Heterogeneous reaction of nitric acid with nitric oxide on glass surfaces under simulated atmospheric conditions

Article abstract of DOI:10.1021/jp040184u

The heterogeneous reaction of nitric acid (HNO₃) with nitric oxide (NO) on borosilicate glass surfaces was studied in a flow system at relative humidity levels in the range 21-86%. Reactant concentrations were kept closer to ambient atmospheric levels as compared to all previous studies of this reaction. Within experimental error, no formation of the proposed reaction products nitrous acid (HONO) and nitrogen dioxide (NO₂) was observed. Upper limits of the reactive uptake coefficients of NO on borosilicate glass surfaces, covered with ～1 monolayer of HNO₃, were determined: γ(NO→HONO) < 4.0 × 10_-11 and γ(NO→NO₂) < 2.5 × 10_-9. These values are significantly lower than previously reported values, which were determined at higher reactant concentrations. Results obtained upon investigation of the secondary heterogeneous reaction of the proposed product HONO with HNO ₃ under identical experimental conditions show that HONO should be observed in the study of the reaction HNO₃ + NO, if it is formed. Thus, the obtained upper limit γ(NO→HONO) is representative for the reaction HNO₃ + NO → HONO + NO₂. Under the assumption that the glass surfaces, typically used in laboratory studies of this reaction, are representative for environmental surfaces, the latter reaction is unimportant for atmospheric HONO formation and for a renoxification of the atmosphere.

Full text of DOI:10.1021/jp040184u

J. Phys. Chem. A 2004, 108, 5793-5799
5793
Heterogeneous Reaction of Nitric Acid with Nitric Oxide on Glass Surfaces under
Simulated Atmospheric Conditions
J o1 rg Kleffmann,* Thorsten Benter, and Peter Wiesen
Physikalische Chemie/FB C, Bergische UniVersit a¨ t Wuppertal, 42097 Wuppertal, Germany
ReceiVed: February 27, 2004; In Final Form: April 26, 2004
3
The heterogeneous reaction of nitric acid (HNO ) with nitric oxide (NO) on borosilicate glass surfaces was
studied in a flow system at relative humidity levels in the range 21-86%. Reactant concentrations were kept
closer to ambient atmospheric levels as compared to all previous studies of this reaction. Within experimental
2
error, no formation of the proposed reaction products nitrous acid (HONO) and nitrogen dioxide (NO ) was
observed. Upper limits of the reactive uptake coefficients of NO on borosilicate glass surfaces, covered with
-
11
-9
∼
1 monolayer of HNO
3
, were determined: γ(NOfHONO) < 4.0 × 10 and γ(NOfNO
2
) < 2.5 × 10 .
These values are significantly lower than previously reported values, which were determined at higher reactant
concentrations. Results obtained upon investigation of the secondary heterogeneous reaction of the proposed
product HONO with HNO
the study of the reaction HNO
representative for the reaction HNO
3
under identical experimental conditions show that HONO should be observed in
+ NO, if it is formed. Thus, the obtained upper limit γ(NOfHONO) is
+ NO f HONO + NO . Under the assumption that the glass surfaces,
3
3
2
typically used in laboratory studies of this reaction, are representative for environmental surfaces, the latter
reaction is unimportant for atmospheric HONO formation and for a “renoxification” of the atmosphere.
1
. Introduction
From recent laboratory studies, the heterogeneous reaction
of HNO3 with NO
Heterogeneous reactions of nitrogen oxides play an important
role in atmospheric chemistry. For example, the heterogeneous
HNO (ads) + NO(g) f HONO + NO2
(2)
3
hydrolysis of N2O5 is a significant sink of NOx in both the
1
2
stratosphere and troposphere, strongly affecting ozone con-
centration and acid rain formation, respectively. Another
example is the heterogeneous reaction of NO2 on humid surfaces
was proposed as a source of HONO in the atmosphere.22,23 In
addition, it was suggested that reaction 2 followed by reaction
3
2
NO + H O f HONO+ HNO
(1)
2
2
3
HNO (ads) + HONO(ads) f H O + 2NO
2
(3)
3
2
is of importance for a “renoxification” of the boundary layer.2
Reaction 2 has been the subject of several other studies.
