Heterogeneous chemistry of HONO on liquid sulfuric acid: A new mechanism of chlorine activation on stratospheric sulfate aerosols

Article abstract of DOI:10.1021/jp952060a

Heterogeneous chemistry of nitrous acid (HONO) on liquid sulfuric acid (H₂SO₄) was investigated at conditions that prevail in the stratosphere. The measured uptake coefficient (γ) of HONO on H₂SO₄ increased with

Full text of DOI:10.1021/jp952060a

J. Phys. Chem. 1996, 100, 339-345
339
Heterogeneous Chemistry of HONO on Liquid Sulfuric Acid: A New Mechanism of
Chlorine Activation on Stratospheric Sulfate Aerosols
Renyi Zhang,* Ming-Taun Leu,* and Leon F. Keyser
Earth and Space Sciences DiVision, Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California 91109
ReceiVed: July 21, 1995; In Final Form: September 26, 1995
Heterogeneous chemistry of nitrous acid (HONO) on liquid sulfuric acid (H2SO4) was investigated at conditions
that prevail in the stratosphere. The measured uptake coefficient (γ) of HONO on H2SO4 increased with
increasing acid content, ranging from 0.03 for 65 wt % to about 0.1 for 74 wt %. In the aqueous phase,
HONO underwent irreversible reaction with H2SO4 to form nitrosylsulfuric acid (NO+HSO4 ). At temperatures
-
below 230 K, NO+HSO4 was observed to be stable and accumulated in concentrated solutions (>70 wt %
-
H2SO4) but was unstable and quickly regenerated HONO in dilute solutions (<70 wt %). HCl reacted with
HONO dissolved in sulfuric acid, releasing gaseous nitrosyl chloride (ClNO). The reaction probability between
HCl and HONO varied from 0.01 to 0.02 for 60-72 wt % H2SO4. In the stratosphere, ClNO photodissociates
rapidly to yield atomic chlorine, which catalytically destroys ozone. Analysis of the laboratory data reveals
that the reaction of HCl with HONO on sulfate aerosols can affect stratospheric ozone balance during elevated
sulfuric acid loadings after volcanic eruptions or due to emissions from the projected high-speed civil transport
(HSCT). The present results may have important implications on the assessment of environmental acceptability
of HSCT.
Introduction
are suggestions of HONO formation due to NO2 hydrolysis on
aqueous droplets17 and reduction of HNO3 by SO2 and by
Surface-catalyzed reactions occurring on sulfate aerosols
enhance ozone destruction by reactive chlorine radicals (Cl and
ClO) in the stratosphere.1,2 Heterogeneous reactions involving
ClONO2, HCl, and H2O, for example, transform the inactive
chlorine reservoir species (i.e., ClONO2 and HCl) into forms
(HOCl and Cl2) that are easily photolyzed to produce atomic
chlorine ;3-5 ozone destruction then occurs in catalytic cycles
initiated by Cl, via the ClO dimer mechanism6,7 or the reaction
between ClO and BrO.8 Hydrolysis of N2O5 on sulfate aerosols
also contributes to the global ozone destruction by deactivating
odd nitrogen species (NOx) during elevated sulfuric acid loading
events.9-11 In particular, this reaction has been proposed to
mitigate the environmental impact of the projected fleet of high-
speed civil transport (HSCT) by converting the nitrogen oxide
effluent into stable HNO3.12-14
bromine ions (Br-) in acidic particles.18
HONO is produced by aircraft emissions due to aircraft
engine combustion. Arnold et al.19 reported in situ measure-
ments of trace species inside a DC-9 contrail around 10 km:
the measured HONO concentrations inside the DC-9 plume were
∼ 5 × 109 molecules cm-3, about 2 orders of magnitude higher
than the background values. In addition, sulfuric acid aerosols
nucleate homogeneously inside aircraft plumes due to SO2
emissions.20 In situ aerosol measurements of Concorde super-
sonic aircraft plumes confirm the existence of high aerosol
densities inside aircraft plumes, with particles most likely
consisting of 70-80 wt % H2SO4.21 Hence, heterogeneous
processes involving HONO on sulfuric acid particles may
influence the plume chemistry, particularly in terms of hydrogen
oxides HOx.
Both homogeneous and heterogeneous chemistry of HONO
could have important implications in atmospheric chemistry.
