J. Phys. Chem. 1996, 100, 13765-13775
13765
Heterogeneous Kinetics of HONO on H2SO4 Solutions and on Ice: Activation of HCl Frederick F. Fenter and Michel J. Rossi* Laboratoire de Pollution Atmosphe´ rique et Sol (LPAS), Swiss Federal Institute of Technology (EPFL), 1015 Lausanne, Switzerland ReceiVed: March 15, 1996; In Final Form: May 17, 1996X
In order to better assess the role of nitrous acid in the chemistry of the upper troposphere and in the plumes of jet aircraft, we have studied the interaction of HONO with sulfuric acid liquid solutions (50-95 wt %) and with ice. The experiments were carried out using a Knudsen cell over the temperature range of 180-200 K for ice and 200-273 K for the solutions of H2SO4. The rate of uptake of HONO on sulfuric acid is found to vary greatly as a function of the weight percent of the solution, with an uptake probability that changes from about 0.1 at 95 wt % to less than 10-3 for solutions under 60 wt %. HONO is adsorbed reversibly onto ice with submonolayer coverage. In the presence of HCl, the formation of ClNO is observed for both heterogeneous systems. The maximum uptake probability of HCl in the presence of HONO on the H2SO4 liquid solutions is observed at 60 wt %. The reaction takes place on ice with greater efficiency compared to H2SO4 solution experiments, which allows for the unambiguous detection of ClNO as the reaction product. We will show the results of experiments conducted over a wide range of initial conditions (reactant concentration, temperature, water partial pressure, substrate preparation) using both steady-state and realtime detection schemes. From the data, we can draw some conclusions concerning the interfacial nature of the reaction mechanism. A few brief comments on the atmospheric implications of our results conclude the discussion.
Introduction Nitrous acid is an important atmospheric trace species whose chemistry remains largely uncharacterized.1,2 In the troposphere and in particular near cities, HONO has been found to accumulate during the nighttime to reach levels as high as 8 ppb just before sunrise.1,3-5 The source of gaseous nitrous acid under these conditions is uncertain; the heterogeneous hydrolysis of nitrogen oxides such as NO2 and N2O3 (from the association reaction of NO with NO2) have been suggested as possible mechanisms.1,6 Because HONO is easily photolyzed in sunlight, it may be responsible for a “pulse” of hydroxyl radicals at sunrise that sets off the diurnal oxidative chain reactions.7,8 In addition, nitrous acid is injected by aircraft into the atmosphere with other pollutants (SO2, NO2, NO, HNO3, etc.) and particulates (sulfuric acid and carbonaceous aerosols, water-ice particles).9,10 The chemistry of wakes and plumes is complex; indeed, an understanding of homogeneous reactions, nucleation processes, and heterogeneous reactivities is required to assess the impact of aircraft emissions on the physical and chemical properties of the atmosphere.10 Under conditions where nitrous acid is produced or accumulates to significant concentrations, the heterogeneous reactions of HONO may become important for the local atmospheric chemistry. In the condensed phase, nitrous acid is known to be an efficient oxidizer11 and might react with other trace species once it is adsorbed onto or dissolved into atmospheric aerosols. A relevant suggestion was made by Burley and Johnston several years ago,12,13 who proposed that the presence of nitrosylsulfuric acid, NO+(HSO4-), in atmospheric aerosol could activate HCl according to the following reaction:
HCl + NO+(HSO4-) f H2SO4 + ClNO
(1)
* To whom correspondence should be directed. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00797-6 CCC: $12.00
Although the subject of several studies and much speculation, the role of nitrosylsulfuric acid (NSA) in atmospheric chemistry is ill-defined. There is some evidence that if NOx is present during the atmospheric oxidation of SO2 to H2SO4, then NSA may form in the evolving aerosol.14 Shortly after the publication of Burley and Johnston, laboratory studies determined that NO and NO2 do not interact with sulfuric acid,15 so the direct interaction between NOx and H2SO4 aerosol probably cannot provide a pathway for NSA formation in the atmosphere. The production of NSA from HONO, however, may be much more efficient because both are formally nitrogen(III) compounds:
HONO + H2SO4 f NO+(HSO4-) + H2O
(2)
Because high concentrations of HONO are often associated with elevated aerosol loading (as in the plume of aircraft), the heterogeneous reactions of nitrous acid may have a role in the partitioning of atmospheric trace species under certain conditions. In this work, we report our experiments on the interaction of HONO with water-ice and with supercooled solutions of sulfuric acid with the aim to better characterize the potential heterogeneous chemistry of nitrous acid in the atmosphere:
HONO(g) + H2O(s) f HONO(ads)
(3)
HONO(g) + H2SO4(liq) f HONO(ads)
(4)
In light of the chlorine-activation scheme proposed by Burley and Johnston, we investigated each of the above processes in the presence of HCl to determine if, even in the absence of the nitrosylsulfuric acid intermediate, a direct reaction between gaseous or adsorbed HONO with HCl is possible:
HONO(g) + H+(Cl-) f ClNO(g) + H2O(s)
(5)
HONO(ads) + H+(Cl-) f ClNO(g) + H2O(s)
(6)
© 1996 American Chemical Society
13766 J. Phys. Chem., Vol. 100, No. 32, 1996
Fenter and Rossi
TABLE 1: Knudsen Cell Parameters reactor parameter volume surface area (total) gas number density sample collision frequency,a orifice diameters escape rate constant (φ ) 1 mm)b escape rate constant (φ ) 4 mm)b escape rate constant (φ ) 8 mm)b escape rate constant (φ ) 15 mm)b
value 3
1830 cm 2000 cm2 (1-1000) × 1010 cm-3 30.4(T/M)0.5 s-1 1, 4, 8, and 14 mm 1.6 × 10-2(T/M)0.5 s-1 2.0 × 10-1(T/M)0.5 s-1 8.0 × 10-1(T/M)0.5 s-1 2.1(T/M)0.5 s-1
a A , the sample surface area, is 15.2 cm2. b These are the experih mentally determined values for the orifices of nominal width.
