis of HN3 will result in the formation of •N3 and HN• radicals, respectively. While HN• is thought to react rapidly with O2 to give NO and HO• (35), the atmospheric fate of •N3 is less certain. The most detailed information regarding the reactivity of this species comes from the work of Hewett and Setser (36). Using a dischargeflow/LIF system, these authors reported that the reactions of •N3 with NO, NO2, and CO all occurred with similar rate coefficients (k3 ) 2.9 × 10-12 cm3 molecule-1 s-1; k4 ) 1.9 × 10-12 cm3 molecule-1 s-1; and k5 ) 1.8 × 10-12 cm3 molecule-1 s-1). Thermodynamically accessible channels for these reactions (based on ∆Hf(N3) ) 99 kcal/mol (33)) are given below: •
N3 + NO f N2O + N2
(3a)
N3 + NO2 f 2NO + N2
(4a)
f 2N2O
(4b)
f N2 + N2O + O•
(4c)
f 2N2 + O2
(4d)
N3 + CO f NCO + N2
(5)
•
•
Given the relative atmospheric abundances of these three reactants, reaction of •N3 with CO would thus dominate. No products of these reactions were determined in the Hewett and Setser study, although likely possibilities (channels 3a, 4a, 4b, and 5) were suggested. Reaction of •N3 with O2 was found to be slow (36), k < 5 × 10-13 cm3 molecule-1 s-1, although this upper limit is not nearly low enough to rule out its importance in the atmosphere. In this work, studies of processes related to the atmospheric destruction and ultimate fate of hydrazoic acid are reported, including measurement of (a) the UV absorption spectrum of HN3 from 215 to 365 nm; (b) its photolysis quantum yield at 351 nm; (c) the rate coefficient for reaction of HN3 with HO•; and (d) the end-products of the reactions of •N3 radical with O2, NO, NO2, and CO. Experiments showed that the diurnally averaged tropospheric lifetime of HN3 is about 1-2 days, with both solar photolysis and reaction with HO• contributing about equally to its removal.
Materials and Methods UV Absorption Measurements. The UV absorption spectrum for HN3 was determined using a diode array spectrometer system that has been described in detail previously (37, 38). Measurements were made in a 90-cm long Pyrex absorption cell equipped with quartz windows. The output from a broadband D2 lamp is first collimated, then passes through
the absorption cell, and is focused onto the entrance slit of a 0.3 m Czerny-Turner spectrograph (equipped with a 300 grooves/mm grating), which disperses the light onto a 1024pixel diode array detector (EG&G Model 1420). With this configuration, each pixel is separated in wavelength by about 0.25 nm, providing coverage from about 220 to 450 nm with a spectral resolution of 0.6 nm. The system was calibrated in wavelength via interpolation between the positions of the emission lines from a low-pressure mercury lamp. Spectra were obtained from the summation of 100-200 exposures of the diode array, each exposure being 0.2 s in duration. Raw spectral data at each pixel, I(λ), obtained in the presence of an HN3 sample, were converted to absorbance (base e) via comparison with a spectrum, Io(λ), recorded with the absorption cell evacuated, i.e., A(λ) ) ln {Io(λ)/I(λ)}. Absorption spectra were smoothed, and the smoothed data were then interpolated in wavelength to obtain absorbance values at 0.5 nm intervals. Gaseous HN3 samples (for the UV measurements and all other studies conducted herein) were obtained by gently heating a mixture of approximately 1 g each of NaN3 and a solid carboxylic acid (glycolic acid or stearic acid) (18, 19). For the UV cross section measurements, the concentration of HN3 in the absorption cell was determined by pressure measurement, with the assumption that the gaseous samples contained no impurities. Measurements of HN3 samples by FTIR spectroscopy revealed no measurable impurities. Multiple fills of the absorption