In the troposphere, nitrous acid is recognized as a key species
in both indoor and outdoor air pollution. The gas-phase
chemistry of HONO plays an important role in the formation
of hydroxyl radicals (OH), since HONO photolyzes rapidly to
produce OH compared to ozone and formaldehyde photolysis.
Because of its rapid photolysis at daytime, elevated concentra-
tions of HONO have normally been observed only at night
ranging up to a few ppbv at polluted sites, but recent observa-
tions reveal that daytime steady state HONO concentrations of
100-500 pptv have been observed in the clean troposphere.15
Despite its potential importance in atmospheric chemistry, the
formation mechanism of HONO is not well established.16
Homogeneous gas-phase reactions involving HOx and NOx are
apparently too slow to account for the observed high levels of
HONO. It is generally believed that heterogeneous processes
are largely responsible for the HONO generation, among which
Recent stratospheric measurements also suggest that pho-
tolysis of HONO could explain the observed anomalous OH
and HO2 concentrations shortly after sunrise,14,22 although the
exact formation mechanism of HONO is still questionable. We
have recently reported laboratory studies of heterogeneous
interaction of peroxynitric acid (HO2NO2, PNA) with sulfuric
acid and excluded the possibility that HONO is formed by
heterogeneous decomposition of PNA on aerosols.23
Burley and Johnston postulated that stratospheric sulfate
aerosols may contain nitrosylsulfuric acid (NO+HSO4-).24 They
suggested that the heterogeneous reaction between HCl and
-
NO+HSO4 may proceed on sulfuric acid particles, releasing
gaseous nitrosyl chloride (ClNO). This process potentially
provides a mechanism for chlorine activation in the stratosphere.
At present, assessments of the role of nitrous acid in
atmospheric heterogeneous chemistry are limited by the lack
of quantitative kinetic information. In this paper we investigate
the interaction of gas-phase nitrous acid with liquid sulfuric acid
and subsequent reaction with HCl to form nitrosyl chloride. We
present laboratory measurements of the HONO uptake coef-
* To whom correspondence should be addressed.
Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0339$12.00/0 © 1996 American Chemical Society
+
+

340 J. Phys. Chem., Vol. 100, No. 1, 1996
Zhang et al.
ficient and the reaction probability of HCl with HONO on
sulfuric acid. Analysis of the laboratory data reveals that, at
elevated sulfuric acid loadings such as that after the eruption
of Mt. Pinatubo, this process could result in an altered chlorine
budget and an increased abundance of active chlorine. Fur-
thermore, our results imply that heterogeneous processes involv-
ing HONO on sulfate aerosols play a key role in regulating the
plume chemistry and can perturb the ozone balance.
Experimental Section
Measurements of heterogeneous reactions were performed
using a fast flow reactor in conjunction with chemical ionization
mass spectrometer (CIMS) detection. The general experimental
apparatus and procedures used here have been described
previously,23,25-27 and only a brief description is given along
with details specific to this work.
A horizontally mounted flow reactor of inner diameter 2.8
cm was doubly jacketed and temperature regulated. Liquid
sulfuric acid films were prepared by totally covering the inside
wall of the flow reactor with acid solutions. The thickness of
the liquid film was estimated to be about 0.1 mm. Temperature
and water partial pressure in the flow reactor were adjusted to
form acid compositions representative of stratospheric sulfate
aerosols. For most experiments reported here, the water partial
pressure was maintained closely at ∼5 × 10-4 Torr. As the
temperature was varied from 207 to 230 K, the acid composition
changed from 60 to 75 wt %, estimated from the vapor pressures
of the H2SO4/H2O binary system.28,29 Typical experimental
conditions were total pressure of 0.4 Torr and carrier gas flow
-
Figure 1. Mass spectrum of SF6 reaction with the effluent from a
HONO bubbler. F-‚HONO was formed by a fluoride ion transfer from
SF6- to HONO. Impurity in the HONO sample was recognized mainly
as NO2 (NO2-, m/e ) 46). Some fluoride ions were also detected in
the CIMS, possibly formed by electron impact of SF6.
reactor was estimated by assuming the same rate coefficient as
that for HNO3 reaction with SF6- and by comparing the relative
signal intensities between the two species under identical
-
conditions.23 HCl and ClNO were detected using SF6 corre-
sponding to F-‚HCl (m/e ) 55) and Cl- (m/e ) 35),
respectively.32 A likely complication in the detection of ClNO
came from the presence of some fluoride ion (F-, m/e ) 19),
velocity of 1700-2000 cm s-1
.