Because the aim of the study is, in part, to aid in the validation of atmospheric chemistry models, we will briefly discuss the probable role of these heterogeneous interactions in the Earth’s atmosphere. While this study was in progress, it came to our attention that Zhang et al. had recently submitted a paper on the HONO/ HCl interaction on sulfuric acid solutions.16 Because they kindly provided a copy of their unpublished manuscript, we will be able to compare our results, obtained using a Knudsen cell, with their experiments conducted using a coated-wall flow reactor. Experimental Details The experiments were carried out using a Teflon-coated Knudsen cell with mass spectrometric detection of gas-phase species. The apparatus has been described in detail in several recent publications,17,18 so we will only briefly describe the experiment and the experimental procedure. The only new aspect of the apparatus is the low-temperature support used to cool and temperature control the condensed substrates (water and sulfuric acid) down to the 180-270 K temperature range. The Knudsen cell is a Teflon-coated glass reactor operated under molecular flow conditions. Gases are introduced either by a steady flow through a glass capillary or by a pulse across a solenoid valve (General Valve, Series 9). In the absence of a reactive surface, the gas exits the cell via a small opening into a differentially pumped vacuum chamber. The residence time of the gas in the cell is determined by the diameter of the exit orifice and can be calculated by simple gas kinetic relations. At the entrance of the upper vacuum chamber, an effusive molecular beam is formed; the beam passes into the lower chamber via a 1-cm-diameter opening, where it is modulated by a chopper (operated typically at 160 Hz) and subsequently detected by a quadrupole mass spectrometer (Balzers QMG 420). The modulated electron multiplier signal is processed by a digital lock-in amplifier (Stanford Research SR850 DSP) and transferred to a personal computer for storage and analysis. The density of a gaseous species in the reactor is determined directly by mass flow calibrations. This is carried out by measuring the pressure drop (with a MKS Baratron pressure transducer) in a closed calibrated volume located behind the entrance capillary. In this way, the flow of molecules into the cell and the mass spectrometer signal can be related to the gas density in the reactor. The parameters for the Knudsen cell reactor used in this study are summarized in Table 1. In Figure 1, the mass spectra for the various gaseous species of this study are presented with the relevant calibration data (found in the caption). In steady-state experiments, a known flow of the reactant is introduced in the Knudsen cell via a capillary while the reactive substrate remains isolated within a second small chamber. The only loss term for the gas is effusion out of the cell, so the density of the reactant can be determined precisely by solving
Figure 1. Summary of the mass spectral data used in this study. A HCl flow of 4.0 × 1013 molecule s-1 from the cell yields 1 mV of signal at m/e 36; for ClNO, a flow of 2.0 × 1014 molecule s-1 gives 1 mV at m/e 35; for HONO, 4.0 × 1014 molecule s-1 results in 1 mV at m/e 47. The flow calibrations are carried out using the lock-in amplified signal of the mass spectrometer, with the effusive molecular beam modulated at 160 Hz.
