Nitrate ion photolysis in thin water films in the presence of bromide ions

Article abstract of DOI:10.1021/jp109560j

Nitrate ions commonly coexist with halide ions in aged sea salt particles, as well as in the Arctic snowpack, where NO₃^- photochemistry is believed to be an important source of NO_y (NO + NO₂ + HONO + ...). The effects of bromide ions on nitrate ion photochemistry were investigated at 298 ( 2 K in air using 311 nm photolysis lamps. Reactions were carried out using NaBr/NaNO₃ and KBr/KNO ₃ deposited on the walls of a Teflon chamber. Gas phase halogen products and NO₂ were measured as a function of photolysis time using long path FTIR, NO_y chemiluminescence and atmospheric pressure ionization mass spectrometry (API-MS). Irradiated NaBr/NaNO₃ mixtures show an enhancement in the rates of production of NO₂ and Br ₂ as the bromide mole fraction (Χ_NaBr) increased. However, this was not the case for KBr/KNO₃ mixtures where the rates of production of NO₂ and Br₂ remained constant over all values of Χ_KBr. Molecular dynamics (MD) simulations show that the presence of bromide in the NaBr solutions pulls sodium toward the solution surface, which in turn attracts nitrate to the interfacial region, allowing for more efficient escape of NO₂ than in the absence of halides. However, in the case of KBr/KNO₃, bromide ions do not appreciably affect the distribution of nitrate ions at the interface. Clustering of Br^- with NO₃^- and H₂O predicted by MD simulations for sodium salts may facilitate a direct intermolecular reaction, which could also contribute to higher rates of NO₂ production. Enhanced photochemistry in the presence of halide ions may be important for oxides of nitrogen production in field studies such as in polar snowpacks where the use of quantum yields from laboratory studies in the absence of halide ions would lead to a significant underestimate of the photolysis rates of nitrate ions.

Full text of DOI:10.1021/jp109560j

ARTICLE
pubs.acs.org/JPCA
Nitrate Ion Photolysis in Thin Water Films in the Presence of
Bromide Ions
Nicole K. Richards,† Lisa M. Wingen,† Karen M. Callahan,† Noriko Nishino,† Michael T. Kleinman,‡
Douglas J. Tobias,*,† and Barbara J. Finlayson-Pitts*,†
†Department of Chemistry, University of California, Irvine, California 92697-2025, United States
‡Department of Medicine, University of California, Irvine, California 92697-1825, United States
S Supporting Information
b
ABSTRACT: Nitrate ions commonly coexist with halide ions
in aged sea salt particles, as well as in the Arctic snowpack,
-
where NO3 photochemistry is believed to be an important
source of NOy (NO þ NO2 þ HONO þ ...). The effects of
bromide ions on nitrate ion photochemistry were investigated
at 298 ( 2 K in air using 311 nm photolysis lamps. Reactions
were carried out using NaBr/NaNO3 and KBr/KNO3 depos-
ited on the walls of a Teflon chamber. Gas phase halogen
products and NO2 were measured as a function of photolysis
time using long path FTIR, NOy chemiluminescence and
atmospheric pressure ionization mass spectrometry (API-MS).
Irradiated NaBr/NaNO3 mixtures show an enhancement in
the rates of production of NO2 and Br2 as the bromide mole
fraction (χNaBr) increased. However, this was not the case for
KBr/KNO3 mixtures where the rates of production of NO2
and Br2 remained constant over all values of χKBr. Molecular
dynamics (MD) simulations show that the presence of bro-
mide in the NaBr solutions pulls sodium toward the solution surface, which in turn attracts nitrate to the interfacial region, allowing
for more efficient escape of NO2 than in the absence of halides. However, in the case of KBr/KNO3, bromide ions do not appreciably
affect the distribution of nitrate ions at the interface. Clustering of Br- with NO3 and H2O predicted by MD simulations for
-
sodium salts may facilitate a direct intermolecular reaction, which could also contribute to higher rates of NO2 production. Enhanced
photochemistry in the presence of halide ions may be important for oxides of nitrogen production in field studies such as in polar
snowpacks where the use of quantum yields from laboratory studies in the absence of halide ions would lead to a significant
underestimate of the photolysis rates of nitrate ions.