4,25
which is proposed to be an important source of nitrous acid
3
-9
26-30
(HONO), in both laboratory systems and the atmosphere. In
It
several recent studies it was demonstrated that the photolysis
of HONO can contribute significantly to OH radical formation
during daytime.1
was reported that the reaction rate is proportional to the HNO3
and NO concentration and to the surface-to-volume ratio,
demonstrating the heterogeneous nature of the reaction. How-
ever, it was also shown that the kinetics of reaction 2 is more
complicated than the stoichiometry implies. For example, in the
0-13
The sources of nitrous acid in the atmosphere are still not
8
,14
completely understood. In addition to direct emission,
26
heterogeneous pathways are most probably responsible for
HONO formation in the atmosphere. Besides HONO formation
by reaction 1, other heterogeneous pathways have been pro-
posed. For example, it has been suggested that HONO is formed
detailed study of Smith the author concluded that the reaction
is autocatalytic in NO2. In addition, a positive water vapor
dependence and a negative temperature dependence in the range
273-303 K was observed. A water vapor dependence of
1
5,16
24
by the heterogeneous conversion of NO2 on soot surfaces.
reaction 2 was also recently reported by Saliba et al. who
However, recent studies demonstrated that this noncatalytic
reaction cannot explain current HONO levels in the atmo-
concluded that the reaction rate reached a maximum at
intermediate humidity levels corresponding to a surface water
coverage of approximately three monolayers.
1
7,18
sphere.
On the basis of correlation studies, it was proposed
that the heterogeneous reaction of NO and NO2 (or N2O3) on
In all studies reported up to now, reaction 2 was investigated
at NOy concentrations, which were orders of magnitude higher
than those prevailing in the atmosphere. In addition, most studies
on reaction 2 were carried out at low relative humidity (RH).
Due to the complex reaction kinetics, an extrapolation of these
results to atmospheric conditions is highly uncertain.
1
9
humid surfaces explains atmospheric HONO formation.
However, field studies20,21 and a laboratory investigation in
which the reaction was studied under humidity levels and NOx
7
concentrations prevailing in the atmosphere, indicate that this
reaction is unimportant.
In the present study, reaction 2 was investigated in a flow
system in the presence of atmospheric relative humidity levels
and NOy concentrations that were much lower compared to all
*
To whom correspondence should be addressed. Tel: +49-202-439-
3
534. Fax: +49-202-439-2505. E-mail: kleffman@uni-wuppertal.de.
1
0.1021/jp040184u CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/16/2004

5794 J. Phys. Chem. A, Vol. 108, No. 27, 2004
Kleffmann et al.
3
Figure 1. Experimental setup for investigation of reactions 2 and 3 and the adsorption of HNO on glass surfaces.
studies reported so far. In addition, reaction 3 was studied using
the same experimental setup to determine whether HONO, one
of the products of reaction 2, can be observed under the
experimental conditions or is rapidly converted to NO2.
of Taira and Kanda31 was modified by mixing diluted solutions
of sodium nitrite and H2SO4 in the temperature controlled
stripping coil (see Figure 1). The HONO purity was higher
compared to that from a bubbler system similar to that reported
by Taira and Kanda, which was also used earlier in our
laboratory. In addition, with the modified setup, the HONO
concentration could be varied much faster with a response time
3
1
2
. Experimental Section
Reaction 2 was investigated in a flow system, which is
(
10-90%) of about ∼2 min, as a result of the higher liquid-
schematically shown in Figure 1. A calibrated mixture of NO
in N2 (Messer Griesheim, 80 ppm; flow meter: Tylan 0-30
mL/min) was diluted with synthetic air (flow controller: Brooks
phase exchange rate of the HONO source. All liquid flows
within the two stripping coils were adjusted by peristaltic pumps
(Ismatec, Reglo 4).
0
-500 mL/min) to achieve mixing ratios in the range 0.5-10
The nitrite and nitrate concentrations in the effluent of the
ppmV. The mixture was humidified (21-86% RH) with
ultrapure water (Milli Pore) in a temperature controlled stripping
coil, which also removed impurities of nitrous acid (HONO)
from the NO mixture. The humidity was calculated under the
assumption that the gas phase was saturated at the temperature
of the stripping coil. Errors of the relative humidity were
estimated from the accuracy of the temperature measurement
to be <2%. Nitric acid (HNO3) was generated by bubbling a
small flow of synthetic air (flow controller: Brooks 0-10 mL/
min) through a mixture containing 1% vol HNO3 (65%) in H2-
SO4 (70%). The impurities NO2 and HONO were found to be
second stripping coil were measured by ion chromatography
Shimadzu, Model 6a) using UV detection at λ ) 209 nm after
(
preconcentration on a Dionex TAC LP1 column. The concentra-
tions of HONO and HNO3 were calculated using the measured
nitrite and nitrate concentrations and the measured liquid and
gas flow rates. The errors of the HONO and HNO3 concentra-
tions were calculated from the accuracy of the nitrite and nitrate
measurements and the errors of the liquid and gas flow rate
determination.