for the steady-state concentration. At the moment when the second chamber is opened, exposing the substrate to the reactive gas, an interaction between the gas and solid is detected as a decrease in the mass spectrometer signal. Once the new steady state is established, we can describe the first-order rate constant of uptake, kI, in terms of the rate constant for effusive loss from the cell, kesc, and the observed signal change:
kI )
( )
Si - 1 kesc Sf
(7)
where Si and Sf are the mass spectrometer steady-state signals before and during the reaction. The application of this equation requires that the uptake process be first order with respect to the gas density, which is routinely verified by showing that kI is independent of the reactant flow rate. In the pulsed-valve experiments, the solenoid valve is opened for a brief period, typically 1 ms, to introduce the reactant. The flux of all gaseous species from the cell is monitored in real time by the mass spectrometer. A control experiment, during which the substrate is isolated from the gas burst, yields a reactant signal that decays exponentially according to its known value of kesc. The sample is subsequently exposed to the reactor volume for the second pulse; from the resulting decay trace, the rate constant for the first-order loss of gaseous reactant can be determined by fitting to a simple exponential decay function. Because an individual pulse of gaseous molecules represents a
The Heterogeneous Kinetics of HONO on H2SO4 small fraction of a surface monolayer and because we are able to follow in real time the evolution of the interaction between gas and condensed phases, the pulsed-valve technique allows us to gain kinetic information in the limit of low sample dosing that is unattainable by the steady-state method. The nitrous acid for this study was synthesized roughly according to the procedure outlined by Cox19 but adapted to our requirements for the low-pressure reactor: 60 mL of 0.2 M NaNO2 solution is cooled to 273 K and stirred in a Tefloncoated round-bottom flask, to which 3-5 drops of 50 wt % sulfuric acid are added. The flask is closed and evacuated before letting the mix of evolving gases into the reactor. The line between flask and reactor consists of 6-mm Teflon tubing and includes a capillary (Teflon, 2-cm length, 0.2-mm diameter) to reduce the pressure followed by a Teflon tube coiled in a lowtemperature bath (200 K). Between the temperature bath and the reactor, a bleed-off valve is mounted to reduce the gas flow if needed. The 200 K bath reduces the water content of the effluent to the milliTorr level required for the Knudsen reactor. Using the mass spectrometer, we found that the gases produced from this source are NO, NO2, and HONO, with FNO ) 10FHONO ) 2FNO2, where F represents the relative rate of flow. This ratio was highly variable and changed, for example, according to the condition of the flask’s Teflon coating and the number of drops of H2SO4 added; in addition, the HONO flux often changed during an experiment. The source typically produced 1015 molecules s-1 of HONO, corresponding to densities of approximately 1012 cm-3 inside the Knudsen cell reactor. Because of the fluxes of NO and NO2 from the source, we monitored HONO by its m/e 47 peak (HONO+) for all the experiments described below. The experiments were carried out using a sample support designed for the study of low-temperature heterogeneous kinetics. The full description of the support will be published separately.20 With the low-temperature support, the substrate under study can be prepared as cold as 150 K. With a programmable temperature controller (Eurotherm), the final temperature and ramping speed of the cooling cycle can be precisely controlled. Taking into consideration the systematic uncertainties of controller drift, thermocouple errors, and temperature gradients in the support, we estimate that the stated temperature of an experiment is known to (2 K. The sulfuric acid solutions were prepared by gravimetric dilution of 95 wt % H2SO4 (Fluka) with demineralized H2O. The concentration was occasionally verified by acid-base titration, from which we determine that the preparation of the solutions is reproducible to (2 wt %. HCl was prepared in small quantities by adding sulfuric acid to solid sodium chloride (Fluka) according to the established procedure.21 Nitrosyl chloride was obtained from Matheson and was used without further purification. Nitrosylsulfuric acid was supplied by Fluka (>97 %, with traces of methylene chloride). Results 1. Uptake of Nitrous Acid on Sulfuric Acid Solutions. HONO is readily taken up by concentrated sulfuric acid, as shown in Figure 2, where the steady-state HONO signal (m/e 47) is shown before, during, and after exposure to (a) 87 wt % and (b) 60 wt % H2SO4 solutions. The interaction is strongly dependent on the H2SO4 concentration. Applying eq 7 to the experimental trace shown in Figure 2b reveals that the rate of HONO uptake is smaller at the end of the exposure (t ) 90 s) than at the start (t ) 25 s), indicating a saturation effect. The downward drift in intensity of the m/e 47 signal during the 120 s of the experiment is due to the instability of the HONO source.
J. Phys. Chem., Vol. 100, No. 32, 1996 13767
a
b
Figure 2. Mass spectroscopic traces showing the interaction of HONO with two concentrations of H2SO4 solution: (a) T ) 270 K and (b) T ) 220 K. O and C refer to the opening and closing of the sample chamber. At OFF, the HONO flow is stopped. Both experiments are conducted using the 4-mm orifice.
