Potential role of the nitroacidium ion on HONO emissions from the snowpack

Article abstract of DOI:10.1021/jp068264g

The effects of photolysis on frozen, thin films of water-ice containing nitrogen dioxide (as its dimer dinitrogen tetroxide) have been investigated using a combination of Fourier transform reflection - absorption infrared (FT-RAIR) spectroscopy and mass spectrometry. The release of HONO is ascribed to a mechanism in which nitrosonium nitrate (NO⁺NO₃ ^-) is formed. Subsequent solvation of the cation leads to the nitroacidium ion, H₂ONO⁺, i.e., protonated nitrous acid. The pathway proposed explains why the field measurement of HONO at different polar sites is often contradictory.

Full text of DOI:10.1021/jp068264g

1
167
2007, 111, 1167-1171
Published on Web 01/31/2007
Potential Role of the Nitroacidium Ion on HONO Emissions from the Snowpack
Stig Hellebust, Tristan Roddis, and John R. Sodeau*
Department of Chemistry, UniVersity College Cork, Ireland
ReceiVed: December 1, 2006; In Final Form: January 12, 2007
The effects of photolysis on frozen, thin films of water-ice containing nitrogen dioxide (as its dimer dinitrogen
tetroxide) have been investigated using a combination of Fourier transform reflection-absorption infrared
(
FT-RAIR) spectroscopy and mass spectrometry. The release of HONO is ascribed to a mechanism in which
+
-
nitrosonium nitrate (NO NO
3
) is formed. Subsequent solvation of the cation leads to the nitroacidium ion,
ONO , i.e., protonated nitrous acid. The pathway proposed explains why the field measurement of HONO
at different polar sites is often contradictory.
+
H
2
NO3- + hν f NO + O
-
(1)
(2)
(3)
Introduction
2
The polar snowpack can have a major influence on the
overlying atmosphere. It does not appear to inhibit emissions
from land and ocean surfaces, as might be thought simplistically;
rather there is increasing evidence that it can act as a source.
Indeed, for some years it has been recognized that impurities
present in Arctic snow can be photolyzed to release reactive
trace gases into the boundary layer. As outlined below, the
chemical mechanisms by which these processes are driven have
been investigated in the laboratory using a number of techniques
to explain unusual field observations. In this regard, one of the
most intriguing recent findings has been the periodic detection
of NOx and HONO emissions from certain sunlit snowpacks
located at the poles. The phenomenon was first encountered
during summertime at Summit in Greenland during the PSE98
campaign by Honrath et al. The findings were subsequently
confirmed by a number of other measurement campaigns. For
example, noon-time HONO fluxes of 5-10 nmol m
found at Summit during 2000 and ca. 40 nmol m
were later determined at Alert in the Canadian High Arctic
during 2000. Beyond the necessity for sunlight to initiate the
releases, the presence of acidified snow also appeared, in the
earliest studies, to be a common factor. Thus the absence of
measurable HONO fluxes in the marine Arctic at Ny- A° lesund
NO3- + hν f NO2- + O
NO2- + H O T HONO + H O
+
3
2
The proposed reaction sequence is analogous to that often
1
1
suggested to proceed in solution. However, the efficiency of
reaction 2 is extremely low with a measured quantum yield (Φ)
of about 0.001. In contrast, reported Φ1 values for NO2
production are some 10-100 times greater than the nitrite-
1
2
forming route. In these experiments, initial excitations into
both the stronger ππ* state and the lower energy, weaker nπ*
state of the nitrate ion have been investigated. Although the
latter is directly populated on the snowpack by solar radiation,
it would also be rapidly formed after absorption into the ππ*
state via the process of internal conversion. In fact, the product
observations are in line with the general expectation that nπ*
photochemistry leads to bond breaking whereas ππ* photo-
chemistry leads to isomerization. In summary, the direct
production of HONO via reaction 3 might be expected to be
negligible at the poles if the photolysis of nitrate ions was indeed
the driving force, especially with the solar fluxes and absorbance
values involved.