’ INTRODUCTION
concentrations are enriched on the Arctic snowpack, with as
many as one bromide ion to 188 chloride ions. Bromine has been
clearly shown to be involved in processes causing severe tropo-
spheric ozone destruction during polar sunrise4-6,41,47,48 and to
impact chemistry at midlatitudes as well.49-54 The source of
atomic and molecular bromine is believed to be the oxidation
of Br- in sea salt aerosol and snow likely initiated by reaction
with O3.4,6,8,9,19,20,22-24
Investigations of surface marine air in the Arctic and at lower
latitudes have provided compelling evidence that inorganic
halogen chemistry can significantly influence the composition
of the marine boundary layer (MBL).1-13 Sea salt aerosols arising
from wave action are a large source of halides that can travel
hundreds of kilometers inland and deposit onto surfaces.14,15
Bromide ions are oxidized by OH radicals and O3, and chloride
by OH radicals to form gaseous halogen species such as Br2, Cl2
and BrCl.4,6,8,16-37 Molecular halogens can then be photolyzed
by actinic radiation38,39 to produce reactive halogen atoms that
impact tropospheric chemistry.7-9,40-42
The conventional view of simple salt solutions has been that
ions are repelled from the air-water interface and prefer bulk
solvation.55 However, a combination of computational56-67,162,163
Despite the fact that bromide ions are a minor component of
sea salt,43 with one bromide ion for every 650 chloride ions, they
are thought to play an important role in MBL chemistry. Particles
less than 1 μm in diameter are often observed to be enriched
in bromide ions44,45 and Koop et al.46 reported that bromide
Special Issue: Victoria Buch Memorial
Received: October 5, 2010
Revised:
December 7, 2010
Published: February 03, 2011
r
2011 American Chemical Society
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and experimental studies18,24,68-82 has shown that Cl-, Br- and
I- can be present at the air-water interface. Furthermore, with
increasing size and polarizability of these anions, the interfacial
concentration increases. There is also evidence that both the
kinetics and mechanisms of interfacial reactions are different
from those in the bulk phase.21,69,70,74,83-94 The presence of
these halides at the air-water interface suggests their oxidation
may take place at the surface without the need for incoming gas
molecules to diffuse into the bulk before reaction. Indeed, there
is evidence for this in the OH þ Cl- and O3 þ Br- reac-
tions.70,87,95
Nitrate ions are typically present in sea salt aerosols and
snowpacks as a result of reactions between halide ions and gas-
eous nitrogen oxides.96-108 These reactions result in the conver-
sion of Cl- and Br- to gaseous halogen products, with a con-
comitant increase in the number of nitrate ions in the condensed
phase. Deposition of inorganic nitrate onto snowpack also
occurs.38,109,110 Calculations of nitrate concentrations on the
Arctic snowpack quasi-liquid layer suggest that it could be as high
as 0.3 μM.111 Nitrate absorbs light in the actinic region above 290 nm
and dissociates via two pathways:
investigates the photochemistry that occurs when nitrate and
bromide ions are present together in a thin film of aerosol par-
ticles deposited on a Teflon substrate. The implications of this
photochemistry are discussed in terms of the role of nitrate ion
photolysis in snowpack as a major source of oxides of nitrogen.
’ EXPERIMENTAL SECTION
Photolysis Experiments. Photolysis experiments were per-
formed in 230 L Teflon reaction chambers constructed using
51 μm thick FEP Teflon, surface area 2.6 m2, and heat sealed
along the edges. The Teflon chambers were coated with salt
solution particles using a 6-jet Collison nebulizer (BGI Inc., Model
CN25) at a backing pressure of 20 psi provided by N2 gas
(Oxygen Service Co., UHP, 99.999%). This process was repeated
three times for 15 min each. Suspended aerosol was evacuated
each time, leaving a thin layer of aerosol particles coating the
walls of the chamber. The approximate thickness of the coating is
0.8 μm, which was calculated by weighing the reaction chamber
dry (no aerosol added) and then weighing after treatment with
aerosol. By use of the mass of the aerosol on the chamber walls
(2.8 g) and densities of NaNO3 solutions,143 the volume of the
solution added to the chamber walls could be estimated. Know-
ing the surface area of the Teflon chamber and the volume of
solution added, the average thickness of the coating was calcu-
lated, assuming it was uniform. The molal concentration of
nitrate ions in the thin films was determined from the measured
relative humidity during photolysis experiments (75% RH) and
the relationship between water activity and molality of nitrate.143
Molarity was calculated from molality on the basis of published
data for the density of NaNO3 solutions.144 By comparing the
initial number of moles of NaNO3 in the film and the number of
moles of NO2 produced during photolysis, we determined that
approximately 0.03% of the nitrate was photolyzed during the
experiment. This value is a lower limit since it does not take into
account losses of NO2, for example, by photolysis.
NO3 þhν T NO2þO-
ð1aÞ
-
-
3
T NO2 þOð PÞ
ð1bÞ
ð2Þ
O-þH2O f OHþOH-
Pathway 1a in bulk aqueous solution at 305 nm and 298 K
produces O- and NO2 with a quantum yield of 0.01.112-116 The
O- reacts rapidly with water to form the hydroxyl radical (OH),
reaction 2.117 Pathway 1b, producing nitrite ion and O(3P),
occurs with an overall quantum yield of 0.001. The overall photo-
lysis quantum yields for both pathways are low due to recombi-
nation of photoproducts confined by a solvent cage.
However, there is increasing evidence that photolysis of
species at various liquid interfaces can have higher quantum
yields than those in the bulk.89-94,118-123 If the nitrate ion has an
incomplete solvent cage, then the photolysis products can more
easily escape the condensed phase into the surrounding atmo-
sphere, preventing recombination and resulting in an increased
overall quantum yield. This enhancement may play a role in
laboratory studies that have shown that NOy (NO þ NO2 þ
HONO þ ...) and OH are readily released during irradiation of
nitrate doped snow and ice.100,121,122,124-131 Furthermore, field
studies show that nitrate photolysis is a source of OH in the
interstitial air within snowpacks and/or the overlying boundary
layer.132-140
After being coated with salt, the Teflon reaction chamber was
filled with synthetic air (Scott-Marrin, Riverside, CA; NOx <
0.001 ppm, SO2 < 0.001 ppm) that had passed through a water
bubbler to yield air with relative humidities for the NaBr/NaNO3
experiments in the range of 70-75% RH and for the KBr/KNO3
experiments of 85-90% RH. These relative humidities, which
were measured using a relative humidity-temperature probe
(Vaisala, HMP 338), are above the mutual deliquescence point of
the mixed salts, which is 57.5 ( 0.6% RH for the sodium salt
mixture and 80.4 ( 0.5% for the potassium salts (Supporting
Information). The coated chamber was irradiated with 14 narrow
band UVB lamps (λ ∼ 311 nm) that overlap with the lowest
electronic absorption band of aqueous nitrate (Figure 1). Each
chamber was equipped with Teflon ports for gas sampling. All
experiments were conducted at 298 ( 2 K, and chambers were
sampled for a total of 150-400 min.