The uptake coefficient was determined from the decay
corresponding to the reactant loss, using the standard cylindrical
flow tube analysis. To account for the reactant radial and axial
gradient in the flow reactor which arose when there was a large
reactant wall loss, the observed first-order rate coefficient was
corrected for gas-phase diffusion restrictions according to the
method suggested by Brown.30 The diffusion coefficients of
HONO and HCl in helium were estimated to be 300 and 350
Torr cm2 s-1 at 220 K, respectively, both with a temperature
-
possibly formed by SF6 fragmentation (Figure 1), since the
reaction between HCl and F- also yielded Cl-. These F-
concentrations, however, were too low to result in any ap-
preciable amount of Cl- and thus did not interfere with the
ClNO detection. As shown in Figure 1, the HONO source
contained an impurity as nitrogen dioxide, NO2 (detected as
NO2-, m/e ) 46), but we did not observe HNO3. (HNO3 can
be detected as F-‚HNO3 using SF6-). Nitric oxide (NO) could
also be present in the HONO sample but was undetectable in
the CIMS due to its small electron affinity. The presence of
NO and NO2, however, did not affect the present results (Zhang,
Leu, and Keyser, unpublished results). Partial pressures of the
reactants (HONO and HCl) were maintained at (3-5) × 10-7
Torr. The low reactant concentrations were essential to
minimize the occurrence of secondary reactions of the product
ions.
dependence of T1.75 31
The systematic error in the present
.
measurements was estimated to be (20%, including uncertain-
ties in acid composition, temperature, pressure, flow rate, and
the correction associated with gas-phase diffusion.
HONO was synthesized by slowly adding ∼5 mL of 0.1 M
NaNO2 to ∼10 mL of 40 wt % H2SO4 which was chilled (at
273 K) and vigorously stirred. The solution containing HONO
was then transferred to a bubbler maintained at 273 K. Gaseous
HONO was added to the flow tube along with a small He flow
(0.1-10.0 cm3 min-1 at STP) and further diluted in the main
He flow (about 300 cm3 min-1 at STP) before contacting the
liquid surface. An unjacketed movable injector was used to
deliver HONO into the flow reactor. At low temperatures
(<230 K), H2O (which was also eluded from the HONO source)
condensed inside the cold injector and, thus, did not change
the composition of the liquid film.
Results
HONO Uptake on Sulfuric Acid. Figure 2 displays the
temporal evolution of the HONO signal as it was exposed to a
sulfuric acid film at three different temperatures. The water
partial pressure used in these experiments was about 5 × 10-4
Torr. The resultant sulfuric acid contents were 72, 65, and 61
wt % at temperatures of 224.6, 213.5, and 209.0 K, respectively.
In 61 wt % sulfuric acid (Figure 2c), the gas-phase concentration
of HONO dropped instantly upon exposure to H2SO4 at 0.5 min,
due to HONO adsorption into the liquid; the signal later returned
to its original value as the film was saturated. Terminating the
exposure at 2 min resulted in an opposite peak due to desorption.
In contrast, loss of HONO in 72 wt % sulfuric acid (Figure 2a)
was more pronounced and time independent, characteristic of
irreversible reactions occurring in the aqueous phase. No
saturation occurred for even longer durations (t > 1 h), nor was
Gas-phase concentrations of the reactants and products were
simultaneously monitored by the CIMS. Figure 1 shows a mass
-
spectrum of SF6 reaction with the effluent from the HONO
bubbler. HONO was detected in the CIMS as F-‚HONO (m/e
-
) 66), produced by a fluoride ion transfer with SF6
,
SF6- + HONO F-·HONO + SF5
(1)
We are not aware of any measurements of the rate coefficient
for reaction 1. The HONO concentration in the neutral flow
+
+

HONO on Liquid Sulfuric Acid
J. Phys. Chem., Vol. 100, No. 1, 1996 341
Figure 4. Uptake coefficient (γ) of HONO on liquid sulfuric acid as
a function of temperature at PH O ) 5.0 × 10-4 Torr. The estimated
acid content (top axis) ranged2from about 65 to 75 wt %, as the
temperature was varied from 213 to 230 K. Each point in the figure is
an average of more than three measurements. The error bars represent
one standard deviation of each determination. The solid line is a linear
least-squares fit through the data. Experimental conditions are similar
to those in Figure 2.