The results of all the experiments over the range 55-95 wt % are summarized in Table 2 and in Figure 3, where the experiments are organized according to the bulk composition of the H2SO4 solution. It is common to classify this type of interaction according to the equilibrium water of the H2SO4 solution. To ensure that the two classification schemes are equivalent, we carried out many of the experiments under an external flow of H2O. For a given composition and temperature, the water flow is adjusted to the level where opening and closing the sample chamber does not cause a deflection in the m/e 18 signal of the mass spectrometer. The steady-state pressure that corresponds to this flow is the equilibrium vapor pressure of the sulfuric acid sample at the established temperature. When the vapor pressure of the H2SO4 solution was determined before an experiment, the values were always within 25% of the literature values. Of more importance, we found that the presence of the external flow of water has no influence on the observed rate of HONO uptake, certainly because the H2SO4 sample is routinely isolated in the sample chamber of small volume until the moment of measurement. The vapor pressure determination confirms that the sample has remained in the solution phase during the cooling. Occasionally, while adjusting the external H2O flow, a “phase transition” was observed in which the water vapor pressure of the liquid solution suddenly changed; this probably coincides with the formation of a frozen hydrate of sulfuric acid. It is interesting to note that, on these samples, the rate of HONO uptake was immeasurably small. Although we did not pursue this point, the observations are interesting in light of recent observations of Zhang et al. on the low reactivity of sulfuric acid monohydrate crystals.22 2. Uptake of HONO on Ice. Nitrous acid is adsorbed reversibly with submonolayer coverage on ice between 180 and 200 K. A typical result is shown in Figure 4, where an ice sample is exposed to gaseous HONO in our reactor. When the
13768 J. Phys. Chem., Vol. 100, No. 32, 1996
Fenter and Rossi
TABLE 2: Experimental Results for HONO Uptake on Sulfuric Acid expt 09ac 09af 10af 10aa 14aa 14aap 14ab 15a 15ap 15ac 15acp 17aa 17aap 22aa 23aa 24aa 24aap 29aap 10ag 27o 26jb 03jl 25ob 11jya 11jyb 08jyb 08jyf 05jyb 19db 20dc 20da
i a [H2SO4]/wt % orifice/mm FHONO T/K
95 95 73 73 87 87 65 80 80 80 80 73 73 67 73 73 73 60 65 65 60 60 73 55 55 55 60 62.5 60 60 65
15 15 4 4 4 15 4 15 4 4 15 4 15 4 4 4 4 4 4 4 4 4 4 1 1 1 4 4 4 4 4
0.8 (15) 0.4 (15) 1.0 (15) 1.2 (15) 0.7 (15) 0.9 (15) 0.6 (15) 2.6 (15) 1.8 (15) 1.2 (15) 1.6 (15) 0.7 (15) 0.9 (15) 1.0 (15) 0.3 (15) 2.4 (15) 2.8 (15) 0.6 (15) 1.0 (15) 0.8 (15) 7.2 (15) 3.2 (15) 1.4 (15) 0.9 (15) 0.8 (15) 2.4 (15) 3.5 (15) 0.8 (15) 0.5 (15) 0.6 (15) 2.4 (15)
273 220 230 230 270 270 235 265 265 250 250 230 230 235 235 226 226 235 235 230 220 220 238 220 220 220 225 230 215 230 235
Si/Sf
γ
59/11 57/10 35/5 46/14 43/3 57/23 56/25 79/40 52/7 31/4 43/22 49/17 62/46 47/22 44/16 56/18 62/23 14/10 77/49 24/10 54/41 47/37 34/13 52/35 42/30 55/43 65/55 20/15 52/36 29/22 37/11
3.0 (-1) 3.2 (-1) 3.8 (-2) 2.5 (-2) 8.5 (-2) 1.0 (-1) 7.9 (-3) 6.8 (-2) 4.1 (-2) 4.3 (-2) 6.7 (-2) 1.2 (-2) 2.4 (-2) 7.3 (-3) 1.1 (-2) 1.3 (-2) 1.1 (-2) 2.6 (-3) 3.6 (-3) 8.5 (-3) 1.8 (-3) 1.7 (-3) 1.0 (-2) 2.6 (-4) 2.1 (-4) 1.4 (-4) 1.2 (-3) 2.1 (-3) 2.8 (-3) 2.0 (-3) 1.5 (-2)
Figure 4. Interaction of HONO with a frozen 0.1 M NaCl solution at T ) 190 K. The same interaction is observed for pure water as well. O and C refer to the opening and closing of the sample chamber. The uptake is rapidly saturated, and if the flow of HONO is stopped (at OFF) and the sample reexposed, the reversibility of the uptake becomes apparent.
a Flow of molecules introduced into cell in units of molecule s-1; values are given with the power of 10 in parentheses; for example, 3.0 (14) ) 3.0 × 1014.
Figure 5. Experimental mass spectrometric trace showing the HCl + HONO reaction on 60 wt % H2SO4 at T ) 225 K. The bar at the bottom gives the quadrupole mass filter setting. O and C refer to the opening and closing of the sample chamber; M refers to a HONO flow on-off cycle. At 60 s, the dosing of the solution begins. The degree of saturation is tested at 130 s by isolating and reexposing the H2SO4 solution. At 200 s, the sample is seen to be saturated with HONO. At 220 s, the H2SO4 is exposed to concurrent flows of HCl and HONO. Three M cycles are carried out to show that the uptake rate of HCl (m/e 36) is strongly affected by the presence of HONO and that no change is observed at m/e 35, as expected for ClNO product formation.
Figure 3. Semilogarithmic plot of the determined uptake probabilities for HONO and ClNO as a function of the H2SO4 concentration. Experimental conditions for each experiment are listed in Tables 2 and 3.
sample chamber is opened, a sudden drop in HONO density is observed, corresponding to an initial uptake probability of about 1 × 10-3. By the time the uptake has saturated about 10 s later, the surface has taken up about 3 × 1014 molecules cm-2, corresponding to about 3% of a formal monolayer. If the sample is isolated, the reactor evacuated, and the sample chamber subsequently reopened to the reactor volume, most of the adsorbed HONO is observed to desorb back into the gas phase.