Measurements of Gas Phase Products. Gaseous NO2 was
measured as a function of time by periodically sampling from the
Teflon chamber and analyzing by either a multiple reflection
White-type optical system145 interfaced to a Fourier transform
infrared spectrometer (Mattson, Infinity 60AR) or a chemilumi-
nescence nitrogen oxides analyzer (ThermoElectron Corp.,
Model 42C). The total optical path length for the White cell
was 28 m and infrared spectra were recorded at a resolution of
0.5 cm-1 with 1024 coadded scans. Both the nitrogen oxides
analyzer and FTIR were calibrated in the range of NO2 levels
Studies by Wingen et al.141 have shown that there are en-
hanced NO2 yields from photolysis of deliquesced nitrate aero-
sols containing chloride ions. This was attributed to the presence
of Cl- at the solution-air interface, which draws the Naþ cation
toward the surface. This in turn attracts the nitrate ions closer to
the interface where they experience a reduced solvent cage. Both
theory and experiment show that bromide ions have a higher
propensity for the interface than chloride ions58,59,61,64,65,67,69,73
and therefore may be expected to have a similar or even greater
effect on nitrate ion photochemistry. Recent high pressure XPS
measurements on aqueous mixtures of nitrate and halide ions
provide experimental confirmation that these halide ions do
indeed draw nitrate ions closer to the interface.142 This paper
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Determination of Deliquescence Relative Humidities.
Relative humidity measurements on saturated solutions of NaBr/
NaNO3, KBr/KNO3 and NaCl/NaNO3 were conducted at
294 ( 0.5 K as a function of mole fraction of NaBr (i.e., bromide
ions), χNaBr, to determine the mutual deliquescence relative
humidity (MDRH) for each mixture. Solutions were placed in a
Vaisala humidity calibrator (HMK15) and sampled consecu-
tively with two relative humidity probes. Relative humidity was
recorded every hour until three consistent measurements were
obtained to ensure equilibration and reproducibility. Saturated
solutions of NaBr, NaCl, NaNO3, MgCl2, LiCl, KBr and KNO3,
which have well-known deliquescence points, were sampled
frequently to test the accuracy of the relative humidity probes.
The measured values were within (0.9% RH of published
values.147-149
Figure 1. Absorption spectrum of dilute aqueous NaNO3 (0.4 M, green
line) and photolysis lamp spectrum (black line).
Chemicals and Solution Preparation. Solutions of the salts
NaBr (Fluka, >99.5%), NaNO3 (Fisher, Certified ACS, >99.0%),
NaCl (Fluka, >99.5%), KBr (EMD Chemicals, >99.4%) and
KNO3 (Fisher, >99.4%) were made using nanopure water
(Barnstead, 18.2 MΩ cm). Experiments on pure NaNO3 and
mixtures of NaBr/NaNO3 were carried out in two ways (1)
varying both Br- and NO3- concentrations while keeping total
molarity and ionic strength constant and (2) varying the bromide
concentration while holding nitrate constant (0.5 M). The total
halide concentrations varied from 0.13 to 4 M. Experiments on
pure KNO3 and mixtures of KBr/KNO3 were performed by
varying the bromide concentration with constant, 0.5 M, nitrate
ion concentration.
detected in the experiments using mixtures of known concentra-
tions of NO2 in nitrogen (see below). Errors in concentrations
are reported as 2s, where s is the sample standard deviation
defined as
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
N
2
i¼0 ðxi-xÞ
s ¼
N-1
where N is the number of samples and was 3-5 depending on
the measurement.146
A dual quadrupole mass spectrometer (Perkin-Elmer Sciex,
API-300) was used to quantify Br2 production as a function of
photolysis time. The Teflon chamber contents were sampled at
∼1 L min-1 for 6 min directly into the API-MS source. Monitor-
ing techniques utilized in this study are single quadrupole (Q1)
and multiple reaction monitoring (MRM), both in negative ion
mode. Q1 scans cover the range m/z 30-500 amu. Molecular
bromine has relative peak intensities of 1:2:1 for m/z 158, 160,
and 162 due to the natural isotopic abundances of 51% 79Br and
49% 81Br. Increased sensitivity and selectivity can be achieved
using MRM in which the first quadrupole is set to transmit m/z
160 due to Br2 and the second quadrupole transmits the 79Br
fragment generated in the collision cell separating the two
quadrupoles. The MRM mode ensures that all fragments of
m/z 79 originated from the parent ion of 160 rather than other
bromine-containing products. The MRM or Q1 signal intensity
is measured during experiments and is calibrated using known
concentrations of gaseous Br2. For each calibration point, a
known volume of Br2 was flowed into a Teflon chamber with a
measured volume of air.
Scanning Electron Microscopy (SEM). The morphology
and elemental composition of thin films of NaBr/NaNO3 and
NaCl/NaNO3 were investigated using a Zeiss Evo LS 15 scan-
ning electron microscope equipped with a Thermo Electron
Corp. UltraDry Silicon Drift energy dispersive X-ray spectros-
copy (EDS) detector. An accelerating electron beam voltage of
8 keV was used. The thin films were prepared on copper stubs
(Ted Pella Inc.) placed inside a Teflon reaction chamber and
subjected to nine cycles of coating with salt solution particles.
SEM images were taken after the last evacuation.
NO2 was synthesized for calibrations by mixing NO gas
(Matheson, 99.5%), purified by passage through a dry ice acetone
bath to remove impurities such as HNO3, with excess O2 (Oxygen
Services Co., 99.993%) for several hours. The NO2 product was
condensed using a dry ice acetone bath, while excess O2 was
pumped away. The purified NO2 was stored in the dark at room
temperature in a glass bulb. The Br2 mixture used for calibrations
was prepared by collecting the vapor over liquid Br2 (Acros,
99.8%), which had been purified by three freeze-pump-thaw
cycles, and diluting with N2 and then storing in a glass bulb.