Figure 2. Variation of HONO signal as it was exposed and not exposed
to 5 cm length of (a) 72 wt % H2SO4 at 224.6 K, (b) 65 wt % H2SO4
at 213.5 K, and (c) 61 wt % H2SO4 at 209.0 K. The injector was moved
upstream at about 0.5 min and returned to its original position at 2
min. Experimental conditions: PHONO ≈ 5 × 10-7 Torr, PHe ) 0.4
0.051, and 0.016, corresponding to the acid contents of 73, 70,
and 65 wt %, respectively.
In Figure 4 are plotted uptake coefficients of HONO in
sulfuric acid against the temperature. In these experiments,
HONO uptake was studied by maintaining a constant water
partial pressure of about 5 × 10-4 Torr and by varying
temperature and allowing water vapor to equilibrate with the
liquid. The top axis labels the estimated sulfuric acid content.
Each point in the figure represents an average of more than
three measurements, with the vertical bars indicating one
standard deviation. The solid line is a linear least squares fit
of the data, which results in an expression of γ ) -0.9123 +
0.00435T for the temperature range 213-226 K. Figure 4
shows that the uptake coefficient decreases with decreasing acid
content: γ approaches 0.1 for about 73 wt % H2SO4, whereas
its value decreases by almost a factor of 5 for 65 wt % H2SO4.
This decline in γ occurs primarily because of evaporation of
HONO into the gas phase. For these experiments, it was
necessary to keep the exposure time of HONO to the liquid
film as short as possible, in order to minimize the saturation
effect and obtain a reproducible loss rate coefficient. In general,
physical solubility of HONO in sulfuric acid may depend on
the temperature and acid composition: it should increase with
decreasing temperature and decreasing acid composition. The
fact that our measured uptake coefficients of HONO in dilute
sulfuric acid ( 65 wt %) are less than that measured on pure
liquid water33 indicates that HONO uptake in less concentrated
sulfuric acid is determined by both aqueous phase reactions and
solubility of HONO. Results of the uptake coefficients of
HONO in sulfuric acid are summarized in Table 1.
Torr, PH O ≈ 5 × 10-4 Torr, and flow velocity ) 1700-1900 cm s-1
.
2
Figure 3. Plot of the HONO signal as a function of reaction distance
on three acid solutions: 73 wt % H2SO4 at 226 K (open triangles), 70
wt % H2SO4 at 220 K (filled circles), and 65 wt % H2SO4 at 213 K
(open squares). The lines are linear least-squares fits through the data.
Experimental conditions are similar to those in Figure 2.
any gas-phase product detected by the CIMS. For the inter-
mediate sulfuric acid content (65 wt %, Figure 2b), some
saturation was evident in the adsorption curve, but the HONO
signal was never completely recovered.
Figure 3 shows the loss of HONO as a function of injector
position as it was exposed to three sulfuric acid solutions: 73
wt % at 226 K (open triangles), 70 wt % at 220 K (filled circles),
and 65 wt % at 213 K (open squares). It is seen in Figure 3
that the HONO signal decreased exponentially as a function of
reaction distance in accord with first-order kinetics. The slope
of the decay curve was employed to derive the first-order rate
coefficient; these coefficients lead to uptake coefficients of 0.10,
Reaction of HCl with HONO. We studied the reaction
probability between HONO and HCl on liquid sulfuric acid.
The experiments were performed by first exposing the liquid
film to HONO vapor at PHONO ≈ 5 × 10-7 Torr for a few
minutes and then measuring the HCl uptake. In concentrated
H2SO4 (>70 wt %), it was virtually impossible to saturate the
liquid film at such a low HONO partial pressure, as noted above.
Because HCl also physically dissolves in sulfuric acid, it is
essential to differentiate the reactive uptake from the physical
+
+

342 J. Phys. Chem., Vol. 100, No. 1, 1996
Zhang et al.