cell were made, and measured UV cross sections from the different samples were indistinguishable. Sample pressures were varied between 0.5 and 4.1 Torr. Photolysis Quantum Yield at 351 nm. The photodissociation quantum yield for HN3 at 351 nm was obtained by subjecting HN3/O2 samples (about 1-1.5 Torr HN3 in 1 atm O2 buffer gas) to multiple shots from a pulsed XeF excimer laser (Lambda Physik Compex 102). The photolysis experiments were conducted in a cylindrical Pyrex cell (25 cm long, 3.5 cm i.d.) equipped with quartz windows, which transmitted about 80% of the 351 nm radiation. The laser pulse energy, defined as the average of the energy before and after the cell as measured with a pyroelectric power meter (Questek P9104), was typically 90 mJ/pulse which corresponds to 1.6 × 1017 photons/pulse at 351 nm. Photolysis experiments were run for about 3 h with the excimer operating at 8 Hz, resulting in a total of roughly 105 laser shots. Pre- and postphotolysis concentrations in the UV cell were determined by FT-IR spectroscopy, using the environmental chamber/FT-IR spectrometer system described below. For prephotolysis measurements, a calibrated (1 L) bulb was filled with the same HN3 sample as the UV cell, and the contents of the calibrated bulb were then swept into the chamber for analysis. Following irradiation, the concentration of HN3 and photoproducts were determined by first expanding the contents of the UV cell into the calibrated bulb and then sweeping the bulb contents into the chamber. Photolysis of HN3 leads to the production of reactive NH radical. The effects of the subsequent chemistry of these species (in particular, the formation of OH and resulting consumption of HN3) was accounted for in the analysis, as described in the results section. Control experiments were also conducted in which the UV cell was filled with HN3/O2 and left to stand for 3 h without irradiation; no loss of HN3 was noted in these experiments. HO• and Cl• Rate Coefficient Measurements and •N3 Reaction Product Studies. Rate coefficient measurements and end-product studies were all carried out in an environmental chamber/Fourier transform infrared spectrometer system, which has been described previously (39). The chamber is 2 m long, is constructed of stainless steel, and has a volume of 47 L. It is interfaced via a set of modified Hanst-type multipass optics to a Fourier transform specVOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Infrared absorption cross sections for HN3 measured in this work. HN3 concentrations were determined manometrically in a 1 L calibrated volume. Trace amounts of CO2 are evident in the spectrum near 2350 cm-1. trometer (Bomem DA3) operating in the infrared. The optics were adjusted to allow sixteen traverses of the cell, thus providing an observational path length of 32.6 m. Infrared spectra were obtained at a spectral resolution of 1 cm-1 over the range 800-3900 cm-1 from the coaddition of 150-200 interferograms, which required 2.5-3.5 min acquisition time. Reaction mixtures were photolyzed through a quartz window located at one end of the chamber using a Xe-arc lamp, equipped with a Corning 7-54 filter, which provided radiation in the 250-400 nm range. For the HO• and Cl• rate coefficient measurements, standard relative rate methodologies were employed (39). For the HO• studies, ethene was used as the reference compound, while HO• generation was from the photolysis of methyl nitrite in the presence of added NO:
CH3ONO + hν f CH3O• + NO
(6)