3. Reaction of HONO with HCl on Liquid H2SO4 Solutions. In a series of experiments, we studied the interaction of HCl in the presence of HONO on H2SO4 solutions to see if ClNO is formed. In some experiments, concurrent flows of HCl and HONO were exposed to the H2SO4 substrate simultaneously; in other experiments, the HCl was added only after the H2SO4 solution had been “doped” with HONO (i.e., exposed to HONO until uptake saturation set in). A reaction in which HCl is converted to ClNO can be detected by the mass spectrometer as a change in the m/e 35 to m/e 36 signal ratio. Because the m/e 35 calibrations for HCl and ClNO are identical (within 10%, cf. Figure 1), the signature of reaction 5 is the decrease at m/e 36 (loss in the HCl steady-state density), with no detectable change at m/e 35; i.e., at m/e 35, the signal change due to the consumption of HCl will be compensated for by the production of ClNO. In Figure 5, we show the results of an experiment conducted with 60 wt % H2SO4; at the end of the trace, it is seen that the presence of HONO significantly enhances the HCl interaction with the H2SO4 substrate. It is important to note that stopping the HONO flow has no influence on the m/e 35 signal, confirming the hypothesis of HCl-to-ClNO signal compensation. For those experiments where the en-
The Heterogeneous Kinetics of HONO on H2SO4
J. Phys. Chem., Vol. 100, No. 32, 1996 13769
TABLE 3: Heterogeneous Kinetics of HONO + HCl in H2SO4 expt
[H2SO4]/ wt %
T/K
orifice/ mm
i a FHONO
i a FHCl
observationsb concurrent flows with strong HONO uptake; no change at m/e 36 concurrent flows with weak HONO uptake; no change at m/e 35, m/e 36 sample dosed with HONO for 10 min before HCl addition; no change observed at m/e 36 sample saturated with HONO before HCl addition; γHCl,T ) 5 × 10-4; no change observed at m/e 35 sample saturated with HONO and HONO flow maintained; γHCl,T ) 7 × 10-4 concurrent flows, first exposure; γHCl,R ) 4 × 10-4 sample saturated with HONO and HONO flow maintained; γHCl,R ) 1.6 × 10-3 sample saturated with HONO and HONO flow maintained; γHCl,T < 2 × 10-4 sample saturated with HONO and HONO flow maintained; γHCl,T ) 2.7 × 10-3; γHCl,R ) 2.1 × 10-3 (estd) sample saturated with HONO and HONO flow maintained; γHCl,R ) 5 × 10-4 concurrent flows, first exposure; γHCl,R ) 1.0 × 10-3 sample saturated with HONO and HONO flow maintained; γHCl,R ) 1.7 × 10-3
1 2 3 4
80 65 73 65
265 233 235 225
4 4 4 4
1.2 (15) 0.8 (15) 0.8 (15) off
1.2 (15) 2.4 (14) 4.8 (14) 1.0 (15)
5 6 7 8 9
65 62.5 60 65 60
225 230 225 230 230
4 4 4 4 4
1.0 (15) 0.8 (15) 1.5 (15) 2.0 (15) 4.0 (14)
4.0 (14) 4.8 (14) 4.0 (14) 3.0 (14) 5.2 (14)
55 60 60
220 225 225
4 4 4
1.0 (15) 3.6 (15) 3.2 (15)
4.0 (14) 3.2 (14) 3.2 (14)
10 11 12
Flow of species into cell in units of molecule s-1. b γHCl,T and γHCl,R refer to the total and reactive uptake coefficients, respectively. These are defined in the text. a
hancement of the HCl uptake in the presence of HONO was observed, a value for the reactive uptake coefficient, listed as γHCl,R in Table 3, was calculated in the following way: The H2SO4 solution is exposed concurrently to HONO and HCl, and the resulting total uptake of HCl (at m/e 36) is referred to as γHCl,T. In a certain number of experiments, the HONO flow was stopped (shown as “M” in Figure 5) to measure the residual HCl interaction with the acid solution, allowing us to separate the total rate of HCl uptake (γHCl,T) into nonreactive and reactive components, the latter listed as γHCl,R in Table 3. All the results are summarized in Table 3. The observations can be summarized in a few points: (1) At concentrations of H2SO4 greater than 65 wt %, no interaction between HCl and the substrate is observed, even for sulfuric acid solutions that had been dosed for long periods with nitrous acid prior to the HCl exposure. (2) A weak interaction is observed at 62.5-65 wt % when the solutions are predosed with HONO and when HONO is flowed concurrently at the time of HCl exposure. The decrease at m/e 36 corresponds to a total uptake probability of γ ) (0.5-1.0) × 10-3; because the signal-to-noise ratio at m/e 35 is poor, ClNO formation cannot be confirmed. Since HCl conversion is not directly observed, the stated value for γ is an upper limit to the probability of reactive uptake. However, uptake coefficients on the order of 10-3 are much greater than those for the HCl interaction alone on sulfuric acid solutions in this concentration range,23 and stopping the HONO flow causes a decrease in the HCl uptake (as observed for the solutions of 60 wt %). These observations provide evidence that the measured uptake may in fact represent a reactive uptake. (3) The greatest interaction is observed at 60%, for which the HCl uptake is stronger and clearly enhanced by the presence of HONO. In addition, the signal at m/e 35 is strong enough to unambiguously state that the loss of HCl signal at this mass is being compensated for by the production of another chlorinecontaining species. On the basis of three experiments, we determine that γ ) (2.0 ( 0.7) × 10-3 for the uptake of HCl in the presence of HONO on 60 wt % H2SO4 that has been previously doped with HONO. 4. Additional Relevant Experiments: H2SO4 Substrate. The product of reactions 5 and 6, ClNO, was found to be reactive with respect to concentrated H2SO4 to yield gaseous HCl. If nitrosylsulfuric acid is the other product, the reaction would be analogous to reaction 2, NSA formation by nitrous acid:
ClNO + H2SO4 f HCl + NO+(HSO4-)
(8)
Figure 6. Mass spectrometric trace illustrating the production of HCl from the ClNO interaction with H2SO4 (87 wt %) at T ) 238 K using the 4-mm orifice. The bar at the bottom gives the quadrupole mass filter setting. Before 70 s, the relevant masses are scanned with only ClNO present. At 70 s, the H2SO4 is exposed and rapid uptake of ClNO is observed (m/e 30). HCl is produced with unitary yield, as seen here by the change at m/e 36. At 170 s, when the ClNO flow is turned off, the HCl production stops.
