A unique method for laboratory quantification of gaseous nitrous acid (HONO) using the reaction HONO + HCL → ClNO + H2O

Article abstract of DOI:10.1021/jp992890e

A novel method for quantifying gaseous HONO in laboratory systems has been developed. The technique uses the reaction of gas phase HONO with an excess of HCl gas to yield nitrosyl chloride (ClNO), which was quantified using FTIR. The gas-phase reaction of HONO with HCl formed ClNO with a stoichiometry of 0.9 ± 0.2 (1σ), i.e., within experimental error of unity. The concentration-time profiles for HONO and ClNO were fitted with a kinetics model, which showed that the reaction HONO + HCl → ClNO + ClNO was slow (k₁ ≤ (1.9 ± 1.3) x 10^-19/mole-sec (2σ) at 297 K. However, the reaction was a useful approach for quantifying HONO in laboratory systems because calibration for ClNO can be readily made. The method is better than UV/visible spectroscopy in that it does away with the more complex data associated with DOAS and other species (e.g., HNO₃), which cannot be measured by DOAS, can be determined. Compared to the denuder and NO_x detector approaches, the method is also more direct and specific.

Full text of DOI:10.1021/jp992890e

J. Phys. Chem. A 2000, 104, 329-335
329
A Unique Method for Laboratory Quantification of Gaseous Nitrous Acid (HONO) Using
the Reaction HONO + HCl f ClNO + H2O
Lisa M. Wingen, William S. Barney, Matthew J. Lakin, Theo Brauers,† and
Barbara J. Finlayson-Pitts*
Department of Chemistry, UniVersity of California, IrVine, IrVine, California 92697-2025
ReceiVed: August 16, 1999; In Final Form: October 29, 1999
Although the formation and reactions of gaseous nitrous acid (HONO) in the atmosphere are of great interest,
it is difficult to accurately measure HONO both in the atmosphere and in laboratory systems. We report a
new technique for quantifying gaseous HONO in laboratory systems. The method utilizes the reaction of gas
phase HONO with an excess of HCl gas to produce nitrosyl chloride (ClNO), which is readily quantified
using FTIR. HONO was formed by flowing N2 over the surface of an aqueous HCl solution and through a
bed of NaNO2, then directly into a 561 L chamber. An excess of gaseous HCl was added to the chamber to
initiate the reaction in N2 at room temperature and 1 atm total pressure. The loss of HONO was followed by
DOAS and FTIR and the formation of ClNO was measured by FTIR. While direct measurement of HONO
by FTIR is limited by uncertainties in the available infrared absorption cross sections, calibration for ClNO
is readily carried out since ClNO can be synthesized with high purity. The stoichiometry for ClNO formed
to HONO reacted was determined to be 0.9 ( 0.2 (1σ). The concentration-time profiles for both HONO and
k1
ClNO were fitted with a kinetics model which gave a rate constant for the reaction HONO + HCl f ClNO
+ H2O of k1 e (1.9 ( 1.3) × 10-19 cm3 molecule-1 s-1 (2σ) at 297 K. This should be taken as the upper limit
for the gas phase reaction since some contribution from heterogeneous reaction at the chamber walls cannot
be conclusively ruled out.
I. Introduction
specific method which requires a HONO reference spectrum44
generated using a high-purity source of HONO. Although
Nitrous acid (HONO) photodissociation has been found to
be the predominant source of OH in the early morning in some
urban areas.1-3 The reaction of HONO on surfaces4-6 to produce
N2O may also contribute to the atmospheric sources of N2O, a
greenhouse gas which also contributes to stratospheric ozone
cycles7,8 and for which there are still large uncertainties in many
of its sources.7,9,10 In addition, HONO has direct and indirect
health effects,11,12 which is of particular concern indoors where
elevated HONO levels have been found.13-17 HONO is also
known to react with secondary and tertiary amines to form
carcinogenic nitrosamines.18
The nighttime concentration of HONO has been measured
to reach up to 10-15 ppb in heavily polluted areas.2,19-22
Potential sources of HONO that have been proposed include
hydrolysis of NO2 on various surfaces,5,13,23-35 reaction of NO2
on soot,36-39 reaction of HO2 with NO2,40 peroxyacetyl nitrate
decomposition, NO2 abstraction of allylic hydrogen atoms, and
direct emission from combustion processes.17,41-43 Despite the
large amount of research which has been devoted to HONO,
these sources remain poorly understood and difficult to study
experimentally due, in part, to problems with quantifying
HONO.
