1143
J. Phys. Chem. 1995, 99, 1143-1150
Gas-Phase UVNisible Absorption Spectra of HOBr and BrzO John J. Orlando* Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado 80303-3000
James B. Burkholder National Oceanic and Atmospheric Administration, Aeronomy Laboratory and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309 Received: August 26, 1994; In Final Form: November 18, 1994@
The gas-phase UV/visible absorption spectrum of HOBr, an important atmospheric trace species, is reported for the first time. The HOBr spectrum, measured over the range 200-420 nm, consists of two absorption = (3.1 f 0.4) x cm2 molecule-') and near 350 nm (0- = (6.1 f bands peaking near 280 nm (amax 1.0) x cm2 molecule-'). Atmospheric photolysis rates for HOBr calculated from these data are found to be significantly slower than previously assumed. The UV/visible absorption spectrum of BrzO, which was used as the HOBr precursor, was also recorded for the first time. The Br20 spectrum has maxima near 200 nm (omax = (2.0 f 0.2) x lo-'' cm2 molecule-') and 314 nm (a- = (2.3 f 0.3) x cm2 molecule-'). These spectra of BrzO and HOBr are compared with those of the analogous chlorine species. In addition, some thermodynamic properties for B r 2 0 and HOBr have been obtained. The equilibrium constant for the reaction Br2O H20 2 HOBr (10, -lo), was determined to be 0.02, yielding AG0,(298 K) = 2.33 kcaY mol for the forward reaction. This equilibrium constant is evaluated in terms of recent reports of AHd298 K) for HOBr.
+
+ HBr - Br, + H,O HOBr + HC1- H,O + BrCl
Introduction
HOBr
Bromine compounds released at the earth's surface have been linked to ozone destruction in various regions of the earth's atmosphere-for example, in the lower ~tratosphere,'-~ in the Antarctic during an ozone hole event,4 and in the lower Arctic troposphere in ~ p r i n g . ~ - ~ Although relatively little is known about its gas-phase chemistry or photochemistry, hypobromous acid (HOBr) is a significant bromine-containing species in the atmosphere. The main route to the formation of HOBr in the atmosphere is reaction of HO2 with BrO:239 HO,
+ BrO
-
+ 0,
HOBr
+ H,O
-
HOBr
+ HNO,
+ 0, -BrO + 0, OH + 0, -.HO, + 0, HO, + BrO - HOBr + 0, HOBr + h v - OH + Br Br
(1)
(2)
on or in aerosol particles also leads to HOBr formation, although its importance is limited to regions of low sunlight and/or high aerosol content, such as Arctic haze events in the troposphere or in the nighttime stratosphere. Destruction of HOBr in the lower atmosphere (below 25 km) is dominated by photolysis: HOBr
+ hv - OH + Br
(3)
with a significant contribution from reaction with O(3P)at higher altitudes:"
+
O(3P> HOBr
-
OH
+ BrO
(4)
Heterogeneous reaction with HBr or HC18,10,12 @
Abstract published in Advance ACS Abstrucrs, January 1, 1995.
0022-3654/95/2099-1143$09.00/0
(6)
may play a role in polar regions. The role of HOBr in the determination of atmospheric odd oxygen levels is of particular significance in the lower stratosphere, where the formation of HOBr (reaction 1) followed by its subsequent photolysis (reaction 3) leads to an important catalytic cycle for ozone
The hydrolysis of BIONO~,~JO BrONO,
(5)
20,
net:
-
(7) (8) (1)
(3)
30,
In addition, its heterogeneous formation and destruction via reactions 2 and followed by photolysis of the Br2 product, has been proposed as an important mechanism involved in the depletion of ozone in the Arctic troposphere during Arctic haze eventsS8 Despite the expectation that photolysis is a significant atmospheric loss mechanism for HOBr, no gas-phase UV/visible spectra are available to date to quantify this process. Atmospheric models have estimated HOBr photolysis rates by assuming the shape of the HOCl spectrum, with a red-shift of 30 nm.l,, The obvious uncertainties in this procedure are compounded by the differences in the shape and magnitude of the HOCl spectra that have been reported in the literature.13-18 Despite the lack of gas-phase UV spectra for HOBr, some spectroscopicand structural information is available. Aqueousphase UV/visible spectra of HOBr have been available for some 5,8,10312
0 1995 American Chemical Society
Orlando and Burkholder
1144 J. Phys. Chem., Vol. 99, No. 4, 1995 time.19 In addition, Schwager and Arkel120 have reported an infrared spectrum for HOBr in a matrix isolation experiment, while gas-phase infrared2' and microwavez2 studies have also been reported. The heat of formation of HOBr has also been a topic of some controversy in the recent literature. The current recommendation13for atmospheric chemistry, AHf" = -19 kcavmol, is based on an estimate by B e n ~ o n .More ~ ~ recently, Monks et al.24 reported a significantly higher value of - 9;: kcal/mol, based on the determination of the ionization potential of HOBr. In addition, McGrath and R ~ w l a n dhave ~ ~ calculated a value of - 14.2 & 1.6 kcaVmol using the Gaussian-2 theory of molecular energies. In this paper, we report the first measurement of the gasphase HOBr UV/visible absorption spectrum. In addition, the UV/visible spectrum of Br2O (used as the HOBr precursor) is reported for the first time. The spectra are compared to the analogous chlorine compounds, C120 and HOC1. The equilibrium constant for the reaction pair Br2O H20 2HOBr is also reported, and is used to determine AHf(298 K) of Br2O relative to that of HOBr. Finally, atmospheric photolysis rates of HOBr are calculated and compared to previously reported values.