1
-
2
-1
h
were
2
-2 -1
h
fluxes
3
However, the fact remains that HONO is formed at the poles
4
was attributed to the presence of local alkaline snow surfaces.
However, recent data obtained at a Southern Hemisphere site
in Browning Pass, Antarctica, indicate that fluxes of nitrous
acid were measured to be close to zero even though both acidic
snow and abundant UV light were present. Clearly, much more
polar field data are required to establish the full conditions for
HONO release.
and its subsequent photochemistry can lead, in principle, to the
formation of OH radicals, above snowpacks in sunlit conditions.3
5
These would be produced either by direct photolysis of the
HONO product in (3) or from O ion reaction with any
-
hydroxonium ions present.
+
-
H O + O f H O + OH
(4)
3
2
In the meantime, a number of laboratory-based experiments
have been undertaken by various research groups to clarify the
potential chemical mechanisms, which may operate in the field.
Both water-ices and artificial snows have been investigated and,
from the results obtained, a photolysis-driven process involving
Another route for the production of OH radicals is the
photolysis of hydrogen peroxide present in snow. In fact, it has
been shown that the pathway is some 10-100 times more
efficient than the corresponding nitrate photolysis route for OH
6
-10
13
acidified nitrate and nitrite ions appears to operate.
The
radical production. Actual field emissions will therefore depend
chemistry is summarized in reactions 1-3.
on relative concentrations of the two potential sources.
1
0.1021/jp068264g CCC: $37.00 © 2007 American Chemical Society

1168 J. Phys. Chem. A, Vol. 111, No. 7, 2007
Letters
The photolysis of artificial snow created from basic nitrate
a reflection-absorption infrared/temperature programmed de-
ion solutions has been reported to lead to greatly enhanced NO
to NO2 yields compared to more acidic counterparts where the
ratio is reversed. This finding is suggested to originate from
the interplay between nitrate ions and their nitrite ion photo-
sorption (RAIRS-TPD) study of NO2 on water-ice.
Experimental Section
6
The experimental setup has been described in detail else-
products. The latter species absorb sunlight to a much greater
1
7
where, and a brief description is given here only. The
measurements were made using an ultrahigh vacuum chamber
extent than the parent in the solar output region between 315
9
and 350 nm. Reactions 1 and 2 would therefore become
(UHV) fitted with vacuum compatible KBr windows in the
superseded by reaction 5 in alkaline environments.
optical path. The vacuum was maintained by an Edwards EO4
cryo-cooled oil diffusion pump, backed by a rotary vane
NO2- + hν f NO + O-
(5)
-8
pump: the resultant base pressure was 10 mbar or better. A
gold-foil substrate was placed in thermal contact with a liquid
nitrogen reservoir through tungsten rods and suspended inside
the vacuum chamber. The foil was heated resistively and cleaned
by heating to 500 K between experiments. For RAIRs measure-
ments of the surface species, a Bruker Vertex 70 FTIR
instrument was used with a liquid nitrogen cooled HgCdTe
detector. IR spectra were collected at 4 cm resolution and 64
scans averaged. The UHV chamber was also fitted with a
quadrupole mass spectrometer (EPIC 300, Hiden Analytical UK)
for gas-phase analysis of chemical release from the frozen
substrates, using electron impact ionization with 70 eV ioniza-
tion potential.
It has also been proposed that the production of the perox-
ynitrite ion, OONO , formed as a result of “cage-recombination”
-
on reaction 1 might also occur. This ion is stabilized in alkaline
6
conditions and can be photolyzed to give NO. The sequence
is shown as reactions 6 and 7.
-
1
-
-
{
NO + O } f OONO
(6)
(7)
2
-
-
OONO + hν f NO + O2
In fact, there is good experimental evidence for the formation
-
Nitrogen dioxide, 99.5% pure, was obtained from Merck Ltd.
and used as received. The water used was of Millipore quality
and degassed by repeated freeze-pump-thaw cycles.