various methods of gaseous HONO synthesis have been
reported,45-47 generation of HONO containing low levels of
impurities, particularly NO2, is difficult. Another group of
methods for HONO quantification involves the use of denuders
and NOx detectors.48,29 Denuder methods rely on the efficiency
and specificity of each denuder and have additional uncertainties
associated with obtaining HONO from the difference between
two numbers which are often very similar in magnitude.Wet
chemical techniques such as ion chromatography or reduction
of dissolved nitrite to NO have also been used.22,46,49 Finally,
atmospheric pressure ionization mass spectrometry has been
used to measure HONO15 via MS-MS of the Cl-‚HONO adduct
formed when CCl4 is added to the discharge region in which
the ions are generated.
We report here a unique procedure for quantification of
HONO in laboratory systems which utilizes the reaction of
gaseous HONO with excess HCl to produce ClNO:
HONO + HCl f ClNO + H2O
(1)
The product nitrosyl chloride can be synthesized in high purity
for calibrations and is easily measured using FTIR.
Several approaches have been applied to measuring gaseous
HONO in laboratory systems and in the atmosphere. Differential
optical absorption spectroscopy (DOAS) is a sensitive and
The reaction of HONO with HCl on sulfuric acid surfaces50-52
or on ice53 has been reported, and on ice the yield of ClNO is
100% with respect to the HONO reacted.53 We show that this
stoichiometry also applies to the reaction 1 and report an upper
limit for the rate constant of reaction 1 in the gas phase at room
temperature.
* To whom correspondence should be addressed: e-mail BJFINLAY@
UCI.EDU; phone (949) 824-7670; fax (949) 824-3168.
† Institut fu¨r Atmospha¨rische Chemie, Ju¨lich, Germany.
10.1021/jp992890e CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/17/1999

330 J. Phys. Chem. A, Vol. 104, No. 2, 2000
Wingen et al.
II. Experimental Section
and IR spectroscopy (0.5 cm-1 resolution). The HONO genera-
tion apparatus described above was used to flow a HONO/N2
mixture into the evacuated chamber to an initial pressure varying
from ∼65 Torr to ∼550 Torr and N2 was added to a total
pressure of 700 Torr. Wall loss of HONO was then monitored
for 15 to 30 min by FTIR and DOAS. The titration experiment
was initiated by flushing an excess of HCl (g) from a passivated
glass cell of known volume into the chamber through a side
panel using N2 to a total pressure of 750 Torr. The amount of
excess HCl used was at least 10 times the initial HONO
concentration, which was determined in real time by DOAS
using the HONO cross sections of Bongartz et al.56,57 The loss
of HONO due to reaction with HCl was followed with time by
DOAS and FTIR, and ClNO formation was followed by FTIR.
The loss of ClNO in the chamber under the experimental
conditions was measured by following it with time after most
of the HONO had reacted so that there was no formation of
ClNO at these longer times. The ClNO and NO2 concentrations
were determined by calibration with pure samples of these
compounds.
Another potential source of ClNO in these experiments is
the reaction of HCl with NO2, present as an impurity in the
synthesis of HONO. To investigate the products of this reaction,
several experiments were carried out in which known concen-
trations of NO2 and HCl were flushed into the chamber, followed
by filling to 1 atm with humid N2 to replicate conditions of a
typical titration experiment. NO2 was followed by DOAS and
by FTIR and ClNO and HNO3 were followed by FTIR.
