Atmospheric Chemistry of HydrazoicAcid (HN3): UV AbsorptionSpectrum, HO• Reaction Rate, andReactions of the •N3 RadicalJ O H N J . O R L A N D O * A N DG E O F F R E Y S . T Y N D A L L

Atmospheric Chemistry Division, National Center forAtmospheric Research, Boulder, Colorado 80305

E R I C A . B E T T E R T O N

Department of Atmospheric Sciences, University of Arizona,P.O. Box 210081, Tucson, Arizona 85721-0081

J O E L O W R Y A N D S T E V E T . S T E G A L L

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 ofhydrazoic acid, HN3, have been studied. The ultravioletabsorption spectrum of HN3 is shown to possess a maximumnear 262 nm with a tail extending to at least 360 nm.The photolysis quantum yield for HN3 is shown to be ≈1at 351 nm. Using the measured spectrum and assuming unityquantum yield throughout the actinic region, a diurnallyaveraged photolysis lifetime near the earth’s surface of 2-3days is estimated. Using a relative rate method, the ratecoefficient for reaction of HO• with HN3 was found to be (3.9( 0.8) × 10-12 cm3 molecule-1 s-1, substantially largerthan the only previous measurement. The atmospheric HN3lifetime with respect to HO• oxidation is thus about 2-3days, assuming a diurnally averaged [HO•] of 106 moleculecm-3. Reactions of •N3, the product of the reaction ofHO• with HN3, were studied in an environmental chamberusing an FTIR spectrometer for end-product analysis.The •N3 radical reacts efficiently with NO, producing N2Owith 100% yield. Reaction of •N3 with NO2 appears togenerate both NO and N2O, although the rate coefficientfor this reaction is slower than that for reaction with NO. Noevidence for reaction of •N3 with CO was observed, incontrast to previous literature data. Reaction of •N3 withO2 was found to be extremely slow, k < 6 × 10-20 cm3

molecule-1 s-1, although this upper limit does notnecessarily rule out its occurrence in the atmosphere.Finally, the rate coefficient for reaction of Cl• with HN3 wasmeasured using a relative rate method, k ) (1.0 ( 0.2)× 10-12 cm3 molecule-1 s-1.

IntroductionOver the past decade, demand for sodium azide (NaN3), theprincipal active ingredient in automobile air bag inflators,has rapidly risen to exceed 5 million kg per year (1). This has

greatly increased the potential for accidental environmentalreleases and for human exposure to this highly toxic material.Aqueous sodium azide is readily hydrolyzed to yield hydrazoicacid, HN3 (pKa 4.7), a volatile substance that partitionsstrongly to the gas phase (KH ) 12 M atm-1) underatmospheric conditions (2). For example, even at concentra-tions as low as 6.5 ppm (m/v) NaN3 in the aqueous phase(pH 6.5) the gas-phase concentration reaches the thresholdlimit value of 0.11 ppmv (as hydrazoic acid gas) so there isinterest in understanding the fate of atmospheric hydrazoicacid. The problem of significant azide releases to theenvironment is not a hypothetical one. For example, the townof Mona, UT was evacuated in 1996 afer a tanker truck hauling80 55-gallon drums of NaN3 overturned (1). The problem ofazide disposal will remain for decades, given the manymillions of kilograms of NaN3 that is currently being carriedby the nation’s automobile fleet (1).

Although the tropospheric fate of HN3 has not been thesubject of systematic study, sufficient data are available toindicate that reaction with OH and photolysis are likelytropospheric removal processes. Hack and Jordan (3) studiedthe reaction of HO• with HN3 via the flash photolysis of H2O2/HN3/He mixtures, with HO• detection via pulsed LIF andreported a value for k1 of 1.3 × 10-12 cm3 molecule-1 s-1.

This would imply a tropospheric lifetime for HN3 of about10 days (for a diurnally averaged [HO•] ) 106 molecule cm-3).Reaction of O(1D) with HN3 appears to occur at essentiallythe gas-kinetic rate, k ) (3.2 ( 1.0) × 10-10 cm3 molecule-1

s-1 (4), while values reported for the rate coefficient for itsreaction 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 anyatmospheric importance.

The UV spectroscopy and photochemistry of HN3 has beenstudied in considerable detail (e.g., refs 9-34), although keydata for assessment of the importance of troposphericphotolysis have yet to be obtained. McDonald et al. (18) havereported absorption cross sections for HN3 throughout thevacuum and near UV (100-325 nm). Vacuum UV measure-ments by Okabe (115-210 nm) (19) and a single wavelengthdetermination at 193 nm (24, 25) indicate that theseMcDonald et al. (18) data may be low by about 20%. Althoughthe McDonald et al. data do not extend beyond 325 nm andindeed may be systematically low, they do indicate thattropospheric photolysis of HN3 could be significant (pho-tolysis lifetime ≈2-3 days).