The gases and vapors were admitted to the chamber through
glass dosing lines using manual or piezoelectric pulse valves
and directed onto the gold substrate held at low temperatures.
of OONO ions upon photolysis of nitrate ions in aqueous
solution albeit at wavelengths much lower than those encoun-
tered in the snowpack and also on a very rapid time scale when
the ππ* state is being directly probed. Hence, femtosecond
transient absorption spectroscopy has shown that the formation
-
of OONO , in its cis-isomer, represents 44% of the primary
-
6
Dosages were measured in langmuir (1 langmuir ) 10 Torr
s ) 1.33 mbar s). For “mixed ices” a co-deposition process
utilizing two or more compounds was developed. It entailed
simultaneous exposure of appropriate vapors from separate
vessels containing the chemicals to the substrate. Mixing ratios
were determined by pre-setting the piezoelectric pulse valve
pulse widths for each reactant at room temperature. The pulse
valves were then activated simultaneously with the desired total
dosing pressure being controlled by the frequency of pulsing.
Typical deposition conditions involved dosing at 70-250
langmuirs. The temperature of the substrate was continually
monitored and was varied between 100 and 500 K during
experiments.
photoproduct when irradiation at 200 nm is performed. Subse-
quent relaxation back to the nitrate ion is measured to be
efficient but some of the isomer dissociates into NO and the
1
4
superoxide ion.
NO3- + hν f ONOO-
(8)
(9)
-
ONOO f NO + O2-
The Φ of photoreaction 8 has been measured in aqueous
solution as 0.1, a value equivalent to that measured for the
12
production of nitrogen dioxide in reaction 1. Furthermore, in
one of few published studies on the photolysis of nitrate ions
on water-ice, it has been shown, by use of FT-IR spectroscopy,
For the photolysis experiments, a xenon arc lamp from
ORIEL (model 60010) was employed as a broad light source
solar mimic. The sample was irradiated through a quartz window
on the chamber.
-
that ammonium nitrate photodecomposes to OONO ions. In
contrast, the photolysis of frozen nitric acid hydrates was shown
to lead only to the formation of molecular nitric acid in a proton-
transfer mechanism.15
The effect of pH on the photochemistry of the system is
certainly complex because the simple equilibrium step shown
in reaction 3 does not describe the system over the full range
of pH encountered on polar frozen surfaces. Hence, aqueous
solutions of nitrite ions are not observed, by UV-vis spectros-
copy, to be free of nitrous acid contributions until a pH of ca.
Results and Discussion
NO2 on Gold. The IR spectra of pure NO2 samples condensed
onto the cold gold substrate, before and after photolysis, are
-
1
shown in Figure 1. Sharp features at 1300 and 785 cm
accompanied by a weaker doublet at 1760/1731 cm- are readily
observable in Figure 1a following deposition of NO2 at 115 K.
These bands are, in fact, due to the D2h symmetric dimer of
nitrogen dioxide, N2O4. They have been previously assigned
1
1
0 is reached; between pH 3 and 10, varying mixtures of the
-
strongly UV-absorbing species HONO along with NO2 ions
are present; and at pHs below 3, protonated HONO, i.e., the
by several researchers as the νs(NO2), δ(NO2), and νas(NO2),
+
16
18-20
nitroacidium ion, H2ONO , appears.
modes respectively.
Indeed, the RAIRS relative band
Undoubtedly, the photolysis of nitrate ions in/on snowpack
is involved in the observed field measurements but, given the
varying field observations and the reactive nature of the nitrogen
dioxide product, its potential involvement in the HONO release
mechanism from frozen surfaces would also appear to be worth
further experimental investigation. Hence, we describe below
intensity measurements are consistent with the D2h-N2O4
molecules being adsorbed to the surface with their central N-N
axis aligned perpendicular to the surface. The dimerization
occurs readily because the equilibrium process, 2NO2 T N2O4,
is highly temperature-dependent and, at the concentrations
employed in these experiments, is pushed efficiently to full
2
1

Letters
J. Phys. Chem. A, Vol. 111, No. 7, 2007 1169
2 2
Figure 2. RAIRS spectra of, from the top down: NO /H O co-
Figure 1. Spectra of NO
2
deposited on gold at 115 K (a) and following
deposited at 110 K (a); same film after 60 min UV photolysis (b);
same film annealed and held at 190 K (c). Capital letters refer to band
groupings as discussed in text.