UV/Visible and IR Analyses. The DOAS approach is based
on using differential cross sections rather than absolute cross
sections. In the case of absolute cross sections, the Beer-
Lambert law applied to complex mixtures of absorbing gases
is given by eq I
Apparatus. The apparatus used to perform the HONO
titration experiments has been described in detail by De Haan
et al.54 Briefly, it is a 561 L stainless steel and aluminum
chamber with two sets of White optics55 for long path UV/
visible and FTIR spectroscopy, (Mattson, Infinity AR). The base
path length of the White cell is 2 m, with all experiments
performed at a total path length of 52.53 ( 0.07 m in both
UV/visible and IR spectroscopy. UV/visible spectroscopy was
carried out using a high-pressure Xenon arc lamp (Oriel, 75
W, O3-free, model 6263) and a grating spectrograph (Jobin-
Yvon, Spex, model HR460) equipped with a diode array detector
(Princeton Instruments, Inc., model RY-1024). The chamber has
a surface area-to-volume ratio of S/V ) 8 m-1 and is evacuable
to ∼1 × 10-3 Torr. It is equipped with a relative humidity gauge
(Vaisala, type HMP432), and the internal walls are coated with
an inert halocarbon wax (Halocarbon Products Corp., Series
1500) to minimize loss of reactants at the walls.
Generation of HONO and Reference Gases. Gaseous
nitrous acid was generated by flowing N2 (Oxygen Service
Company, ultrahigh purity, 99.999%) over the surface of an
aqueous solution of 5.2 M HCl (prepared from Fisher HCl,
37.5%, 12.1 M) held at 0 °C with an ice bath. HCl (g) and
water vapor in N2 flowed upward through a glass cell with a 9
cm diameter porous glass frit holding a bed of ∼20 g NaNO2
powder (Fisher, 99.6%) moistened with Nanopure water (18.2
MΩ‚cm). The optimum flow rate was = 2.8 L min-1, with lower
flow rates yielding more NO2 impurity in the output. In some
of the experiments, gaseous HONO was synthesized using
“super free-flowing” NaNO2 (Aldrich, 99.5%). In these cases,
no water was added to the surface of the powder and lower
flow rates were used, resulting in less NO2 and water vapor,
but some unreacted HCl, in the output. HONO flowed directly
from the generation apparatus into the chamber through a port
in a side panel.
I(λ) ) I (λ) × exp[-L( σ N + σunkNi)]
(I)
∑
o
i i
Nitrosyl chloride was synthesized using a mixture of ∼100
Torr Cl2 (Matheson, 99.5%) and a slight excess (>200 Torr)
of NO (Matheson, 99%). The mixture was first condensed at
77 K in the coldfinger of a 5 L bulb, then allowed to warm to
room temperature. Reaction to form ClNO occurs primarily in
the liquid phase as the mixture warms. After several such
reaction cycles, the gaseous ClNO was condensed at 195 K with
a dry ice-acetone bath and the excess NO was pumped away.
The purity was tested using FTIR with a 32.0 or 52.5 m path
length. No evidence of NO was found in the spectrum. NO2
was sometimes observed at very low levels, <2%. The ClNO
calibration was performed by adding a measured pressure of
ClNO to the chamber and adding N2 to a total pressure of ∼1
atm. Successive dilutions were made by pumping the chamber
down to a known total pressure and rediluting with N2 to 1
atm.
NO2 was synthesized by combining an excess of O2 (Oxygen
Service Co., 99.993%) with NO which had been first passed
through a trap at 195 K to remove impurities such as HNO3.
The NO2 was purified by condensing the mixture at 195 K using
a dry ice-acetone bath and pumping away the excess O2.
Gaseous HCl (Scott Specialty Gases, 99.995%) was used as
a reactant in the titration. Reference spectra of HNO3 were
obtained using dry HNO3 prepared using the vapor over a 2:1
mixture of liquid H2SO4:HNO3 (H2SO4, Fisher, 95.7%, 18 M;
HNO3, Fisher, 70%, 15.8 M).
where I(λ) is the light intensity at λ after passing through the
sample, Io(λ) is the lamp intensity after travelling the total path
length (L) with no absorbers, σi is the absolute absorption cross
section for each species, i, Ni is the concentration of each
species, and σunk represents Rayleigh scattering and broad
absorptions from unknown species. UV/visible spectra can be
further described as the combination of a broad, continuous
absorption feature and highly wavelength-dependent, structured
absorption bands. Thus, eq I can be further separated giving eq
II58
I(λ) ) I (λ) × exp(-L σ′ N ) ×
∑
o
i
i
exp(-L[ (σBN + σunkNi)) (II)
∑
i
i
where σi ) σ′ +σB, in which σ′ represents the cross sections
i
i
for the structured portion of speciies i, i.e., the differential cross
section, and σBi represents the cross sections for the broad
portion of species i. The structured spectral features provide
the specificity needed to identify and quantify each compound,
and hence only the differential cross sections are used in the
DOAS approach.
