Temperature and pressure dependent rate coefficients for the reaction of Hg with Br and the reaction of Br with Br: A pulsed laser photolysis-pulsed laser induced fluorescence study

Article abstract of DOI:10.1021/jp054688j

A pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to study the recombination of mercury and bromine atoms, Hg + Br + M → HgBr + M (1) and the self- reaction of bromine atoms, Br + Br + M → Br₂ + M (2). Rate coefficients were determined as a function of pressure (200-600 Torr) and temperature (243-293 K) in nitrogen buffer gas and as a function of pressure (200600 Torr) in helium buffer gas at room temperature. For reaction 1, kinetic measurements were performed under conditions in which bromine atoms were the reactant in excess concentration while simultaneously monitoring the concentration of both mercury and bromine. A temperature dependent expression of (1.46 ± 0.34) × 10 ^-32 × (T/298)-^(1.86±1.49) cm⁶ molecule^-2 s^-1 was determined for the third-order recombination rate coefficient in nitrogen buffer gas. The effective second-order rate coefficient for reaction 1 under atmospheric conditions is a factor of 9 smaller than previously determined in a recently published relative rate study. For reaction 2 we obtain a temperature dependent expression of (4,31 ± 0.21) × 10^-33 × (T/298)- ^(2.77±0.30) cm⁶ molecule^-2 s ^-1 for the third-order recombination rate coefficient in nitrogen buffer gas. The rate coefficients are reported with a 2σ error of precision only; however, due to the uncertainty in the determination of absolute bromine atom concentrations and other unidentified systematic errors we conservatively estimate an uncertainty of ± 50% in the rate coefficients. For both reactions the observed pressure, temperature and buffer gas dependencies are consistent with the expected behavior for three-body recombination.

Full text of DOI:10.1021/jp054688j

J. Phys. Chem. A 2006, 110, 6623-6632
6623
Temperature and Pressure Dependent Rate Coefficients for the Reaction of Hg with Br and
the Reaction of Br with Br: A Pulsed Laser Photolysis-Pulsed Laser Induced Fluorescence
Study†
Deanna L. Donohoue, Dieter Bauer, Brandi Cossairt, and Anthony J. Hynes*
DiVision of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Science,
UniVersity of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149
ReceiVed: August 19, 2005; In Final Form: NoVember 29, 2005
A pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to study the
recombination of mercury and bromine atoms, Hg + Br + M f HgBr + M (1) and the self-reaction of
bromine atoms, Br + Br + M f Br2 + M (2). Rate coefficients were determined as a function of pressure
(200-600 Torr) and temperature (243-293 K) in nitrogen buffer gas and as a function of pressure (200-
600 Torr) in helium buffer gas at room temperature. For reaction 1, kinetic measurements were performed
under conditions in which bromine atoms were the reactant in excess concentration while simultaneously
monitoring the concentration of both mercury and bromine. A temperature dependent expression of (1.46 (
0.34) × 10-32 × (T/298)-(1.86(1.49) cm6 molecule-2 s-1 was determined for the third-order recombination rate
coefficient in nitrogen buffer gas. The effective second-order rate coefficient for reaction 1 under atmospheric
conditions is a factor of 9 smaller than previously determined in a recently published relative rate study. For
reaction 2 we obtain a temperature dependent expression of (4.31 ( 0.21) × 10-33 × (T/298)-(2.77(0.30) cm6
molecule-2 s-1 for the third-order recombination rate coefficient in nitrogen buffer gas. The rate coefficients
are reported with a 2σ error of precision only; however, due to the uncertainty in the determination of absolute
bromine atom concentrations and other unidentified systematic errors we conservatively estimate an uncertainty
of (50% in the rate coefficients. For both reactions the observed pressure, temperature and buffer gas
dependencies are consistent with the expected behavior for three-body recombination.
Introduction
(AMDEs) have been observed in the Arctic,4,5 Antarctic,6,7
Greenland8 and Norway.9 The observations of AMDEs suggest
that, at least under certain circumstances, mercury can undergo
fast atmospheric transformations.
In the atmosphere mercury exists primarily in its elemental
form, Hg(0), which until recently was thought to be unreactive
in the gas phase. Hg(0) is insoluble in water and has a low
deposition rate. Estimates of the atmospheric lifetime of Hg(0)
vary, but it is believed to be in the range of six months to one
year. Mercury is a known toxin, and over the past decade there
have been observations of increased mercury levels in Arctic
lake waters,1 Arctic animal populations,2 and Arctic indigenous
human populations.3 With no known local sources of mercury
and no known physical or chemical process that could concen-
trate mercury in the Arctic, these high levels of mercury were
an enigma and a significant health concern. Recent observations
in polar regions suggest that under certain conditions, probably
as a result of reaction with halogens, Hg(0) can be rapidly
oxidized to a form which has a much higher deposition rate.
Thus interest in understanding the detailed mechanism of
atmospheric mercury transformation has grown.
The first direct evidence of a fast atmospheric transformation
of Hg(0) was observed in Alert Canada. In 1995, Schroeder et
al.4 conducted continuous surface-level measurements of total
gaseous mercury. They discovered that with polar sunrise, there
were frequent episodic events in which total gaseous mercury
was almost completely depleted from the surface air. During
these depletion events, a very strong correlation between the
depletion of Hg(0) and ozone was observed. Following this
initial study Atmospheric Mercury Depletion Episodes
AMDEs seem to correlate well with tropospheric ozone
depletion. Dramatic episodic depletions in tropospheric ozone
have been recorded in polar regions over the last several years.
Typically, these events take place in the spring after polar sunrise
and can result in a reduction of ozone concentrations from 40
ppb to less than 1 ppb over the course of a single 24 h period.
Early measurements10 revealed a large increase in the levels of
filterable bromine compounds that coincided with the ozone
depletion events. Direct spectroscopic observations have shown
that large increases in bromine monoxide, BrO, concentrations
coincide with these depletion events; it seems clear that a
catalytic cycle involving BrO plays a role in these ozone
depletion episodes.11 It is reasonable to suspect that similar
halogen chemistry is driving AMDEs. The reactions of Hg(0)
with BrO and Br have been suggested as a potential mechanism.
However, there are few kinetic data available for rate coefficients
of elemental mercury with halogen radicals, making it difficult
to model AMDEs.
Additionally, there is evidence that an unknown gas-phase
oxidation process influences concentrations of mercury in the
marine boundary layer.12,13 In a recent modeling study Hedge-
cock and co-workers13 suggested that Br atoms are the primary
oxidant of Hg(0) in the marine boundary layer and they calculate
a typical lifetime of about 10 days. The implications of this for
† Part of the special issue “David M. Golden Festschrift”.