In contrast to HONO and HNO3, the concentrations of NO
and NO2 were found to be almost unaffected by the stripping
coils, due to the much lower solubilities and low reactivities of
these compounds. Accordingly, the NO2 concentration was
measured downstream of the second stripping coil by a Luminol
NOx monitor (Unisearch, LMA-3D). The instrument was
calibrated at NO2 mixing ratios of 0-20 ppbV during blank
experiments under the same conditions, i.e., [NO] and RH, as
in the experiments with HNO3 + NO. NO2 was obtained from
Messer Griesheim as a 2.09 ppmV premix-gas balanced with
N2. The error of the NO2 concentration was calculated from
the accuracy of the NO2 calibration mixture, specified by Messer
Griesheim, and the statistical errors of the calibration curve.
<0.1% after the HNO3 source was allowed to run for a day.
NO and HNO3 were mixed in a PFA T-piece leading to a final
HNO3 mixing ratio of ∼600 ppbV at a total standard gas flow
rate of 400-450 mL/min (T ) 298 K, p ) 760 Torr).
The PFA T-piece was directly attached to a glass coil
-
1
(
borosilicate glass, l ) 160 cm, 0.2 cm i.d., S/V ) 19.6 cm ),
which was used as the reactive surface. It was cleaned prior to
each experiment. The glass coil was connected to a second
stripping coil in which soluble compounds, e.g., HONO and
HNO3, were removed by ultrapure water. The sampling ef-
ficiencies of the stripping coil for HONO and HNO3, measured
by an additional stripping coil, were found to be >97% and
>
99.9%, respectively, under the experimental conditions ap-
For studying reaction 2 the glass coil was first flushed with
HNO3 mixtures overnight. After determining the concentrations
of HNO3 and the upper limits for the impurities of HONO and
NO2 in the HNO3 mixtures, different amounts of NO were
added. Because the NO contained about 0.5% NO2 small
amounts of HONO were formed by reaction 1 in the flow system
behind the humidifier and in the second stripping coil. Accord-
ingly, the signals for HONO and NO2 were also measured for
plied. For the investigation of reaction 3 also a different
-1
borosilicate glass coil (l ) 58 cm, 0.2 cm i.d., S/V ) 20.0 cm )
and a borosilicate glass flow tube (lrxn ) 65 cm, 1.7 cm i.d.)
with a much lower surface-to-volume ratio of 0.72 cm ,
including the transfer line to the second stripping coil, were
employed. For the generation of HONO, a source similar to
that reported by Taira and Kanda31 was used. The HONO source
-
1

Reaction of Nitric Acid with Nitric Oxide on Glass
J. Phys. Chem. A, Vol. 108, No. 27, 2004 5795
TABLE 1: Summary of Experimental Conditions and
Results for the Investigation of Reaction 3a
1
7
RH
%)
HNO
(ppbV)
3
HONO
(ppbV)
∆NO
2
10 k(3)het
3
-1
(
(ppbV)
(cm s cm)
2
2
2
3
3
3
3
4
4
4
5
52.8
52.8
5
5
5
5
5.3
5.3
5.3
3.9
3.9
3.9
3.9
9.3
9.3
9.3
2.8*
700
690
650
1000
1020
1020
1045
735
740
750
2360
2410
2330
630
615
620
150
160
160
285
290
295
50
170
315
20
3.1
10.2
17.9
1.3
15.0 ( 3.3
15.1 ( 2.8
15.0 ( 2.7
10.9 ( 3.3
11.7 ( 2.4
11.8 ( 2.1
10.7 ( 1.8
6.7 ( 1.7
7.8 ( 1.6
6.1 ( 1.2
7.1 ( 1.2
7.2 ( 1.0
7.0 ( 1.4
6.0 ( 1.3
5.7 ( 1.7
5.9 ( 1.4
6.1 ( 3.7
5.3 ( 2.2
5.4 ( 2.0
6.5 ( 2.1
5.8 ( 1.1
5.8 ( 0.9
5.6 ( 0.8
2.9 ( 1.0
2.9 ( 0.7
2.3 ( 0.6
58
4.1
195
385
87
290
570
270
270
28
595
91
300
97
13.8
25.2
2.5
10.0
15.3
9.7
27.2
2.6
13.5
1.9
6.6
0.5
1.6
3.3
1.1
3.0
10.1
19.7
1.5
4.8
7.6
Figure 2. Adsorption of HNO
3
on the glass coil surface as a function
of relative humidity (T ) 296 ( 1 K).