+
-
Experimental Section The ultimate goal of the experiments reported here was to measure the UVhisible absorption cross sections of HOBr from which its atmospheric photolysis rates could be calculated. However, our synthesis of HOBr required the use of Br2O as a precursor, a molecule for which very little was known. Hence, the initial phase of the experiment involved the development of synthesis procedures for B1-20, its identification using chemical ionization mass spectrometry (CIMS), and the quantitative determination of its UV/visible absorption spectrum using a diode array spectrometer. Following the characterization of BrzO, experiments were conducted in which Br2O was reacted with H20 to produce HOBr. Fourier transform infrared absorption measurements were conducted to confirm HOBr production from this chemistry and, in the final phase of the experiments, measurements of the UVhisible spectrum of HOBr were carried out. Following a description of the UVhisible diode array spectrometer which was central to these experiments, details of the preparation and characterization of BrzO and HOBr will be presented. UVNisible Absorption Spectrometers. The UVhisible spectrometer system located at the NOAA Aeronomy Laboratory was used for most measurements and has been described previously.26 Briefly, the collimated output of a 30 W D2 lamp passed through an absorption cell (either 25 or 36 cm in length) and was focused onto the entrance slit of a spectrograph equipped with a 600 grooves/mm grating, which dispersed the radiation onto a 1024 element diode array detector. Experiments were conducted with a 400 p m entrance slit, providing a spectral resolution of about 0.4 nm. In this configuration, the spectrometer recorded spectra over 84 nm intervals. The spectral intervals used were different for each experiment and will be detailed later. A wavelength calibration was performed using the emission lines from a low pressure Hg lamp. For measurements at long wavelengths (A > 295 nm), a Pyrex window was placed in front of the spectrograph to limit scattered radiation from shorter wavelengths. Some survey spectra were recorded using the UVhisible absorption system at NCAR. Briefly, this consists of a Dz lamp, a 90 cm long Pyrex absorption cell equipped with quartz windows. a spectrograph equipped with either a 150 grooves/
mm grating (spectral coverage of about 250 nm, resolution 1.5 nm) or a 600 grooves/" grating (65 nm coverage, 0.4 nm resolution), and a 1024 element blue-intensified diode array detector. Absorption spectra were calculated as the natural logarithm of the ratio of the light intensity with the cell empty (I,,) to the cell full (0: A = In(IdT). Each measurement of I or I, consisted of a summation of 10-25 exposures of the diode array and required 2-5 s to complete. Spectra were stored on a PC computer for further manipulation. Details of spectral subtraction procedures and absorption cross section determinations will be presented as they pertain to the specific experiments conducted. Synthesis and Characterization of BrzO. Br2O was synthesized via three different methods, all of which relied on the reaction of Br2 with HgO, by analogy with the C12/C120 system:14-18,27-30 2Br2
+ HgO - Br,O + HgBr,
(9)
The simplest method of generating Br2O involved the direct addition of HgO to an absorption cell. Enough HgO was added to partially cover the bottom of the absorption cell, the cell was evacuated, and Br2 gas (1-10 Torr) was added. Very little conversion of Br2 to BrzO occurred (about l%),and the Br2O concentration reached steady state within a few seconds of the addition of Br2 to the cell. This method had the advantage of providing a constant source of Br2O for long periods of time (at least 1 h), and was useful for obtaining survey spectra. The disadvantages of this system were that high levels of Br2 were always present, rendering measurements of BrzO absorption spectra at wavelengths greater than 380 nm impossible. Also, the photolysis of such large quantities of Br2 with either the D2 lamp or with room light led to the catalytic destruction of BrzO, limiting its concentration in the absorption cell. In addition, the presence of HgO in the cell allowed little flexibility-reference spectra of pure Br2 had to be recorded in a different absorption cell. The second method involved the generation of BrzO in a round bottom flask. In this method, HgO powder (5-10 g) was added to a 500 mL round-bottom flask, and the flask was evacuated. Br2 gas (50- 100 Torr) was then added to the flask to generate BrzO. The gaseous contents of the flask, containing Br2O but mostly Br2, were then transferred to either the CIMS instrument or to an absorption cell. The conversion of Br2 to Br2O was found to occur very quickly but again with limited yield. The Br2O yield was found to increase as approximately the square of the amount of Br2 added to the flask, in accord with the stoichiometry of reaction 9. While this method of Br2O generation provided more flexibility than method one, the main drawback was again the presence of large Brz impurities. The third method for generation of gaseous BrZO provided the cleanest source of BrzO, with Br2 impurities minimized. In this method, Br2 was again added to the evacuated round-bottom flask as described for method two. The gases in the flask were then pumped through a second trap held at liquid N2 temperature. This procedure was repeated several times to obtain sufficient quantities of Br2O in the cold trap. The temperature of the trap was then raised to about -40 "C, and Brz was removed by pumping. Toward the end of the pumping procedure, about 2 h, a white solid was observed in the trap. The trap was then warmed rapidly to room temperature, and the gases were transferred to the absorption cell. In this fashion, mixtures of BrzO and Br2 were obtained but with substantially less Br2 (30-50%) than was obtained using the previously described procedures.