10.1021/jp054688j CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/08/2006

6624 J. Phys. Chem. A, Vol. 110, No. 21, 2006
Donohoue et al.
Figure 1. Experimental setup for the PLP-PLIF system to detect Hg atoms by one-photon LIF and Br atoms by two-photon LIF, including optical
and flow system configurations.
chemistry on a global scale are unclear; because the precise
mechanism of mercury transformation is unknown.
In an effort to elucidate the role of Br atoms in AMDEs and
in the MBL, we have made direct measurements of the rate
coefficient for the reaction of elemental mercury with bromine
atoms, reaction 1, as a function of temperature and pressure in
similar to the apparatus employed in the previously reported
determination of the rate coefficient for mercury with chlorine
atoms.25
Bromine atoms were produced by pulsed laser photolysis
(PLP) of molecular bromine. The temporal profiles of both
bromine atoms and mercury atoms were monitored by two- and
one-photon laser induced fluorescence (LIF), respectively. The
experimental configuration is detailed in Figure 1. The experi-
ments were conducted in a temperature controlled Pyrex reaction
vessel. Four mutually perpendicular sidearms with quartz
windows were attached to the center of the vessel. The
photolysis and the probe lasers were overlapped using dichroic
mirrors and then propagated through two of the cell’s sidearms,
perpendicular to the gas flow. The temperature of the reaction
vessel was controlled by a circulating methanol bath while the
windows were constantly flushed with dry air to prevent
condensation. A thermocouple was inserted into the reaction
zone through a vacuum seal, allowing measurement of the gas
temperature under the precise pressure and flow conditions of
the experiment. Experiments were carried out under “slow-flow”
conditions. The gas velocity was maintained at approximately
26 cm s-1, to completely replace the gas mixture in the reaction
zone between the laser pulses. All flows were monitored using
calibrated mass flow controllers. The pressure in the reaction
cell was monitored with a capacitance manometer.
Hg + Br + M f HgBr + M
(1)
nitrogen and helium buffer gases. Kinetic measurements were
performed with bromine as the reactant in excess concentration,
while temporal profiles of both reactants were monitored by
LIF. These measurements require an accurate determination of
the Br atom concentration; thus we must account for the loss
of Br atoms by the bromine atom recombination. Consequently,
we also measured the rate coefficient for the recombination of
bromine atoms, reaction 2, under similar experimental condi-
Br + Br + M f Br2 + M
(2)
tions. Reaction 2 has been studied several times before;14-19
the results from this study will be compared to the previous
results below.
There have been three prior experimental20-22 and two
theoretical determinations23,24 of the rate coefficient for reaction
1. We report rate coefficients that are significantly smaller than
that in a recently published relative rate study,21 but in
reasonable agreement with the two previous theoretical deter-
minations. An additional unpublished relative rate determina-
tion22 reported two rate coefficients for two different reference
molecules. The preferred value in that study was significantly
faster than the rate reported in this work. However, the second
rate coefficient reported in that study is in good agreement with
the one in this work.
Bromine atoms were produced by photolysis of molecular
bromine using the 532 nm, third harmonic of a Nd:YAG laser.
Br2 + hν f Br + Br
(3)
An output power of approximately 500 mJ per pulse resulted
in bromine atom concentrations ranging from 2.5 × 1015 to 40
× 1015 molecules cm-3. The photolysis of molecular bromine
at 532 nm from the 3Π1u and 2Π0u+ bonding states to the 1Π1u
repulsive exited state leads to the formation of two bromine
atoms. Some of the resulting bromine atoms were electronically
excited; however, these excited species were rapidly deactivated
to the 2P03/2 ground state,26 resulting in a quantum yield for the
photolysis of molecular bromine19 of 2.
The buffer gas flowed through a mercury bubbler at room
temperature. This produced stable mercury concentrations,
which ranged from 5 × 1011 to 20 × 1011 molecules cm-3 under
our flow conditions.
Experimental Section
The reaction between gaseous elemental mercury and bromine
atoms was studied by pulsed laser photolysis-pulsed laser
induced fluorescence (PLP-PLIF) as a function of pressure and
temperature in nitrogen and helium buffer gas. Experiments were
conducted at three temperatures, 293, 263 and 243 K, and three
pressures, 200, 400 and 600 Torr. The experimental design is

Rate Coefficients for the Reactions of Hg or Br with Br
J. Phys. Chem. A, Vol. 110, No. 21, 2006 6625
Elemental mercury and molecular bromine concentrations
were monitored in situ by UV photometry using the 253.7 and
365 nm lines from a mercury lamp, respectively. The reaction
mixture was passed through an absorption cell and the lamp
output was split with a dichroic beam splitter and detected by
two interference filter/photomultiplier (PMT) combinations and
each absorbance was recorded. The molecular bromine con-
centrations were determined using a 1 m cell and the literature
cross-section of molecular bromine27 at 365 nm of 1.258 ×
10-19 cm2. The line width of the mercury absorption line is
narrower than the broadened output of the mercury lamp;
therefore, the cross-section depends on the line width of the
lamp and requires a determination of the effective absorption
cross-section. The effective cross-section of mercury determined
in this system was 1.36 × 10-14 cm2 for absorbencies less than
0.7, whereas a polynomial relationship was used for absorben-
cies greater than 0.7. The methods used to determine this cross-
section and the relevant plot have been previously reported.25
During kinetic measurements an absorption path length of 100
cm was used.
When both mercury and bromine were passed through the
absorption cell, a significant reduction of the mercury concen-
tration was observed, and the bromine concentration was left
unchanged. This observation was expected because the bromine
concentration was 3 orders of magnitude larger than the mercury
concentration. These observations indicate that there is a
heterogeneous reaction occurring within the system, most likely
on the walls of the cell. Therefore, to reduce the affects of this
heterogeneous chemistry, the absorption cell was placed after
the LIF cell and the mercury and bromine gases were mixed
immediately before the LIF cell. By reducing the mixing time
and cell wall surface area before the detection zone, this loss
of mercury was minimized.
Figure 2. Excitation scheme for the two-photon LIF of Br atoms. The
excitation transition is at 2 × 261 nm with the subsequent fluorescence
transition near 845 nm.
of the laser before reaching the detection volume, and any
variation in the laser power. The averaged laser power was used
for the calculation of the bromine atom production.
Fluorescence was detected by two PMTs positioned perpen-
dicular to both the direction of propagation of the laser beams
and the direction of the gas flow. The PMT signals were
typically averaged for 50 laser pulses by a 500 MHz digital
scope and logged on a computer. The temporal profiles of the
LIF signals were then constructed by varying the delay time
between the photolysis and probe lasers using a digital delay
generator.
