308 nm photolysis of nitric acid in the gas phase, on aluminum surfaces, and on ice films

Article abstract of DOI:10.1021/jp909867a

We have studied the photolysis of nitric acid (HNO₃) in the gas phase at 253 and 295 K, on aluminum surfaces at 253 and 295 K, and on ice films at 253 K, by using 308 nm excimer laser photolysis combined with cavity ring-down spectroscopy. We monitored both the ground-state NO₂ and the electronically excited NO₂, NO₂*, produced from the HNO₃ photolysis. NO₂* + OH is a predominant photolysis pathway (if not the only photolysis pathway) from the gas-phase photolysis of HNO₃ at 308 nm. The NO₂* quantum yields from the HNO₃ photolysis on aluminum surfaces are 0.80 ± 0.15 at 295 K and 0.92 ± 0.26 at 253 K, where errors quoted represent 2σ measurement uncertainty. The corresponding NO ₂* quantum yield from the HNO₃ photolysis on ice films is 0.60 ± 0.34 at 253 K. The 308 nm absorption cross sections of HNO₃ on Al surfaces and on ice films have been directly measured. Absorption cross sections of HNO₃ on Al surface at 308 nm are (4.19 ± 0.17) x 10^-18 and (4.23 ± 0.45) x 10^-18 cm²/molecule at 253 and at 295 K, whereas the corresponding absorption cross section of HNO₃ on ice films is (1.21 ± 0.31) x 10^-18 cm²/molecule at 253 K (errors quoted represent 2σ measurement uncertainty). Atmospheric implications of the results are discussed.

Full text of DOI:10.1021/jp909867a

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J. Phys. Chem. A 2010, 114, 2561–2568
2561
308 nm Photolysis of Nitric Acid in the Gas Phase, on
Aluminum Surfaces, and on Ice Films
Chengzhu Zhu,†,‡ Bin Xiang,† Liang T. Chu,† and Lei Zhu*,†
Wadsworth Center, New York State Department of Health and Department of EnVironmental Health Sciences, State
UniVersity of New York, Albany, New York 12201-0509, and School of Resources and EnVironmental Engineering,
Hefei UniVersity of Technology, Hefei 230009, People’s Republic of China
ReceiVed: October 14, 2009; ReVised Manuscript ReceiVed: January 16, 2010
We have studied the photolysis of nitric acid (HNO3) in the gas phase at 253 and 295 K, on aluminum surfaces
at 253 and 295 K, and on ice films at 253 K, by using 308 nm excimer laser photolysis combined with cavity
ring-down spectroscopy. We monitored both the ground-state NO2 and the electronically excited NO2, NO2*,
produced from the HNO3 photolysis. NO2* + OH is a predominant photolysis pathway (if not the only photolysis
pathway) from the gas-phase photolysis of HNO3 at 308 nm. The NO2* quantum yields from the HNO3 photolysis
on aluminum surfaces are 0.80 ( 0.15 at 295 K and 0.92 ( 0.26 at 253 K, where errors quoted represent 2σ
measurement uncertainty. The corresponding NO2* quantum yield from the HNO3 photolysis on ice films is 0.60
( 0.34 at 253 K. The 308 nm absorption cross sections of HNO3 on Al surfaces and on ice films have been
directly measured. Absorption cross sections of HNO3 on Al surface at 308 nm are (4.19 ( 0.17) × 10-18 and
(4.23 ( 0.45) × 10-18 cm2/molecule at 253 and at 295 K, whereas the corresponding absorption cross section of
HNO3 on ice films is (1.21 ( 0.31) × 10-18 cm2/molecule at 253 K (errors quoted represent 2σ measurement
uncertainty). Atmospheric implications of the results are discussed.
1. Introduction
state and the electronically excited state of NO2, respectively.