Numerous photochemistry and photodissociation dy-namics studies of HN3 have been conducted at wavelengthsranging from 308 nm into the vacuum UV (e.g., refs 13-17,19-34). The major photolysis products appear to be NH andN2 at all wavelengths studied (13, 14, 20-27, 29, 30, 32-34),although a minor process to form H and N3 has also beenobserved at 193, 248, and 266 nm (28, 31-33):

The HN• photoproduct is formed exclusively in the exciteda1∆ electronic state at long wavelength (λ g 248 nm) (25), inkeeping with spin conservation rules, although other elec-tronic states (A,b,c) have been detected at 193 nm and below

Corresponding author phone: (303)497-1486; fax: (303)497-1411;e-mail: orlando@acd.ucar.edu.

HO• + HN3 f products (1)

HN3 + hν f HN• + N2 (2a)

f H• + •N3 (2b)

Environ. Sci. Technol. 2005, 39, 1632-1640

1632 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005 10.1021/es048178z CCC: $30.25 2005 American Chemical SocietyPublished 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 unitquantum yields are generally assumed at longer wavelengths.Absolute quantum yields for H-atom formation have beenmade at three wavelengths (28, 32): φ193 ) 0.14; φ248 ) 0.20;and φ266 ) 0.04. No H-atom quantum yield data are availableat longer wavelengths, although H-atom production occursto at least 280 nm (33), and the energy threshold for thisprocess is near 325 nm.

Photolysis experiments at higher [HN3] and at higher totalpressures have also been carried out. Kodama (18), forexample, photolyzed mixtures of HN3 (6.6 × 104 ppmv) in ≈0to 0.8 atm of Xe buffer gas at 313 nm. Chain reactions wereobserved (ΦN2 ) 4.85; λ ) 313 nm) which were thought toinvolve the reaction of the HN• photoproduct with additionalHN3, an unlikely path in the atmosphere. In earlier studiesof a similar nature, Beckman and Dickinson (11, 12) reportedΦHN3 ) 3.6 (λ ) 190 or 254 nm).

Reaction with OH and photolysis of HN3 will result in theformation 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 detailedinformation regarding the reactivity of this species comesfrom the work of Hewett and Setser (36). Using a discharge-flow/LIF system, these authors reported that the reactionsof •N3 with NO, NO2, and CO all occurred with similar ratecoefficients (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 reac-tions (based on ∆Hf(N3) ) 99 kcal/mol (33)) are given below:

Given the relative atmospheric abundances of these threereactants, reaction of •N3 with CO would thus dominate. Noproducts of these reactions were determined in the Hewettand Setser study, although likely possibilities (channels 3a,4a, 4b, and 5) were suggested. Reaction of •N3 with O2 wasfound to be slow (36), k < 5 × 10-13 cm3 molecule-1 s-1,although this upper limit is not nearly low enough to rule outits importance in the atmosphere.

In this work, studies of processes related to the atmo-spheric destruction and ultimate fate of hydrazoic acid arereported, including measurement of (a) the UV absorptionspectrum of HN3 from 215 to 365 nm; (b) its photolysisquantum yield at 351 nm; (c) the rate coefficient for reactionof HN3 with HO•; and (d) the end-products of the reactionsof •N3 radical with O2, NO, NO2, and CO. Experiments showedthat the diurnally averaged tropospheric lifetime of HN3 isabout 1-2 days, with both solar photolysis and reaction withHO• contributing about equally to its removal.

Materials and MethodsUV Absorption Measurements. The UV absorption spectrumfor HN3 was determined using a diode array spectrometersystem that has been described in detail previously (37, 38).Measurements were made in a 90-cm long Pyrex absorptioncell equipped with quartz windows. The output from abroadband D2 lamp is first collimated, then passes through

the absorption cell, and is focused onto the entrance slit ofa 0.3 m Czerny-Turner spectrograph (equipped with a 300grooves/mm grating), which disperses the light onto a 1024-pixel diode array detector (EG&G Model 1420). With thisconfiguration, each pixel is separated in wavelength by about0.25 nm, providing coverage from about 220 to 450 nm witha spectral resolution of 0.6 nm. The system was calibratedin wavelength via interpolation between the positions of theemission lines from a low-pressure mercury lamp. Spectrawere obtained from the summation of 100-200 exposuresof the diode array, each exposure being 0.2 s in duration.Raw spectral data at each pixel, I(λ), obtained in the presenceof an HN3 sample, were converted to absorbance (base e) viacomparison with a spectrum, Io(λ), recorded with theabsorption cell evacuated, i.e., A(λ) ) ln {Io(λ)/I(λ)}. Absorp-tion spectra were smoothed, and the smoothed data werethen interpolated in wavelength to obtain absorbance valuesat 0.5 nm intervals.

Gaseous HN3 samples (for the UV measurements and allother studies conducted herein) were obtained by gentlyheating a mixture of approximately 1 g each of NaN3 and asolid carboxylic acid (glycolic acid or stearic acid) (18, 19).For the UV cross section measurements, the concentrationof HN3 in the absorption cell was determined by pressuremeasurement, with the assumption that the gaseous samplescontained no impurities. Measurements of HN3 samples byFTIR spectroscopy revealed no measurable impurities.Multiple fills of the absorption cell were made, and measuredUV cross sections from the different samples were indis-tinguishable. Sample pressures were varied between 0.5 and4.1 Torr.