Br Atom LIF Detection. The relative Br atom concentration
was monitored by a two-photon LIF excitation scheme described
previously in the literature.29 This excitation scheme involves
The initial bromine atom concentration produced by pho-
4
the two-photon excitation of the spin forbidden 5p D03/2-4p5
tolysis was determined from28
2P0 transition near 261 nm with subsequent fluorescence
3/2
4
4
detection near 855 nm from the 5p D03/2-5s P5/2 transition,
as shown in Figure 2. The 261 nm probe laser, with typical
output powers in the range 85-300 µJ, was generated by
frequency doubling the 522 nm output from a tunable dye laser
(Spectra Physics PDL3). The fluorescence was detected by a
PMT with both an interference filter centered at 850 nm and a
750 nm cutoff filter to eliminate laser stray light. A 60 cm focal
length lens was used to focus the laser beam into the detection
volume, resulting in a detection limit of 2.0 × 1013 molecules
cm-3 for measurements in high pressures of nitrogen.
Br2/AL)
[Br] ) [Br2]‚QY‚(1 - exp-(P /h)(c/λ)(σ
)
(I)
L
where QY is the quantum yield of reaction 3, PL is the in laser
power in joules, h is Planck’s constant, c is the speed of light
in cm s-1, σBr is the absorbance cross-section at 532 nm in
2
cm2, λ is the laser wavelength in cm, and AL is the area of the
laser in cm2. The photolysis laser has a nominally Gaussian
profile; hence the beam passed through a ceramic aperture,
located 2 m from the reaction zone, to cut off the edges of the
beam. The variation in the fluence across the beam profile at
the reaction zone was determined by measuring the power
passing through a 0.05 cm pinhole ceramic aperture. The power
meter/aperture combination was placed on a linear translation
stage and the transmitted photolysis laser power was recorded
in 0.05 cm steps giving the power variation across the horizontal
diameter of the beam. From the observed beam profile the
effective laser diameter in the reaction volume was determined
to be 0.6 ( 0.05 cm. Over this diameter the beam profile has
an approximately homogeneous or “top hat” power profile with
a maximum deviation of (20% from the mean. The homogene-
ity of the beam was confirmed by evaluating the difference
between laser power measurements conducted with and without
an additional 0.487 cm aperture, which was located ap-
proximately 40 cm from the reaction zone. The ratio of the
powers was in good agreement with that calculated from the
ratio of the areas, suggesting that there are no significant hot
spots in the center of the beam. The laser power was measured
before and after the LIF cell for each decay. This was done to
account for reflection loss on the windows, the small absorption
Hg Atom LIF Detection. The relative Hg concentration was
3
monitored by exciting the 6p P0 -6s2 1S0 transition at 253.7
1
nm. The excitation beam was generated by frequency doubling
the 507 nm output of a dye laser (Spectra Physics PDL3), which
was then passed through a variable attenuator and reduced to a
laser power of approximately 5 µJ, to avoid saturation of the
6p 3P0 -6s2 1S0 transition, and thus reduce the laser stray light.
1
Resonance fluorescence was observed by a PMT with an
interference filter centered at 254 nm. A lens was used to adjust
the diameter of the probe beam to about 0.3 cm, approximately
half the size of the photolysis laser. This size was found to give
best results for the detection of mercury while minimizing
diffusion related problems. For the mercury LIF the detection
limit was less than 4 × 1010 molecules cm-3 for low-pressure
helium measurements and 2 × 1011 molecules cm-3 for
measurements conducted in high pressures of nitrogen.
Results
Measurements of Hg + Br + M f HgBr + M (M ) He,
N2). Direct determination of rate coefficients for the reactions

6626 J. Phys. Chem. A, Vol. 110, No. 21, 2006
Donohoue et al.
of gaseous elemental mercury presents a significant experimental
challenge due to the low vapor pressure of mercury. The low
vapor pressure makes it difficult to study the kinetics of this
system using a traditional approach with the stable reactant in
pseudo-first-order excess for anything other than reactions with
very fast rate coefficients. To overcome these difficulties, we
made kinetic measurements under conditions in which bromine
atoms were the reactant in excess while simultaneously monitor-
ing the concentration of both reactants.
The rate coefficient for the recombination of mercury and
bromine atoms, reaction 1, was determined with Br atom
concentrations typically 5000 times larger than the mercury
concentration. Both mercury and bromine atom concentrations
were monitored by LIF. The Br atom concentration was varied
between 2.5 × 1015 and 40 × 1015 molecules cm-3, and Hg
concentrations were in the range (5-20) × 1011 molecules cm-3
.
At the Br atom concentrations required to observe a signifi-
cant loss of mercury atoms, the bromine atom recombination
reaction, reaction 2, resulted in a significant decrease in Br atom
concentration on the time scale of the mercury atom decays.
Because the Br atom concentration was not constant, a simple
pseudo-first-order decay, i.e., an exponential decay, of mercury
atoms was not observed. Instead, the mercury temporal profiles
were fit by numerical integration, and the observed bromine
temporal profiles were analyzed assuming simple second-order
kinetics.
Figure 3. Typical bromine atom temporal profiles, shown for
measurements conducted in 400 Torr N2 at 243 K.
The temporal profiles of the bromine and mercury atoms were
characterized by
d[Hg]
dt
) -k1[Br][Hg][M]
(II)
d[Br]
dt
) -2k2[Br]2[M] - k1[Br][Hg][M]
(III)
Because the concentration of mercury was at least 3 orders of
magnitude smaller than the initial Br atom concentration,
k1[Hg] should be at least 1 order of magnitude smaller than
k2[Br]; therefore, the second term in eq III is negligible and
results in the simplified equation
Figure 4. Typical mercury atom temporal profiles, shown for
measurements conducted in 400 Torr N2 at 243 K.
by simulating the measured decays using the derived values of
k1′ and k2′ in the ACUCHEM program.30
d[Br]
) -2k2[Br]2[M]
(IV)
The numerically integrated fits to the observed mercury
temporal profiles are shown as lines in Figure 4 and the second-
order rate coefficients, k1′, obtained in He and N2 are listed in
Table 1. Molecular nitrogen quenched the mercury fluorescence
signal efficiently; therefore, the fluorescence yield and thus the
S/N ratio degraded with increasing pressure. This was most
noticeable in the 243 K dataset; however, the overall accuracy
of the pressure dependent rate data should not have been
significantly affected by this reduction of the S/N ratio.