TABLE 1: Summary of the Uptake Coefficient of HONO in
Sulfuric Acida
temp H2SO4,
(K)
temp H2SO4,
(K)
wt %b
γ ( 1σ
wt %b
γ ( 1σ
213.5
216.4
218.5
218.6
218.6
220.4
65.3
67.4
68.7
68.8
68.8
69.9
0.016 ( 0.0009 222.6
0.040 ( 0.0078 222.6
0.033 ( 0.0012 224.6
0.026 ( 0.0154 224.7
0.037 ( 0.0067 226.1
0.059 ( 0.0119
71.3
71.3
72.3
72.4
73.0
0.072 ( 0.0193
0.060 ( 0.0054
0.048 ( 0.0085
0.047 ( 0.0054
0.091 ( 0.0162
a The experiments were performed by maintaining constant water
partial pressure at ∼5 × 10-4 Torr and by regulating temperature
between 213 and 226 K. Each point is an average of more than three
measurements. Experimental conditions are PHe ) 0.4 Torr, V ) 1700-
2000 cm s-1, and PHONO ≈ 5 × 10-7 Torr. b Estimated from the
temperature and water partial pressure.
Figure 6. Plot of the HCl signal as a function of reaction distance on
three HONO-doped sulfuric acid solutions: 71 wt % at 223 K (open
squares), 68 wt % at 217 K (filled circles), and 62 wt % at 209 K
(open triangles). The lines are linear least-squares fits through the data.
Experimental conditions are similar to those in Figure 5.
Figure 5. Variation of the HCl signal when exposed to a 3.5 cm length
of HONO-doped sulfuric acid film at 222 K. The disappearance of
HCl upon exposure to H2SO4 at ∼0.7 min was accompanied by the
appearance of ClNO. The exposure was terminated at ∼2 min. The
acid content of the film was estimated to be ∼71 wt %. Experimental
conditions: PHONO ≈ 5 × 10-7 Torr, PHCl ) 3 × 10-7 Torr, PHe ) 0.4
Figure 7. Reaction probability (γ) of HCl with HONO dissolved in
sulfuric acid. The reaction probability was obtained from the observed
HCl decay rate, when HCl was exposed to HONO-doped sulfuric acid.
Each point in the figure is an average of more than three measurements.
The error bars represent one standard deviation of each determination.
Experimental conditions are similar to those in Figure 5.
Torr, PH O ≈ 5 × 10-4 Torr, and flow velocity ) 1700-1900 cm s-1
.
2
uptake of HCl, when calculating the first-order loss coefficient.
In general, HCl physical uptake in sulfuric acid depends on both
the temperature and acidity: for temperatures between 210 and
230 K and acid contents between 60 and 75 wt % the time
scale for saturating an acid film with HCl vapor would typically
be less than a few minutes.5,33 Thus, as long as sufficient time
is allowed for the system to reach equilibrium, the reaction
probability between HONO and HCl can be accurately deter-
mined on the basis of the HCl decay. HCl vapor was introduced
into the flow reactor through an unjacketed injector.
Figure 5 shows that loss of HCl on 71 wt % H2SO4 is
irreversible and time-independent, which notably differs from
the HCl uptake in absence of HONO.5,34 The disappearance
of HCl upon exposure to H2SO4 inversely correlated with the
appearance of ClNO, demonstrating that ClNO was liberated
into the gas phase via the reaction of HCl with HONO dissolved
in sulfuric acid,
of 10 because the rate coefficient for SF6- reaction with ClNO
is about an order of magnitude smaller than that of SF6- reaction
with HCl.32
Figure 6 is a semilog plot of measured HCl signal versus
injector position. In all the three cases, the decay of HCl
followed the first-order rate law. The observed first-order
coefficients for HCl gave reaction probabilities of 0.022, 0.016,
and 0.017, corresponding to 71, 68, and 62 wt % H2SO4,
respectively. Thus, the reaction probability between HONO and
HCl on sulfuric acid did not change significantly over the
composition range 62-71 wt %.