TABLE 4: Experimental Results for ClNO Uptake on Sulfuric Acid expt
[H2SO4]/ wt %
orifice/ mm
i a FClNO
T/K
Si/Sf
γ
20o 23ob 25oa 25of 26ob 26oc 26oc 26od 26of 26oh 26ohp 26ob
62 87 73 87 80 95 95 95 95 95 95 80
4 15 4 4 4 15 4 15 4 4 15 4
4.5 (15) 4.8 (15) 4.4 (15) 3.6 (15) 5.2 (15) 6.8 (15) 5.3 (15) 6.8 (15) 5.2 (15) 12.0 (15) 12.0 (15) 4.8 (15)
220 265 238 238 269 280 280 280 280 280 280 225
50/50 35/30 40/37 38/10 41/34 157/75 127/13 38/19 45/4.5 40/3 40/19 23/17
[HCl]0, the HONO uptake depends on the rate at which HCl is adsorbed onto the surface. Comparing Figures 8 and 9 shows the effect of doubling the HCl flow on the HONO uptake rate when [HONO]0 > [HCl]0; in particular, the rise time to steady-state production of ClNO, as well as the overall reaction rate at steady state, is seen to scale with the HCl flow rate. Under these conditions, the rate of HONO uptake is limited by the amount of HCl at the surface. At steady state, there is quantitative conversion of HCl and HONO into ClNO; i.e., for each HCl molecule adsorbed, there is one of HONO taken up and one of ClNO produced. (3) When
Figure 8. (a) Mass spectrometric trace showing the interaction of HONO and HCl on ice (T ) 190 K) with production of ClNO. The bar at the top gives the quadrupole mass filter setting, where A refers to a m/e 35-36-35 cycle. The dotted line represents the drift of the HONO source during the experiment. Before 95 s, the initial contributions due to HONO and HCl are shown. At 95 s, the ice is exposed. A strong uptake of HCl and an initially very weak HONO interaction are apparent. Within 100 s after the start of the exposition, m/e 35 production is observed. In b, the mass spectrometer signals are smoothed, interpolated, and converted into flows to facilitate the interpretation of the data.
the HCl flow is at least a factor of 2 greater than the HONO flow, the rate of HONO uptake increases to an upper limiting value that is independent of the HCl density. The rate of HONO uptake under these conditions was studied at 180, 190, and 200 K, and the results are shown in Figure 10. There is good agreement between the experiments conducted with the 4-mm and 15-mm orifices, indicating that the uptake of HONO is a pseudo-first-order process under these conditions. The reaction appears to have only a slight dependence on temperature. In order to gain some additional insight into the reaction between HONO and HCl on ice, we studied the interaction of HONO with frozen aqueous solutions of HCl; the results are presented in Table 6. We found that HONO is taken up very efficiently on “frozen” solutions prepared in the 0.1-10 M concentration range and that ClNO is produced with unitary yield. By inspecting the HCl/H2O phase diagram,25 with the observed rate of HCl flow from the frozen sample and with the known temperature, we can infer the thermodynamic state of the substrate. As shown in Table 6, for all except the most concentrated sample, the HCl flow from the sample is about 1.0 × 1014 molecules s-1 with the 4-mm orifice (and about a factor of 4 greater with the 8-mm orifice), corresponding to a steady-state pressure of 3 × 10-6 Torr. This is very near the value for the ice solution coexistence line at 190 K. For the most concentrated solution, the higher rate of HCl desorption is consistent with the vapor pressure at 190 K given by the HCl trihydrate solution coexistence line. This indicates that the
The Heterogeneous Kinetics of HONO on H2SO4
J. Phys. Chem., Vol. 100, No. 32, 1996 13771
a
Figure 10. Uptake coefficients for the HONO uptake on ice as a function of temperature, determined using the 4-mm and the 14-mm orifice. The experiments were conducted with an excess of HCl. Experimental conditions are summarized in Table 5.
b
TABLE 6: Experimental Results for HONO Uptake on Frozen HCl Solutions (T ) 190 K) expt 30n 01d 04da 04db 12d(c) 07da 07db
[HCl]/M orifice/mm 10.0 1.0 0.1 0.01 0.1 0.1 0.1
4 4 4 4 4 8 8
i a FHONO
1.4 (15) 1.6 (15) 1.4 (15) 0.8 (15)
o b FHCl
4.4 (14) 1.2 (14) 0.9 (14) 0.9 (14) 1.2 (14) 1.6 (15) 6.0 (14) 3.2 (15) 4.0 (14)
Si/Sf
γ
30/2.5 35/2 28/2.5 42/21 P.V. 36/6.5 38/9
7.0 (-2) 1.0 (-1) 7.0 (-2) 6.4 (-3) 3.0 (-2) 1.1 (-1) 8.2 (-2)
a Flow of HONO into the cell in units of molecule s-1 b . Flow of HCl from the frozen sample in molecule s-1. c Pulsed-valve experiment shown in Figure 12.