2
0 min UV irradiation (b) and 70 min UV irradiation (c). The inset
-
1
shows expanded view of the region 2300-1800 cm . Arrows in the
inset indicate bands at 2217 and 1961 cm , discussed in text.
-
1
observed previously for nitrogen dioxide dimers. For example,
reactive zeolite surfaces apparently promote the reaction by
autoionisation as do polluted water films.
association.22 Indeed, no spectral evidence for the presence of
NO2 molecules on the surface was obtained.
28
29
After photolysis of the low-temperature films described above,
three main sets of IR bands were recorded (Figure 1b). The
NO2 on Water-Ice. Co-deposition of NO2 and water vapor
onto the gold foil substrate held at 110 K resulted in the IR
spectrum shown in Figure 2. Hence, before photolysis a strong
grouping of bands with similar absorbance values appear at
-
1
first consists of two peaks observed at 1961 and 2217 cm .
Their relative absorbances depend upon exposure time with the
latter band clearly growing at the expense of the other. Such
behavior indicates that secondary photochemistry is occurring,
and indeed, after prolonged photolysis time, only the higher
wavenumber peak of the two is seen to remain (Figure 1c). This
photochemistry is readily explainable in terms of the initial
production of the D′-N2O4, dinitrogen tetroxide dimer,
ONONO2. This species is subsequently photolyzed to produce
-1
1
765/1745, 1269, and 765 cm (group A in Figure 2), although
the latter feature is somewhat obscured by the broad water
librational mode. Such a RAIRS spectrum indicates that the D2h-
N2O4 molecules are not ordered on the cold surface; rather, they
should be considered as an amorphous dispersion within the
-1
water-ice matrix. Much weaker features at 1866 and 1595 cm
+
(group B in Figure 2) are again observed and represent the
presence of very small amounts of NO and N2O3. Weak bands
the nitrosonium ion, NO . The solid-phase infrared spectra have
been reported previously for both these species with the band
-
1
-
1
at 1303 and 1422 cm are also apparent and due to the
formation of the nitrate ion.
at 1961 cm corresponding almost exactly to that determined
20,23-26
fortheD′-isomertrappedinalow-temperatureargonmatrix.
A second group of IR bands are measured at 1420, 1337,
One hour photolysis of the ice film results in almost the
complete loss of the strong grouping of IR bands. In addition,
-
1
1
035, and 823 cm ; these can all be assigned to nitrate ions.
Finally, a third set of bands, much weaker in intensity than the
-1
new bands appear at 1430, 1341, and 1044 cm (group C in
-
1
above features, are observed at 1864 and 1590 cm . They
represent, respectively, clear diagnostics of the formation of
-1
Figure 2). Very weak bands at 3525, 3225, and 2748 cm are
also measurable in absorbance difference spectra. A rather weak
NO and subsequently the asymmetric form of N2O3.27 Both
species are expected to be formed at low concentrations in the
chemical system investigated here from homolytic photolysis
of any of the nitrogen oxides present.
17
-1
band at 2230 cm is also apparent and is both blue-shifted
-1
+
and broader than the feature at 2217 cm assigned to NO in
the water-free experiments. Such a spectral change is a good
indication of the bare ion becoming solvated or linked to another
species. Indeed, the blue-shift behavior has a clear analogy with
The above results indicate that photolysis of nitrogen dioxide
deposited on a low-temperature gold surface proceeds as
follows:
30
the isoelectronic CO and CO-H2O systems. Here matrix
-1
isolated CO displays its IR fundamental at 2138 cm , whereas
-
1
its water-partnered counterpart absorbs at 2149 cm .