Absorption Spectra of HOBr and Br2O
J. Phys. Chem., Vol. 99, No. 4, 1995 1145
To identify BrzO as a product of the reaction of Br2 with HgO, a chemical ionization mass spectrometer was emp l ~ y e d . ~Br2O ~ , ~was ~ synthesized in the round-bottom flask, as described above (method two). The gaseous contents of the flask were then flowed through a needle valve into the ionmolecule reactor region of the instrument, where reaction with SF6- took place. A small portion of the flow through the reactor passed through a small orifice, and ions were detected using a quadrupole mass filter. Attempts to characterize Br2O via infrared absorption were unsuccessful. Spectra were recorded using a BOMEM DA3.002 Fourier transform ~pectrometer.~~ The infrared light source was a heated graphite rod. The spectrometer was equipped with a KBr beamsplitter and a liquid He cooled Cu:Ge detector. Measurements were made over the range 700-1500 cm-', at a spectral resolution of 0.1 cm-'. Br20 was generated by method one, the addition of Br2 gas to a 20 cm long Pyrex absorption cell that contained HgO. The fundamental Br-0 stretches for this species are located near 500 ~ m - ' ,out ~ ~of the range of our spectrometer. Therefore, our attempts were limited to observations of weak combination and overtone bands. This fact, coupled with the inability to generate large concentrations of Br20, led to a lack of success in the detection of any absorption features due to Br2O. For measurements of its UVhisible spectrum, Br2O was synthesized using method three, and samples containing mixtures of Br2 and BrzO were introduced to the 20 cm long absorption cell. Spectra were recorded over a series of spectral intervals (190-255, 220-300, 235-318, 310-390, 350-430, and 380-460 nm). Reference spectra of Br2 were also recorded over these wavelength intervals. The Br20 spectrum was constructed by overlapping these spectra, starting at long wavelengths and moving to shorter wavelengths. The Br2O spectra were obtained following subtraction of absorption due to Br2, which was greatest at long wavelengths (A > 380 nm). Subtractions of Br2 were made at the long wavelengths by assuming that BrzO absorption was zero at 440 nm and longer. This procedure was adopted because it provided flat baselines in the residual Br20 spectra (absorbances of less than 5 x at wavelengths beyond 440 nm. Spectra recorded at successively shorter wavelengths were then normalized in the region of overlap with the previous spectrum, until a full spectrum from 200 to 440 nm was obtained. Because Br2O could not be obtained free of Brz contamination, it was not possible to determine the Br2O concentration via absolute pressure measurements. Instead, the Br20 concentration was obtained from its quantitative conversion to Br2 as follows. Mixtures of BrzO and Brz, obtained via method three above, were added to the absorption cell and a spectrum was recorded. Then, the gases in the cell were irradiated with a small desk lamp to effect the conversion of Br20 to Br2. Typically, the mixture was photolyzed for 2-3 s with the desk lamp, the lamp was turned off, and a new absorption spectrum was recorded. Absorptions due to Br2O and Brz in each spectrum were again determined by spectral subtraction, assuming that BrzO did not absorb beyond 440 nm. Absorption cross sections for Br20 were then determined by assuming a 1:l conversion of Brz0 to Br;?-i.e., the increase in Br2 concentration (determined at 420 nm) in successive spectra could be equated to the decrease in the absorption (at 314 nm) due to BrzO. Synthesis and Characterization of HOBr. HOBr was obtained from the reaction of Br20 with H20, by analogy to the C12O/H2O/HOC1 equilibrium system:
Br,O
14-18329,30
+H20
t*
2HOBr
(10, -10)
Both IR and UV/visible measurements of HOBr were conducted, with the procedure for HOBr production being slightly different in each case. For the IR measurements, HgO powder was added to the bottom of a 15 cm long pyrex absorption cell, equipped with AgCl windows. HgO powder ( e 3 g) was added to the bottom of the absorption cell, and the cell was evacuated. Then, HzO ( e 1 0 Torr) and Br2 ( ~ 2 5 - 5 0 Torr) were added to the cell., This method provided a source of HOBr that was constant for a period of about 20 min, enough time to record 3-5 coadditions at 0.04 cm-' resolution. After this time, the cell was evacuated and refilled with Brz and H20 and 5 more coadditions were acquired. This procedure was repeated until about 25 coadditions were acquired in total. Spectral measurements were made in the 3500-3800 cm-' region ( V I fundamental of HOBr) and the 1100-1250 cm-l region ( 7 2 fundamental of HOBr). For the 3500 cm-l region, the spectrometer was equipped with an InSb detector and CaF2 beam splitter, while a liquid He cooled Cu:Ge detector and a KBr beamsplitter were used for