Computations. Molecular dynamics (MD) simulations were
performed to investigate ion partitioning and the environment of
nitrate at the liquid-vapor interface of aqueous NaNO3, KNO3,
mixtures of NaBr/NaNO3 and mixtures of KBr/KNO3. The sur-
face of aerosol droplets was simulated using a slab geometry118,150
of water and ions with dimensions of 30 Å ꢀ 30 Å ꢀ 100 Å with
three-dimensional, periodic boundary conditions.151 Simulations
were run in the NVT ensemble with a temperature (300 K)
regulated by a Berendsen thermostat.152 The MD trajectories
were generated using Sander in the AMBER 8 suite,153 which has
a modified calculation of induced dipole to avoid well-known
problems with polarization catastrophe.154 The time step was
1 fs, and trajectory data were recorded every ps. Particle-mesh
Ewald, a method for computing the interaction energies in periodic
systems, was used to calculate electrostatic interactions with the
real-space part of Ewald sum and the Lennard-Jones interactions
truncated at 12 Å.155,156 The polarizable POL3 water model157,158
was employed to simulate water molecules. Water bond lengths
and angles were constrained using SHAKE algorithm.158 Nitrate
ions were modeled using parameters by Thomas et al.159 which
were modified from Lennard-Jones parameters from Minofar
et al.160 and polarizability parameters by Salvador et al.161 The
sodium and bromide parameters were adapted from the work
of Berkowitz and co-workers.162,163 Potassium parameters were
UV/Visible Spectra. The UV absorption spectra of aqueous
NaNO3 (0.5 M), KNO3 (0.5 M), 8:1 NaBr/NaNO3 (4 M/
0.5 M) and 8:1 KBr/KNO3 (4 M/0.5 M) solutions were
obtained using UV-visible grade quartz cells of 1 mm path
length and a Cary 50 UV-visible spectrophotometer at 0.5 nm
resolution.
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Figure 2. NO2 production during photolysis experiments of pure NaNO3 and mixtures of NaBr/NaNO3 at 75% RH and 298 K in air, where χNaBr is the
mole fraction of NaBr (a) at a constant total ion concentration, (b) same as (a) but normalized to the initial [NO3-], (c) with constant 0.5 M nitrate ion
concentration and increasing amounts of Br-. Error bars are 2s of replicate experiments.
taken from Chang et al.164 Force field parameters are provided in
the Supporting Information (Table S1). Solutions of 2M NaBr,
2M NaNO3, 4 M NaNO3 and mixed NaBr and NaNO3 solutions
(total 4 M) were simulated using a slab containing 864 water
molecules. Each simulation was equilibrated for 3 ns, and an
additional 5 ns were used for analysis. The potassium series of
simulations was performed at the same concentrations as the
sodium series solutions with the exception of the simulation of
neat aqueous KNO3, which was performed at 2 M. The potas-
sium series of simulations was equilibrated for 3 ns and a further 8
ns were used for analysis.
measured to be ΔBr2/ΔNO2 = 0.25 ( 0.03 (2s). A search for
other bromine-containing products such as BrNO, BrNO2, and
BrONO2 was made using long-path FTIR, but no such species
were observed. In addition, using Q1 mass spectra the ratio of the
fragment at m/z 79 to the parent peak for Br2 at m/z 160 was
consistent with that of an authentic sample of Br2, suggesting
there were no significant amounts of other bromine-containing
gaseous products.
The formation of NO2 is expected from the direct photolysis
-
of NO3
:
NO3 þ hν T NO2þO-
ð1aÞ
-
In aqueous solutions, O- reacts with water, with a lifetime of
’ RESULTS AND DISCUSSION
∼0.2 ns:
The impact of bromide ions on nitrate ion photolysis was
investigated using two approaches: (1) holding the total number
of ions constant while varying the relative amounts of bromide
and nitrate and (2) using constant initial amounts of nitrate, with
increasing amounts of bromide. The first case is analogous to the
atmospheric situation where halide ions in sea salt particles are
replaced by nitrate during transport in air,40,98,99,165 but inter-
pretation of the data requires normalizing due to the differences
in nitrate concentrations. The second approach avoids the need
for normalization, thus making comparisons more straightfor-
ward, but is less representative of atmospheric conditions.
The formation of NO2 as a function of photolysis time for
NaBr/NaNO3 mixtures at a constant total number of ions is
shown in Figure 2a. The production of NO2 increases as the
relative amounts of nitrate ions increase and bromide ions decrease.
To compare NO2 production rates per nitrate ion, the data were
normalized to the initial concentration of NO3- present in the
O- þ H2O f OHþOH-
k ¼9:3 ꢀ 107 L mol-1 s-1 ðref 117Þ
ð2Þ
Bromide ions are well-known to be oxidized by OH radicals,
generating Br2:
Br- þ OH f BrOH-
k ¼1:1 ꢀ 1010 L mol-1 s-1 ðref 28Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
-
BrOH- þ Br- f Br2 þOH-
k ¼1:9 ꢀ 108 L mol-1 s-1 ðref 28Þ
2Br2- f Br3 þBr-
-
k ¼2:4 ꢀ 109 L mol-1 s-1 ðref 166Þ
Br3- f Br2 þ Br-
nebulized solution. Figure 2b shows that NaNO3 (χNaBr = 0) in
-
the absence of bromide produced the least NO2 per initial NO3
.
k ¼5 ꢀ 107 s-1 ðref 167Þ
The production of NO2 became more pronounced as χNaBr
increased beyond 0.5. As seen in Figure 2c, results similar to
Another possibility is reaction of Br- with O-:
Br- þ O- f products
-
Figure 2b were obtained using constant NO3 concentrations
(0.5 M) and increasing amounts of Br-.
k ¼1:8 ꢀ 108 L mol-1 s-1 ðref 168Þ
ð7Þ
Figure 3 shows that the rate of NO2 production (Figure 3a)
using constant [NO3-] (data from Figure 2c) and the corre-
sponding production of gas phase Br2 (Figure 3b) both increase
with χNaBr. The rate of Br2 release during photolysis experiments
ranged from 0.57 to 1.1 ppb min-1 after the initial induction
period. The ratio of Br2 to NO2 at the end of the experiments was
While the products of reaction 7 do not appear to have been
identified, they may include O2- þ Br, with subsequent reac-
tions of the bromine atoms generating Br2. Although reaction 7 is
not generally considered in bromide oxidation in bulk aqueous
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Figure 3. (a) Rates of production of NO2 as a function of χNaBr and (b)
Br2 production as a function of time. Symbols correspond to different
molar ratios for mixtures of NaBr/NaNO3 at 70-75% RH at 298 K in air
and a constant initial NO3- concentration of 0.5 M. The open square is
for pure NaNO3. All error bars are 2s.