Taking the natural logarithm of both sides, eq II becomes
Io
ln
) L σ′ N + L (σBN + σunkNi)
(III)
( )
∑
∑
i
i
i
i
HONO Titration. A total of six titration experiments were
performed. Continuous, simultaneous HONO measurements
were made using UV/visible spectroscopy (0.27 nm resolution)
I
which is of the form

Quantification of Gaseous Nitrous Acid
F(λ) ) (a × S (λ)) + P(λ)
J. Phys. Chem. A, Vol. 104, No. 2, 2000 331
(IV)
∑
i
i
i
where Si(λ) are reference absorption spectra of known concen-
tration, ai are scaling factors for the reference spectra, and P(λ)
is a polynomial to fit the broad features. Because the recorded
spectra consist of the sum of the broad features and the
structured features of the absorber(s), the spectra in the form
of eq IV must be fitted mathematically to determine the
contributions of each species to the total spectrum. This was
accomplished by minimizing the residuals between each ex-
perimental spectrum and a linear combination of the reference
spectra of all the contributing species to obtain a model
spectrum, F(λ), based on eq IV.59 HONO and NO2 concentra-
tions were determined by fitting UV/visible spectra with HONO
and NO2 reference spectra and a third-order polynomial in the
340-380 nm region. The published cross sections of Bongartz
et al.56,57 were used for HONO after first converting to 0.27
nm resolution by convolution with the instrument function of
the grating spectrograph.
Equation IV can be applied to IR spectroscopy as well
because rovibrational spectra are structured, while the baseline
is treated as a broad feature. Hence, IR spectra were analyzed
using eq IV with polynomials of zero to second order to simulate
the baseline and with reference spectra for HONO, HNO3, and
H2O. The use of an aqueous solution of HCl for the synthesis
of HONO produced a significant amount of water vapor in the
chamber so that the relative humidity during the experiments
varied from ∼10% to 30%. This water vapor caused interfer-
ences in the IR spectra in both HONO and ClNO regions. The
interfering rotational structure from water vapor was accounted
for by fitting the experimental spectra to a reference water
spectrum taken at a similar water vapor concentration.
Figure 1. Infrared spectra showing (a) the loss of HONO (1263 cm-1
)
and (b) the increase in ClNO (1799 cm-1) with time upon addition of
excess HCl (7.93 × 1014 molecules cm-3) during a typical experiment
-1
(#2). The absorption cross section for ClNO at 1799 cm
with 0.5
cm-1 resolution determined in these studies using authentic ClNO was
(1.08 ( 0.11) × 10-18 cm2 molecule-1, base 10 (2σ).
III. Results and Discussion
Typical IR spectra showing the loss of HONO and production
of ClNO are shown in Figure 1. The HONO band centered at
1263 cm-1 (ν3, trans-HONO) is shown in Figure 1a at t ) 0,
as well as at 1 h and 3 h after HCl addition. Figure 1b shows
the corresponding production of ClNO (ν1, centered at 1799
cm-1) at the same times. UV/visible spectra showing HONO
loss at approximately the same times are shown in Figure 2.
The fitting procedure described above was applied to the spectra
in Figures 1 and 2, followed by subtraction of the interfering
compounds leaving the HONO and ClNO spectra shown.
Stoichiometry of Reaction 1. For each experiment, the
change in the concentrations of ClNO (determined by FTIR)
and HONO (determined by DOAS) were calculated relative to
the time when HCl was added. The increase in [ClNO] was
plotted against the decrease in [HONO] for equal time intervals
and the slope obtained for each experiment. Using a numerical
integration program, ACUCHEM,60 the HONO and ClNO time
traces were modeled with wall losses set to experimentally
observed values as well as with wall losses set to zero.