The third-order recombination rate coefficients were then
determined from linear fits of the plots of the second-order rate
coefficients, k1′, versus the concentration of N2 or He, as shown
in Figure 5. The plots show the expected linear dependence of
rate coefficient versus concentration, indicating that the reaction
was in the low pressure, third-order regime, as might be expected
for an atom-atom recombination. Assuming that the recombi-
nation rate coefficients are in the low pressure limit, the effective
second-order rate coefficient should be zero at zero pressure.
Consequently, the third-order recombination rate coefficients,
k1, have been calculated by forcing the plots through the origin.
The difference between the forced and unforced slopes was
under 7% and within the precision of the fit. The third-order
recombination rate coefficients, k1, are listed in Table 2 and
dt
For each experimental condition, temporal profiles of bromine
and mercury atoms were measured using LIF. Typical sets of
temporal profiles of each atom are shown in Figures 3 and 4.
Under each set of conditions, i.e., a fixed pressure, temperature
and initial bromine atom concentration, the effective second-
order rate coefficient, k2′, for the recombination of bromine
atoms was calculated from
1
[Br]t
1
) 2k2′t +
(V)
[Br]0
which assumes that first-order losses by diffusion and reaction
with impurities are negligible. Substituting [Br] into eq II gives
d[Hg]
dt
1
) -k1′[Hg]
(VI)
(
)
2k2′t + (1/[Br]0)
This equation was numerically integrated to give the best fit to
the measured mercury profiles and hence a value for k1′, the
effective second-order rate coefficient for the recombination of
Hg with Br. The numerical integration procedure was checked

Rate Coefficients for the Reactions of Hg or Br with Br
J. Phys. Chem. A, Vol. 110, No. 21, 2006 6627
Figure 5. Variation of the effective second-order rate coefficients for
the recombination Hg and Br atoms, k1′, with pressure.
Figure 6. Arrhenius plot of the third-order rate coefficients for the
recombination of Hg and Br atoms, k1, in N2 and He.
TABLE 1: Second-Order Rate Coefficients for the
concentration was a critical component in measuring the rate
coefficient for the mercury and bromine recombination reaction.
The relative concentration profile was determined with good
precision using LIF. However, the initial Br atom concentration
was calculated and was, we believe, the largest source of
systematic error in the reported rate coefficient for reaction 1.
We can, however, make some assessment of the accuracy of
this calculation by comparing our measured bromine atom
recombination rate coefficients, which also depend on the
accuracy of the calculation of the initial Br atom concentration,
with literature values. As shown in Figure 3, bromine atom
temporal profiles were monitored by LIF with the concentration
typically followed to 5-20% of the initial bromine atom signal.
Under each set of experimental conditions, i.e., a fixed pressure,
temperature and initial bromine atom concentration, the effective
second-order rate coefficient, k2′, for the recombination of Br
atoms was calculated from the Br temporal profile using eq VIII,
Recombination of Mercury and Bromine Atoms, k1′
1013(k1′ ( 2σ error)
gas
N2
T (K)
P (Torr)
(cm3 molecule-1 s-1
)
243
200
400
600
200
400
600
200
400
600
1.63 ( 0.17
2.96 ( 0.52
5.12 ( 1.77
1.32 ( 0.05
2.95 ( 0.20
4.37 ( 1.15
1.04 ( 0.15
1.70 ( 0.34
2.92 ( 0.67
263
293
He
293
200
400
600
0.30 ( 0.03
0.55 ( 0.03
0.81 ( 0.19
TABLE 2: Third-Order Rate Coefficients for the
Recombination of Mercury and Bromine Atoms, k1,
Determined in This Work at 293 K in He and 243, 263, and
293 K in N2, with the Resulting Temperature Dependent
Expression for N2
1
[Br]t
1
) 2k2′t +
(VIII)
[Br]0
1032(k1 ( 2σ error)
gas
He
N2
T (K)
(cm6 molecule-2 s-1
)
again assuming a negligible first-order loss due to reaction with
impurities or diffusion. Linear fits of plots of 1/[Br] vs time
give the effective second-order recombination rate coefficient,
k2′. Figure 7 shows a series of plots for the reciprocal of absolute
bromine atom concentration versus time. This provides an
indication of the precision of the data sets, which were obtained
at the same temperature and pressure. Each plot should have
the same slope independent of initial Br atom concentration.
The data shown in Figure 7 were taken at 243 K in 400 Torr
nitrogen buffer gas with initial Br atom concentrations ranging
from 1.8 × 1016 to 3.8 × 1016. The plots demonstrated good
linearity and gave an average second-order recombination rate
coefficient of (1.08 ( 0.17) × 10-13 cm3 molecule-1 s-1 where
the uncertainty is a 2σ error of precision. To ensure that the
addition of mercury did not affect the observed Br atom decay,
experiments were conducted in the presence and absence of
mercury. The temporal profiles and derived rate coefficients
were identical within the precision of the measurements. This
was expected because the Hg concentration was at least 3 orders
of magnitude smaller than the initial Br atom concentration.
The values of the effective second-order rate coefficients
together with 2σ errors are summarized in Table 3.
293
0.42 ( 0.02
243
263
293
2.06 ( 0.18
1.98 ( 0.07
1.43 ( 0.13
(1.46 ( 0.34) × 10-32 × (T/298)-(1.86(1.49)
plotted in Figure 6. The temperature dependent expression for
reaction 1 in nitrogen is given by eq VII reported with 2σ errors
k1,N (243-298K) )
2
-(1.86(1.49)
T
298
(1.46 ( 0.34) × 10-32
×
(VII)
(
)
of precision only. However, due to uncertainty in the calculation
of absolute Br atom concentrations, which is discussed below,
and other unidentified systematic errors, we conservatively
estimate the error in the rate coefficient to be (50%. The
observed behavior is consistent with a three-body recombination,
demonstrating a positive pressure dependence, an inverse
temperature dependence, and a slower rate coefficient in helium
than in nitrogen.
Measurements of Br + Br + M f Br2 + M (M ) He,
N2). The determination of temporal profiles of Br atom
The third-order recombination rate coefficients were then
determined from linear fits of the plot of second-order rate

6628 J. Phys. Chem. A, Vol. 110, No. 21, 2006
Donohoue et al.
Figure 7. Second-order rate coefficient plot for Br atom, shown for
measurements conducted in 400 Torr N2 at 243 K.
Figure 9. Arrhenius plot of the third-order rate coefficients for the
recombination of Br atoms, k2, in N2 and He. Literature values are
shown for comparison.