Riffault et al.10 used laser-induced fluorescence (LIF) to determine
Nitric acid (HNO3) is a major atmospheric oxidation product
of NOx (NOx ) NO + NO2).1,2 Although its gas-phase photolysis
in the troposphere is slow,3,4 the photolysis rate for HNO3 deposited
on ground and vegetation surfaces to form HONO and NOx has
been reported5 to be 1-2 orders of magnitude faster than that in
the gas phase. The photolysis of HNO3 adsorbed on ground surfaces
has been proposed as a major daytime source of HONO in low-
NOx environments.5–7 To understand the difference between the
nitric acid photolysis rate in the gas phase and the rate of photolysis
on surfaces, the Zhu group has recently determined the UV
absorption cross sections of surface-adsorbed HNO3 in the 290-330
nm region8 through the use of Brewster angle cavity ring-down
spectroscopy. The study8 showed that the surface absorption cross
sections of HNO3 are at least 2 orders of magnitude higher than
the cross section values of the nitric acid vapor, in the wavelength
region studied. Since the product channels and quantum yields
associated with the HNO3 photolysis on the surface have not been
directly measured, further investigations are necessary to character-
ize the photolysis quantum yields of surface-adsorbed HNO3.
The gas-phase photolysis of nitric acid in the actinic UV
region (λ g 290 nm) can occur through the following pathways:
the OH quantum yield from the photolysis of HNO3 vapor; they
obtained an OH quantum yield of 1.05 ( 0.29 at 308 nm photolysis
wavelength. However, OH quantum yield determination alone
would not tell whether OH is a coproduct of the NO2 channel or
a coproduct of the NO2* channel. An earlier HNO3 photodisso-
ciation dynamics study at 193 nm by Jacobs et al.11 found that the
OH radicals formed are vibrationally and rotationally cold but
translationally hot, suggesting HNO3 photolysis occurring on a
repulsive potential energy surface. On the basis of the results of
this HNO3 photodissociation dynamics study, we infer that the
majority of the nitrogen dioxide molecules formed from the HNO3
photolysis at 308 nm are vibrationally cold but electronically hot
in order to account for the energy release beyond products
translational energy following the 308 nm photolysis of HNO3.
Our postulate is supported by the work of MacLeod et al.12 who
monitored the NO2* fluorescence emission (the onset of emission
occurs near 400 nm and has a maximum around 558 nm) from
the 248 nm photolysis of HNO3 vapor and assumed a quantum
yield of unity for the NO2* + OH channel from the HNO3
photolysis and used that to calibrate the NO2* quantum yield from
the pernitric acid (HO2NO2) photolysis at 248 nm. Assuming that
the photolysis of HNO3 adsorbed on Al surfaces or on ice films
breaks the same bonds in HNO3 as does gas-phase photolysis, we
would like to determine whether the 308 nm photolysis of adsorbed
HNO3 on Al or ice surfaces proceeds through the following
photolysis pathways:
HNO3 + hV f OH + NO2
f OH + NO2*
(λ e 604 nm)
(λ e 381 nm)
(1g)
(2g)
f HONO + O(3P)
(λ e 393 nm) (3g)
HNO3(ad) + hV f OH(g/ad) + NO2(g/ad)
(1s)
where photochemical thresholds were calculated from the corre-
sponding enthalpy changes.9 NO2 and NO2* represents the ground
f OH(g/ad) + NO2*(g/ad) (2s)
* To whom correspondence should be addressed. E-mail: zhul@
wadsworth.org. Phone: 518-474-6846. Fax: 518-473-2895.
† State University of New York.
The products formed from the 308 nm photolysis of adsorbed
HNO3 are expected to carry excess energy. This excess energy
‡ Hefei University of Technology.
10.1021/jp909867a  2010 American Chemical Society
Published on Web 02/02/2010

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2562 J. Phys. Chem. A, Vol. 114, No. 7, 2010
Zhu et al.
may break weak bonds between the photolysis products and the
surface, resulting in the release of the photolysis products, such as
NO2* and NO2, into the gas phase.