Control experiments showed that photolysis of HN3 was negligible on the time scale of a typical kinetics run. Similar methodologies were employed to determine the Cl• rate coefficient, with Cl2 photolysis as the Cl• source and both acetone and methyl chloride as the reference compounds. These experiments were conducted in 700-710 Torr synthetic air, with initial concentrations in the chamber as follows: [Cl2] ) (3.8-5.8) × 1015 molecule cm-3; [HN3] ) (2.1-3.5) × 1014 molecule cm-3; [acetone] ) (4-5) × 1014 molecule cm-3; or [CH3Cl] ) (1.4-1.8) × 1015 molecule cm-3. Acetone and methyl chloride were monitored primarily near 1220 and 1350 cm-1, respectively. For these experiments, which could be conducted on shorter time scales than the OH experiments, wall losses were found to be negligibly slow. For studies of the products of the reactions of •N3, reaction of Cl• with HN3 was used as the •N3 source reaction in most cases:
CH3O• + O2 f CH2O + HO2•
(7)
Cl2 + hν f Cl• + Cl•
(9)
HO2• + NO f HO• + NO2
(8)
Cl• + HN3 f •N3 + HCl
(10)
Experiments were carried out in synthetic air at 700-720 Torr total pressure, with initial concentrations of species in the chamber as follows: [CH3ONO] ≈ 4 × 1015 molecule cm-3; [NO] ≈ 5.5 ×1014 molecule cm-3; [C2H4] ≈ 3.5 × 1014 molecule cm-3; [HN3] ≈ (2-4) × 1014 molecule cm-3. Quantification of ethene was done primarily using the strong absorption feature centered at 950 cm-1. Quantification of HN3 was accomplished using the absorption features at 2140 and 1150 cm-1. A calibrated infrared absorption spectrum for HN3 is shown in Figure 1. The heterogeneous loss of HN3 in the chamber was found to be significant on the time scale of a typical relative rate determination, and thus this process had to be corrected for. Upon filling the chamber with the mixture just described, the decay of HN3 and C2H4 was monitored in the dark for a period of about 30 min, in the presence of UV light for another 25-30 min, and then again in the dark for a further 20-30 min. While C2H4 was found to be stable during the dark periods, HN3 loss occurred with a first-order rate coefficient in the range of (1-3) × 10-5 s-1. Some conditioning of the cell with time was evident, as HN3 decays were typically faster before photolysis than after. For determination of the relative rate coefficient, HN3 loss during the photolysis period was corrected by the average of the loss rate before and after photolysis. The magnitude of the correction was about 20%. 1634
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FIGURE 2. UV absorption spectrum (solid line) for HN3 measured in this work. Data from ref 18 are given at selected wavelengths (open circles). The action spectrum for HN3 near the earth’s surface is shown in arbitrary units as the dashed line.
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In these experiments, mixtures of Cl2 (typically ≈4 × 1015 molecule cm-3) and HN3 (≈3 × 1014 molecule cm-3) were photolyzed in 1 atm N2 in the presence of one or more reactants (NO, NO2, CO, or O2). HO•-initiated oxidations of HN3 were also carried out, involving the photolysis of CH3ONO (≈4 × 1015 molecule cm-3), NO (3-5 × 1014 molecule cm-3), and HN3 (2-4 × 1014 molecule cm-3). Further details regarding reaction conditions for various experiments are provided in the results section of the manuscript. Chemicals were obtained from the following sources: glycolic acid (99%, Aldrich), stearic acid (Chem. Service), sodium azide (Sigma), Cl2 (Matheson, UHP), ethene (Linde, C.P.), acetone (Sigma-Aldrich, 99.9+%, HPLC grade), methyl chloride (Matheson), CO (Linde, CP grade), NO (Linde), NO2 (from reaction of NO with O2), N2 (boil-off from a liquid N2 Dewar), O2 (U.S. Welding, UHP). Methyl nitrite, CH3ONO, was synthesized from the dropwise addition of sulfuric acid to saturated solutions of sodium nitrite in methanol (40) and stored in dry ice between uses. Gases were used as received, while acetone was degassed by several freeze-pump-thaw cycles before use.