D -N O (ads) + hν f ONONO
2
(10)
(11)
2
h
2
4
Upon annealing to temperatures between 150 and 210 K a
number of important spectral changes are observed. The largest
changes are observed at temperatures above 180 K when many
of the IR bands begin to be lost. Of particular note is the fact
+
-
3
ONONO + hν f NO + NO
2
Annealing the surface to 210 K causes the IR spectrum of
the photolyzed thin film to change abruptly to one consisting
-
1
that near 205 K the 2230 cm feature is lost entirely. These
observations clearly indicate that some chemicals are emitted
from the surface and possibly transformed by the warming
process
-
1
of a single strong band at 1386 cm accompanied by two
-
1
weaker features at 2229 and 1035 cm . At 220 K the smaller
features are lost, leaving only one strong band remaining at 1374
-
1
To clarify the nature of the emissions, the mass spectrometer
was employed to monitor the gas phase in parallel to the
collection of the RAIRS data. A solid water-ice surface was
first deposited and warmed to 210 K. This control experiment
led to the detection of only one signal at m/z ) 18, which is, of
course, due to H2O. Photolyzed N2O4 ice films were then
cm . These shifts originate with a change in phase, from
amorphous to crystalline, for the nitrosonium nitrate species
formed in reaction 11. The transition has been discussed in detail
24
previously by Givan and Loewenschuss.
Thermal analogues to the photoprocesses discussed above,
which lead to the formation of the nitrosonium ion, have been

1170 J. Phys. Chem. A, Vol. 111, No. 7, 2007
Letters
Figure 4. RAIRS spectra of, from the top down: NO
2
deposited at
1
10 K and photolyzed for 70 min with a top layer of water ice added
subsequently (a); same film annealed and held at 190 K (b) and 200 K
c). Nitric acid deposited at 115 K and annealed to 200 K (d) is shown
Figure 3. Thermal desorption profiles of NO
photolysis of D2h-N in water ice. The mass spectral signal m/z )
7 has been corrected for the contribution from water using the water
y 2
and H O subsequent to
2 4
O
(
1
for comparison.
fragmentation pattern 74% m/z ) 18, 17% m/z ) 17. The peak positions
of m/z ) 17 are 170, 200, and 210 K. The peak position of m/z ) 47
is seen at 205 K. The peak positions of the water signal (m/z ) 18) are
Taken together, the IR and mass spectrometry results indicate
that the following mechanism operates in a water-ice thin film:
190 (shoulder), 200, and 210 K, and the peak position of m/z ) 30 is
205 K with a minor peak at 197 K.
+
+
NO + H O f {(H O)NO}
(12)
(13)
(14)
2
2
warmed and led to detectable gas-phase fragments with m/z )
7, 30, 46, and 47. These being in addition to the signature
1
+
+
{
(H O)NO} + H O f HONO + H O
2 2 3
signal of water detected at m/z ) 18. In parallel with the IR
measurements, the largest increase in signal begins at about 180
K. The most likely identities for the fragments in the system
+
-
+
-
H O + NO f {H O NO }
3
3
3
3
+
+
+
studied here are OH (m/z ) 17), NO (m/z ) 30), NO2 (m/z
+
+
In the overall process, NO produced on the ice surface by
photolysis of the nitrogen dioxide dimer becomes solvated with
water and is then hydrolyzed to HONO, which is efficiently
released to the gas phase. In contrast, the nitrate component is
retained on the surface as NAT. The chemistry is presumably
in addition to the simple photodesorption event, producing NO2
in the gas phase, observed by Rieley et al. when wavelengths
)
46) and HONO (m/z ) 47).
To investigate the processing in more detail, a temperature
programmed desorption (TPD) experiment was then carried out
between 150 and 340 K, employing a constant heating rate of
approximately 0.8 K/s. The results indicate that the gas-phase
signal for the m/z ) 17 and 30 fragments increase gradually
from 150 K, with the former maximizing at about 170 K. They
increase further when the m/z ) 18 (water signal) starts
increasing around 170 K. At about 205 K the signals for the
fragments m/z ) 17, 30, 46, and 47 reach a maximum: this
3
1
with λ > 360 nm were used.