Photolysis Quantum Yield at 351 nm. The photodisso-ciation quantum yield for HN3 at 351 nm was obtained bysubjecting HN3/O2 samples (about 1-1.5 Torr HN3 in 1 atmO2 buffer gas) to multiple shots from a pulsed XeF excimerlaser (Lambda Physik Compex 102). The photolysis experi-ments were conducted in a cylindrical Pyrex cell (25 cm long,3.5 cm i.d.) equipped with quartz windows, which transmittedabout 80% of the 351 nm radiation. The laser pulse energy,defined as the average of the energy before and after the cellas 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 runfor about 3 h with the excimer operating at 8 Hz, resultingin a total of roughly 105 laser shots. Pre- and postphotolysisconcentrations in the UV cell were determined by FT-IRspectroscopy, using the environmental chamber/FT-IR spec-trometer system described below. For prephotolysis mea-surements, a calibrated (1 L) bulb was filled with the sameHN3 sample as the UV cell, and the contents of the calibratedbulb were then swept into the chamber for analysis. Followingirradiation, the concentration of HN3 and photoproducts weredetermined by first expanding the contents of the UV cellinto the calibrated bulb and then sweeping the bulb contentsinto the chamber. Photolysis of HN3 leads to the productionof reactive NH radical. The effects of the subsequentchemistry of these species (in particular, the formation ofOH and resulting consumption of HN3) was accounted forin the analysis, as described in the results section. Controlexperiments were also conducted in which the UV cell wasfilled 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 measurementsand end-product studies were all carried out in an envi-ronmental chamber/Fourier transform infrared spectrometersystem, which has been described previously (39). Thechamber is 2 m long, is constructed of stainless steel, andhas a volume of 47 L. It is interfaced via a set of modifiedHanst-type multipass optics to a Fourier transform spec-

•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)

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trometer (Bomem DA3) operating in the infrared. The opticswere adjusted to allow sixteen traverses of the cell, thusproviding an observational path length of 32.6 m. Infraredspectra were obtained at a spectral resolution of 1 cm-1 overthe range 800-3900 cm-1 from the coaddition of 150-200interferograms, which required 2.5-3.5 min acquisition time.Reaction mixtures were photolyzed through a quartz windowlocated at one end of the chamber using a Xe-arc lamp,equipped with a Corning 7-54 filter, which provided radiationin 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 referencecompound, while HO• generation was from the photolysis ofmethyl nitrite in the presence of added NO:

Experiments were carried out in synthetic air at 700-720Torr total pressure, with initial concentrations of species inthe chamber as follows: [CH3ONO] ≈ 4 × 1015 molecule cm-3;[NO] ≈ 5.5 ×1014 molecule cm-3; [C2H4] ≈ 3.5 × 1014 moleculecm-3; [HN3] ≈ (2-4) × 1014 molecule cm-3. Quantificationof ethene was done primarily using the strong absorptionfeature centered at 950 cm-1. Quantification of HN3 wasaccomplished using the absorption features at 2140 and 1150cm-1. A calibrated infrared absorption spectrum for HN3 isshown in Figure 1.

The heterogeneous loss of HN3 in the chamber was foundto be significant on the time scale of a typical relative ratedetermination, 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 aperiod of about 30 min, in the presence of UV light for another25-30 min, and then again in the dark for a further 20-30min. While C2H4 was found to be stable during the darkperiods, HN3 loss occurred with a first-order rate coefficientin the range of (1-3) × 10-5 s-1. Some conditioning of thecell with time was evident, as HN3 decays were typically fasterbefore photolysis than after. For determination of the relativerate coefficient, HN3 loss during the photolysis period wascorrected by the average of the loss rate before and afterphotolysis. The magnitude of the correction was about 20%.

Control experiments showed that photolysis of HN3 wasnegligible on the time scale of a typical kinetics run.

Similar methodologies were employed to determine theCl• rate coefficient, with Cl2 photolysis as the Cl• source andboth acetone and methyl chloride as the reference com-pounds. These experiments were conducted in 700-710 Torrsynthetic air, with initial concentrations in the chamber asfollows: [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 near1220 and 1350 cm-1, respectively. For these experiments,which could be conducted on shorter time scales than theOH experiments, wall losses were found to be negligibly slow.

For studies of the products of the reactions of •N3, reactionof Cl• with HN3 was used as the •N3 source reaction in mostcases:

In these experiments, mixtures of Cl2 (typically ≈4 × 1015

molecule cm-3) and HN3 (≈3 × 1014 molecule cm-3) werephotolyzed in 1 atm N2 in the presence of one or morereactants (NO, NO2, CO, or O2). HO•-initiated oxidations ofHN3 were also carried out, involving the photolysis ofCH3ONO (≈4 × 1015 molecule cm-3), NO (3-5 × 1014 moleculecm-3), and HN3 (2-4 × 1014 molecule cm-3). Further detailsregarding reaction conditions for various experiments areprovided 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), methylchloride (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 acidto saturated solutions of sodium nitrite in methanol (40) andstored in dry ice between uses. Gases were used as received,while acetone was degassed by several freeze-pump-thawcycles before use.