Figure 4. (a) Rates of production of NO2 as function of χKBr and (b)
Br2 production as a function of time. Symbols correspond to different
molar ratios for mixtures of KBr/KNO3 at 85-90% RH at 298 K in air
and a constant initial NO3- concentration of 0.5 M. The open squares
are for pure KNO3 at 85% RH. All error bars are 2s.
solutions due to the rapid conversion of O- to OH, it cannot be
discounted a priori at the interface where the effective water
concentration transitions from that in the liquid to the gas phase.
The data in Figure 3b show an induction time for the appear-
ance of Br2 in the gas phase. This is attributed to the sensitivity to
pH of the cycling of bromine compounds between the gas and
aqueous phases.9 The generation of hydroxide ions in reaction 2
provides a mechanism for hydrolysis of Br2 and the deprotona-
tion of HOBr in the aqueous phase to BrO-:
Br2 þ OH- f HOBr þ Br-
k ¼7:0 ꢀ 109 L mol-1 s-1 ðref 169Þ
ð8Þ
HOBr þ OH- T BrO- þ H2O
pka ¼8:8 ðref 26Þ ð9Þ
Figure 5. UV/visible spectra of 0.5 M NaNO3 (red dashed line), 0.5 M
KNO3 (blue line), 4 M NaBr/0.5 M NaNO3 (yellow line), and 4 M
KBr/0.5 KNO3 (green line). Errors are 2s.
HOBr þ Br- þ Hþ T Br2 þ H2O
k ¼1:6 ꢀ 1010 L mol-1 s-1 ðref 169Þ
ð10Þ
The further oxidation of some NO2 by OH in the film and
secondary heterogeneous chemistry involving water170 generates
HNO3, causing acidification of the film, the conversion of BrO-
to HOBr and the generation of Br2 via reaction 10. The delay in
production of gas phase Br2 reflects the time to acidify the film
from the initial pH of 5.5 to the point that Br2 is formed. This was
confirmed by experiments in which the pH of the nebulizing
solution was lowered from 5.5 to 3 by addition of HNO3, which
showed no delay in bromine production. As seen in Figure 2b,c,
NO2 is generated faster for the larger mole fractions of Br-,
leading to more rapid acidification and generation of Br2. The
measured yield of Br2 relative to NO2 is half that expected if every
photolysis event via (1a) led to Br2. This is likely due to retention
of bromine products in the film, e.g., BrO-. Gas phase HOBr
cannot be detected as its parent ion by API-MS, but any HOBr
remaining in the film could also contribute to the lack of mass
balance for the bromine products.
Similar experiments were carried out for mixtures of KBr and
KNO3 at relative humidities between 85 and 90%. These were
above the MDRH of 80.4 ( 0.5% (Supporting Information) so
the mixtures should be deliquesced, which was confirmed by
visual inspection. Figure 4a shows the initial rate of NO2 pro-
duction as a function of χKBr. In contrast to the NaBr/NaNO3
mixtures, the rate of NO2 production is independent of the bro-
mide mole fraction. In this experimental system, it was difficult to
obtain reproducible and stable water vapor concentrations above
90% RH, so the data point for pure KNO3 in Figure 4a (open
squares) was obtained at 85% RH, below the KNO3 deliques-
cence relative humidity of 92.2 ( 0.4%. The rate of NO2 pro-
duction from solid KNO3 will be less than that for the deli-
quesced salt since surface passivation will occur,171,172 and hence,
the rate of NO2 production for χKBr = 0 in Figure 4a is lower than
expected for deliquesced KNO3. Figure 4b shows that the
formation of gaseous Br2 is also independent of χKBr. In short,
there does not appear to be a significant impact of added bromide
ions on either NO2 or Br2 production for the potassium salts, in
contrast to the sodium salts.
One possible source of the difference between the sodium and
potassium salts is a difference in the UV absorption spectra that
results in more rapid photolysis for KNO3. Hudson et al.173
showed that as nitrate solutions of calcium, magnesium and
sodium salts became more concentrated, the lowest electronic
absorption peak (n f π*) shifted to higher energies and also
decreased in intensity as a function of nitrate concentration and
the nature of the cation. On the other hand, Wingen et al.141
reported no noticeable shift in the absorption spectra of the 9:1
NaCl/NaNO3 mixture (χNaCl = 0.9) over that of pure NaNO3.