The ground state and the electronically excited states of NO2
can be monitored by their vibronic absorption and/or emission
in the visible region. Since there are four electronic states of
NO2 in the visible region (2A1, 2B2, 2B1, and 2A2) and each one
can interact with the other three electronic states, the visible
spectra of NO2 are very complex. Detailed description about
the visible spectra of NO2 can be found in the “Spectral Atlas
of Nitrogen Dioxide, 5530 to 6480 Å” by Hsu et al.13 and the
references within. The ground-state NO2 can be monitored by
its near UV-vis absorption14–19 in the 255-600 nm region with
peak absorption occurring at about 400 nm. Most of the
oscillator strength of the visible spectrum for the ground state
2
of NO2 comes from the X A1-2B2 transition.13 The excited
NO2 exhibits structured UV-vis absorption bands in the
350-650 nm region.14 In the study of Davidson et al.,14 the
excited NO2 was formed by heating the NO2 sample to 124 °C,
and the excited NO2 absorption spectrum was attributed to that
of the vibrationally excited ground-state NO2. As the wave
functions of the electronically excited NO2 and the high
vibrational levels of the ground-state NO2 are strongly mixed,
the excited NO2 spectrum in the 350-650 nm region can be
that of the electronically excited NO2 or that of the vibrationally
excited ground-state NO2, depending upon how the excited NO2
was generated. The electronically excited NO2 has “anoma-
lously” long radiative lifetime possibly caused by the interelec-
tronic level mixing with the ground state.20 Although there is a
strong transition moment between the vibrational levels of the
electronically excited NO2 and the high vibrational levels of
the ground-state NO2, the high vibrational levels of the ground
state have essentially no transition strength to the lowest
vibrational levels of the ground state. The lifetimes of the
electronically excited NO2 vary21 in the range of 55-90 µs as
the excitation wavelength is varied in the 3980-6000 Å range.
In this paper, we report results obtained from the 308 nm
excimer laser photolysis of HNO3 in the gas phase at 295 and
253 K, on Al surfaces at 295 and 253 K, and on ice films at
253 K, using cavity ring-down spectroscopy.22,23 The NO2*
quantum yields from the HNO3 photolysis on Al surfaces and
on ice films have been measured. In addition, the adsorbed
HNO3 absorption cross sections on Al surfaces and on ice films
at 308 nm have been directly determined. Atmospheric implica-
tions of the results are discussed.
Figure 1. Schematics of the stainless steel cells used for the gas-phase
study (above) and for the surface study (below).
cavity. The inner diameter of the jacketed gas cell is 4 in., and
the cell has an inner volume of 2.4 L. For this gas cell
configuration, the photolysis products detected by cavity ring-
down spectroscopy were those formed from the gas-phase
photolysis.
The stainless steel flow cell used for studying HNO3 pho-
tolysis on Al surfaces and on ice films (the surface-study cell)
has been described in detail elsewhere.27 It has the form of a
hollow rectangular prism (see Figure 1). The length of the cell
is 55 cm, and the cell’s cross section is 2.54 cm × 2.54 cm. A
pair of high-reflectance cavity mirrors vacuum-sealed the two
ends of the cell. Fused-silica windows were mounted to the front
and back of the cell, for transmission of the photolysis beam.
A pair of rectangular Al reflectors (∼47% measured reflectivity
inside the cell at 308 nm) were mounted inside the cell, along
its length. The photolysis beam entered the cell through the
window on the front side. The photolysis beam was subsequently
bounced back and forth by the Al reflectors, a number of times,
before it exited the cell through a window. The probe laser beam
entered the cell along the main optical axis. A fraction of the
probe laser pulse was injected into the cavity through the front
mirror, and the intensity decay of this fraction inside the cavity
was measured by monitoring the weak transmission of light
through the rear mirror, with a photomultiplier tube (PMT). The
amplified PMT signal was fitted to a single-exponential decay
function, from which the ring-down time constant and the total
loss per optical pass were calculated.
2. Experimental Technique
An excimer laser operating at 308 nm was used to photolyze
HNO3 in the gas phase, on Al surfaces, and on ice films; the
photolysis products formed from the HNO3 photolysis were then
probed by cavity ring-down spectroscopy. A stainless steel
reaction cell for gas-phase study (the gas cell) has been described
in detail elsewhere.24–26 It is shown in Figure 1 to enable
comparison with a cell used for the surface studies. Only the
essential features of the gas cell are summarized here. The gas
cell has a double-walled configuration to facilitate variation of
the cell temperature; it was vacuum-sealed by a pair of high-
reflectance cavity mirrors at both ends. The output from the
photolysis laser was propagated into the reaction cell at a 15°
angle to the main cell axis, through a side arm. The probe laser
pulse, delayed relative to the photolysis laser pulse and used to
detect NO2* (at 552.57 nm) and NO2 (at 352 nm),14–16 was
introduced into the cell along the main optical axis. The probe
laser beam overlapped the photolysis beam at the center of the
The surface-study cell is equipped with various reagent ports,
a pumping port, and pressure- and temperature-measurement
ports, all located on top of the cell. A rectangular viewport,
made of transparent plastic (acrylic) and installed on the top,
along the main optical path, allowed us to view the ice film
growth conditions and the photolysis beam. The bottom of the
cell had a cooling block, which was connected to a low-
temperature bath/circulator (Neslab ULT-80; ethanol was used
as the coolant). Temperature at the Al reflector surface was
measured with a thermocouple intact. Helium was bubbled
through a distilled water bubbler at 295 K, and water-vapor/
He mixture was admitted to the cell via four feedthroughs placed
at the bottom of the cell (Figure 1). At the other end of each
feedthrough was a length of stainless steel tubing, 13-14 cm
3
long, /16 in. o.d. diameter. On each length of tubing, a row of
precision holes was drilled, with 1 cm spacing. Four lengths of
stainless steel tubing were used to form two sets of internally

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Photolysis of Nitric Acid
J. Phys. Chem. A, Vol. 114, No. 7, 2010 2563
mounted water vapor outlets directed toward two Al surfaces.