In Figure 7, values of reaction probability calculated from
experiments such as those displayed in Figure 6 are presented
as function of temperature at PH O ≈ 5 × 10-4 Torr. The HCl
2
vapor pressure was maintained at 3 × 10-7 Torr. Contrary to
the uptake coefficient of HONO on sulfuric acid, which
increases monotonically with the acid content (Figure 4), the
reaction probability between HCl and HONO may attain a
minimum at compositions around 65 wt %. A likely explanation
is that HCl solubility increases at low temperatures and in dilute
HCl(g) + HONO(aq) ClNO(g) + H2O(l)
(2)
Calibration of the relative detection sensitivities for HCl and
ClNO in the CIMS gave a ClNO yield of nearly unity due to
HCl loss. Note that the ClNO signal was multiplied by a factor
+
+

HONO on Liquid Sulfuric Acid
J. Phys. Chem., Vol. 100, No. 1, 1996 343
TABLE 2: Summary of the Reaction Probability of HCl
with HONO Dissolved in Sulfuric Acida
HONO uptake was barely measurable, indicating very small
solubility of HONO in sulfuric acid. The laboratory observa-
-
tions also showed that NO+HSO4 was stable in concentrated
temp H2SO4,
(K)
temp H2SO4,
(K)
wt %b
γ ( 1σ
wt %b
γ ( 1σ
H2SO4 (>72 wt %) and accumulated in the liquid (Figure 2c)
at temperatures below ∼230 K. We observed no precipitation
of solid nitrosylsulfuric acid crystals when continuously expos-
ing HONO to H2SO4 over 1 h, probably due to small HONO
concentrations used in the experiments (typically about 5 × 10-7
Torr). Solubility of NO+HSO4- in sulfuric acid was estimated
to be about a few percent by weight for stratospheric condi-
tions.24
207.9
209.1
213.2
217.3
60.8
61.7
65.1
68.0
0.020 ( 0.0049 218.6
0.018 ( 0.0030 220.4
0.011 ( 0.0040 222.6
0.016 ( 0.0010
68.8
69.9
71.3
0.016 ( 0.0008
0.019 ( 0.0014
0.020 ( 0.0019
a The experiments were performed by maintaining constant water
partial pressure at ∼ 5 × 10-4 Torr and by regulating temperature
between 208 and 223 K. Each point is an average of more than three
measurements. Experimental conditions are PHe ) 0.4 Torr, V ) 1700-
2000 cm s-1, PHONO ≈ 5 × 10-7 Torr, and PHCl ) 3 × 10-7 Torr.
b Estimated from the temperature and water partial pressure.
In Figure 4 the uptake coefficient (γ) of HONO increases as
the temperature increases (at a fixed H2O partial pressure) or
as the acid content increases. This apparently reflects instability
of NO+HSO4- in dilute sulfuric acid and subsequent regenera-
tion and evaporation of HONO, consistent with the ionic
mechanism noted above.
sulfuric acid (the Henry’s law solubility constant of HCl
increases by more than an order of magnitude for the acid
compositions from 75 to 65 wt % at a constant water partial
pressure)5,34 and, thus, leads to an increase in γ at low
temperatures. Alternatively, it is also plausible that the low
values of γ are due to scatter in the data. Results of γ’s for
reaction 2 are listed in Table 2.
A first-order rate coefficient (kI) for the reaction of HONO
with H2SO4 can be determined from the measured uptake
coefficient together with the estimated Henry’s law solubility
constant (H) and liquid-phase diffusion coefficient (Dl)36,37
γ ≈ 4RTH kID
Discussion
x
l
(7)
HONO Uptake on Sulfuric Acid. In general, HONO uptake
in sulfuric acid exhibited two distinct features, dependent on
the acidity: HONO uptake in more dilute sulfuric acid (<61
wt %) appeared to comprise several steps, including adsorption,
saturation, and desorption; loss of HONO in more concentrated
solutions (>72 wt %) was completely irreversible and time-
independent. Transition between these two types of uptake
occurred in the intermediate solutions (i.e. 61-72 wt %). These
observed behaviors may be associated with the formation of
nitrosylsulfuric acid (NO+HSO4-) and subsequent ionic reac-
tions in the liquid, as proposed and documented at room
temperature.