measurements near 1200 cm-'. For the study of the HOBr UVhisible spectrum, Br2O/Br:! mixtures were prepared in the round-bottom flask (method two above), about 2-5 Torr of this gas mixture was added to the 25 cm absorption cell, and an absorption spectrum was recorded. A reference spectrum for BrzO was obtained for this mixture following the quantitative subtraction of Br2, using our prior knowledge of the shape of the Br20 spectrum. H20 vapor (about 5-10 Torr) was then added to the cell to convert Br20 to HOBr, and a second absorption spectrum was recorded immediately after the addition of the H20. Changes in the spectrum were noted, due to a decrease in [BrzO] and an increase in [HOBr]. Quantitative spectral subtraction of Br2 and Br2O absorptions were then carried out to obtain the residual HOBr spectrum. Subtraction of Br2 was done by assuming that only Brz absorbed at wavelengths beyond 440 nm, or from prior knowledge of the shape of the Br2O spectrum. Subtraction of Br2O was carried out using the vibrational structure observed in its spectrum between 345 and 390 nm. This experiment was repeated over several spectral intervals (215-300, 325-410, and 300-380 nm) to obtain the full shape of the HOBr spectrum. The absolute cross section for HOBr at 280 nm was also obtained from the above experiment. The loss of Br2O upon addition of H20 was used to determine the amount of HOBr produced, assuming the stoichiometry of reaction 10, A[HOBr] = 2A[Br20]. The concentrations of Br2O and HOBr present following addition of a known H20 concentration were also used to obtain the equilibrium constant:
K , , = [HOBr]2/[Br,01[H201 Results and Discussion
BrzO Identification via Chemical Ionization Mass Spectrometry. To identify Br2O as a product of the reaction of Br2 with HgO, CIMS experiments were conducted on pure Br2 and on BrZ which had been exposed to HgO. Both spectra show ion peaks due to the reactant SF6- (mass 146 and 148), Brz(mass 158, 160, and 162), and Br2.F- (mass 177, 179, and 181). The Br2'F- ion is obtained via F- transfer from sF6-, as observed in the reaction of SF6- with numerous other species:32 SF6-
+ Br2
-
Br,'F-
+ SF,
(11)
The ion peaks at mass 193, 195, and 197, which are obtained only when HgO is present, are identified as BrZOF-, and show
1146 J. Phys. Chem., Vol. 99, No. 4, 1995
Orlando and Burkholder n in .
4 x 1 0.”
3 006
-
I
8
N
E Y
z
F
E v)
004
*
In m
0 02
0
!z
u
0 00 1
360
380 400 WAVELENGTH (nm)
420
Figure 2. Conversion of BrZO to BrZ via visible photolysis. Successive absorption spectra show an increase in absorption due to Br2 and a decrease in absorption due to BrzO. 0 WAVELENGTH (nm)
Figure 1. UVhisible absorption spectrum of BnO, obtained from the reaction of Brz with HgO. Absorption due to Brz impurity has been subtracted out of the spectrum. Shown for comparison is the absorption spectrum of C120. that Br20 is obtained as a product of the BrZ/HgO reaction system:
SF,-
+ B r 2 0 - Br20’F- + SF,
(12)
UVNisible Absorption Spectrum of Br2O. The overall shape of the Br2O spectrum (after subtraction of contributions due to Br2) was obtained by normalizing spectra recorded over different wavelength intervals in the region of their overlap and is displayed in Figure 1. The spectrum of BrZ0 is characterized by a strong narrow absorption band peaking near 200 nm and a weaker feature with a maximum at 314 nm. This weaker feature possesses a broad shoulder to the long wavelength side, with measurable absorption extending to about 440 nm. A weak vibrational band progression superimposed on the absorption continuum between about 340 and 380 nm is also observed. The spacing of the vibrational bands is about 500 cm-’, consistent with a stretching fundamental of Br20.34 Absorption cross sections for Br2O were obtained by stoichiometric conversion of Br20 to Br2 using visible photolysis, as described in the experimental section. With each successive irradiation, the absorption due to BrzO was found to decrease, while that of Brz increased as shown in Figure 2. The mechanism for this conversion is likely as follows:
+ Br Br + Br,O - Br2 + BrO BrO f BrO- Br + Br + O2 - Br2 + 0, Br2
+ hv
+
Br
(13) (14) (15a) (1 5b)
Occasionally, BrO was observed in UV spectra recorded immediately after photolysis, indicating its role as an intermediate in the Br2O to Br2 conversion. After acquiring a series of such spectra, the concentration of Br2 in each was obtained by spectral subtraction. The decrease of absorption due to Br2O
0
1
2
3
4
5
6
7x10‘’
A[Br,] (molecule cm.’)
Figure 3. Determination of the Br2O absorption cross section at 314 nm. Plotted is the decrease in absorption due to Br2O at 314 nm, as a function of the amount of Brz produced, as a mixture of Br20Brz is photolyzed with a visible light source. The different symbols are used to distinguish between different experiments.