Results and Discussion UV Absorption Spectrum of HN3 Our HN3 UV absorption spectrum is plotted in Figure 2, and the data are tabulated
TABLE 1: Ultraviolet Absorption Cross Sections for HN3 Measured in This Work wavelength (nm)
absorption cross section (cm2 molecule-1)
wavelength (nm)
absorption cross section (cm2 molecule-1)
215 217 219 221 223 225 227 229 231 233 235 237 239 241 243 245 247 249 251 253 255 257 259 261 263 265 267 269 271 273 275 277 279 281 283 285 287 289
3.61E-19 2.89E-19 1.97E-19 1.39E-19 1.01E-19 7.49E-20 5.75E-20 4.88E-20 4.60E-20 4.61E-20 4.63E-20 4.73E-20 5.09E-20 5.62E-20 5.95E-20 6.12E-20 6.35E-20 6.93E-20 7.54E-20 7.65E-20 7.73E-20 7.82E-20 8.01E-20 8.59E-20 8.69E-20 8.34E-20 8.21E-20 8.06E-20 7.86E-20 7.78E-20 7.47E-20 7.00E-20 6.70E-20 6.34E-20 5.74E-20 5.07E-20 4.45E-20 3.89E-20
291 293 295 297 299 301 303 305 307 309 311 313 315 317 319 321 323 325 327 329 331 333 335 337 339 341 343 345 347 349 351 353 355 357 359 361 363 365
3.40E-20 2.96E-20 2.56E-20 2.22E-20 1.90E-20 1.64E-20 1.40E-20 1.19E-20 1.02E-20 8.64E-21 7.35E-21 6.22E-21 5.22E-21 4.48E-21 3.74E-21 3.12E-21 2.60E-21 2.17E-21 1.85E-21 1.53E-21 1.26E-21 1.08E-21 8.72E-22 7.43E-22 6.57E-22 5.00E-22 3.81E-22 3.53E-22 2.82E-22 2.35E-22 2.03E-22 1.61E-22 1.08E-22 9.02E-23 9.34E-23 5.27E-23 4.76E-23 3.57E-23
in 2 nm intervals in Table 1. Uncertainties are estimated to be (7% near the maximum of the spectrum (dominated by the uncertainty in the measurement of the HN3 partial pressure, with lesser contributions from uncertainty in temperature and path length). Uncertainties increase at longer wavelength (e.g., to about (20% near 360 nm) due to increasing uncertainty in the absorbance measurement. The spectrum shows a broad local maximum near 262 nm, and the onset of a stronger maximum at wavelengths shorter than 215 nm. Measurable absorption extends beyond 360 nm. The spectrum of HN3 has previously been measured quantitatively by McDonald et al. (18) over the range 100325 nm. Their data (estimated from their Figure 3) are shown at a few wavelengths in Figure 2. However, measurements by Okabe (115-210 nm) (19) and Rohrer and Stuhl (193 nm only) (25) suggest that the McDonald et al. data may be low by about 15-20%. A similar discrepancy between our data and those of McDonald et al. is also evident in Figure 2. The scaling of the McDonald et al. (18) data suggested by Rohrer and Stuhl (25) (by the ratio of the two 193 nm cross sections) provides a cross section at 248.5 nm (6.8 × 10-20 cm2 molecule-1) that is within 2% of our data point at this wavelength. Thus, a combination of the Okabe (19) data in the vacuum UV (which agrees with the Rohrer and Stuhl (25) data at 193 nm) with our longer wavelength data appears to provide an accurate representation of the HN3 absorption spectrum over the range 110-360 nm.