Figure 9. (a) Mass spectrometric trace showing the same interaction as Figure 8 but with the HCl flow approximately doubled at T ) 190 K. The bar in the middle gives the quadrupole mass filter setting, where A refers to a m/e 35-36-35 cycle. The dotted line represents the drift of the HONO source during the experiment. Procedure identical to that shown in Figure 8. In b, the mass spectrometer signals are smoothed, interpolated, and converted into flows to facilitate the interpretation of the data.
TABLE 5: Heterogeneous Kinetics of HONO + HCl on Ice uptakeb expt
i a FHCl
i a FHONO
HCl
HONO
o31a o31b ot1 3na 3nc 3ne 3nf 6nb 7na 7nb 8na 8nb
1.4 (14) 3.7 (14) 1.9 (14) 1.0 (15) 1.0 (15) 3.6 (15) 3.1 (15) 7.2 (14) 3.6 (15) 3.6 (15) 4.6 (15) 4.6 (15)
1.2 (15) 1.4 (15) 1.2 (15) 2.4 (15) 4.8 (14) 6.0 (14) 2.0 (15) 1.0 (15) 1.2 (15) 8.0 (14) 6.4 (14) 4.0 (14)
8.0 (13) 2.4 (14) na 9.6 (14) 9.6 (14) 3.3 (15) 2.9 (15) 7.0 (14) 3.4 (15) 3.0 (15) 2.4 (15) 2.4 (15)
1.0 (14) 2.4 (14) 2.0 (14) 8.0 (14) 3.2 (14) 5.2 (14) 1.6 (15) 5.2 (14) 1.1 (15) 6.4 (14) 3.2 (14) 1.6 (14)
o a FClNO T/K orifice/mm
1.0 (14) 3.0 (14) 1.4 (14) 1.0 (15) 3.8 (14) 4.6 (14) 1.4 (15) 6.4 (14) 9.3 (14) 8.0 (14) 4.0 (14) 2.0 (14)
190 190 190 190 190 190 190 180 180 200 180 200
4 4 4 4 4 4 4 4 4 4 15 15
a The superscripts refer to the measured flow into (i) or the detected flow out of (o) the cell. b The uptake is the difference between the flow into the cell and the flow out of the cell (Fi - Fo).
reaction interface for all experiments listed in Table 6 is probably a liquid layer containing concentrated aqueous HCl. The uptake probability for these substrates, varied over several orders of magnitude in HCl concentration, can be described by γ ) 0.07
Figure 11. Interaction of HONO with a frozen (T ) 190 K) 0.1 M HCl solution. The 4-mm orifice was in place. The bar at the bottom gives the quadrupole mass filter setting, where A refers to a m/e 3536-35 cycle. At 30 s, the substrate is exposed, and the uptake of HONO (m/e 47) is evident. Based on the m/e 35, 36, and 49 signals, the ClNO production can be separated from the HCl outgassing. The m/e 49 signal (ClN+) is an important confirmation that ClNO is the reaction product.
( 0.04. On frozen 10-2 M solutions, some interaction is still observed, although the rate of HONO uptake is greatly reduced. On more dilute solutions, no reaction takes place. The results for an experiment conducted on 10-1 M HCl solution are shown in Figure 11. An important secondary aspect of the experiment shown in Figure 11 is that the chlorine-containing product of the reaction between HCl and HONO on ice is confirmed to be ClNO by the identification of the weak ClN+ fragment at m/e 49. The ratio of m/e 49 to m/e 35 for the product is found to
13772 J. Phys. Chem., Vol. 100, No. 32, 1996
Fenter and Rossi
Figure 12. Pulsed-valve experiment for the HONO interaction with 0.1 M HCl solution (T ) 190 K), conducted using the 4-mm orifice. In a, a pulse corresponding to about 3 × 1015 molecules of HONO is injected at 0.5 s with no reactive surface present. The mass filter is set to m/e 47. The subsequent first-order decay is in good agreement with the known value of kesc for HONO. In b, the experiment is repeated in the presence of the frozen HCl solution. The observed first-order decay rate constant for HONO uptake is given. In c, the mass filter is set to m/e 35 to follow the real-time production of ClNO.