3.9
3.9
3.9
4.9
pure humidified NO mixtures at the same NO mixing ratios as
in the experiments with HNO3, for which the PFA T-piece with
the HNO3 source and the glass coil were removed from the
flow system. The amount of HONO and NO2 formed by reaction
54.9
54.9
315
620
28
5
5
5
5
7
7
7
7.2**
2
was calculated after subtraction of the signals of the pure NO
7.2**
7.2**
7.2**
8.4
8.4
8.4
84
mixtures. The errors of the formed HONO and NO2 were
calculated from the errors of the HONO and NO2 concentrations
of both the reaction mixture and the pure NO mixture.
280
550
120
390
770
305
715
720
730
In separate experiments, reaction 3 was also studied using
the setup shown in Figure 1 with HONO and HNO3 mixing
ratios in the range 30-770 and 150-2400 ppbV, respectively.
In addition, the relative humidity was varied between 25 and
a
Relative humidity (RH), initial HNO
3
and HONO mixing ratios,
amount of NO
2
formed, and heterogeneous rate constant for reaction 3
-
1
in a borosilicate glass coil (l ) 160 cm, S/V ) 19.6 cm , trxn ) 0.62
7
8%. The rate constant of reaction 3 was calculated from the
s, T ) 296 ( 1 K). (*): length of the glass coil: 58 cm (S/V ) 20.0
NO2 formed after subtraction of the blank signal, when the glass
coil was removed from the system. The error of the rate constant
was calculated from the accuracy of the concentrations of
HONO, HNO3 and NO2 and the errors of trxn and S/V; see eq
I. To demonstrate that reaction 3 is a heterogeneous process,
the surface-to-volume ratio of the reactive surface was changed
by a factor of ∼30 by using a flow tube with a larger inner
diameter made of the same type of glass as used before.
-
1
cm , trxn ) 0.22 s, T ) 296 ( 1 K); (**): flow tube (lrxn ) 65 cm,
-1
S/V ) 0.72 cm , trxn ) 61 s, T ) 295 ( 1 K).
The adsorption was also studied for different HNO3 mixing
ratios at ∼50% RH. Upon increasing the HNO3 mixing ratio
from 145 to 2250 ppbV, the amount of adsorbed HNO3
increased only by a factor of 2 (cf. Figure 2).
In summary, for the experimental conditions applied in the
In additional experiments, the amount of HNO3 adsorbed on
the glass coil was measured by ion chromatography at different
humidities (0.5-83% RH) and different HNO3 mixing ratios
study of reactions 2 and 3 a surface coverage of HNO of ∼1
3
monolayer was determined.
3.2. Investigation of Reaction 3. In several previous studies
it was concluded that only small steady-state concentrations of
HONO were observed for reaction 2 due to the much faster
secondary reaction 3 leading to NO2 as the final product.2
To examine whether the small upper limit for HONO formation
by reaction 2 (see section 3.3) was influenced by secondary
chemistry, reaction 3 was also investigated. Because reaction 3
was found to be heterogeneous and thus dependent on the
(
145-2250 ppbV) by flushing the coil several times with
ultrapure water.
2-25,30
3
. Results and Discussion
.1. HNO3 Adsorption on the Glass Coil. Because reactions
3
2
and 3 are proposed to be heterogeneous processes, the amount
27
of HNO3 adsorbed on the surface is of potential importance for
a mechanistic interpretation of the reactions. For this reason,
the adsorption of HNO3 was studied for the same glass coil,
which was used for studying reactions 2 and 3 for different
humidities and HNO3 concentrations.
surface properties, the same experimental setup as for the study
of reaction 2 was used.