(at 314 nm) could then be related to the increase in the Br2 concentration, as shown in Figure 3. Assuming the stoichiometry of the conversion is 1:l (i.e.. for each BrzO that is lost a Br, is produced), then the slope of Figure 3, divided by the path length of the cell, yields the absorption cross section for Br2O at 314 nm. A least-squares fit to this data yields an cm2 molecule-’ absorption cross section of (2.3 f 0.3) x for BrzO at 3 14 nm, which can then be used to place the entire BrzO spectrum into a quantitative framework (see Figure 1 and Table 1). The BrzO absorption cross sections reported here should be treated as an upper limit. Loss of Br2O without stoichiometric Br2 formation would lead t o a systematic overestimation of the Br2O cross section. As expected, there is a great deal of similarity between the spectrum of BrzO and its chlorine analog, C120 (see Figure l).15,16,27.28 Both spectra possess a strong absorption band i n the deep UV (C120: A,, = 171 nm, a,,, = 1.74 x lo-’’ cm2 BrzO: Amax = 200 nm, a,,, = 2.0 x lo-’’ cm2 molecule-’). with the Br2O peak being stronger and shifted to longer wavelength. The near-UV absorption features are also qualitatively similar in shape, both possessing a shoulder to long wavelengths. The Br2O absorption (Amax = 314 nm, amax = 2.3
J. Phys. Chem., Vol. 99, No. 4, 1995 1147
Absorption Spectra of HOBr and Br20
I
0 25
220
260
240
400~10.~’
,
280
0 7
1:: P a
O 3
0 2
0 1
00 300
320
360
360
340
0 30
$0
I
10
0 05
340
360
360
400
WAVELENGTH (nm)
200
250
300
350
400
WAVELENGTH (nm)
Figure 5. UVlvisible absorption spectrum of HOBr obtained in this work. Small squares, measured data. Solid line, parametrized fit to the measured data. Also shown for comparison is the aqueous phase spectrum of HOBr from ref 19.
Figure 4. Spectra of HOBr, obtained following the addition of water vapor to Br2O. Thin solid line, observed spectrum. Dotted line, Br20 spectrum. Dashed line, BrZ spectrum. Thick solid line, HOBr spectrum (obtained from subtraction of BrZO and Br2 from the observed spectrum).
x cmz molecule-’) is again shifted to longer wavelengths and is somewhat more intense than the corresponding C120 absorption (A- = 255 nm, , a = 2.0 x cm2 molecule-’). The vibrational structure observed in the Br20 spectrum is not evident in C120. IR Absorption Spectra of HOBr. Infrared measurements of HOBr were undertaken to identify it as a product of the reaction of Br20 with H20. As described in the Experimental Section, Br2 and H20 were added to a 15 cm long absorption cell containing HgO powder. Measurements were made in both the v1 and v.2 regions of the HOBr spectrum. Absorptions due to HOBr were identified by comparison with previously reported line position^,^^,^^ which were obtained by flowing Br2O over an aqueous slurry of HgO to form HOBr. HOBr was indeed identified in our spectra, indicating that HOBr could be generated from the reaction of water vapor with BrzO. UVNisible Absorption Spectrum of HOBr. As detailed previously, conversion of BrzO to HOBr via reaction 10 was used to obtain spectra of HOBr. A spectrum of Br20B1-2 was recorded, H20 was added, and a second spectrum was recorded. Quantitative subtraction of BrZ and Br2O from the second spectrum yielded the residual HOBr spectrum. Experiments of this type were conducted over overlapping spectral ranges (215300, 295-380, and 330-415 nm) to obtain the entire HOBr spectrum, as shown in Figure 4. The stoichiometry of reaction 10 was used to obtain the absolute absorption cross section for HOBr. The HOBr cross section at 280 nm, the peak of the HOBr spectrum, can be obtained from the following relationship:
where A[BrzO] is the difference in BrzO concentration before
and after H20 addition (as obtained from analysis of the spectra recorded before and after H20 addition) and AHOB~,Z~O is the absorbance at 280 due to HOBr in the residual spectrum. Eight experiments of this type were conducted (covering the 270355 nm range) yielding an average gHOBr,280 of (3.1 f 0.4) x cm2 molecule-’, where the quoted uncertainty is the la uncertainty in the average value. As discussed previously, the absorption cross sections for Br2O are based on the assumption of unit conversion of Br2O and Br2. If this assumption were not valid and the Br2O absorption cross sections were indeed lower, then the HOBr absorption cross sections would decrease proportionally. The HOBr UVhisible absorption cross sections obtained using this procedure are displayed in Figure 5 and listed in Table 1. The spectrum consists of two absorption bands, the stronger band peaking near 280 nm (a,= = 3.1 x cm2 molecule-’), and the weaker band peaking near 350 nm (am,= 6.1 x cm2 molecule-’). To facilitate the incorporation of these absorption cross section data into atmospheric models, we have conducted a least-squares fit of the observed spectrum to the sum of three Gaussians as follows: 3
a(H0Br) = Z A n exp(-I?,,@,, - 2)’) n= 1
Initially, all nine parameters (An,B,, and A,,, n = 1-3) were allowed to float in the fit. For simplicity, the three A,, were then fixed to the integer value nearest the fit value and the A,, and B, were allowed to float in a second fit. The parameters obtained are listed in Table 2 and reproduce the observed spectrum between 245 and 395 nm with an average deviation of less than 2% (see Figure 5). Atmospheric photolysis rates of HOBr obtained from the measured spectrum and the parameterized spectrum are identical. It should be stressed that the parametrized fit has no physical significance; it is provided merely to aid the incorporation of the HOBr absorption cross sections into atmospheric models.