Results and DiscussionUV Absorption Spectrum of HN3 Our HN3 UV absorptionspectrum is plotted in Figure 2, and the data are tabulated

FIGURE 1. Infrared absorption cross sections for HN3 measured inthis work. HN3 concentrations were determined manometrically ina 1 L calibrated volume. Trace amounts of CO2 are evident in thespectrum near 2350 cm-1.

CH3ONO + hν f CH3O• + NO (6)

CH3O• + O2 f CH2O + HO2• (7)

HO2• + NO f HO• + NO2 (8)

FIGURE 2. UV absorption spectrum (solid line) for HN3 measuredin this work. Data from ref 18 are given at selected wavelengths(open circles). The action spectrum for HN3 near the earth’s surfaceis shown in arbitrary units as the dashed line.

Cl2 + hν f Cl• + Cl• (9)

Cl• + HN3 f •N3 + HCl (10)

1634 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005

in 2 nm intervals in Table 1. Uncertainties are estimated tobe (7% near the maximum of the spectrum (dominated bythe uncertainty in the measurement of the HN3 partialpressure, with lesser contributions from uncertainty intemperature and path length). Uncertainties increase atlonger wavelength (e.g., to about (20% near 360 nm) due toincreasing uncertainty in the absorbance measurement. Thespectrum shows a broad local maximum near 262 nm, andthe onset of a stronger maximum at wavelengths shorterthan 215 nm. Measurable absorption extends beyond 360nm. The spectrum of HN3 has previously been measuredquantitatively by McDonald et al. (18) over the range 100-325 nm. Their data (estimated from their Figure 3) are shownat a few wavelengths in Figure 2. However, measurementsby Okabe (115-210 nm) (19) and Rohrer and Stuhl (193 nmonly) (25) suggest that the McDonald et al. data may be lowby about 15-20%. A similar discrepancy between our dataand those of McDonald et al. is also evident in Figure 2. Thescaling of the McDonald et al. (18) data suggested by Rohrerand 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 thiswavelength. Thus, a combination of the Okabe (19) data inthe vacuum UV (which agrees with the Rohrer and Stuhl (25)data at 193 nm) with our longer wavelength data appears toprovide an accurate representation of the HN3 absorptionspectrum over the range 110-360 nm.

Some diffuse structure is evident in both our measurementand that of McDonald et al. (18) in the 240-290 nm region.As noted by McDonald et al., this structure can be attributedto two 1600 cm-1 progressions offset from each other byabout 600 cm-1. They assigned the 1600 cm-1 mode to theν2 N-N-N asymmetric stretch in the upper electronic stateand the 600 cm-1 frequency to an upper state N-N-Nbending mode (most likely ν6).

HN3 Quantum Yield Determination at 351 nm. Usingthe 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 wasdetermined to be 2.14 ( 0.25 (uncertainties given throughoutthis paper are 1σ). N2O, the only measurable photoproduct,had an appearance quantum yield of 0.86 ( 0.15. Theseobservations are consistent with a near-unity (1.07 ( 0.15)quantum yield for the primary photodissociation processyielding HN•, coupled with subsequent loss of a second HN3

via reaction with OH:

The near-quantitative production of N2O during photolysis(i.e., one N2O produced for every two HN3 moleculesconsumed) is strong evidence for the near-quantitativeconversion of HN radicals into NO, since R3a appears to bethe only logical source of N2O in the system.

Our spectrum and quantum yield determination can beconvolved with solar flux data to calculate a theoreticaltropospheric photolysis rate constant for HN3 (jHN3, s-1). Forthese calculations, diurnally averaged sea-level surface solarflux data corresponding to a 40 °N, mid-summer day wereused (41). The retrieved action spectrum (dashed line, Figure2) shows that maximum atmospheric photolysis is centerednear 320 nm. Integration under this curve, assuming aquantum yield of unity for the entire actinic region, yieldsan 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

TABLE 1: Ultraviolet Absorption Cross Sections for HN3Measured in This Work

wavelength(nm)

absorptioncross section

(cm2 molecule-1)wavelength

(nm)

absorptioncross section

(cm2 molecule-1)