TABLE 4: Third-Order Rate Coefficients for the
Recombination of Bromine Atoms, k2, Determined in This
Work at 293 K in He and 243, 263, and 293 K in N2, with
the Resulting Temperature Dependent Expression for N2
1033(k2 ( 2σ error)
gas
He
N2
T (K)
(cm6 molecule-2 s-1
)
293
1.43 ( 0.17
243
263
293
7.53 ( 1.03
6.18 ( 0.89
4.46 ( 0.86
(4.31 ( 0.21) × 10-33 × (T/298)-(2.77(0.30)
temperature dependent expression is given by
k2,N (243-298K) )
2
Figure 8. Variation of the effective second-order rate coefficients for
the recombination of Br atoms, k2′, with pressure.
-(2.77(0.30)
T
298
(4.31 ( 0.21) × 10-33
×
(IX)
(
)
TABLE 3: Second-Order Rate Coefficients for the
Recombination of Bromine Atoms, k2′
In eq IX the uncertainties are measures of 2σ error of precision
only. As we discuss in detail below, we estimate an uncertainty
of (50% in the accuracy of the rate coefficient, due principally
to the uncertainty in the calculation of absolute Br atom
concentrations. Overall, the data showed the expected behavior
for a three-body recombination, a positive pressure dependence,
an inverse temperature dependence, and a higher deactivation
efficiency for nitrogen relative to helium.
Potential Sources of Systematic Error. As we have noted
above, the variation of the effective second-order rate coef-
ficients with pressure should show a linear dependence that
passes through the origin. In fact, the bromine atom recombina-
tion reaction data consistently show slight negative offset, which
may be indicative of a systematic error in the calculation of the
bromine atom concentration. It should be noted that these offsets
were relatively small, and in most cases the fits that were forced
through the origin passed within the 2σ errors of precision
associated with each data point.
To calculate the initial bromine atom concentration, eq I was
employed using the absorption cross-section of bromine mol-
ecules at 355 nm, the molecular bromine concentration, the
average laser power, and the laser diameter. The error associated
with the first two parameters in eq I should be less than 5%.
As discussed previously, there is some uncertainty in the
calculation of the effective diameter and homogeneity of the
laser beam and hence the fluence. This was assessed by
1014(k1′ ( 2σ error)
gas
N2
T (K)
P (Torr)
(cm3 molecule-1 s-1
)
243
200
400
600
200
400
600
200
400
600
4.66 ( 0.87
10.8 ( 1.7
19.2 ( 4.1
3.06 ( 0.25
8.63 ( 0.96
14.4 ( 4.1
2.44 ( 0.71
4.80 ( 1.31
9.71 ( 3.26
263
293
He
293
200
400
600
0.96 ( 0.22
1.64 ( 0.22
3.00 ( 0.70
coefficients, k2′, versus concentration of N2 or He, as shown in
Figure 8. As is the case for the Hg + Br recombination, the
data show a good linear dependence of the effective second-
order rate coefficient on pressure. However, we see a consistent,
negative offset. As for reaction 1, the third-order rate coefficients
were obtained by forcing all fits through the origin. The forced
plots pass through most of the error bars associated with each
data point whereas the difference in the slopes of the forced
and unforced fits varied from 8% to 25%. The third-order
recombination rate coefficients, k2, are listed in Table 4 and
plotted in Figure 9. For the data in nitrogen, the resulting

Rate Coefficients for the Reactions of Hg or Br with Br
J. Phys. Chem. A, Vol. 110, No. 21, 2006 6629
TABLE 5: Comparison of Literature Data for Third-Order Rate Coefficients for the Recombination Bromine Atoms, k2
gas
N2
T (K)
298
300-1225
298-373
298
P (Torr)
k2 (cm6 molecule-2 s-1
)
ref
∼500
(9.46 ( 0.94) × 10-33
14
15
16
18
19
∼100
(4.55 ( 0.55) × 10-33 a
(9.1 ( 0.83) × 10-33
(8.3 ( 0.30) × 10-33
(1.1 ( 0.11) × 10-32
2.2-3.2
∼2.25
293
243-293
750-5250000
200-600
(4.31 ( 0.21) × 10-33 × (T/298)-(2.77(0.30)
this work
He
300-1225
298-373
300-1500
298
293
293
100-300
2.2-3.2
760
1.6 × 10-33 × (T/298)(-1.26 ( 0.04)a
(3.31 ( 0.27) × 10-33
15
16
17
18
31
19
1.725 × 10-33 × (T/298)-0.68 × exp(-0.21(kcal/mol)/RT)a
(2.81 ( 0.50) × 10-33
∼2.25
750-5250000
750-5250000
200-600
(4.25 ( 0.41) × 10-33
(3.31 ( 0.78) × 10-33
293
(1.43 ( 0.17) × 10-33
this work
a Rate coefficients were corrected to include factor of 2 for second-order rates.
measuring the variation in the fluence across the width of the
beam. We estimate the error in the diameter determination to
be less than 10%. However, the error associated with the
calibration of the laser power meter, homogeneity within the
beam profile, and shot to shot variability increase the uncertainty
and result in an estimated error of (20% in the fluence.
Therefore, we believe that (30% represents a conservative
overall estimate of the uncertainty in the Br atom concentrations.
A second source of systematic error could be the influence of
secondary chemistry on the bromine atom temporal profiles.
Molecular bromine does not thermally dissociate at the tem-
peratures in this study. No experimental data are available for
reactions of HgBr and we expect it to react with both Br and
Br2; however, its concentration is much lower than that of [Br]
and it should not affect Br atom profiles. However, the fate of
HgBr is a significant uncertainty in assessing the role of Br in
AMDEs. Considering the typical errors of precision in the data
and the possibility of other small, but unidentified, systematic
errors, we believe that (50% represents a reasonable estimate
of the overall uncertainty in the measured rate coefficients.
All of these studies reported agreement with the earlier Ip et al.
study,15 without accounting for the difference of a factor of 2
in the defined second-order rate coefficient, when, in fact, the
reported rate coefficients for the later studies are a factor of
2-3 faster than the Ip et al. study.
In assessing potential sources of systematic error, we can
identify two possible complications in our experimental ap-
proach. The first is additional loss of bromine atoms by reaction
with impurities. However, any additional reaction, which
resulted in the loss of bromine atoms, would increase the
observed rate. Because the rate that we observed is slower than
that in the previous studies, this cannot account for the observed
discrepancy. The second possible complication in our system
is the over- or underestimation of the Br atom concentration.
Because we must determine absolute Br atom concentrations,
an error in this determination could affect the resulting rate
coefficient. However, the errors associated with our determi-
nation were previously discussed and resulted in a maximum
estimated error of (30%; therefore this should not account for
the discrepancy. We therefore agree with the earlier work of Ip
et al.15 and Clarke et al.17
Mercury and Bromine Atom Recombination. Three previ-
ous experimental20-22 and two theoretical determinations23,24
have reported values for reaction 1 and these results are
compared with the current work in Table 6 and in Figure 10.