During the experiments significant amounts of NO were
2
formed (see Table 1). The rate of NO formation increased
2
linearly with the HONO and the HNO concentration as well
3
With ∼600 ppbV HNO3 present at relative humidity levels
as with the surface-to-volume ratio (S/V), which is in good
agreement with the study of Kaiser and Wu. Thus, for the
data evaluation, reaction 3 was treated as a second-order surface
reaction. The rate constant k(3)het was calculated using eq I
1
4
-2
27
of 0.5-78% a surface coverage of ∼(2-3) × 10 cm was
determined, which corresponds to approximately one monolayer
of HNO3 on the surface.25 Under these conditions the amount
of HNO3 adsorbed on the surface does not significantly depend
on the relative humidity in the range 0.5-60% (cf. Figure 2).
Only for higher relative humidities was an increase of the surface
coverage observed, which is readily explained using the results
reported by Saliba et al.24 The authors demonstrated that below
[HNO
ln
HNO ] - [HONO] [HNO ] [HONO] t S/V
]
[HONO]
1
3
t
0
1
1
k(3)het
)
[
3
0
0
3 0
t
rxn
(I)
∼
50% RH HNO3 remains undissociated on the surface and
ionizes at higher relative humidity levels, leading to the observed
higher adsorption.
The measured concentrations [HNO3]0 and [HONO]0, which
were determined during the blank experiments when the glass
coil was removed, were not used here, because the changes

5796 J. Phys. Chem. A, Vol. 108, No. 27, 2004
Kleffmann et al.
Figure 4. Typical plot of the HONO mixing ratio as a function of the
NO mixing ratio for pure NO (blank) and for reacting HNO
3
+ NO
Figure 3. Humidity dependence of the heterogeneous rate constant
k(3)het (T ) 296 ( 1 K).
mixtures. ([HNO ] ) 750 ppbV, RH ) 30%, T ) 298 ( 1 K).
3
caused by reaction 3 were smaller than the precision of the
HNO3 and HONO measurements. The values of the initial
concentrations [HNO3]0 and [HONO]0 were calculated from the
measured concentrations [HNO3]t and [HONO]t, after reaction
time trxn in the glass coil, and from the amount of NO2 formed
at trxn, taking into account the stoichiometry of reaction 3. In
Table 1, all experimental results for reaction 3 are summarized.
Within experimental error the rate constant of reaction 3 is
independent of the concentrations of HONO and HNO3 and of
the surface-to-volume ratio at constant humidity. In contrast,
for increasing relative humidity a significant decrease of the
rate constant k(3)het was observed (cf. Figure 3). An exponential
fit of the data points yields eq II for k(3)het
Figure 5. HONO and NO
corrected for the blank signals of pure NO as a function of the NO
mixing ratio. ([HNO ] ) 750 ppbV, RH ) 30%, T ) 298 ( 1 K).
2 3
mixing ratios in the reaction HNO + NO
2
96(1K
-16
-2
k(3)het
) 3.39 × 10 exp(-3.19 × 10 RH)
3 -1
3
(cm s cm) (II)
0
.5 and 10 ppmV was added to HNO3 mixtures of 350-750
Under the experimental conditions applied when studying
reaction 2, i.e., the reaction surface, relative humidity, [HNO3],
and reaction time, the rate constant of reaction 3 is determined
ppbV at constant relative humidity. In addition, blank experi-
ments with NO present but in the absence of HNO3 were
performed at the same humidity and NO mixing ratios. As an
example, the HONO formation for an experiment at 30% relative
humidity is shown in Figure 4. The HONO concentration
increases linearly with the NO concentration for both pure NO
and the HNO3 + NO mixture.
After subtraction of the blank signals, i.e., HONO and NO2
in pure NO mixtures, from signals obtained with the reacting
HNO3 + NO mixtures, mixing ratios in the range (0.15 ppbV
for HONO and (4 ppbV for NO2 were determined. Figure 5
shows the results from an experiment conducted at 30% RH.
The experimental uncertainty was significantly larger than the
detection limits of the instruments of ∼0.05 and 0.2-0.6 ppbV
for HONO and NO2, respectively. This was caused by the
subtraction of relatively large signals obtained for HONO and
NO2 from the pure NO mixtures. Within the experimental
accuracy the corrected HONO and NO2 levels were almost
independent of the NO concentration present (cf. Figure 5).