215 3.61E-19 291 3.40E-20217 2.89E-19 293 2.96E-20219 1.97E-19 295 2.56E-20221 1.39E-19 297 2.22E-20223 1.01E-19 299 1.90E-20225 7.49E-20 301 1.64E-20227 5.75E-20 303 1.40E-20229 4.88E-20 305 1.19E-20231 4.60E-20 307 1.02E-20233 4.61E-20 309 8.64E-21235 4.63E-20 311 7.35E-21237 4.73E-20 313 6.22E-21239 5.09E-20 315 5.22E-21241 5.62E-20 317 4.48E-21243 5.95E-20 319 3.74E-21245 6.12E-20 321 3.12E-21247 6.35E-20 323 2.60E-21249 6.93E-20 325 2.17E-21251 7.54E-20 327 1.85E-21253 7.65E-20 329 1.53E-21255 7.73E-20 331 1.26E-21257 7.82E-20 333 1.08E-21259 8.01E-20 335 8.72E-22261 8.59E-20 337 7.43E-22263 8.69E-20 339 6.57E-22265 8.34E-20 341 5.00E-22267 8.21E-20 343 3.81E-22269 8.06E-20 345 3.53E-22271 7.86E-20 347 2.82E-22273 7.78E-20 349 2.35E-22275 7.47E-20 351 2.03E-22277 7.00E-20 353 1.61E-22279 6.70E-20 355 1.08E-22281 6.34E-20 357 9.02E-23283 5.74E-20 359 9.34E-23285 5.07E-20 361 5.27E-23287 4.45E-20 363 4.76E-23289 3.89E-20 365 3.57E-23

FIGURE 3. Relative rate of decay of HN3 (after correction forheterogeneous loss) versus that of ethene in the presence of HO•.

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

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Center. In these experiments, HN3/air mixtures (containedin a Teflon bag) were exposed to solar radiation over thecourse of 2 days, and the HN3 temporal profile was monitoredvia FTIR spectroscopy. Although HN3 decay was clearlyobserved during sunlight hours, these experiments are notconsidered quantitative at this time due to the presence ofpatchy cloud cover and the potential for HN3 loss viaunwanted processes (i.e, heterogeneous loss, reaction withOH). Further experiments of this type, to provide a quantita-tive comparison with the laboratory cross section andquantum yield data, are planned.

Rate Coefficient for Reaction of HO• with HN3. The ratecoefficient for reaction of HO• with HN3 was determinedrelative to the well-established rate coefficient for reactionof HO• with C2H4 (8.2 × 10-12 cm3 molecule-1 s-1, 298 K, 1atm total pressure (42, 43)).

Relative rate data, after correction for heterogeneous lossof HN3 as described earlier, are displayed in Figure 3 andyield a rate coefficient ratio k1/k12 ) 0.48 ( 0.05. Themagnitude of the heterogeneous correction was on the orderof 20%. Incorporating a 10% uncertainty in k12, a value of k1

) (3.9 ( 0.8) × 10-12 cm3 molecule-1 s-1 is determined. Wealso note that other species potentially reactive with HN3 aregenerated in these experiments, namely O-atoms (via pho-tolysis of NO2) and O3 (via recombination of O with O2). Bothof these species react slowly with HN3 (4, 35) and will notcontribute significantly to its loss.

Hack and Jordan (3) reported the only other measurementof k1. They monitored the decay of HO• via pulsed laser-induced fluorescence, following the flash photolysis ofmixtures of H2O2 and HN3 in He buffer gas. Their value, (1.3( 0.2) × 10-12 cm3 molecule-1 s-1, is considerably smallerthan ours; reasons for this discrepancy are not obvious.Similarities in the reactivity of HN3 and HBr might beexpected, given the near identical H-X bond strengths (369kJ mol-1) in the two species (33, 42, 43) and the reasonablecorrelation between HO• rate coefficients and H-X bondstrengths in the hydrogen halides (and pseudohalides). Thisholds true in a qualitative way, with k1 being about a factorof 2 lower than the rate coefficient for reaction of HO• withHBr (42).

Using our value for k1 and a diurnally averaged tropo-spheric HO• concentration of 1 × 106 molecule cm-3, thelifetime for HN3 with respect to HO• reaction can be estimatedto be ≈2-3 days.

Rate Coefficient for Reaction of Cl• with HN3. Relativerate data for R10 versus the two reference reactions, R13 andR14, are shown in Figure 4.

Least-squares analysis of these data yield the followingrate coefficient ratios, k10/k13 ) 0.44 ( 0.06 and k10/k14 ) 2.19( 0.25. Although there is some discrepancy in the literatureregarding the value of k13 (values range from about (1.8-3.1)× 10-12 cm3 molecule-1 s-1), recent data (39, 44, 45) arecentered at (2.2 ( 0.4) × 10-12 cm3 molecule-1 s-1. The valueof k14 ) (4.9 ( 0.8) × 10-13 cm3 molecule-1 s-1 seems to bewell established (42, 43). Combining these reference ratecoefficient data with our measured ratios yields values for

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 valuek10 ) (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), allobtained using flow tube methodologies. While there is veryreasonable agreement between the various room-temper-ature measurements, which range from (9-13) × 10-13 cm3

molecule-1 s-1, these measurements were of an indirectnature, involved substantial occurrence of, or correction for,secondary reactions, and/or required modeling of fairlycomplex reaction systems, and thus uncertainties on the orderof (25% typically apply. Our measurement of k10 via acompletely different methodology, one that should be freeof any complication from secondary reactions, providesconfirming 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 reactionof HO• with these two species, Cl• reacts more slowly withHN3 than with HBr (5, 35), in this case by about a factor of5.