1148 J. Phjs. Chem., Vol. 99, No. 4, 1995
Orlando and Burkholder
TABLE 1: Absorption Cross Sections of BrzO and HOBr (10-l8 cm2) wavelength wavelength (nm) o(Br2O) a(H0Br) (nm) a(Br7O) a(H0Br) 196 198 200 202 204 206 208 210 212 214 216 218 220 222 224 226 228 230 232 234 236 238 240 242 244 246 248 250 252 254 256 258 260 262 264 266 268 270 272 274 276 278 280 282 284 286 288 290 292 294 296 298 300 302 304 306 308 310 312 3 I4 316 318 320
17.4 19.3 20.0 19.5 18.2 16.1 13.4 10.9 8.44 6.42 4.88 3.68 2.90 2.34 1.99 1.74 1.56 1.43 1.31 1.19 1.08 0.969 0.869 0.767 0.674 0.587 0.514 0.443 0.385 0.336 0.308 0.275 0.249 0.230 0.214 0.207 0.206 0.206 0.217 0.238 0.265 0.305 0.361 0.429 0.516 0.626 0.745 0.883 1.04 1.21 1.38 1.55 1.72 1.88 2.01 2.12 2.21
3
a(HOBr) = xA,,e-B"Wn-W)2 1
0.0400 0.0408 0.0430 0.0463 0.0489 0.0508 0.0513 0.0554 0.0551 0.0569 0.0575 0.0585 0.0611 0.0610 0.0598 0.0593 0.0589 0.0589 0.0578 0.0570 0.0544 0.0532 0.0511 0.0487 0.0453 0.0441 0.0394 0.0368 0.0332 0.0302 0.0280 0.0249 0.0201 0.0177 0.0167 0.0147 0.0114 0.0102 0.0063 0.0018 0.0029 0.0029
Figure 6. Comparison of the measured HOBr spectrum with the gasphase spectrum of HOCl red-shifted by 45 nm. The HOCl spectrum is that measured by Burkh01der.l~
Although this is apparently the first report of a gas-phase spectrum of HOBr in the literature, an aqueous-phase spectrum19 is available for comparison (see Figure 5). The aqueous phase spectrum is seen to possess the same qualitative shape as the gas-phase spectrum, although the short-wavelength band is about 15% stronger and is blue shifted by about 15 nm compared to the gas-phase spectrum. The weaker band in the aqueous phase spectrum is also shifted to shorter wavelength relative to the
gas-phase spectrum and appears more like a shoulder than a distinct absorption feature. It is also interesting to compare the gas-phase HOBr spectrum with that of HOCI, not only because of the obvious similarity in the two molecules but also because a red-shifted HOCl spectrum has been used in atmospheric models to estimate HOBr photolysis rates. As seen in Figure 6, the HOCIl4 and HOBr spectra are very similar in shape with the HOBr spectrum shifted 45 nm to the red of the HOCl spectrum. The intensity of the 280 nm absorption feature in the HOBr spectrum is about 50% higher than the corresponding feature in HOCI, while the weaker bands are essentially of equal intensity. It should be noted here that there has been considerable debate in the literature regarding the shape and magnitude of the HOCl spectrum, with the measurements essentially falling into two groups. The HOCl spectrum shown in Figure 6 is that measured by Burkholder14 and is essentially identical to that originally reported by Knauth.16 The other group of HOCl spectral measurements, reported independently by Molina and Molina,l5 Permien et a1.,I8and Mishalanie et al.," and currently recommended for use in atmospheric models13 does not appear consistent with our measured HOBr spectrum. In all known cases, the spectrum of a Br-containing molecule has a peak absorption cross section that is higher than its chlorine analog. This would not be the case if the currently recommended HOCl spectrum is compared to our HOBr spectrum.
2.26
2.30 2.30 2.28 2.25 2.20
0.0043 0.0080 0.0143 0.0232 0.0336 0.0453 0.0590 0.0736 0.0912 0.110 0.129 0.151 0.174 0.196 0.219 0.240 0.259 0.278 0.292 0.302 0.308 0.310 0.307 0.301 0.290 0.273 0.255 0.234 0.211 0.187 0.164 0.141 0.119 0.0997 0.0837 0.0702 0.0588 0.0511 0.0445 0.0407 0.0395
322 324 326 328 330 332 334 336 338 340 342 344 346 348 350 352 354 356 358 360 362 364 366 368 370 372 374 376 378 380 382 384 386 388 390 392 394 396 398 400 402 404 406 408 410 412 414 416 418 420 422 424 426 428 430 432
TABLE 2: Parametrization of HOBr UV Absorption Spectrum"
2.15
2.10 2.06 2.01 1.99 1.96 1.94 1.94 1.94 1.94 1.94 1.95 1.93 1.93 1.91 1.87 1.86 1.77 1.78 1.64 1.67 1.49 1.52 1.36 1.36 1.21 1.20 1.10 1.05 0.988 0.906 0.873 0.797 0.764 0.704 0.665 0.621 0.576 0.532 0.487 0.447 0.401 0.363 0.324 0.283 0.243 0.216 0.186 0.155 0.126 0.0985 0.0802 0.0596 0.0388 0.0227 0.0092
v
,I=
parameter
n=l
).n
268 0.113 0.00349
A!, B,,
12
=2
''
350 0.062 0.000822
286 0.262 0.00279
Cross sections are in units of nanometers. 400x10
n=3
cm:.