be identical to that of the calibration sample of ClNO, as can be seen in Figure 1. We investigated the interaction of gaseous nitrous acid with samples containing neutral chloride to see if these might also be able to form gaseous ClNO. In one experiment, HONO was found not to react with a frozen aqueous solution that had been saturated with NaCl at room temperature and cooled to 190 K. When the experiment was repeated for a 10-1 M NaCl solution, the HONO interaction was identical to that for pure ice, as shown in Figure 4. In a third experiment, 2 g of NaCl crystals (approximately 200 µm in diameter) was exposed to HONO in the Knudsen reactor, and no interaction was observed. Thus, the lack of reactivity on the frozen NaCl solution is exactly what would be expected if the solution separates into ice and precipitated NaCl crystals upon cooling. In a final series of experiments, we used pulsed-valve introduction of HONO into the reactor to study the real-time kinetics of the HONO interaction with a 10-1 M HCl solution that had been cooled to 190 K. The results of these experiments are shown in Figure 12. In panel a, the decay of a HONO pulse in the absence of the reactive surface is shown, with the firstorder decay rate constant corresponding to kesc of HONO for the 4-mm aperture. In b, the trace is recorded under the same conditions with the reactive substrate open to the reactor volume. The rate constant of 2.3 s-1 corresponds to an uptake coefficient of 3.0 × 10-2; this value is consistent with the steady-state values and has been included in the average value of (7.6 ( 2.5) × 10-2 listed in Table 5. In panel c, the rise and fall of the ClNO product signal is shown (with a doubled time scale). The agreement between the decay rate constant of the signal (0.40 s-1) with kesc for ClNO (0.42 s-1) provides another confirmation that the product of the reaction is indeed ClNO. It is, in addition, important to note that the agreement between the pulsed-valve and the steady-state experiments demonstrates that the latter were unaffected by “saturation”; i.e., the coverage of surface chloride is not significantly depleted on the time scale of the measurement. Discussion In the previous section, we presented experimental results for the interaction of nitrous acid, alone and in the presence of HCl, with a variety of heterogeneous surfaces. Two broad conclusions that can be drawn from the experiments are that
(1) HONO itself is taken up efficiently in concentrated H2SO4 solutions, only weakly and reversibly on ice, and not at all on solid sodium chloride and that (2) HCl activation by HONO in the presence of an active substrate takes place with high probability on ice but only to a lesser extent on H2SO4 solutions over a limited range of concentrations. Table 7 gives a summary of the rate constants and uptake coefficients determined in this work. In this section, we discuss more generally the nature of these interactions, comparing, when possible, our results with those of the previous study by Zhang et al.16 and the general implications of our study for atmospheric chemistry. The efficient uptake of HONO by concentrated sulfuric acid solutions had been previously characterized by Zhang et al.16 The values they report extend over the concentration range of 65-75 wt % and in general are found to be larger than those of this study. A line is drawn in Figure 3 to compare the two data sets. The uncertainties overlap near the 65 wt % point, and the discrepancy approaches a factor of 2-3 at 75 wt %. It is important to note that the data (as plotted in Figure 3) show scatter that is greater than the experimental uncertainty. The reason for the large scatter may be related to our observation that a “phase transition” leading to the probable formation of a frozen hydrate of sulfuric acid can dramatically suppress the rate of HONO uptake. If small islands of frozen hydrate form at the surface of the solution, the resulting inhomogeneous mixture might be characterized by a different rate of overall uptake. The data of Zhang et al. show similar scatter (the broken line in Figure 3 is the linear fit selected by the authors). The onset of an observable HONO uptake rate at 55 wt % is interesting in light of a recent Henry’s law constant (KH) determination for the dissolution of HONO in H2SO4 as a function of acid concentration by Becker et al.26 They find that KH decreases exponentially between 0 and 53 wt %; as the H2SO4 is further concentrated, the effective KH increases very rapidly, which the authors convincingly attribute to the onset of the HONO/NO+ equilibrium in solution. It is interesting to note that the observable rate of HONO uptake under our conditions corresponds to the acidity where NO+ formation begins to greatly enhance the amount of gaseous HONO that can be dissolved into the H2SO4 solution. We observed a heterogeneous reaction between HCl and HONO on sulfuric acid only over a very limited range of H2SO4 concentrations. The uptake probability for HCl on the
The Heterogeneous Kinetics of HONO on H2SO4
J. Phys. Chem., Vol. 100, No. 32, 1996 13773
TABLE 7: Summary of Kinetic Measurements process
substratea
kI, s-1
γ
no. of expts
HONO uptake HONO uptake HONO uptake HONO uptake HONO uptake HONO uptake HONO uptake HONO uptake HONO uptake HONO + HClb HONO uptakec HONO + HClc HONO HONO ClNO ClNO ClNO ClNO ClNO HCl + NSA HCl + NSA HONO + HCl(sol) HONO + HCl(sol)
95% SA 87% SA 80% SA 73% SA 67% SA 65% SA 62.5% SA 60% SA 55% SA 60% SA ice ice NaCl (10-1 M) NaCl(s) 95% SA 87% SA 80% SA 73% SA 62.5% SA NSA (s) NSA (sol) 0.1-10 M HCl 10-2 M HCl
24.5 7.3 4.3 ( 1.1 1.4 ( 0.7 5.7 (-1) 6.9 ( 3.5 (-1) 1.7 (-1) 1.6 ( 0.6 (-1) 1.9 ( 0.6 (-2) 1.6 ( 0.4 (-1)
3.1 (-1) 9.3 (-2) 5.5 ( 1.4 (-2) 1.8 ( 0.9 (-2) 7.3 (-3) 8.8 ( 4.5 (-3) 2.1 (-3) 2.0 ( 0.7 (-3) 2.5 ( 0.8 (-4) 1.8 ( 0.4 (-3) γo ≈ 1.0 (-3) 6 (-2)FHCl