ω
where R is the gas constant (0.082 L atm mol-1 K-1), T is the
temperature, and ω is the thermal velocity of HONO. The
liquid-phase diffusion coefficient for HONO in sulfuric acid
was obtained from a cubic cell model:38 for 70 wt % H2SO4 at
220 K, the value for Dl was 1.5 × 10-8 cm2 s-1. The Henry’s
law constant for HONO in sulfuric acid was estimated to be
∼102 M atm-1 for 70 wt % H2SO4 at 220 K.39 Thus, we
calculated a value ∼3 × 106 s-1 for kI for 70 wt % H2SO4 at
220 K using eq 7. According to the theory of gas-particle
I
x
reactions, the diffusoreactive length (l ) Dl/k ), characteris-
It has been previously realized that equilibrium of N(+III)
species in highly acidic H2SO4 at room temperature may involve
complex aqueous reactions between nitrogen species and sulfuric
acid. Deschamps reported absorption spectra between 214 and
tic of the effective depth within the liquid over which the
reaction occurs, was calculated to be ∼6 × 10-4 µm.4,36 This
estimate for l is small enough so that no limitation to the uptake
of HONO is expected on small particles. In addition, it is
worthwhile to point out that the Kelvin barrier, which arises
from a pressure increase on a curved surface, for condensational
growth of small particles40 plays no role in limiting the HONO
uptake and reaction with H2SO4. Thus, the measured uptake
coefficients should be applicable to the stratosphere.
300 nm for 70-96 wt % H2SO4 containing 1 mol L-1 dissolved
- 35
NO+HSO4
:
the spectrum showed only the nitrosonium ion
(NO+) in 96 wt % H2SO4; the amounts of H2ONO+ (hydrated
nitrosonium ion) and NO+ were equal in 88 wt % H2SO4; H2-
ONO+ dominated in 70.5 wt % H2SO4, with a trace amount of
HONO but no NO+. The proposed ionic mechanism is24,35
Reaction of HCl with HONO. The reaction mechanism
between HCl and HONO dissolved in sulfuric acid may also
vary with the acid composition, depending on the N(+III)
species available in the liquid. Hence, it is likely that HCl reacts
with the hydrated nitrosonium ion in dilute solutions
HONO + H2SO4 NO+HSO4- + H2O
(3)
(4)
(5)
(6)
-
-
NO+HSO4
NO+ + H2O H2ONO+
H2ONO+ + H2O H3O+ + HONO
NO+ + HSO4
HCl + H2ONO+ H3O+ + ClNO
(8)
but with nitrosylsulfuric acid in concentrated solutions
-
HCl + NO+HSO4
H2SO4 + ClNO
(9)
-
NO+HSO4 is an ionic species soluble in H2SO4. In concen-
trated sulfuric acid, NO+HSO4- accumulates in the liquid and,
upon reaching saturation, precipitates out as solid crystals to
permit further accumulation.
Also, at least for the case in concentrated H2SO4 (>70 wt %),
the reaction between HCl and HONO dissolved in sulfuric acid
occurs very near the gas-liquid interface, owing to exceedingly
small HCl solubility. Using an estimated Henry’s constant of
∼102 M atm-1 for HCl in 70 wt % H2SO4 at 220 K,5,34 the
first-order rate coefficient due to HCl reaction with HONO is
∼2 × 105 s-1; this results in a value of ∼2 × 10-3 µm for the
diffusoreactive length. Therefore, the correction needed to apply
the results from bulk liquids to small aerosols is expected to be
small.
Indeed, the process depicted in Figure 2c may not simply
reflect the physical uptake (i.e., a reversible process), but rather
a complex ionic reaction sequence (such as the one described
-
above) with HONO being the end product. NO+HSO4 is
known to be unstable in excess water and reacts with H2O to
form the N(+III) species H2ONO+ and HONO. Our experi-
ments with more dilute H2SO4 (less than 61 wt %) showed that
+
+

344 J. Phys. Chem., Vol. 100, No. 1, 1996
Zhang et al.