are in units of
,
300
200
100
0
200
250
300
350
400
WAVELENGTH (nm)
J. Phys. Chem., Vol. 99, No. 4, 1995 1149
Absorption Spectra of HOBr and BrzO An attempt to produce HOBr with only small Br2 and Br2O spectral interferences, via the hydrolysis of BrON02, was also carried out. BrON02 (1-2 Torr) was added to the 25 cm absorption cell and H20 vapor (5-10 Torr) was added. Hydrolysis of BrON02, presumably on the surfaces of the cell, occurred rapidly (in ~5 s). However, the major absorber observed in the UV spectrum was Br20, not HOBr, due to the rapid equilibration of HOBr, H20, and Br2O (reaction 10, - 10). Thus, this method was not pursued further as a source of HOBr. Thermodynamic Properties of HOBr and BrzO. Analysis of the experiments above in which Br2O was converted to HOBr via addition of H20 were consistent with the assumption that equilibrium had been reached. HOBr was formed rapidly following the addition of H20 (in less than the time required to record a UVhisible spectrum, about 5 s) and its concentration (and that of Br2O) was found to be constant for minutes afterward. Determination of the concentrations of Br20 and HOBr in the presence of a known (excess) amount of H20 yielded an equilibrium constant for the reactions 10 and -10 of 0.02 f 0.002 (precision only). From the relation AG = -RT In Kp, the equilibrium constant can be used to determine the free energy change for reaction 10, 2.33 kcdmol. With the knowledge of the AHf of H20, -57.81 kcdm01,'~and assuming AS x 0, the following relationship can be obtained: M f ( B r 2 0 ) - 2AHdHOBr) = 55.5 kcal/mol At present, the AHf of Br20 is unknown, while that of HOBr is uncertain. However, we note that the AHf of HOBr is probably in the range -924 to -1425 kcavmole, yielding a BrzO AHf in the range 27-38 kcavmol. An independent method was used to estimate the heats of formation of HOBr and Br2O by comparison with their chlorine analogues. We note that Br bonds are typically 10 k 3 kcdmol weaker than the analogous C1 bond.13 Using this relationship and the known AH4298 K) of C120 and HOCl,13 AH4298 K) for Br20 and HOBr are estimated to be 33 and - 11kcdmol, respectively. These values are consistent with our measurement of Klo and with the recently reported values for AHd298 K) of HOBr.24,25 The equilibrium constant for the reaction pair C1,O iH20
-
2HOC1
(16, -16)
is 0.1 k 0.01,14-16329,30 yielding AG",(298 K) for this process of 1.4 kcal mol-'. The similarity in the energetics for K16 and Klo (within 1 kcal mol-') is not surprising given the aforementioned 10 kcal mol-' difference between Br bonds and the corresponding C1 bond. That is, the difference in AH4298 K) between C120 and BrzO, which involves two X - 0 bonds, would be expected to be approximately twice the difference in AHf(298 K) between HOCl and HOBr, which involves only one X-0 bond. Atmospheric Implications of the HOBr Spectrum. The UV/visible absorption cross sections of HOBr were used to determine its rate of photolysis in the atmosphere, assuming a photodissociation quantum yield of unity. At 17 km, for July 4th at 30' N with an 0 3 column of 305 Dobson units, the diurnally averaged HOBr photolysis rate is determined to be 5 x s-l (lifetime x h). For the same conditions at ground level, the HOBr photolysis rate was found to be about a factor s-l (lifetime 1 h). As stated earlier, of two slower, 2.4 x HOBr photolysis rates have been estimated in atmospheric using recommended HOCl cross sections red-shifted by 30 nm. Using the same solar fluxes as above and the redshifted spectrum yields a photolysis rate at 17 km of 8 x lop4 s-l, some 60% faster than that based on our spectrum.