Lengths of stainless steel tubing were placed in the bottom of
the cell, and the water vapor was sprayed at 45° to the normal,
so as to optimize the condensation of water vapor on the Al
surfaces. Ice films were formed by vapor deposition on the Al
surfaces at 253 K. This cell configuration allowed us to study
not only HNO3 photolysis on ice films but also HNO3 photolysis
on Al surfaces in the absence of ice films. The typical ice film
thickness was estimated about 10 µm calculated from the mass
flow rate of water vapor (measured by a Teledyne-Hastings
mass flowmeter), the ice deposition time, the bulk density28,29
(0.63 g/cm3) of vapor-deposited ice, and the geometric area of
the ice films deposited.
The inner surfaces of both gas and surface-study cells, and
the stainless steel joints on the gas transport line, were coated
with halocarbon wax (series 1500; Halocarbon Products Corp.)
or treated with halocarbon grease (Halocarbon 25-5S grease;
Halocarbon Products Corp.), to minimize decomposition of
HNO3 on cell surfaces and on stainless steel joints. The vapor-
deposited ice is polycrystalline at 253 K.30 The ice film deposited
on the Al surface was uniform to the naked eye and was freshly
prepared in every experiment. Following the generation of ice
films, HNO3 was introduced into the cell under slow-flow
conditions. The vapor pressure of ice at 253 K is about 0.75
Torr.31 The total pressure in the cell for the ice film experiments
was the vapor pressure of ice at 253 K, plus the added HNO3
pressure. In the absence of ice films, the total pressure in the
surface-study cell was equal to the admitted HNO3 vapor
pressure.
Once the 308 nm excimer laser radiation was introduced into
the surface-study cell, the photolysis of HNO3 in the gas phase
and the photolysis of adsorbed HNO3 on the surface occurred
simultaneously. The absorption of the probe beam at 552.57
nm by electronically excited NO2, obtained through measure-
ment of the cavity losses with and without a photolysis pulse,
was the sum of the contributions both from the photolysis of
HNO3 in the gas phase and on surface. The NO2* absorption
produced from the HNO3 photolysis on surface was obtained
by subtracting the NO2* absorption from the gas-phase HNO3
photolysis from the total NO2* absorption. A pulse/delay
generator was used to vary the delay time between the firing of
the photolysis laser and the firing of the probe laser. Quantum
yield measurements in the gas phase and on Al surfaces were
made at a laser repetition rate of 0.1 Hz, whereas measurements
on ice film surfaces were made at 1 Hz. The spectrum scan
was performed at a laser repetition rate of 1 Hz.
Figure 2. Cavity ring-down absorption spectrum of the product in
the 550-560 nm region, after the gas-phase photolysis of 1.50 Torr of
HNO3 at 308 nm and at 295 K. The spectrum was recorded at a
wavelength interval of 0.05 nm.
probed for possible HNO3 photolysis products using cavity ring-
down spectroscopy under each condition. The ground-state NO2
was monitored14–16 at 352 nm. The electronically excited NO2
was probed14 at 552.57 nm. As stated in the Experimental
Technique section, there is a very small amount of (unavoidable)
NO2 impurity (e0.05%) in the HNO3 sample, but we did not
observe transient ground-state NO2 formation immediately after
the HNO3 photolysis. Upon the basis of the magnitude of the
NO2* absorption generated from the HNO3 photolysis, the lack
of observation of the transient ground-state NO2 signal from
the HNO3 photolysis, and the OH quantum yield of 1 from the
HNO3 photolysis at 308 nm,10 we conclude that NO2* + OH is
the predominant photolysis pathway (if not the only photolysis
pathway) from the HNO3 photolysis in the gas phase at 308
nm. Results from the HNO3 photolysis in the gas phase, on Al
surfaces, and on ice films are described in individual sections
below.