From the data shown in Figure 5 reactive uptake coefficients
γrxn can be calculated. The reactive uptake coefficient is the
ratio between the reactive collisions of NO (ωrxn) divided by
the total number of wall collisions of NO (ωgaskinetic) per unit
surface and time
-
17
3
-1
to be in the range k(3)het ) (2-15) × 10
cm s cm (85-
2
1% RH). For comparison, the second-order gas-phase rate
2
7
constants reported earlier in the studies of Kaiser and Wu,
2
8
32
Streit et al., England and Cocoran, and Wallington and
Japar33 were converted to heterogeneous rate constants using
the corresponding surface-to-volume ratio. Values of k(3)het of
-
17 27
-17 28
-17 32
(
2.5-8.6) × 10
,
4.8 × 10
,
2.7 × 10
,
and 5.0 ×
-18 33
3
-1
27
10
,
cm s cm were determined. Kaiser and Wu reported
that the rate of reaction 3 depended on the surface properties.
For example, a 3.5 times higher rate constant was observed when
27
a new untreated reactor was used. With the exception of the
study of Wallington and Japar,33 all literature values are in
excellent agreement within the range of rate constants obtained
in the present study, which confirms that reaction 3 is indeed a
heterogeneous process. Wallington and Japar33 determined the
rate constant from the observed decay of HONO. However, the
authors reported a much faster decay of HNO3, which could
not be explained by the HNO3 wall loss. Because possible
HONO formation processes, for example, by reaction 1, were
not taken into consideration, the value determined in this study
appears to be too low.
It is concluded that only a minor fraction of the HONO
formed by reaction 2 is converted into NO2 by reaction 3 using
identical experimental conditions as applied in the study of
reaction 2; see below for details.
ωrxn
[NO]rxnV
coil
4
γrxn
)
)
(III)
(
)(
)
ωgaskinetic
Scoil∆trxn [NO]ini Vj NO
3
.3. Investigation of Reaction 2. Reaction 2 was investigated
in different experiments at relative humidities in the range 21-
6%. In each set of experiments, NO at mixing ratios between
Vcoil and Scoil denote the volume and surface of the glass coil,
respectively, ∆trxn is the reaction time of a gas molecule in the
glass coil and Vj NO is the mean velocity of NO molecules. Due
8

Reaction of Nitric Acid with Nitric Oxide on Glass
J. Phys. Chem. A, Vol. 108, No. 27, 2004 5797
TABLE 2: Summary of Experimental Conditions and
Results for the Investigation of Reaction 2a
1
1
10
RH
T
HNO
3
range NO
(ppbV)
10 γ(NOf 10 γ(NOf
HONO) NO
(%) (K) (ppbV)
2
)
21.3 303
30.3 298
43.3 303
49.5 299
54.1 297
61.3 297
78.9 301
85.9 299
710
760
615
550
350
550
550
560
1250-9600 -5.1 ( 7.7
0.4 ( 5.0
4 ( 26
620-5050
5.7 ( 4.5
3.4 ( 4.5
1450-9500
-10.5 ( 8.6
-11 ( 43
640-5100 -2.9 ( 3.8
510-4350
630-5100 -1.5 ( 3.5
1400-9200 0.2 ( 8.3
1.9 ( 6.0
-34 ( 17
2.1 ( 6.7
620-5000 -3.1 ( 12.8 42 ( 57
a
3
Relative humidity (RH), temperature, HNO mixing ratio, range
of NO mixing ratios, and reactive uptake coefficients of NO for reaction
Figure 7. Comparison of literature data for uptake coefficients of NO
2
0
in a borosilicate glass coil (l ) 160 cm, S/V ) 19.6 cm-1, trxn
)
3 3
on surfaces saturated with HNO as function of the HNO concentration
.5-0.7 s). The error limits for the uptake coefficients represent the
with the upper limit obtained in the present study.
statistical precision (2σ) of the linear least-squares fits as shown in
Figure 5.
for NO2 formation is caused by the lower sensitivity of the NO2
instrument and the higher blank values for NO2.
From the values of the rate constant of reaction 3 (see section
3
.2) it is calculated that only 0.5-4% of the HONO (86-21%
RH) possibly formed in reaction 2 is converted into NO2 by
reaction 3. Thus, the much lower value of γ(NO f HONO) <
-
11
4
× 10
is representative for reaction 2 and not significantly
influenced by secondary chemistry.
Reaction 2 was investigated in several other studies.2
2-30
The
rate of the reaction was found to be proportional to [HNO3]
26-30
26,27
and [NO]
and to the surface-to-volume ratio.