Comparison of the two model runs showed that wall loss effects
were negligible until ∼54 min; thus, all stoichiometry plots were
obtained using data from e54 min. A slope of unity indicates
that one ClNO forms for every HONO destroyed, i.e., a one-
to-one stoichiometry for reaction 1. The slopes obtained for these
stoichiometry plots are shown in Table 1 (column 4) for all
experiments, with the average equal to 0.9 ( 0.2 (1σ). Although
there is low precision between experiments, on average one
ClNO molecule forms for every HONO molecule reacted. This
is consistent with the results of Fenter and Rossi,53 who observed
Figure 2. UV/visible spectra of gaseous HONO corresponding to
approximately the same times as the IR spectra shown in Figure 1. At
t ) 0 [HONO]o ) 7.6 × 1013 molecules cm-3, at t ) 1 h [HONO] )
3.6 × 1013 molecules cm-3, and at t ) 3 h [HONO] ) 9.3 × 1012
molecules cm-3. Spectra are offset on y-axis for clarity.
a one-to-one stoichiometry for reaction of HONO with HCl on
an ice substrate.
Other possible sources of ClNO in this system were consid-
ered. The reaction of NO2 impurity, present in the HONO, with
HCl was examined
2NO2 + HCl f ClNO + HNO3
To test this possibility, concentrations of HCl, NO2, and H2O
typical of those in the titration experiments were introduced

332 J. Phys. Chem. A, Vol. 104, No. 2, 2000
Wingen et al.
Figure 3. Typical plots of ln(aHONO) using the IR data versus time
(experiment 3). Circles (O) are for the 790 cm-1 peak, inverted triangles
(3) are for the 852 cm-1 peak, and squares (0) are for the 1263 cm-1
peak. The symbols do not lie on top of each other because different
reference HONO spectra were used for each infrared peak.
into the chamber and the formation of ClNO followed with time.
Under conditions similar to those of experiments 1-6, only
small yields of ClNO, <2% relative to the ClNO formed during
a titration experiment with similar concentrations of NO2, were
produced. Thus, reaction of NO2 with HCl cannot be a
significant source of ClNO in these HONO + HCl experiments.
While some contribution from other unrecognized secondary
reactions cannot be ruled out with certainty, no additional
products were observed in the infrared spectrum.
Kinetics of Reaction 1. The time dependence of HONO and
ClNO during the reaction of HONO with HCl can be used to
measure the rate constant for reaction 1. Reactions 1-3 describe
this system:
k1
HONO + HCl
98 ClNO + H2O
(1)
k2
ClNO
9
8 products
(2)
(3)
k3
HONO
98 products
The time dependence of HONO is described by its loss by
reaction with HCl, reaction 1, as well as its loss to the walls,
reaction 3. Although there may be a small contribution from
photodissociation of HONO and ClNO by the Xe arc lamp used
for DOAS, these are small compared to the total loss; for
example, the net rate of loss of ClNO at ∼10% RH is not
significantly different with or without the DOAS beam. HCl
was used in excess so that its concentration remained ap-
proximately constant (e6% loss over the times studied here).
Thus, the integrated rate equation for HONO is given by eq V
o + k3)t
[HONO] ) [HONO]oe-(k [HCl]
(V)
1
where k1 is the second-order rate constant for the reaction of
HONO with HCl, k3 is the first-order rate constant for HONO
loss in the chamber, [HCl]o is the initial HCl concentration, and
[HONO]o is the initial HONO concentration before addition of
excess HCl. To obtain k1 from eq V, ln(aHONO), where aHONO
are the scaling factors in eq IV, was plotted versus time, the
slope of which is equal to -(k1[HCl]o+ k3). The first-order loss
of HONO, k3, was obtained experimentally by following HONO
by FTIR and DOAS before HCl was added for each experiment.
[HCl]o is known for each experiment, thus k1 can be determined
for each experiment from the slope after addition of HCl.