However, to probe this possibility, the UV-visible absorption
spectra of aqueous solutions of NaBr/NaNO3 and KBr/KNO3
were recorded. Figure 5 shows the molar absorptivities (base 10)
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Table 1. Composition of MD Simulations (864 Water
Molecules)
-
χBr-
no. of Br-
no. of NO3
no. of Naþ or Kþ
0 (2 M)
0 (4 M)
0.1
36
72
65
36
18
7
36
72
72
72
72
72
7
36
54
65
0.5
0.75
0.9
for 0.5 M NaNO3 (red line), 0.5 M KNO3 (blue line), 4 M NaBr/
0.5 M NaNO3 mixture (yellow line), and 4 M KBr/0.5 M KNO3
(green line) in the region of the n f π* transition. Both NaNO3
and KNO3 have molar absorptivities centered at 302 nm. The
4 M NaBr/0.5 M NaNO3 (χNaBr = 0.9) shows a 2 nm shift toward
the blue (300 nm) while the 4 M KBr/0.5 M KNO3 (χKBr = 0.9)
shows no significant shift. The absolute values of the molar
absorptivities are slightly smaller, but still within 10%, of those of
the dilute solutions. These small changes clearly cannot be respon-
sible for increased production of NO2 from NaNO3 in the pres-
ence of bromide.
Figure 6. Density profiles for MD simulations of the air-water inter-
face of (a) 4 M NaNO3 and mixed aqueous solutions of NaBr/NaNO3
for mole fractions of (b) χNaBr = 0.5 and (c) χNaBr = 0.9, (d) 2 M KNO3
and 4 M mixed aqueous solutions of KBr/KNO3 for mole fractions of
(e) χKBr = 0.5 and (f) χKBr = 0.9.
There are some differences in the aqueous concentrations for
the KBr/KNO3 and NaBr/NaNO3 mixtures. The MDRH for
NaBr/NaNO3 was measured to be 57.5 ( 0.6% RH (Supporting
Information) and experiments were carried out well above this at
70-75% RH. However, the MDRH for KBr/KNO3 mixture was
measured to be 80.4 ( 0.5% RH (Supporting Information), and
experiments were able to be carried out reproducibly at 85-90%
RH. Because the KBr/KNO3 experiments were carried out at a
RH approaching the MDRH, the KBr/KNO3 mixtures were
more concentrated relative to the NaBr/NaNO3 mixtures. This
may have contributed to the somewhat higher rate of gas phase
NO2 generation from the KBr/KNO3 solutions (Figure 4a com-
pared to Figure 3a) and the lack of an induction time for Br2
production (Figure 4b) since acidification of the thin film will be
faster as well. However, these differences do not account for
increasing production of NO2 as χNaBr increased.
Previous molecular dynamics simulations of the NaCl/NaNO3
system141 suggested that enhanced NO2 production was attri-
butable at least in part to interfacial Cl- attracting Naþ, which in
turn drew nitrate ions closer to the interface. In this region,
enhanced photochemistry can result from the incomplete solvent
shell around NO3-, which enhances loss of NO2 to the gas phase
and makes its recombination with O- less efficient. To explore
such interactions in the systems studied here, MD simulations of
mixtures of NaNO3 and NaBr were performed with the same
ratio of corresponding ions as in experimental results for the
constant total ion experiments shown in Table 1. Each density
profile was normalized so the area under each curve is 0.5, and
then shifted along the z-axis so that the water Gibbs dividing
surface (GDS) is located at z = 0 Å. Figure 6a shows that in the
absence of Br-, nitrate ions prefer bulk solvation as reported
earlier.61,159,160,174,175 However, as bromide ions are added
(Figure 6b,c), nitrate ions are drawn closer to the interface into
a region of reduced solvent cage through a double-layer effect
similar to that for the NaCl/NaNO3 system.141 Recent high
pressure XPS experiments on liquid jets containing nitrate and
halide ions confirm this effect.142
Figure 7. Density profiles for NO3- ions in 4 M mixtures of (a) KBr/
KNO3 and (b) NaBr/NaNO3 as a function of χBr- from 0.1 to 0.9.
shows that when bromide ions are added, nitrate ions are only
minimally drawn closer to the interface. The saturation concen-
tration of pure KNO3 is 3.3 M;176 thus MD simulations for pure
KNO3 were conducted at 2 M, rather than 4 M for as for NaNO3.
In neat NaNO3 simulations at 2 M (data not shown) approxi-
mately 8% of nitrate ions were within 1 ( 0.5 nm compared to
5% for the 4 M solutions. Thus, if a 4 M KNO3 solution could be
simulated, the concentration of nitrate ions at the interface would
likely be smaller than that in Figure 6.
Figure 7 compares the density profiles for NO3- in mixtures of
KBr/KNO3 and NaBr/NaNO3, respectively, for which χBr-
=
0.1-0.9. In contrast to the sodium salts, the addition of bromide
ions to KNO3 is predicted to have a relatively small impact on the
nitrate ion distribution. This is consistent with our experimental
results where addition of Br- had no significant effect on the
nitrate ion photochemistry for the potassium salts, but in the case
of the sodium salts, the generation of the photoproducts NO2
and Br2 increased. Considerably more contact-ion-pairing
was observed (with no solvent molecules between ions) for
the sodium nitrate (3 Naþ per NO3-) versus potassium nitrate
(2 Kþ per NO3-). This results from the smaller radius of Naþ
causing stronger electrostatic attractions.
The lack of dependence of the rates of NO2 and Br2 produc-
tion on χKBr for the potassium salts is due to most of the photo-
lysis occurring in the bulk where there is efficient trapping of
Figure 6 shows the results of MD simulations for pure KNO3
(Figure 6d) and the KBr/KNO3 mixtures (Figure 6e,f). Figure 6e,f
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OH/O- by Br- and the production of NO2 and Br2 is limited by
the rate of photolysis of NO3-. Note that the film is sufficiently
thin, average thickness 0.8 μm, that the entire film is irradiated
and photolysis is not limited by strong nitrate ion absorption as in
studies of bulk systems.177,178 In the case of the sodium salts,
however, nitrate is drawn toward the interface much more
strongly (Figures 6b,c and 7b) where trapping by the solvent
cage becomes incomplete. In short, the experimental and theo-
retical data argue for a significant contribution of enhanced
overall quantum yields due to bromide ions at the interface in
the case of the sodium salts.