In addition,
the reaction was reported to be dependent on the relative
2
4,26
26
humidity
and on the NO2 concentration. Due to the
complex reaction kinetics any extrapolation of laboratory results
2
2-25,30
to atmospheric conditions
is highly uncertain. In the
Figure 6. Reactive uptake coefficients γ(NOfHONO) and γ-
present work, much lower NOy concentrations as compared to
all other studies were used at atmospheric relative humidity
levels. An upper limit of the reactive uptake coefficient of γ-
2
(NOfNO ) as a function of relative humidity. The errors bars represent
the statistical precision (2σ) of the linear least-squares fits as shown in
Figure 5. (HNO mixing ratio ) 350-750 ppbV, T ) 300 ( 3 K).
3
-
11
(
NOfHONO) < 4 × 10
is derived for reaction 2. For
comparison with literature data, the available rate constants were
converted to reactive uptake coefficients using the corresponding
surface-to-volume ratios and under the assumption of a first-
order NO dependence of reaction 2. With the exception of the
experiments of Saliba et al. and Rivera-Figueroa et al. in
which the gas phase concentrations of nitric acid were not
explicitly specified, most studies were performed under equi-
to the small uptake coefficients (see below), limitation by gas-
phase diffusion to the walls was not considered here. Reactive
uptake coefficients γ(NOfHONO) and γ(NOfNO2) were
determined corresponding to the stoichiometry of reaction 2,
i.e., formation of one molecule of HONO and NO2 per reacted
NO molecule. To obtain a higher accuracy, γ(NOfHONO) and
γ(NOfNO2) on saturated glass surfaces at [HNO3] ≈ 600 ppbV
were calculated from the slopes (mHONO ) ∆[HONO]/∆[NO]
2
4
25
2
6-30
librium conditions;
i.e., high gas-phase concentrations of
HNO3 were in equilibrium with the adsorbed HNO3. In the two
and mNO ) ∆[NO2]/∆[NO]) of the least-squares fits shown as
2
2
4,25
former studies,
a different approach was used: First, the
an example in Figure 5
reaction surfaces were dosed with HNO3 at very high gas-phase
1
7
-3
concentrations of about 10 molecules cm , and then some
of the adsorbed HNO3 was removed by evacuating the reactors.
Because the gas-phase concentrations of HNO3 were not
specified, both studies could not be included in the comparison.
In Figure 7, the reactive uptake coefficients from previous
studies are plotted in double logarithmic format as functions of
the HNO3 concentration. In addition, the upper limit determined
in the present study is given. Clearly, the reactive uptake
coefficient of NO is decreasing with decreasing HNO3 concen-
tration in accordance with the first-order HNO3 dependence of
reaction 2, as observed in most studies. All data are reasonably
well described by a linear fit yielding a slope of 1.01 ( 0.07
mHONO(mNO2)4Vcoil
γ(NOfHONO(NO )) )
(IV)
2
∆
treac Vj NOScoil
Equation IV is applicable under the assumption that reaction 2
is first order in NO, which has been observed in most previous
studies of this reaction. The obtained reactive uptake coefficients
for all experiments are listed in Table 2 and plotted in Figure
6
as a function of the relative humidity.
Mean values of the uptake coefficients of
-
11
γ(NOfHONO) ) (-0.2 ( 3.7) × 10
and
28
(
cf. Figure 7). In the study of Streit et al., significantly smaller
-
9
γ(NOfNO ) ) (0.2 ( 2.3) × 10
2
uptake coefficients compared to all other values are calculated.
This discrepancy was already discussed in the paper of Svensson
30
(
error limit 1σ) are determined, leading to upper limits of γ-
and Ljungstr o¨ m and remains to be resolved. However, because
the reaction was reported to be dependent on the relative
-
11
-9
(NOfHONO) < 4 × 10
and γ(NOfNO2) < 2.5 × 10
2
4,26
26
26,27
for the experimental conditions applied. Both coefficients are
independent of the relative humidity. The much higher value
humidity,
autocatalytic in [NO2] and heterogeneous,
deviations from the fit shown in Figure 7 may be explained by

5798 J. Phys. Chem. A, Vol. 108, No. 27, 2004
Kleffmann et al.
the experimental conditions applied in the Streit et al.28 study,
e.g., different surface properties and relative humidities.