Quantification of Gaseous Nitrous Acid
J. Phys. Chem. A, Vol. 104, No. 2, 2000 333
Figure 4. Time profiles for HONO measured by DOAS (diamonds ())) and ClNO by FTIR (circles (O)) during typical experiments: (a) experiment
3, (b) experiment 1. Errors (2σ) for ClNO and HONO were derived from the error in the peak fitting procedure and are equal to or less than the
size of the data points. Regression of the ClNO data is shown as solid lines.
Figure 3 shows typical plots of ln(aHONO) versus time using
the IR peaks at 790, 852, and 1263 cm-1. Table 1 summarizes
the values obtained for k1 by FTIR (column 5) using this
approach. Column 6 of Table 1 gives the values of k1 obtained
using the same analysis but using the DOAS HONO data. The
average values for k1 using the two measurement methods for
HONO, k1 ) (1.9 ( 1.4) × 10-19 cm3 molecule-1 s-1 (2σ) by
FTIR and k1 ) (1.9 ( 1.4) × 10-19 cm3 molecule-1 s-1 (2σ)
using DOAS, are in excellent agreement, although the error bars
for each are large. The overall rate constant using both analytical
techniques for HONO analysis is k1 ) (1.9 ( 1.3) × 10-19
cm3 molecule-1 s-1 (2σ). Values of k3 ranged from (1.8-6.5)
× 10-5 s-1 in these experiments; this represented from 6 to
39% of the total HONO loss in individual experiments. As
expected, the largest percentages were in the runs in which there
was unreacted HCl present initially in the HONO.
Karlsson and Ljungstro¨m61 studied the reverse reaction, ClNO
hydrolysis, and reported an upper limit for the rate constant for
reaction -1
k-1
ClNO + H2O 8 HONO + HCl
(-1)
to be k-1(296 K) e (7.4 ( 2.4) × 10-22 cm3 molecule-1 s-1
.
Using the calculated value of the equilibrium constant of K1(296
K) ) 1.25 × 104 based on the published Gibbs free energy for
the more stable trans-HONO,62 the upper limit for the rate
constant of the forward reaction 1 is thus k1(296 K) e 9.3 ×

334 J. Phys. Chem. A, Vol. 104, No. 2, 2000
Wingen et al.
10-18 cm3 molecule-1 s-1. Our measured value of k1 ) (1.9 (
1.3) × 10-19 cm3 molecule-1 s-1 (2σ) for a temperature of 297
( 1 K is consistent with this reported upper limit. As discussed
below, a heterogeneous contribution to reaction 1 cannot be
ruled out, and hence this value should be taken as an upper
limit to the gas phase rate constant.
HONO Quantification. The time dependence of the con-
centration of ClNO is determined by its production in reaction
1 and the wall loss in reaction 2. The integrated rate equation
for ClNO, with [HCl] constant, is given by eq VI
flowing” NaNO2 was used in the HONO generation, resulting
in some unreacted HCl passing through the generator and into
the chamber before the titration began. The regression fits in
these experiments also showed the largest deviation from the
ClNO data. The average of the percent deviations, however, is
close to zero, indicating that the [HONO]o values determined
by the titration deviate both positively and negatively, i.e., there
is no apparent systematic error.
The gas-phase reaction of HONO with HCl to produce ClNO
has not been reported in the literature to our knowledge.
However, the heterogeneous reaction of HONO with HCl on
ice and concentrated sulfuric acid solution substrates has been
studied by several groups with a one-to-one stoichiometry as
found in the present studies.50-53 While the spectroscopic
techniques used here detect only gaseous species, we investi-
gated a possible contribution from a wall reaction by carrying
out three titration experiments in a 7.6 L, glass long path cell
k1[HONO]o[HCl]o
1
o
3
2
[ClNO] )
(e-(k [HCl] + k )t - e-k t) +
k2 - k1[HCl]o - k3
[ClNO]o (VI)
In eq VI, k2 is the rate constant for first-order ClNO loss and
[ClNO]o is the initial ClNO concentration. The slow hydrolysis
of ClNO, reaction -1, was treated as first-order ClNO loss
included in the parameter k2 due to the high concentrations of
water used. While [ClNO]o should theoretically be zero, in some
cases some unreacted HCl passed through the HONO generation
apparatus, causing small amounts of ClNO to form before the
addition of excess HCl. The value of [HONO]o is treated as
unknown and is obtained by regression analysis of ClNO data.