Previous studies of the photolysis of bulk nitrate-bromide
solutions reported that generation of gas phase bromine products
(likely Br2) was independent of the bromide ion concentration
-
above a ratio of Br-/NO3 of 0.02.177 Similar results for the
production of nitrite ions were observed by Das et al.179 who
argued that bromide ions actually scavenge OH inside the solvent
shell. Abbatt et al.178 measured the Br2 yields from the photolysis
of ice containing mixtures of bromide, chloride and nitrate and
reported that the Br2 production was independent of the initial
bromide ion concentration. All of these studies were on bulk
solutions or ice, consistent with our KBr/KNO3 experiments
where bulk chemistry predominates.
Figure 8. Top view of a snapshot of a Br--NO3- ion pair in a mixture
with χNaBr = 0.9. Key: green = Naþ, orange = Br-, purple = NO3-, blue
While the combination of our experimental data and the MD
simulations is consistent with an important role for interface
photochemistry, it also suggests that bromide ions can impact
bulk phase chemistry as well. Thus, the ratio of the rate of NO2
= Owater, white = Hwater
.
NO2
production in the presence of bromide ions (RBr- ) to that in
The MD simulations also provide insight into potential impacts
bromide ions on the bulk phase photochemistry. The calcula-
NO2
its absence (RNO
-
) can be expressed as a combination of
3
contributions from the interface region and the bulk:
tions show that in the mixture with χNaBr = 0.9, 20% of the nitrate
-
ions in the simulation have a bromide ion within 3.8 Å of NO3
.
NO2
Br-
int
R
½φ Nint þ φbBr-NbBr-
ꢁ
Br- Br-
Figure 8 shows a typical top-view snapshot of the nitrate-
¼
ðIÞ
-
NO2-
b
½φ -Nb -ꢁ
bromide ion coordination in this mixture. Clustering of NO3
R
NO3
NO3
NO3
with Br- and H2O is evident and occurs both in the bulk and near
In eq I, φint and φb are the quantum yields for nitrate photolysis in
the interface region and the bulk respectively, and Nint and Nb are
the number of available nitrate ions in each region. The numera-
tor refers to the solutions where Br- is present and the denom-
inator to pure NaNO3 solutions. It is assumed that the interface
region does not contribute significantly to the production of NO2
in the absence of bromide compared to the bulk because of the
relatively small concentrations of nitrate ions near the interface
(Figure 6a). For the χNaBr = 0.9, the experimentally measured
increase in NO2 production was a factor of 2.4 ( 0.2 (2s) higher
than that for χNaBr = 0. Taking the quantum yield in the bulk
solution to be 0.011 ( 0.001,112-116 the thickness of the film to
be 800 nm and that of the interface region to be 1 nm (( 0.5 nm
with respect to the GDS), a quantum yield at the interface of
10-13 would be required to generate the measured increase in
NO2 if the nitrate ion concentration was the same in the bulk and
at the interface. However, the MD simulations suggest that the
concentration of nitrate ions in the interface is a factor of 7 ( 3
(2s) higher for χNaBr = 0.9 mixtures than for pure NaNO3. This
factor arises from the integrated density profile area of nitrate
ions within ( 0.5 nm of the GDS, which is 31 ( 6% and 4.7 (
1.4% (2s) of the total nitrate ions for χNaBr = 0.9 and χNaBr = 0,
respectively. Taking this increase in concentration in the inter-
face region into account, the calculated quantum yield at the
interface drops to 1.7 ( 0.9. This is within experimental error of 1,
which is the maximum allowed by definition.39
the interface. A concerted process in which the excited NO3
-
transfers an O- to adjacent water which simultaneously transfers
the remaining OH radical to Br- seems reasonable under these
conditions.
If indeed a concerted process occurs, our experimental data
and the MD simulations can be used to estimate enhancement in
the bulk phase photochemistry, given reasonable assumptions
about the interface. Thus, the solvation environment around
nitrate ions was investigated for the χNaBr = 0.9 mixture by
calculating the number of water molecules around NO3- in the
presence of bromide as function of depth into the slab. The
solvent cage at the interface was reduced by approximately half,
with 12 water molecules at the GDS compared to 22 water
molecules surrounding nitrate in the bulk. Therefore, using a
quantum yield at the interface of 0.5, a bulk phase quantum yield
of 0.02 in the presence of bromide ions is needed to obtain an
overall enhancement in NO2 production of 2.4 ( 0.2 (2s) that
was measured experimentally. This represents a doubling of the
bulk phase quantum yield compared to the value of 0.01 reported
for bulk aqueous phase photochemistry of nitrate ions due to the
presence of bromide. Thus the combined experimental and
theoretical data suggest that bromide ions markedly impact
nitrate photochemistry at the interface via reduced solvent cage
effects and increased interfacial nitrate concentrations. Further
impacts that result from ion coordination may also play a role in
the photochemistry in the bulk and at the interface as well.112-116
If the interface region is taken as 1.0-1.5 nm and the quantum
yield in this region as being in the range 0.25-1.0, then the
However, these data do not rule out an enhancement of
photochemistry in the bulk as well, as suggested by Das et al.179
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The Journal of Physical Chemistry A
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interface contributes approximately 12-46% to the enhance-
ment in NO2 production.
suggests that bromide ions should enhance the photochemistry
at small mole fractions as well. One potential source of the dis-
crepancy is the segregation of bromide and nitrate salts during
the preparation of the thin films. If nitrate and bromide segregate
during preparation of the thin film and do not remix during the
final rehumidification step, bromide would not be available in
the vicinity of OH generation, and the NO2 production would be
similar to that for pure NaNO3. As described above, the chamber
was filled with nebulized particles three times, with pumping of
the chamber contents after the first three treatments. As the
relative humidity in a film with two salt species is lowered below
the MDRH, the salt with a concentration above the eutonic point
composition will become saturated and a crystalline phase can
form, leaving a shell of eutonic point composition surrounding
a core.180-183
To probe for possible segregation of bromide and nitrate ions,
SEM images were acquired on samples prepared in a manner
described above and similar to that used in the NO2 experiments.