FIGURE 3. Relative rate of decay of HN3 (after correction for heterogeneous loss) versus that of ethene in the presence of HO•. Some diffuse structure is evident in both our measurement and that of McDonald et al. (18) in the 240-290 nm region. As noted by McDonald et al., this structure can be attributed to two 1600 cm-1 progressions offset from each other by about 600 cm-1. They assigned the 1600 cm-1 mode to the ν2 N-N-N asymmetric stretch in the upper electronic state and the 600 cm-1 frequency to an upper state N-N-N bending mode (most likely ν6). HN3 Quantum Yield Determination at 351 nm. Using the 351 nm absorption cross section measured in this work (2.03 × 10-22 cm2 molecule-1) and the fractional loss of HN3 upon exposure to excimer irradiation (46% loss for 9 × 104 excimer pulses), the quantum yield for HN3 loss was determined to be 2.14 ( 0.25 (uncertainties given throughout this paper are 1σ). N2O, the only measurable photoproduct, had an appearance quantum yield of 0.86 ( 0.15. These observations are consistent with a near-unity (1.07 ( 0.15) quantum yield for the primary photodissociation process yielding HN•, coupled with subsequent loss of a second HN3 via reaction with OH:
HN3 + hνfHN• + N2
(2a)
HN• + O2 f HO• + NO
(11)
•
HO + HN3 f •N3 + H2O
(1)
•
N3 + NO f N2 + N2O
(3a)
net: 2HN3 + hν + O2 f 2N2 + N2O + H2O The near-quantitative production of N2O during photolysis (i.e., one N2O produced for every two HN3 molecules consumed) is strong evidence for the near-quantitative conversion of HN radicals into NO, since R3a appears to be the only logical source of N2O in the system. Our spectrum and quantum yield determination can be convolved with solar flux data to calculate a theoretical tropospheric photolysis rate constant for HN3 (jHN3, s-1). For these calculations, diurnally averaged sea-level surface solar flux data corresponding to a 40 °N, mid-summer day were used (41). The retrieved action spectrum (dashed line, Figure 2) shows that maximum atmospheric photolysis is centered near 320 nm. Integration under this curve, assuming a quantum yield of unity for the entire actinic region, yields an approximate 1/jHN3 lifetime of 2-3 days. Preliminary experiments to directly determine the HN3 solar photolysis rate were carried out at the Denver Federal VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Center. In these experiments, HN3/air mixtures (contained in a Teflon bag) were exposed to solar radiation over the course of 2 days, and the HN3 temporal profile was monitored via FTIR spectroscopy. Although HN3 decay was clearly observed during sunlight hours, these experiments are not considered quantitative at this time due to the presence of patchy cloud cover and the potential for HN3 loss via unwanted processes (i.e, heterogeneous loss, reaction with OH). Further experiments of this type, to provide a quantitative comparison with the laboratory cross section and quantum yield data, are planned. Rate Coefficient for Reaction of HO• with HN3. The rate coefficient for reaction of HO• with HN3 was determined relative to the well-established rate coefficient for reaction of HO• with C2H4 (8.2 × 10-12 cm3 molecule-1 s-1, 298 K, 1 atm total pressure (42, 43)).
HO• + HN3 f •N3 + H2O
(1)
HO• + CH2dCH2 + M f HOCH2CH2 + M
(12)
Relative rate data, after correction for heterogeneous loss of HN3 as described earlier, are displayed in Figure 3 and yield a rate coefficient ratio k1/k12 ) 0.48 ( 0.05. The magnitude of the heterogeneous correction was on the order of 20%. Incorporating a 10% uncertainty in k12, a value of k1 ) (3.9 ( 0.8) × 10-12 cm3 molecule-1 s-1 is determined. We also note that other species potentially reactive with HN3 are generated in these experiments, namely O-atoms (via photolysis of NO2) and O3 (via recombination of O with O2). Both of these species react slowly with HN3 (4, 35) and will not contribute significantly to its loss. Hack and Jordan (3) reported the only other measurement of k1. They monitored the decay of HO• via pulsed laserinduced fluorescence, following the flash photolysis of mixtures of H2O2 and HN3 in He buffer gas. Their value, (1.3 ( 0.2) × 10-12 cm3 molecule-1 s-1, is considerably smaller than ours; reasons for this discrepancy are not obvious. Similarities in the reactivity of HN3 and HBr might be expected, given the near identical H-X bond strengths (369 kJ mol-1) in the two species (33, 42, 43) and the reasonable correlation between HO• rate coefficients and H-X bond strengths in the hydrogen halides (and pseudohalides). This holds true in a qualitative way, with k1 being about a factor of 2 lower than the rate coefficient for reaction of HO• with HBr (42). Using our value for k1 and a diurnally averaged tropospheric HO• concentration of 1 × 106 molecule cm-3, the lifetime for HN3 with respect to HO• reaction can be estimated to be ≈2-3 days. Rate Coefficient for Reaction of Cl• with HN3. Relative rate data for R10 versus the two reference reactions, R13 and R14, are shown in Figure 4.