Greig et al.20 used flash photolysis combined with absorption
spectroscopy to study reaction 1 at temperatures 393-448 K
in 200 Torr CF3Br resulting in a rate coefficient of 2.82 × 10-13
cm3 molecules-1 s-1, with a reported error of a factor of 3. The
rate coefficient obtained is not directly comparable to those
reported here, due to temperature and buffer gas differences;
however, using the temperature dependent expression reported
in this work, we predict a recombination rate coefficient in 200
Discussion and Comparison with Previous Work
Bromine Atom Recombination. The bromine atom recom-
bination reaction, k2, has been determined in both helium and
nitrogen in several studies.14-19,31 The results of these studies,
including the specific pressure and temperature regimes used
are outlined in Table 5. When evaluating these studies, it is
essential to identify the relationship used to determine the rate
coefficient. In the review of literature we found that three
relationships were used to evaluate k2, eqs IV, X, and XI.
d[Br2]
) k2[Br]2[M]
(X)
dt
Torr nitrogen and 393 K of 5.7 × 10-14 cm3 molecule-1 s-1
.
d[Br]
dt
This is approximately a factor of 4.5 slower than the rate
coefficient reported by Greig et al. This difference in the rate
coefficients might be explained by the difference in the third
body efficiencies of CF3Br versus N2. Hippler et al. found C3F8
to be a factor of 6.5 more efficient as a third body than N2 in
a study of Br atom recombination.19
) -k2[Br]2[M]
(XI)
Equations IV and X should result in an equivalent k2 rate
coefficient, whereas eq XI will result in an expression of the k2
rate coefficient that is a factor of 2 too fast. Therefore, two
studies15,17 had to be corrected for this factor of 2 to be
comparable to our second-order rate coefficients. Once cor-
rected, these two rate coefficient agree, within error limits, with
our rate determination for the Br atom self-reaction in both
helium and nitrogen.
Additionally, we would identify two particularly significant
sources of systematic error in the Greig et al. study. First, the
system was a static system where a gas mixture undergoes
repeated flashes. This experimental approach increases the
possibility of secondary chemistry, product photolysis and
interfering species. Second, to determine the rate coefficient for
reaction 1 it was necessary to determine the absolute mercury
bromide (HgBr) concentration. Mercury bromide concentrations
On the other hand the rates determined by Strong et al.,14
DeGraff et al.,16 Clyne et al.,18 and Hippler et al.19,31 are
consistently a factor of 2-3 faster than our rate determination.

6630 J. Phys. Chem. A, Vol. 110, No. 21, 2006
Donohoue et al.
ref
TABLE 6: Reported Rate Coefficients for the Recombination of Mercury and Bromine Atoms, k1
gas
N2
T (K)
P (Torr)
k1
units for k1
298
298
298
298
180-400
243-293
760
760
760
760
760
760
(3.2 ( 0.3) × 10-12
3.0 × 10-13
(cm3 molecule-1 s-1
(cm3 molecule-1 s-1
(cm3 molecule-1 s-1
(cm3 molecule-1 s-1
(cm3 molecule-1 s-1
(cm6 molecule-2 s-1
(cm6 molecule-2 s-1
(cm3 molecule-1 s-1
)
)
)
)
)
)
21
22
22
23
24
this work
9.7 × 10-13
1.01 × 10-12 exp(209.03/T)
1.1 × 10-12 × (T/298)-2.37
(1.46 ( 0.34) × 10-32 × (T/298)-(1.86(1.49)
He
243-293
393-448
760
200
(4.2 ( 0.2) × 10-33
2.82 × 10-13
)
)
this work
20
CF3Br
were determined by monitoring the C2F6 concentration as a
proxy for bromine atom concentrations and assuming that all
the bromine atoms were converted to mercury bromide. The
C2F6 was formed via the self-reaction of CF3, a product of the
flash photolysis of CF3Br. By using C2F6 concentrations as a
proxy for HgBr concentrations, the Grieg study assumes that
all the Br atoms which were formed in the initial photolysis of
the CF3Br species, react with an Hg atom. Greig et al. assumed
that the formation of mercury(II) bromide (HgBr2) via reaction
4 was insignificant. However, if the rate coefficient calculated
Ariya et al.21 also reported a rate coefficient of (3.2 ( 0.3)
× 10-12 cm3 molecules-1 s-1 for reaction 1, which is a factor
of 9 faster than the rate determined in this work. In contrast to
their Cl atom work, Ariya et al. used a single reference reaction,
Hg(0) with 1-butene, to measure the relative rate of reaction 1.
Kinetic studies on the reaction of Hg(0) and 1-butene are limited,
with only one study referenced.32 Additionally, that study used
a relative rate technique and noted that the observed rate
coefficient depended on the O2 partial pressure, indicating a
complex reaction mechanism. In the Ariya et al. study the rate
observed depended on the concentration of the reference
molecule, the concentration of the OH scavenger, and the
identity of the buffer gas. The observed reaction coefficient for
reaction 1 varied by a factor of 3 as the buffer gas was varied
between nitrogen and air. To obtain linear relative rate plots in
the Ariya et al. study, they added large amounts of an OH
scavenger (cyclohexane), leading to an enhancement in the
absorption of reactant on the cell walls. Ariya et al. reported
that the primary complication to their system was enhanced
removal of the reference compound by reaction with OH or
loss on the cell walls. Any additional loss of the reference
compound would produce an underestimate of the rate coef-
ficient. However, the rate coefficient obtained in that work
exceeds both our directly measured rate coefficient and two
theoretical estimates of the rate coefficient. We feel that a more
plausible explanation would be additional loss of mercury, either
by heterogeneous reaction or possibly by gas-phase reaction with
an oxygenated species, because any additional process that
removed mercury would generate the observed overestimate.
The second relative rate study, a technical report by Spicer
et al.,22 was performed in a 17.3 m3 environmental chamber
using two reference molecules dimethyl sulfide (DMS) and
propene (C3H6) in a buffer gas of air. Spicer et al. reported
values of 3.0 × 10-13 cm3 molecules-1 s-1 for the rate
coefficient relative to DMS and 9.7 × 10-13 cm3 molecules-1
s-1 for the rate coefficient relative to propene. They gave greater
weight to the higher value because the results obtained using
the DMS reference were more variable. Again the factor of 3
variation of the measured relative rate on the identity of the
reference compound implies that the measured rates were
influenced by secondary or heterogeneous chemistry.