Previously published model calculations1s2 have employed s-l in photolysis rates for HOBr in the range (1-1.5) x the lower stratosphere. In some model runs, Poulet et al.' s-'), employ even faster HOBr photolysis rates (=3 x based on an unpublished study of the HOBr spectrum by Schindler. It is evident then that in all cases these models have been overestimating the photolysis rates for HOBr and hence underestimating its atmospheric mixing ratio. Using the new absorption cross sections for HOBr in a 2-D shows daytime concentrations of HOBr and BrON02 to be about equal, with the mixing ratio of BrO being some 4-5 times higher, whereas previous models indicate BrONOz levels to be substantially higher than those of HOBr. The photolysis rate calculations conducted above used roomtempearture HOBr absorption cross sections. However, the temperature dependence of the absorption cross sections should have a minimal effect on the photolysis rate. HOBr photolysis in the lower stratosphere will occur in the region between about 300 and 380 nm, i.e., over the region covered by the 350 nm absorption feature. While the shape of this 350 nm absorption feature is likely to change somewhat with temperature, the integrated intensity is expected to be relatively constant. Since the actinic flux in this wavelength region is relatively flat, these changes in the shape of the absorption feature will have little effect on the calculated photolysis rates. The longer HOBr lifetime may have important consequences for the ozone-destroying catalytic cycle represented by reactions 1, 3,7, and 8. At present, the rate-limiting step in this scheme is believed to be the formation of HOBr, reaction 1. However, the slower photolysis rate for HOBr brings the time scales for reactions 1 and 3 into closer competition. It is also important to compare other potential atmospheric loss processes with photolysis in view of the slower photolysis rates reported here. The reaction of O(3P) with HOBr (4) has been reported to be fast, k4 = (2.5 f 0.4) x lo-" cm3 molecule-' s-'.ll Using typical O(3P) concentration^'^ at 20, 25,30, and 35 km yields HOBr loss rates with respect to reaction 2.5 x 1.0 x and 2.5 x s-l, 7 of 5 x respectively. Thus, photolysis will be the main loss process at low altitudes ( z < 25 km), while reaction with O(3P) will dominate above 30 km. Heterogeneous loss on sulfate aerosol in the stratosphere, postulated recently by Abbatt,12 may also be important but only under conditions of high aerosol loading (postvolcanic) and low irradiation. The rate constant for reaction of OH with HOBr has not been measured but would have to approach the gas-kinetic limit to compete with photolysis or reaction 7 as a loss process for HOBr. This seems an unlikely possibility, especially given the low rate constant for OH with HOCl ( k = 5 x cm3 molecule-' s-l).13
Conclusions In this paper, we have synthesized Bra0 from the reaction of Br2 with HgO, and determined its gas-phase W h i s i b l e absorption cross sections for the first time. The BrZO was used as a precursor in the synthesis of HOBr, for which we also report the first gas-phase UVhisible spectrum. The equilibrium H20 2HOBr, was constant for the reaction pair, Br20 determined to be 0.02, and this value was used to constrain the AHf of Brz0 to the range 27-38 kcaVmol (based on AHf of -9 to -14 k c d m o l for HOBr). Finally, the UVhisible absorption cross sections for HOBr were used to determine its atmospheric photolysis rates. Its lifetime with respect to photolysis at ground level appears to be about 1 h and decreases to about 0.5 h in the lower stratosphere.
+
-
1150 J. Phys. Ckem., Vol. 99, No. 4, 1995 Acknowledgment. The authors would like to acknowledge L. Greg Huey of NOAA for conducting the CIMS measurements, Geoffrey Tyndall of NCAR for many helpful discussions and help in the earlier phases of the work carried out at NCAR, and A. R. Ravishankara of NOAA and Guy Brasseur and Claire Granier of NCAR for useful discussions regarding this project. This work was partially supported by NASA Upper Atmospheric Research Program, under separate contracts to NCAR and NOAA. NCAR is partially supported by the NSF. References and Notes (1) Yung, Y. L.; Pinto, J. P.; Watson, R. T.: Sander. S. P. J. Atmos. Sci. 1980, 37, 339. (2) Poulet, G.; Pine, M.; Maguin, F.; Ramaroson, R.; Le Bras, G. Geophys. Res. Lett. 1992, 19, 2305. (3) Garcia, R. R.; Solomon, S. J. Geophys. Res. 1994, 99, 12,937. (4) McElroy, M. B.: Salawitch, R. J.; Wofsy, S. C.; Logan, J. A. Nature 1986, 321, 759. (5) Barrie, L. A,; Bottenheim, J. W.; Schnell, R. C.; Crutzen, P. J.; Rasmussen, R. A. Nature 1988, 334, 138. (6) Bottenheim, J. W.; Barrie, L. A,; Atlas, E.; Heidt, L. E.; Niki, H.; Rasmussen, R. A,; Shepson, P. B. J . Geophys. Res. 1990. 95, 18,555. (7) McConnell, J. C.; Henderson, G. S.; Bame, L.; Bottenhiem, J.; Niki, H.; Langford, C. H.; Templeton, E. M. J. Nature 1992, 355. 150. (8) Fan, S.-M.; Jacob, D. J. Nature 1992, 359, 522. (9) Bridier, I.; Veyret. B.; Lesclaux, R. Chem. Phys. Lett. 1993, 201, 563. (IO) Hanson, D.; Ravishankara. A. R. NATO ASZ Ser. 1993. 17, 281. (11) Monks, P. S.; Nesbitt, F. L.; Scanlon, M.; Stief, L. J. J. Phys. Chem. 1993, 97, 11699. (12) Abbatt, J. P. D. Geophys. Res. Lett. 1994, 21, 665. (13) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Molina, M. J.; Hampson, R. F.; Kurylo. M. J.; Howard, C. J.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling; Evaluation Number 10. NASA JPL Publ. 1992. No. 92-20. (14) Burkholder, J. B. J. Geophys. Res. 1993, 98, 2963.
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