Reactions of the •N3 Radical. A series of experiments wascarried out to determine the products of the reactions of •N3

with various species (itself, NO, NO2, CO, and O2) and todetermine semiquantitatively the rate coefficients or at leastthe relative rates for these reactions. For convenience, resultsobtained in our work and in previous studies are summarizedin 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 wasefficiently destroyed in these experiments, the only productobserved in the infrared was N2O with a molar yield of only(3.1 ( 0.4)% (Figure 5, open circles). The small N2O yieldobserved 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 largeyields of observable products suggests the involvement of•N3 self-reaction as an important removal process for N3,resulting in the formation of N2, which is undetectable withour apparatus:

Although the rate coefficient for R15 has not been firmlyestablished, it appears to be e 2 × 10-12 cm3 molecule-1 s-1

(36). Box model simulations of the experiments (conducted

HO• + HN3 f •N3 + H2O (1)

HO• + CH2dCH2 + M f HOCH2CH2 + M (12)

Cl• + HN3 f •N3 + HCl (10)

Cl• + CH3C(O)CH3 f CH3C(O)CH2 + HCl (13)

Cl• + CH3Cl f CH2Cl + HCl (14)

FIGURE 4. Relative rate of decay of HN3 versus those of acetone(solid circles) and methyl chloride (open circles) in the presenceof Cl-atoms.

•N3 + •N3 f 3N2 (15)

1636 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005

using the Acuchem software package (46)) indicate that reac-tion of Cl• with •N3 cannot be ignored under these conditions:

The fate of NCl• is not known in this system but may involveformation of NCl3 a compound whose major IR absorptionbands (<800 cm-1) likely occur outside our accessible range.

Finally, we note that in this case, where no reactive species(NOx, CO) have been added to the chamber, the lifetime ofN3 may be sufficiently long (perhaps a few seconds, asindicated by box modeling studies) to allow for someheterogeneous removal at the chamber surface.

We conducted numerous experiments with NO, (0.7-4.2) × 1014 molecule cm-3, added to the standard Cl2/HN3

mixtures. Experiments were conducted with either N2 orsynthetic air as the buffer gas at a total pressure of 700-720Torr. The major product observed in these experiments wasN2O, with a molar yield of 91 ( 12%, independent of thenature of the buffer gas (solid circles, Figure 5). This essentially1:1 conversion of HN3 to N2O, observed under all conditionsin which NO was present, is consistent with the quantitativeoccurrence of R3a:

Losses of NO were also measured. In N2 buffer, the lossof NO was found to be identical to the N2O production (Figure5, open squares), again consistent with the quantitativeoccurrence of R10 and R3a. When air was used as the buffergas, losses of NO exceeded those of N2O, and NO2 productionwas also observed. This extra conversion of NO to NO2 likelyresults from the occurrence of the well-known (35) three-body reaction, R19:

Reasonable correspondence was obtained between theloss of NO in air, corrected by the amount of NO2 formed,and the observed N2O. Hewett and Setser (36) determineda rate coefficient of k3 ) (2.9 ( 0.3) × 10-12 cm3 molecule-1

s-1. They surmised that N2 and N2O were the most likelyproducts of the reaction, a result now confirmed by theexperiments conducted herein.

Another of the reactions studied by Hewett and Setser(36) was the reaction of •N3 with CO, for which they reporteda rate coefficient of (1.8 ( 0.2) × 10-12 cm3 molecule-1 s-1

and proposed NCO and N2 as the most reasonable reactionproducts. When standard Cl2/HN3/N2 reaction mixtures werephotolyzed in our chamber in the presence of CO, (3.5-7)× 1014 molecule cm-3, only a very small (≈4%) yield of N2Owas observed. Although HN3 was efficiently consumed, nochange in [CO] was observed nor was any other productdetected. These findings are suggestive of a CO-catalyzedrecombination of •N3, via R5a, and R20 and/or R21, resultingin large yields of undetectable N2:

However, we present evidence below that suggests thatR5 may be considerably slower than reported by Hewett andSetser (36). Thus, in these Cl2/HN3/CO/N2 experiments, it ispossible that NCO is not formed at all, and •N3 loss iscontrolled by its self-reaction and/or its reaction with Cl•,again leading to large yields of N2 or other undetectableproducts. The trace yield of N2O in these experiments is againlikely the result of the presence of minor NOx impurities inthe chamber.

To assess the relative rate of the reaction of •N3 with NOand CO, mixtures of Cl2 (≈3.5 × 1015 molecule cm-3), HN3

(≈3 × 1014 molecule cm-3), CO ((1-35) × 1014 molecule cm-3),and NO (≈1 × 1014) were photolyzed in the chamber. Theinitial [NO]:[CO] ratio was varied from 1:1 to 1:25. Given thesimilar values for k3 and k5 reported by Hewett and Setser(36), one would expect significant production of NCO viaR5a under all of these conditions, and essentially exclusiveNCO formation at the highest [CO]:[NO] ratios employed.Reaction of NCO with NO, R22, is thought (47-50) to generatenear-equal yields of N2O and CO2.