HgBr + Br f HgBr2
(4)
by Goodsite et al.24 for reaction 4 is correct, then reaction 4
could be a significant sink for bromine atoms. Thus the estimate
of the [HgBr] could be too large, and this would result in the
overestimation of the rate coefficient for reaction 1 in the Greig
et al. determination.
Two more recent studies have utilized relative rate techniques
to study reaction 1 at 1 atm and at room temperature. Ariya et
al. reported rate coefficient for the reactions of Hg(0) with Cl2,
Cl, Br2 and Br in a recent study.21 We have discussed the rate
coefficient obtained for the Hg(0) + Cl reaction previously.25
In that case five different reference molecules were used,
obtaining results that differed by a factor of 270 in the measured
relative rates together with a strong nonlinearity of the relative
rate plot when determined in a bath gas of air. Ultimately, a
series of eight measurements were made using a single reference
reaction, and the reported rate coefficient was determined from
this small subset of the data. The rate coefficient that was
obtained was a factor of 50 faster than the rate coefficient we
obtained using a direct technique that was essentially identical
to that reported here. We attributed the difference to presence
of secondary chemistry in the relative rate system.
In addition to the experimental work described above, there
have been two theoretical determinations of the rate coefficient.
Khalizov et al.23 determined the recombination rate coefficient
for reaction 1 using electronic structure calculations to obtain
both molecular parameters and the capture rate or high-pressure
limit. Once this high pressure limit was obtained, Khalizov et
al. determined a pressure dependent rate coefficient by assuming
a strong collisional deactivation. To compare this with the
observed data, it is essential to consider the mechanism of a
three-body recombination. A three-body recombination consists
of an initial collision that generates an excited complex, reaction
5. A portion of the excited complex will directly decompose
back into reactants, reaction 6, whereas the other portion
Figure 10. The second-order rate coefficients for the recombination
of Hg and Br atoms, k1 at 760 Torr, for this work in N2 and He and
literature values in N2.

Rate Coefficients for the Reactions of Hg or Br with Br
undergoes a collision and is stabilized, reaction 7. The calculated
J. Phys. Chem. A, Vol. 110, No. 21, 2006 6631
the viability of using the relative rate method to determine
kinetic rate coefficients for mercury halogen reactions. For
reaction 2, the self-reaction of bromine atoms, we obtain results,
which are in agreement with the early experimental determi-
nation of Ip et al.15 and the theoretical determination of Clarke
et al.17 but are somewhat slower than more recent studies
To evaluate the importance of the recombination of elemental
mercury and bromine atoms, an effective second-order rate
coefficient of 4.6 × 10-13 cm3 molecules-1 s-1 was calculated
from the reported temperature dependent expression for Arctic
conditions, 260 K and 760 Torr. Assuming a peak concentra-
tion33 of bromine atoms of 107-108 cm-3 the lifetime of
mercury due to bromine is between 2.5 days and 6 h. This means
reaction 1 could play a significant role in AMDEs. However,
the importance of the recombination of mercury and bromine
atoms, reaction 1, will depend on the stability and reactivity of
the HgBr species. Further studies into the reactivity of HgBr
are ongoing.
Hg + Br f HgBr*
HgBr* f Hg + Br
(5)
(6)
(7)
HgBr* + M f HgBr + M
pressure dependent rate coefficient reported by Khalizov et al.
made the physically unrealistic assumption that every collision
of the buffer gas with the initially formed energized HgBr*
complex deactivated the complex to produce a stable HgBr
molecule that cannot dissociate to products. If the initial
calculation of the capture rate coefficient, reaction 5, is accurate,
this assumption should produce the maximum possible recom-
bination rate coefficient under each set of conditions. They
obtained a rate coefficient of 2.07 × 10-12 cm3 molecule-1 s-1
at 298 K, 760 Torr.
The second theoretical study of reaction 1 was carried out
by Goodsite et al.24 This study employed the RRKM theory
using a master equation formulation to predict the rate coef-
ficient for several mercury reactions of interest. In this work
the rate of deactivation of HgBr* is calculated by assigning the
energy of HgBr* into a series of energy grains and assuming
that the average energy removed by each collision with N2 was
400 cm-1. The rate coefficient obtained using this approach was
1.1 × 10-12 cm3 molecules-1 s-1. This more physically realistic
energy transfer model produces a rate coefficient that is a factor
of 2 slower than the study of Khalizov et al. However, both
studies reported rate coefficients that were slower than the
experimental rate coefficient reported in the Ariya study.
Goodsite et al. addressed the large discrepancy with the Ariya
et al. measurement and found that to obtain the experimental
rate coefficient, the bond energy in HgBr had to be increased
by 30 kJ mol-1 over the current experimental data of 74.9 kJ
mol-1. Because the error limit of the experimental determination
of the bond energy is only (4 kJ mol-1, the authors concluded
that the Ariya et al. rate coefficient was overestimated.
The determination of the reaction coefficient for reaction 1
at 298 K and 760 Torr of 3.6 × 10-13 cm3 molecules-1 s-1
from this work reflects a rate coefficient that is a factor of 3
and factor of 6 slower than the two theoretical studies. As noted
above, the strong collision assumption is normally physically
unrealistic and should give an upper limit to the rate coefficient.
Our results suggest that the 400 cm-1 energy removal parameter
of Goodsite et al. is a little too large. Incorporation of a slightly
smaller value would produce a result in good agreement with
our experimental value.
Finally, Hedgecock et al.13 reported a lifetime of mercury in
the MBL of 10.5 days. This lifetime assumes that removal by
reaction 1 is the dominant process in the conversion of elemental
mercury to reactive gaseous mercury, with reaction with OH
and ozone playing an important but lesser role. To calculate
this lifetime, Hedgecock et al. assumed a steady-state Br
concentration of [Br] ) 3.1 × 105 molecules cm-3 and used
the rate coefficient reported by Ariya et al.,21 this results in a
lifetime of elemental mercury due to reaction with bromine
atoms of 11.5 days. If we perform the same calculation using
the rate coefficient for reaction 1 determined in this work at
760 Torr and 298 K, we find the lifetime of mercury due to the
reaction with bromine increases to 104 days. Using the
Hedgecock et al. lifetimes of 133 days for reaction with OH
and 578 days for reaction with O3, we obtain an overall lifetime
of 53 days for mercury in the MBL. The factor of 5 increase in
the lifetime of Hg(0) using our rate coefficient for reaction 1
highlights the need for direct determination of rate coefficients
for Hg(0) reactions to elucidate the overall biogeochemical
cycling of mercury.
Acknowledgment. This research has been supported by a
grant from the U.S. Environmental Protection Agency’s Science
to Achieve Results (STAR) program. The clarity of the paper
was improved by the comments of the two anonymous referees.