Figure 9 shows the element maps for bromine and nitrogen for
χΝaBr 0.5 and 0.9. Segregation of the bromide and nitrate is
clearly seen for χNaBr = 0.5 but not for χNaBr = 0.9. This is
consistent with the eutonic point composition for NaBr/NaNO3
which was measured in the present studies to correspond to
χNaBr = 0.95 (Supporting Information). Drying of a solution with
a composition similar to that of the eutonic composition will not
lead to segregation of the salts, as confirmed by the SEM of the
χNaBr = 0.9. However, for the χNaBr = 0.5, NaNO3 would
crystallize out first because the nitrate ion concentration is much
higher than its eutonic point composition. As drying continues, a
salt mixture with χNaBr = 0.95 will be formed, leading to regions
of NaNO3 segregated from a mixed salt with high Br- content,
consistent with the element images in Figure 9. It is note-
worthy that the eutonic point composition for NaCl/NaNO3
In contrast, the bromide ions in the KBr/KNO3 MD simula-
tions (Figure 7a) do not affect the distribution of nitrate ions at
the interface nearly as dramatically as for the NaBr/NaNO3
system. Also, calculations in Table 2 show that ion coordination
is much less important for the potassium salts as expected for the
larger cation, thus the concerted mechanism involving ion pairs
in the bulk does not appear to be significant. As a result, the rate
of nitrate photolysis remains unaffected by the presence of
bromide ions for Kþ solutions (Figure 4a).
The observation of an enhancement in NO2 production
mainly at χNaBr above 0.6 is in contrast to previous measurements
for NaCl:NaNO3 mixtures where a 40% enhancement in NO2
was observed even at a χNaCl of 0.1.141 The predicted enrichment
of bromide ions at the interface compared to chloride ions58-61
Table 2. Percentage of NO3- Coordinated with Br- within
3.8 Å
χBr-
% NO3- in contact ion pairs with Br-
χNaBr
1.3
9.2
13
0.1
0.5
0.75
0.9
20
χKBr
0.8
4.1
6.7
7.4
0.1
0.5
0.75
0.9
Figure 9. Element maps for mixtures of NaBr/NaNO3: (a) χNaBr = 0.5, (b) χNaBr= 0.9. The left side is Br, and the right side is N.
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is χNaCl = 0.4 at 67% RH (Supporting Information and ref 183).
Thus the previous studies of NaCl/NaNO3 mixtures141 were
carried out under conditions that were not far removed from the
eutonic point where segregation of the halide and nitrate is not
significant. This was confirmed by SEM (data not shown). Thus,
the lack of significant enhancement in the NO2 production at
χNaBr < 0.6 is likely due to incomplete mixing of the nitrate and
bromide salts. However, at χNaBr approaching the eutonic com-
position, segregation does not occur and humidification above
the MDRH gives a well mixed solution of bromide, nitrate, and
sodium ions. Under conditions where the salts do not become
segregated such as the atmosphere, it is expected that bromide
ions will lead to enhanced nitrate ion photochemistry over the
full range of halide ion concentrations as was observed for
chloride ions.141
In summary, these data show that the overall quantum yields
for nitrate ion photolysis in thin aqueous films containing
sodium bromide are enhanced compared to those without
sodium bromide. The increased rate of photolysis is a combi-
nation of reduced solvent cage effects and increased interfacial
concentrations of nitrate, as well as a likely contribution from a
concerted process in the bulk and at the interface due to ion
coordination which brings NO3- and Br- into close proximity.
A similar enhancement in NO2 production was observed in
earlier studies of NaCl/NaNO3 mixtures and attributed to
double layer formation in which interfacial chloride attracted
sodium closer to the solution surface, which in turn pulled
nitrate toward the surface.141 Whether a concerted mechanism
involving ion coordination also contributed in the case of the
chloride salts is currently under investigation. Interestingly,
these effects are not observed for KBr/KNO3 where nitrate
ions reside predominantly in the bulk and there is less ion
coordination.
’ AUTHOR INFORMATION
Corresponding Author
*B.J.F.P.: e-mail, bjfinlay@uci.edu; telephone, (949) 824-7670;
fax, (949) 824-2420. D.J.T.: e-mail, dtobias@uci.edu; telephone,
(949) 824-4295.
’ ACKNOWLEDGMENT
It is a bittersweet honor to contribute this paper to the Victoria
Buch Memorial Issue: bitter because we lost Viki so prematurely,
yet it is an honor to memorialize one of the real “giants” in the
field. Viki was an internationally acclaimed leader whose integra-
tion of theory with experiments has provided unparalleled,
molecular-level insights into a number of systems that play
critical roles in atmospheric chemistry. Her no-nonsense ap-
proach to science and generous sharing of her insights over the
years are legendary and are already sorely missed by the
chemistry community around the world. This research was
supported by the National Science Foundation (Grant nos.
0431312, 0909227, and 0836735). N.K.R. is grateful for partial
support from a DOE GAANN fellowship. We thank Professor
Sergey Nizkorodov for assistance with eutonic point measure-
ments and the staff of the Carl Zeiss Center for Excellence in
Electron Microscopy at the University of California, Irvine, for
assistance and access to the facilities. Additionally, we thank
Professor John Hemminger, Dr. Theresa McIntire, and Christopher
Dilbeck for insightful conversations and Dr. James N. Pitts Jr. for
helpful comments on the manuscript.
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S
Supporting Information. Discussion of the mutual deli-
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’ NOTE ADDED AFTER ASAP PUBLICATION
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