Cl• + HN3 f •N3 + HCl
(10)
Cl• + CH3C(O)CH3 f CH3C(O)CH2 + HCl
(13)
Cl• + CH3Cl f CH2Cl + HCl
(14)
Least-squares analysis of these data yield the following rate coefficient ratios, k10/k13 ) 0.44 ( 0.06 and k10/k14 ) 2.19 ( 0.25. Although there is some discrepancy in the literature regarding the value of k13 (values range from about (1.8-3.1) × 10-12 cm3 molecule-1 s-1), recent data (39, 44, 45) are centered at (2.2 ( 0.4) × 10-12 cm3 molecule-1 s-1. The value of k14 ) (4.9 ( 0.8) × 10-13 cm3 molecule-1 s-1 seems to be well established (42, 43). Combining these reference rate coefficient data with our measured ratios yields values for 1636
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FIGURE 4. Relative rate of decay of HN3 versus those of acetone (solid circles) and methyl chloride (open circles) in the presence of Cl-atoms. k10 of (0.97 ( 0.20) × 10-12 cm3 molecule-1 s-1 and (1.07 ( 0.20) × 10-12 cm3 molecule-1 s-1, from which a final value k10 ) (1.02 ( 0.20) × 10-12 cm3 molecule-1 s-1 can be obtained. There are a number of determinations of k10 in the literature (5-8), including one temperature-dependent study (8), all obtained using flow tube methodologies. While there is very reasonable agreement between the various room-temperature measurements, which range from (9-13) × 10-13 cm3 molecule-1 s-1, these measurements were of an indirect nature, involved substantial occurrence of, or correction for, secondary reactions, and/or required modeling of fairly complex reaction systems, and thus uncertainties on the order of (25% typically apply. Our measurement of k10 via a completely different methodology, one that should be free of any complication from secondary reactions, provides confirming evidence for a value of k10 of (1.0 ( 0.2) × 10-12 cm3 molecule-1 s-1. As discussed above, similarities in reactivity between HN3 and HBr might be expected. As in the case of the reaction of HO• with these two species, Cl• reacts more slowly with HN3 than with HBr (5, 35), in this case by about a factor of 5. Reactions of the •N3 Radical. A series of experiments was carried out to determine the products of the reactions of •N3 with various species (itself, NO, NO2, CO, and O2) and to determine semiquantitatively the rate coefficients or at least the relative rates for these reactions. For convenience, results obtained in our work and in previous studies are summarized in Table 2. The simplest experiments involved the photolysis of Cl2 (≈4 × 1015 molecule cm-3)/HN3 (≈2 × 1014 molecule cm-3) mixtures in the presence of 700 Torr N2. Although HN3 was efficiently destroyed in these experiments, the only product observed in the infrared was N2O with a molar yield of only (3.1 ( 0.4)% (Figure 5, open circles). The small N2O yield observed possibly arises from the presence of small NOx impurities in the chamber (reaction of NO2 or NO with •N3 would eventually lead to N2O, see below). The lack of large yields of observable products suggests the involvement of •N self-reaction as an important removal process for N , 3 3 resulting in the formation of N2, which is undetectable with our apparatus: •
N3 + •N3 f 3N2
(15)
Although the rate coefficient for R15 has not been firmly established, it appears to be e 2 × 10-12 cm3 molecule-1 s-1 (36). Box model simulations of the experiments (conducted
TABLE 2: Summary of Available Data Regarding Rate Coefficients and Products for Reactions of N3 Radicals with Atmospherically Relevant Species reactant
reaction number
rate coefficient (cm3 molecule-1 s-1)
products
10-12
NO NO2
R3 R4
2.9 × (ref 36) 1.9 × 10-12 (ref 36); less than k3/3 (this work)
CO O2
R5 R25
1.8 × 10-12 (ref 36); less than k3/100 (this work)