Stratospheric Implications. Although detailed assessments
of atmospheric implications for HONO heterogeneous reactions
require simulations by full atmospheric models, we present here
simple calculations to illustrate the major importance. For
typical midlatitude stratospheric conditions (i.e., temperatures
between 220 and 230 K and sulfate aerosol compositions
between 70 and 80 wt %), heterogeneous processing of the
chlorine reservoir species ClONO2 and HCl is very inefficient.3-5
We calculate the production rate of ClNO due to the reaction
of HCl with HONO on sulfate aerosols and compare this value
with the well-established gas phase conversion process between
hydroxyl radical (OH) and HCl in the lower stratosphere (∼100
mb or 16 km). The loss rate of gaseous HONO onto sulfate
aerosols is given by
The laboratory observations showed that HONO uptake on
sulfuric acid consisted of the adsorption/saturation processes
in dilute sulfuric acid but was completely irreversible in
concentrated sulfuric acid, consistent with the formation of
-
nitrosyl sulfuric acid NO+HSO4 and the N(+III) species H2-
-
ONO+ in the liquid. The results also showed that NO+HSO4
was stable and accumulated in concentrated solutions (>70 wt
% H2SO4) at temperatures below 230 K but was unstable and
quickly regenerated HONO in dilute solutions (<70 wt %).
Heterogeneous reaction between HCl and HONO dissolved in
sulfuric acid was also investigated. Gaseous nitrosyl chloride
was identified to be the reaction product. Reaction probabilities
between HCl and HONO ranged from 0.01 to 0.02 for 60-72
wt % H2SO4. Analysis of the laboratory data reveals that the
reaction of HCl with HONO on sulfate aerosols can provide a
mechanism for chlorine activation and, subsequently, affect
stratospheric ozone balance, during elevated sulfuric acid
loadings after volcanic eruptions or due to emissions from the
projected high-speed civil transport (HSCT). Hence, the present
results may have important implications on the environmental
impact of aircraft emissions.
d[HONO]/dt ) -1/4γAω[HONO]
(10)
where A is the surface area density of sulfate aerosols and ω is
the thermal velocity of HONO. The uptake coefficient of
HONO in sulfuric acid is taken to be 0.07. The HONO
concentration, ∼107 molecules cm-3, is based on measurements
reported by Arnold et al. at around 10 km.19 (Note that this
value is not constrained by stratospheric measurements. Ad-
ditionally, the formation mechanism of atmospheric HONO is
currently not well defined.16) We assume that the production
rate of ClNO is approximately equal to the loss rate of HONO
(i.e., d[HONO]/dt ≈ -d[ClNO]/dt), since HCl (with concentra-
Acknowledgment. The research was performed at the Jet
Propulsion Laboratory, California Institute of Technology, under
a contract with the National Aeronautics and Space Administra-
tion (NASA). We thank H. S. Johnston for helpful discussions
and C. A. Brock and J. C. Wilson for providing information on
aerosol measurements inside aircraft plumes.
tions of ∼2 × 109 molecules cm-3 41
) is far more abundant than
HONO in the stratosphere (i.e., the rate-limiting step for reaction
2 is HONO incorporation into the aerosols). Using the aerosol
surface densities of 6 × 10-9 and 10-6 cm2 cm-3 for background
aerosols and volcanic aerosols after the eruption of Mt.
Pinatubo,42-44 the ClNO production rates are 34 and 5600
molecules cm-3 s-1, respectively. The value corresponding to
the Mt. Pinatubo aerosol condition is about 10 times larger than
the conversion rate between OH and HCl (∼5 × 102 molecules
cm-3 s-1).41 In the stratosphere, ClNO photodissociates rapidly,
with a photolysis lifetime of ∼560 s (ref 45), and hence, the
reaction between HCl and HONO on sulfate aerosols could
result in an increased abundance of reactive chlorine under
elevated sulfuric acid loadings.
The fate of HONO emitted by the projected HSCT is
governed by both photolysis and heterogeneous loss. Sulfuric
acid particles nucleate homogeneously inside aircraft plumes
due to SO2 emissions.20,21 Using a mean embryo radius of 0.4
nm and a particle number density of 109 cm-3 inside a newly
formed aircraft plume,20 the characteristic lifetime of HONO
due to loss on sulfate aerosols (τ ) 4/(γAω)) would be about
90 s, considerably shorter than the photolysis lifetime of HONO
(∼820 s).45 Thus, the heterogeneous process can lead to
removal of a significant portion of gaseous HONO from aircraft
effluent and largely regulate the plume chemistry (in terms of
hydrogen oxides, HOx). This is particularly true for nighttime
emissions when HONO photolysis ceases. Our results dem-
onstrate that sulfate aerosols can act as a temporary reservoir
for HONO emitted by the HSCT and further interact with HCl,
once available, to release ClNO. This process can affect the
stratospheric ozone balance.
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