3.2. Photolysis of HNO3 in the Gas Phase at 295 and 253
K. Figure 2 is a cavity ring-down absorption spectrum of the
product in the 550-560 nm region after the 308 nm gas-phase
photolysis of HNO3 at 295 K. This is a spectral region with
high NO2* absorption14 and low absorption from the ground-
state NO2 (the previous literature NO2* spectroscopic study14
was done at 1.5 nm resolution). The observation of transient
product from the HNO3 photolysis at 308 nm, in a spectral
region that NO2* exhibits strong absorption, suggests that NO2*
is a 308 nm photolysis product of HNO3. A similar product
absorption spectrum was also obtained following the gas-phase
HNO3 photolysis at 253 K. The cavity ring-down spectrometer
was tuned to the NO2* absorption maximum at 552.57 nm, and
the total absorption resulting from the gas-phase HNO3 pho-
tolysis was measured as a function of time.
High-purity HNO3 was prepared by vacuum distillation of a
3:2 mixture of sulfuric acid (98%; Mallinckrodt Baker) and nitric
acid (70%; Mallinckrodt Baker) at 273 K into a trap cooled at
ethanol/dry ice temperature (195 K).32 Five successive distil-
lations were conducted, to purify the sample. To further reduce
NO2 impurity before each experiment, we purged NO2 from
the liquid HNO3 bubbler, by flowing N2 carrier gas through the
bubbler for about 30 min;33 we then pumped the liquid HNO3
bubbler for 10 min in the absence of N2 flow. The NO2 impurity
in the purified nitric acid vapor was determined by monitoring
the NO2 absorption18 in the 448-452 nm region, using cavity
ring-down spectroscopy. The NO2 impurity was determined to
be less than 0.051%.
Shown in Figure 3 is a transient round-trip absorption profile
measured at 552.57 nm following the 308 nm photolysis of 0.75
Torr of HNO3 at 295 K. Figure 3 shows that the probe laser
absorption at 552.57 nm decreases rapidly with delay time,
between the firing of a photolysis and a probe laser, at less than
60 µs, and then the probe laser absorption is nearly independent
of time. Since both NO2* and NO2 absorb probe laser beam at
552.57 nm with respective absorption cross sections of 9.4 ×
10-19 and 8.9 × 10-20 cm2/molecule at 295 K (cross section
measurement for NO2* at 552.57 nm is described later in this
section), the following kinetic scheme was used to fit the
3. Results and Discussion
3.1. General Features. We investigated the 308 nm pho-
tolysis of HNO3 in the gas phase at 295 and 253 K, on Al
surfaces at 295 and 253 K, and on ice films at 253 K; we then

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2564 J. Phys. Chem. A, Vol. 114, No. 7, 2010
Zhu et al.
the ACUCHEM simulation program.35 The radiative decay rate
constant of NO2* (kNO2*), the NO2* + HNO3 reaction rate
constant (kNO2*+HNO3), σNO2·HNO3, and the initial NO2* concentra-
tion ([NO2*]0) were used as input parameters. Initial values of
kNO2*, kNO2*+HNO3, and σNO2·HNO3 were given to the program, and
the simulated absorption profiles were compared with the
experimental results. Numerical values of kNO2*, kNO2*+HNO3, and
σ
NO2·HNO3 were subsequently adjusted so as to optimize the fit.
The following extracted values are obtained: kNO2* of 1.8 × 104
s-1 at 295 K, kNO2*+HNO3 of 1.1 × 10-13 cm3 molecule-1 s-1 at
295 K, and σNO2·HNO3 of 1.8 × 10-18 cm2/molecule at 552.57
nm. The extracted value of kNO2* corresponds to a radiative
lifetime of 56 µs, which agrees well with the reported lifetimes21
for electronically excited NO2 of 55-90 µs. Temporal absorp-
tion profiles at 552.57 nm from the photolysis of HNO3 at three
pressures are well-fitted by the extracted kNO2*, kNO2*+HNO3, and
σNO2·HNO3 values at 295 K. The quality of the fit is shown in
Figure 3 for temporal absorption profile generated from the
photolysis of 0.75 Torr of HNO3 at 295 K.