TABLE 2: Summary of Available Data Regarding Rate Coefficients and Products for Reactions of N3 Radicals with AtmosphericallyRelevant Species

reactantreactionnumber rate coefficient (cm3 molecule-1 s-1) products

NO R3 2.9 × 10-12 (ref 36) N2O + N2 (≈100%), this workNO2 R4 1.9 × 10-12 (ref 36); less than k3/3 (this work) 2NO + N2 (major, this work); 2N2O (<30%, this work);

N2 + N2O + O (<60%, this work); 2N2 + O2(minor/negligible, this work)

CO R5 1.8 × 10-12 (ref 36); less than k3/100 (this work) NCO + N2 (speculative, ref 36)O2 R25 <1 × 10-13 (ref 36); < 6 × 10-20 (this work) N2O + NO, N2 + NO2, and/or N2 + NO + O (speculative)

FIGURE 5. Product yields observed from the Cl-atom-initiatedoxidation of HN3, plotted as a function of HN3 consumption. Filledcircles, N2O produced in experiments with NO added; open squares,loss of NO in experiments with NO added; filled triangles, N2Oproduced in experiments with NO2 added; open circles, N2Oproduction in the absence of added NOx. See text for detailedexperimental conditions.

Cl• + •N3 f NCl• + N2 (16)

NCl• + Cl2 f NCl2 + Cl• (17)

NCl2 + Cl2 f NCl3 + Cl• (18)

Cl• + HN3 f •N3 + HCl (10)

•N3 + NO f N2 + N2O (3a)

NO + NO + O2 f NO2 + NO2 (19)

•N3 + CO f NCO + N2 (5a)

NCO + NCO f CO + CO + N2 (20)

NCO + •N3 f CO + 2N2 (21)

VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1637

Thus, in our Cl2/HN3/CO/NO experiments, increasing theratio of [CO] to [NO] should result in an increase in the yieldof CO2, via R5a and R22b, and a concomitant decrease in theyield of N2O. However, in no case was any CO2 productionor CO consumption observed in our experiments. Further-more, yields of N2O remained high (g85%), indistinguishablefrom those observed when only NO was added to the system.Thus, we find no evidence for the production of NCO in thissystem, and thus no evidence for the occurrence of a reactionbetween •N3 and CO. From the lack of CO2 production in theexperiment with the highest [CO]:[NO] ratio, we estimatethat the value for k5 to produce NCO is at least 100 timesslower than the value of k3. It should be noted that this findingis obtained under the assumption of approximately equalrate coefficients for the two branches of R22, as reported inrefs 47-50. Should higher yields of N2O relative to CO2 bethe case, the picture would be blurred considerably (notethat R5a followed by R22a is indistinguishable from R3a). Atthe very least then, our data are not consistent with thecombined results of refs 36 and 47-50. Given the substantialevidence for CO2 production in R22 (47-50), obtained byvarious independent investigators using different techniques,it seems most likely that the value of k5 has been overesti-mated in ref 36.

The next reaction studied was that of •N3 with NO2, forwhich Hewett and Setser (36) reported a rate coefficient of(1.9 ( 0.2) × 10-12 cm3 molecule-1 s-1. As noted in theIntroduction, multiple reaction channels are possible, leadingto the production of NO, N2O, N2, O2, and/or O-atoms. Weconducted three types of experiments which can provideinformation regarding the rate coefficient and/or mechanismof R4. First, standard Cl2/HN3 mixtures were photolyzed inthe presence of (1.7-2.8) × 1014 molecule cm-3 NO2 (“type1” experiments). Second, experiments were conducted inwhich both NO and NO2 were added to the standard Cl2/HN3 reaction mixtures (“type 2”). Finally, experiments wereconducted in which HO• was used to initiate the oxidation(“type 3”). While NO2 is not initially present in these “type3” experiments, it is generated in the HO• source chemistry(see R6-8 above) and thus its concentration builds up as anexperiment progresses. As shown in Figure 5 (solid triangles),N2O was clearly a major product in the “type 1” experiments,although its appearance profile is curved and its initial yieldis clearly less than unity. The production of NO was alsonoted in these experiments, which could arise from thephotolysis of NO2 or from R4.

In both the “type 2” and “type 3” experiments (i.e., when NOis present in the initial mixture), the yield of N2O was nearunity, and its appearance profiles were found to be linear.

Because of the multiple pathways for conversion of NO2

to NO and the multiple potential sources of N2O, i.e., R3,R4b, and R4c, a quantitative assessment of the rate coefficientand branching ratios for R4 is not possible. However, throughbox model simulations of the experiments, some conclusionscan be drawn. First, the less than unity yield of N2O early inthe “type 1” experiments clearly precludes channel (4b) frombeing a major channel and precludes a dominant contribu-tion from R4c. Simulations suggest an N2O yield from R4 ofabout 40 ( 20%si.e., a branching ratio of no more than 30%to (4b), or 60% via (4c), or some combination of the two.