The experimental ClNO data were fitted to eq VI using
regression analysis in which the value of [HONO]o was
optimized, while [HCl]o, k1, k2, and k3 were fixed at the values
measured in that experiment. The value of k2 in eq VI was
determined by plotting ln[ClNO] versus time for later times in
the experiment when g95% of the initial HONO had reacted.
Under these conditions, new formation of ClNO was minimal
and ClNO loss was dominant. Values of k2 ranged from (0.7-
2) × 10-5 s-1 in these experiments, which corresponded to
4-11% of the total measured loss rates for HONO. The effect
of changes in HCl concentration during an experiment was
examined by modeling the system with and without HCl wall
loss.60 The small change in HCl concentration over the time of
an experiment (e6%) did not significantly affect the value of
[HONO]o determined by regression (<2%).
Figure 4 shows typical experimental data for the decrease in
HONO (determined by DOAS) and production of ClNO
(determined by IR) with time. Also shown is the regression of
the ClNO data using eq VI with [HONO]o treated as an
adjustable parameter whose value is determined by optimizing
the fit to the data. In these experiments, the fitting procedure
gave [HONO]o ) 6.6 × 1013 molecules cm-3 (Figure 4a) and
[HONO]o ) 9.4 × 1013 molecules cm-3 (Figure 4b), in good
agreement with the values of 6.8 × 1013 and 1.1 × 1014
molecules cm-3, respectively, determined by DOAS.
(S/V ) 40 m-1). The rate constant obtained for reaction 1 in
(7.6L cell)
this cell was found to be k1
= (3 ( 2) × 10-19 cm3
molecule-1 s-1 (2σ), within experimental error of that obtained
in the much larger chamber, suggesting that a surface reaction
does not predominate in these experiments. However, given the
relatively large errors in our values of k1 and that similar
reactions of HCl, e.g., with ClONO2 and N2O5, occur at least
in part on surfaces,63 a heterogeneous component cannot be ruled
out.
IV. Summary
The gas-phase reaction of HONO with HCl has been shown
to form ClNO with a stoichiometry of 0.9 ( 0.2 (1σ), i.e., within
experimental error of unity. Although the reaction is slow, k1
e (1.9 ( 1.3) × 10-19 cm3 molecule-1 s-1 (2σ) at 297 K, it
has been shown to be a useful method for quantifying HONO
in laboratory systems, since calibration for ClNO can be readily
carried out. While the direct use of the available IR cross
sections for HONO in the literature is a possible alternative,
there are significant discrepancies in the reported IR cross
sections.64 This method has the advantage over UV/visible
spectroscopy that it does not require the more complex data
analysis associated with DOAS and allows the simultaneous
determination of the other species, such as HNO3, which cannot
be measured by DOAS. It is also a more direct and specific
method than the denuder and NOx detector approaches.
Acknowledgment. The authors thank Jorg Meyer for
glassblowing the experimental apparatus used for HONO
generation and Jochen Stutz, M. T. Leu, and P. Shepson for
helpful discussions. We also thank the California Air Resources
Board (Contract No. 97-311) and National Science Foundation
(Award No. ATM-9496240) for support of this research.
Table 1 summarizes the initial HONO concentrations obtained
by the titration method (column 7) as well as those determined
by DOAS (column 3) using the cross sections of Bongartz et
al.56,57 The values determined by the titration method deviate
from those determined by DOAS in individual experiments by
-26 to +23% (column 8). The largest deviations occur for those
experiments in which unreacted HCl was present initially from
the HONO generation; in these experiments the initial HONO
concentrations were also smaller so that the absolute increase
in ClNO on adding excess HCl was also smaller. In addition,
the differences in the first-order rates of HONO loss before and
after adding excess HCl were smaller. However, the average
deviation over all experiments is only 1%, but with a large
uncertainty (39%, 2σ). The experiments with the highest
deviations, experiments 4-6, were those in which dry, “free-
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