The NO2* quantum yields from the nitric acid photolysis at 308
nm, at 295 and 253 K, were determined from the ratio of the NO2*
concentration produced in the photolysis/probe laser overlap region
to the absorbed photon density in that region. The overlap region
in the cell for the gas-phase study can be thought of as a rectangular
prism: the prism’s center coincides with that of the cell, its width
and height are defined by the dimensions of the photolysis beam,
and its length is defined by the length over the overlap region.
The length over the overlap region was calculated using (beam
width)(tan 15°)-1, where 15° is the crossing angle between the
pump and probe laser beams. The length of the overlap region is
defined by (beam width)(sin 15°)-1. A 12 mm wide × 6 mm tall
rectangular aperture was placed in the photolysis beam path before
the beam’s entrance to the cell.
Figure 3. Time profile of the total round-trip absorption measured at
552.57 nm following the 308 nm gas-phase photolysis of 0.75 Torr of
HNO3 at 295 K. Symbols (b) denote experimental data. Curves shown
are the kinetic simulations using the ACUCHEM software. See text
for details.
The photolysis beam is absorbed by HNO3 over the entire level
arm through which it travels. The absorbed photolysis photon
density in the overlap region can be derived from (i) the difference
between the transmitted photolysis beam energies entering (Ein)
and leaving (Eout) that region, (ii) the individual photon energy (hc/
λ) at the photolysis wavelength (λ), and (iii) the volume (V) of the
overlap region, via the following equations:
experimental temporal round-trip absorption profile at 552.57
nm:
NO2* ) NO2
(4)
(5)
NO2* + HNO3 ) NO2 ·HNO3
absorbed photon density ) (Ein - Eout)/((hc/λ)V)
absorption(t) ) 2lσNO2*[NO2*] + 2lσNO2[NO2] +
2lσNO2·HNO3[NO2 ·HNO3]
(7)
(6)
V )
(beam width)(beam height)(length of the rectangular prism)
where NO2 ·HNO3 represents the adduct formed from the NO2*
+ HNO3 reaction; l is the length of the photolysis/probe laser
overlap region (more detailed description of the photolysis/probe
) (beam width)2(beam height)(tan15°)-1
laser overlap region is given later in this section); σNO2*, σNO2
,
The energy of the photolysis beam entering or leaving the overlap
region can be calculated from the energy of the incident photolysis
beam entering the cell (E0), the nitric acid absorption cross section
(σ) at 308 nm and the density (n) of nitric acid in the cell, and the
absorbing path length, through application of Beer’s law:
σNO2·HNO3 denote the respective absorption cross sections of
NO2*, NO2, and NO2 ·HNO3 at 552.57 nm; [NO2*], [NO2], and
[NO2 ·HNO3] represent the respective concentrations of NO2*,
NO2, and NO2 ·HNO3. The formation of an NO2 ·HNO3 adduct
has been invoked from the reaction of NO2* + HNO3 rather
than the direct quenching of NO2* by HNO3 (NO2* + HNO3
) NO2 + HNO3) in order to explain the absorption value at
552.57 nm as a function of time (the derivation of the cross
section value for the NO2 ·HNO3 adduct will follow; it is 1.8 ×
10-18 cm2/molecule at 552.57 nm, which is 20-fold the cross
section value for the ground-state NO2 (8.9 × 10-20 cm2/
molecule) at the same wavelength). Also, the NO2 ·HNO3 adduct
was previously observed in the IR region.34 Transient absorption
profiles at 552.57 nm from the photolysis of 0.5, 0.75, and 1.5
Torr of HNO3 were compared with the values calculated by
Ein ) E0 exp(-σnl1)
Eout ) E0 exp(-σnl2)
(8)
(9)
where l1 is the distance between the photolysis beam entrance and
the beginning of the overlap region and l2 is the distance between
the photolysis beam entrance and the end of the overlap region.
The incident photolysis beam energy was measured by a calibrated