Note Added in Proof. In Rate coefficient for gas-phase
oxidation of elemental mercury by bromine and hydroxyl
radicals. Sommar, J.; Ga˚rdfelt, K.; Feng, X.; Lindquist, O. Paper
presented at the 5th International Conference on Mercury as a
Global Pollutant, Rio de Janeiro, 1999, a value of (2.8 ( 0.8)
× 10-13 cm3 molecule-1 s-1 was reported for the rate coefficient
for reaction 1 using a relative rate technique.
We should note that this is the first study of reaction 1 that
has systematically varied the temperature, pressure and buffer
gas. As we note above, our observations are entirely consistent
with the behavior expected for a three body recombination and
our rate agrees well with the two theoretical determinations.
References and Notes
(1) Lockhart, W. L.; Wilkinson, P.; Billeck, B. N.; Danell, R. A.; Hunt,
R. V.; Brunskill, G. J.; Delaronde, J.; St Louis, V. Biogeochemistry 1998,
40, 163.
(2) Wagemann, R.; Innes, S.; Richard, P. R. Sci. Total EnViron. 1996,
186, 41.
(3) Wheatley, B.; Wheatley, M. A. Sci. Total EnViron. 2000, 259, 23.
(4) Schroeder, W. H.; Anlauf, K. G.; Barrie, L. A.; Lu, J. Y.; Steffen,
A.; Schneeberger, D. R.; Berg, T. Nature 1998, 394, 331.
(5) Lindberg, S. E.; Brooks, S.; Lin, C. J.; Scott, K. J.; Landis, M. S.;
Stevens, R. K.; Goodsite, M.; Richter, A. EnViron. Sci. Technol. 2002, 36,
1245.
(6) Ebinghaus, R.; Kock, H. H.; Temme, C.; Einax, J. W.; Lowe, A.
G.; Richter, A.; Burrows, J. P.; Schroeder, W. H. EnViron. Sci. Technol.
2002, 36, 1238.
Conclusions
We have reported recombination rate coefficients for the
reaction of mercury and bromine atoms, k1, together with the
self-reaction of bromine atoms, k2. In both cases the rate
coefficients show pressure and temperature dependencies, as
well as third body deactivation efficiencies, which are consistent
with a three-body recombination. For reaction 1, the recombina-
tion of bromine with mercury, we obtain rate coefficients that
are slower than previously reported rate coefficients. The
discrepancy observed between this work and the relative rate
studies together with the variability in those studies questions

6632 J. Phys. Chem. A, Vol. 110, No. 21, 2006
Donohoue et al.
(7) Temme, C.; Einax, J. W.; Ebinghaus, R.; Schroeder, W. H. EnViron.
Sci. Technol. 2003, 37, 22.
(22) Spicer, C. W.; Satola, J.; Abbgy, A. A.; Plastridge, R. A.; Cowen,
K. A. Kinetics of Gas-Phase Elemental Mercury Reactions with Halogen
Species, Ozone, and Nitrate Radical under Atmospheric Conditions; Florida
Department of Environmental Protection: 2002.
(23) Khalizov, A. F.; Viswanathan, B.; Larregaray, P.; Ariya, P. A. J.
Phys. Chem. A 2003, 107, 6360.
(24) Goodsite, M. E.; Plane, J. M. C.; Skov, H. EnViron. Sci. Technol.
2004, 38, 1772.
(25) Donohoue, D. L.; Bauer, D.; Hynes, A. J. J. Phys. Chem. A 2005,
109, 7732.
(26) Oldman, R. J.; Sander, R. K.; Wilson, K. R. J. Chem. Phys. 1975,
63, 4252.
(27) Maric, D.; Burrows, J. P.; Moortgat, G. K. J. Photochem. Photobiol.
AsChem. 1994, 83, 179.
(28) Rodgers, M. O.; Asai, K.; Davis, D. D. Appl. Opt. 1980, 19, 3597.
(29) Simeonsson, J. B.; Sausa, R. C. Spectrochim. Acta Pt. B-At.
Spectrosc. 1994, 49, 1545.
(30) Braun, W.; Herron, J. T.; Kahaner, D. ACUCHEM/ACUPLOT;
National Bureau of Standards: Gaithersburg, MD, 1986.
(31) Hippler, H.; Luu, S. H.; Teitelbaum, H.; Troe, J. Int. J. Chem. Kinet.
1978, 10, 155.
(32) Barnes, I.; Bastian, V.; Becker, K. H.; Overath, R.; Zhu, T. Int. J.
Chem. Kinet. 1989, 21, 499.
(8) Skov, H.; Christensen, J. H.; Goodsite, M. E.; Heidam, N. Z.;
Jensen, B.; Wahlin, P.; Geernaert, G. EnViron. Sci. Technol. 2004, 38, 2373.
(9) Berg, T.; Bartnicki, J.; Munthe, J.; Lattila, H.; Hrehoruk, J.; Mazur,
A. Atmos. EnViron. 2001, 35, 2569.
(10) Barrie, L. A.; Bottenheim, J. W.; Schnell, R. C.; Crutzen, P. J.;
Rasmussen, R. A. Nature 1988, 334, 138.
(11) Sander, R.; Vogt, R.; Harris, G. W.; Crutzen, P. J. Tellus Ser.
B-Chem. Phys. Meteorol. 1997, 49, 522.
(12) Hedgecock, I. M.; Pirrone, N.; Sprovieri, F.; Pesenti, E. Atmos.
EnViron. 2003, 37, S41.
(13) Hedgecock, I. M.; Pirrone, N. EnViron. Sci. Technol. 2004, 38, 69.
(14) Strong, R. L.; Chien, J. C. W.; Graf, P. E.; Willard, J. E. J. Chem.
Phys. 1957, 26, 1287.
(15) Ip, J. K. K.; Burns, G. J. Chem. Phys. 1969, 51, 3414.
(16) DeGraff, B. A.; Lang, K. J. J. Phys. Chem. 1970, 74, 4181.
(17) Clarke, A. G.; Burns, G. J. Chem. Phys. 1972, 56, 4636.
(18) Clyne, M. A. A.; Woon-Fat, A. R. J Chem. Soc., Faraday Trans.
2 1973, 69, 412.
(19) Hippler, H.; Schubert, V.; Troe, J. J. Chem. Phys. 1984, 81, 3931.
(20) Greig, G.; Gunning, H. E.; Strausz, O. P. J Chem. Phys. 1970, 52,
3684.
(21) Ariya, P. A.; Khalizov, A.; Gidas, A. J. Phys. Chem. A 2002, 106,
7310.
(33) Boudries, H.; Bottenheim, J. W. Geophys. Res. Lett. 2000, 27, 517.