Second, although NO2 to NO conversion is dominated byNO2 photolysis in many cases, some experiments suggest alarge yield of NO must be obtained from R4, and thus a largebranching to channel (4a) and/or (4c) is likely. (Note thatR4c can lead indirectly to NO via production of O-atomsfollowed by the occurrence of R24). Third, nitrogen massbalance arguments, which do not allow for large yields of N2

from R4, can be used to preclude a large contribution fromchannel (4d). Finally, we conclude that the total ratecoefficient k4 is likely about a factor of 3 or more smallerthan k3, given that a unity yield of N2O was obtained in “type2” experiments, even when the initial [NO2]:[NO] ratio wasas high as 4:1. Although not a unique solution, all experimentsconducted can be simulated reasonably well with k4a ≈ 6 ×10-13 cm3 molecule-1 s-1 and k4b ≈ 2 × 10-13 cm3 molecule-1

s-1. Inclusion of a minor occurrence of R4c in the box modelwith a compensatory reduction in k4a and k4b also providesadequate simulations of the experimental results.

The last reaction studied was that of •N3 with O2, a reactionfor which Hewett and Setser (36) reported a rate coefficientupper limit of 1 × 10-13 cm3 molecule-1 s-1. The reactionappears to have three energetically favorable reaction chan-nels

Our experiments consisted of the photolysis of mixturesof Cl2 (≈4 × 1015 molecule cm-3)/HN3 (≈1014 molecule cm-3)in 750-1050 Torr O2/N2 diluent, with the O2 partial pressurevaried from 10 to 1000 Torr. The only product observed inthese experiments was N2O. Although its yield did increasewith O2 partial pressure (from about 8% at 10 Torr O2 to 30%at 1000 Torr), this increase was less than linear in [O2],suggesting that reaction of •N3 with O2 was not the source ofall of the observed N2O. From box model simulations of thehigh [O2] experiments, under the assumption that all of theN2O formed derived from R25, an upper limit of 6 × 10-20

cm3 molecule-1 s-1 was found for k25. This upper limit is notdependent on the mechanism for R25, as each of the threereaction channels listed above gave similar N2O yields in themodel simulations. The derived upper limit is howeversomewhat dependent on the value used in the model for k15,the rate coefficient for self-reaction of •N3 (faster removal of•N3 via R15 requires a larger value for k25 to generate theobserved N2O). While Hewett and Setser have reported anupper limit for k15 of 1.5 × 10-12 cm3 molecule-1 s-1, theupper limit we report for k25 still applies for values of k15 ashigh as 10-11 cm3 molecule-1 s-1.

Atmospheric Fate of HN3. Due to its low Henry’s lawconstant and moderate pKa, aqueous azide is easily trans-ferred to the gas phase as hydrazoic acid (2). The dataobtained in this study show that removal of HN3 from thelower troposphere will be controlled by photolysis and byreaction with HO• with an overall lifetime of about 1-2 days.This implies that a hydrazoic acid plume could be sufficientlylong-lived to be advected many kilometers downwind froma spill, assuming that other more rapid sinks are not available.

Solar photolysis of HN3 and reaction with HO• will leadto the production of the HN• and •N3 radicals, respectively.Although these radicals are likely to have little furtheratmospheric impact, it is interesting to consider their ultimatefate, and thus that of the parent HN3. The HN• radical hasbeen shown to react rapidly with O2 to generate HO• and NO.(35) The kinetic database for •N3 is, however, not sufficientlymature to allow a confident assessment. The work conductedherein, in conjunction with rate data from Hewett and Setser

NCO + NO f N2O + CO (22a)

NCO + NO f CO2 + N2 (22b)

NO2 + hν f NO + O• (23)

O• + NO2 f NO + O2 (24)

•N3 + O2 f N2O + NO (25a)

f N2 + NO2 (25b)

f N2 + NO + O (25c)

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(36), shows that reactions of •N3 with NO or NO2 (leading inlarge part to N2O) may contribute. However, we are as yetunable to rule out reaction of •N3 with either CO or O2 asbeing of atmospheric relevance. Our work indicates that k5

may be considerably slower than measured by Hewett andSetser, but given the typical atmospheric predominance of[CO] over [NOx], R5 may still contribute. Even our upperlimit for k25 leads to a lifetime for •N3 with respect to O2

reaction of a few seconds, a time scale that is comparableto •N3 removal by a few ppbv of NOx. Given the existence ofsome discrepancies between our work and that of Hewettand Setser (36), further direct (time-resolved) kinetic studiesof the •N3 radical, perhaps using tunable diode lasertechniques for product detection, would be productive.

AcknowledgmentsThe National Center for Atmospheric Research is operatedby the University Corporation for Atmospheric Researchunder the sponsorship of the National Science Foundation.The authors are grateful to Richard Ross and Jim Hoban(U.S. EPA NEIC) for their assistance with the solar photolysisexperiments and to J.-F. Lamarque and David Hanson ofNCAR for their critical reading of the manuscript. One of us,E.A.B., would like to thank to Ms. Diana A. Love, Director,U.S. EPA NEIC, for providing financial and technical supportthat made possible his sabbatical leave from the Universityof Arizona.

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Received for review November 19, 2004. Accepted November23, 2004.

ES048178Z

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