4.2. Reaction 3. For the heterogeneous reaction 3 rate
-
17
3
-1
constants in the range k(3)het ) (2-15) × 10
cm s cm
have been determined in the humidity range 85-21% RH for
HONO and HNO3 mixing ratios in the range 30-770 and 150-
2400 ppbV, respectively. It follows that reaction 3 is an
unimportant HONO sink in the atmosphere under the assumption
that borosilicate glass is a representative proxy for environmental
4
. Atmospheric Implication
.1. Reaction 2. Due to the dependence of the reactive uptake
4
coefficient of NO for reaction 2 on the HNO3 concentration as
shown in Figure 7, it is concluded that the upper limit of γ-
-
11
-17
3
(
NOfHONO) < 4 × 10 , determined in this study at the
surfaces. Even for an upper limit of k(3) ) 15 × 10
cm
het
-
1
lowest NOy concentrations reported so far, is representative for
reaction 2 under atmospheric conditions. The value should be
strictly considered as an upper limit, because even in the present
study, the NOy concentrations used were still much higher than
values typically observed in the urban atmosphere. It follows
that reaction 2 is insignificant for both heterogeneous HONO
formation and possible “renoxification” processes in the atmo-
s
cm, a boundary layer height of 100 m, and a maximum
HNO mixing ratio of 50 ppbV, only 0.05% of the HONO
3
4
3
present will be converted by reaction 3 during an 8 h night.
34
5. Conclusion
The heterogeneous reaction of HNO3 with NO was studied
on borosilicate glass surfaces in a flow system at various relative
humidities (21-86% RH) and concentrations closer to atmo-
spheric levels as compared to all previous studies reported in
the literature. An upper limit of the reactive uptake coefficient
of NO on glass surfaces, covered with ∼1 monolayer of HNO3,
sphere, in good agreement with the conclusion of Svensson an
Ljungstr o¨ m.30 However, this conclusion is only valid under the
assumption that the glass surfaces, which were used in most
studies are representative for environmental surfaces, as has been
9
proposed, e.g., in the studies of Finlayson-Pitts et al. and
-
11
Rivera-Figueroa et al.25
of γ(NOfHONO) < 4.0 × 10 is determined for reaction 2.
This value is significantly lower compared to studies performed
at higher nitrogen oxide concentrations. In the present study
also the heterogeneous reaction of HONO with HNO3, reaction
3, was studied under identical experimental conditions. The
second-order rate constant is decreasing with increasing humid-
ity and is found to be in good agreement with literature values.
From the measured rate constants of reaction 3 it is concluded
In contrast hereto, reaction 2 was recently proposed to be of
2
2,23
potential importance for atmospheric HONO formation
and
24,25
for a “renoxification” of the atmosphere.
In the studies of
Saliba et al.24 and Rivera-Figueroa et al., reactive uptake
25
-9
-
8
coefficients of NO in the range 10 to 10 were obtained at
higher reactant concentrations. However, even these values are
1
order of magnitude lower than the reactive uptake coefficients
-
11
-
8
-6
that the upper limit for HONO formation of <4.0 × 10
is
of NO2 of 10 to 10 for reaction 1 as obtained in the
laboratory for atmospheric humidity levels.
of Rivera-Figueroa et al., it was speculated that uptake
coefficients of 10
4,5,6,7
representative for reaction 2 and that this value is not signifi-
cantly influenced by secondary chemistry, i.e., reaction 3. In
the case that the reaction kinetics observed for glass surfaces is
similar to that on environmental surfaces, reaction 2 appears to
be not of importance for atmospheric HONO formation and for
a “renoxification” of the atmosphere.
In the study
25
-
9
-8
to 10
for reaction 2 could be of
importance, due to a high BET surface of the ground. However,
this argument would also hold for reaction 1, turning the latter
into a much stronger HONO source. This argument is supported
by field measurements in which significant HONO formation
Acknowledgment. The financial support by the European
Commission in the framework V program, Contract No. EVK2-
CT-1999-00025 (NITROCAT) is gratefully acknowledged.
2
0,21
was observed in the atmosphere in the absence of NO.
In
7
addition, in a study by Kleffmann et al., heterogeneous HONO
formation by reaction 1 was not affected when high concentra-
tions of NO were added to NO2 mixtures in a quartz glass reactor
under relative humidity and NO2 concentration levels prevailing
in the atmosphere. Because it can be expected that high amounts
of HNO3 formed by reaction 1 from several prior experiments
were adsorbed on this reactor surface, as observed by other
References and Notes
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(
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,9
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