As a caveat, the minor release of HONO observed between
50 and 170 K in the TPD experiments can be explained by
1
hydrolysis of the small amount of the asymmetric form of N2O3
present in the experiments, as previously discussed by Finlayson-
+
being the same temperature at which the IR band for the NO
species is lost. The experimental behavior is summarized in
Figure 3. As an additional note, desorption of a sample film
consisting purely of a mixture of NO and NO2 gives two peaks
in the m/z ) 30 signal, at approximately 130 and 160 K.
2
9
Pitts.
ONNO + H O f HONO + HONO
(15)
2
2
An alternative method of producing the cold thin films was
also employed. Here nitrosonium nitrate was formed initially
by photolysis of a D2h-N2O4 film and a layer of water-ice was
deposited on top. To promote mixing, the solid was then
annealed to temperatures between 190 and 200 K. RAIRS
spectra were taken at all stages, and the most noteworthy infrared
features observed in addition to bands, which can be assigned
to the nitrate ion, were two strong bands at 3430 and 1130 cm .
These features are similar to those monitored after annealing a
nitric acid/water film to 200 K, as shown in Figure 4.
The important question arises: does the photochemistry
discussed above have a relevance to the snowpack observations?
On ice surfaces, the monomer NO2 has an adsorption enthalpy
32
of -22 kJ/mol, whereas the dimer N2O4 has an adsorption
33
enthalpy of around -40 kJ/mol However, it remains a question
of debate whether typical ppb concentrations of NO2 from
airborne sources of pollution could be transformed to the dimer
efficiently on cold, polar surfaces. Thus from the equilibrium
data the fraction of N2O4 formed in the gas phase in the ppb
-1
-7
-4
NO2 range would be ∼10 at 270 K and ∼10 at 210 K.
Neglecting differences in entropic contributions to the adsorption
equilibrium constant between NO2 and N2O4, one can subse-
quently calculate that the fraction of N2O4 on a surface would
-
1
From previous RAIRS studies, the absorptions at 3430 cm
-
1
and 1130 cm can be taken as evidence for the formation of
nitric acid trihydrate (NAT). The bands can be assigned to the
ν1,3(H2O) and ν2(H3O ), oxonium ion modes of NAT, respec-
+
-3
32
only range from 10 at 270 K to 0.4 at 210 K. On the other
hand, computer modeling calculations have shown that nitrogen
dioxide dimer species are significantly more strongly bound to
1
5
tively. They persist at temperatures higher than that at which
+
the NO species is lost, i.e., above ∼205 K. The continued
-
1
34
presence of the main nitrate ion feature around 1425 cm ,
surfaces containing -OH groups than monomer counterparts.
+
despite the loss of NO implies that the oxonium ion then takes
The fact remains that when sufficient concentrations of
nitrogen dioxide are present in snowpack regions, the surface
over the role of counterion.

Letters
J. Phys. Chem. A, Vol. 111, No. 7, 2007 1171
chemistry described in the current experiments could operate
to act as the HONO and NO release mechanism. Such a pathway
may well be important as the quantum yield of reaction 2 is so
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2
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for low-temperature N2O4 have been measured to be consider-
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(
11) Mack, J.; Bolton, J. J. Photochem. Photobiol. A-Chem. 1999, 128,
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1.
3
50 nm.35 A photolysis mechanism would then appear to be
(
possible from the above results, although a hydrolytic dispro-
portionation of NO2 into nitrite and nitrate ions might also occur,
as previously discussed.10
One potential source of significant quantities of frozen
nitrogen dioxide originates from the photolysis of nitrate ions
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(
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(
(
+
are present, the solvated NO may, in preference, act as a
(
nitrosating agent,36 leading to little or no HONO release. Such
constrained micro-environments have been proposed to drive
other polar-related chemistry, e.g., the release of interhalogens
to the troposphere, although it remains to be seen whether they
exist in actual snowpacks.37
8
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(
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Acknowledgment. We acknowledge financial support from
INTAS, the EU Marie Curie programme, and the Norwegian
Research Council. Inputs from Dr. M. Ammann and Dr. A
Grammas are also gratefully acknowledged.
1
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