7779
J. Phys. Chem. 1993,97, 7779-7785
Uptake of HCI in Water Ice and Nitric Acid Ice Films Liang T. Chu, Ming-Tam Leu,' and Leon F. Keyser Earth and Space Sciences Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 109 Received: February 26, 1992; In Final Form: April 26, 1993
The uptake of HCl in water ice and nitric acid ice films has been investigated in a flow reactor interfaced with a differentially pumped quadrupole mass spectrometer. These studies were performed under experimental conditions that may mimic the polar stratosphere. The HCl uptake in ice films at 188 and 193 K was determined to be in the range of 8.7 X 1013 to 1.8 X 1015molecules/cm2 (if the geometric area of the flow reactor, 290 cm2, was used in the calculation) when HCl partial pressures of 7 X 10-8 to 6 X 1 0 6 Torr were used. On the basis of a model which accounts for the total surface area of the films, the true surface density could be a factor of 25 lower than that calculated by the geometric area. A slightly higher uptake was observed at the lower temperature of 188 K. The uptake of HC1 in ice was significantly enhanced by using an HCl partial pressure greater than 1 X Torr. The observation was found to be consistent with the formation of the hexahydrate or the trihydrate of HC1 according to the phase diagram of the HCl/H20 system. The uptake of HCl in nitric acid ice at 188 K was determined to be in the range of 8.0 X 1013to 5.3 X lOI4 molecules/cm2 at a HCl partial pressure of 4.5 X lo-' Torr. Measurement of both H N 0 3 and H2O vapor pressures was made to positively identify the formation of nitric acid trihydrate (NAT) surface according to the phase diagram of the HNOs/ H2O system. The HCl uptake in N A T is comparable to that in water ice in the present experiment, but significantly smaller than the previously reported values by Mauersberger and his co-workers. Implications of these results for the heterogeneous chemistry of the polar ozone depletion are briefly discussed.
I. Introduction Since the discovery of the Antarctic ozone hole, heterogeneous reactions on the surfaces of polar stratospheric clouds have been thought to play an important role in the depletion of ozone.l-lo Two main classes of clouds have been described: type I consists of nitric acid-water ice at about the composition of the trihydrate (NAT) and type I1 consists of water ice with small amounts of acidic impurities. Heterogeneous reactions on these ices are responsible for converting inactive chlorine (for example, HCl or ClON02) to active forms (Clor ClO), whichsubsequently remove ozone by catalytic reactions. There are three HCl reactions
--
+ ClONO,(g) C12(g) + HNO,(s) HCl(s) + N20,(g) ClNO,(g) + HN03(s) HCl(s) + HOCl(g) Cl,(g) + H20(s) HCl(s)
(1) (2)
(3) that are particularly important in the production of active chlorine. Thus, the uptake of HCl by nitric acid and water ice substrates is an important parameter needed to understand the heterogeneous chemistry. Recently Wolff et al.11 studied the incorporation of HCl into ice crystals by using X-ray analytical techniques. They reported that HCl incorporation into ice crystals is limited at low temperatures and determined the HC1uptake to be 0.0018-0.009 in units of mole fraction. Hanson and Mauersbergerl2J3 performed a series of mass spectrometric studies of HCl vapor pressures over ice crystals in a static vacuum chamber and concluded that the solubility of HCl in ice is in the range of 2 X 10-5 to 1 X 1 V at a partial HCl pressure of 10-7 Torr and 200 K. Marti et al.14 also reported a value of less than 2 X 10-5 in their study. There are a few studies on the solubility of HCl in nitric acid ices. Hanson and Mauersbergerl2J3reported that the solubility of HC1 in NAT is about 0.0035-0.005 at HC1 partial pressures of 10-7-106 Torr and at temperatures of 185-200 K. Marti et al.14 confirmed these results and further suggested that the enhancement of HCl uptake in NAT is due to the presence of 0022-365419312097-1779$04.00/0
HN03. An experimentaltechnique similar to that used for water ice was used in their studies. In an investigation of reaction 1, Moore et al.15studied the uptake of HCl in HN03-HzO ices at higher HCl partial pressures and reported that the uptake is in the range of 4 X to 0.15 at HCl partial pressures of lV-10-3 Torr, depending on the acidic composition of the substrate. While this work was in progress, Hanson and Ravishankara16 studied HCl uptake by H20 ice. They used a chemical-ionization mass spectrometer and monitored HC1 by the ion-molecule reaction F- + HC1- C1- HF. They obtained a value of 5 X 1014molecules/cm2 with an ice film thickness of 10 pm at 191 K. They also reported a factor of 2 increase from 3 to 30 pm. In addition, they found that the HCl uptake is independent of P(HC1) over the range of 7 X 1W-2 X 10-6Torr. Another study of this system was carried out by Abbatt et al.,17who used a flow reactor coupled with an electron-impact ionization mass spectrometer. They carried out four experiments at 201 K with HCl partial pressures in the range from 1 X 10-6 to 4 X 1od Torr. In one experiment the ice film thickness was 35 pm and the uptake was measured to be 3 X l O I 5 molecules/cm2. Three other experiments were performed with thinner ice films (5-10 pm) and the uptake was found to be 1 X loL5molecules/cm2,which is a factor of 3 smaller than that found for the thicker substrate. Using a similar technique, Abbatt and Molina18also reported the HCl uptake on the surface of NAT. They found that the uptake is a strong function of water vapor pressure. In order to resolve the discrepancies in the existing literature values, we investigated HCl uptake by using our recently constructed, highly sensitive quadrupole mass spectrometer. In the sections that follow, we describe the experimental methods used to characterize the substrates and to measure the uptake. The results are then summarized, discussed, and compared with previous measurements. Finally, we briefly discuss the possible implications of our results for polar ozone depletion.
+
II. Experimental Section The uptake measurement was carried out in a tubular flow reactor coupled to a differentially pumped quadrupole mass 0 1993 American Chemical Society
Chu et al.
7780 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 ROTARY PUMP
I REACTANTS INLET
HELIUM INLET
PREA
WATER VAPOR INLET
L COUNTER
Figure 1. Schematic diagram of the experimental apparatus. The sliding injector was used to deposit either water ice or NAT on the wall of the flow reactor. HCI was admitted alternately into the reactor through the reactant inlets located at the downstream and the upstream ends as shown. A differentially pumped quadrupole mass spectrometer was employed as a detector for monitoring HCI, H20, and HNOl concentrations.
spectrometer (QMS). The experimental apparatus is shown in Figure 1. As mentioned in section I, the mass spectrometer was recently constructed and a detailed description is given in this section. The flow reactor and its accessories have been used previously in our investigation6JJg of reactions 1 and 2 and will be described only briefly. The flow reactor was constructed of borosilicate glass, and its dimensions are 2.8 cm inside diameter and 33.0 cm in length. The geometric area of the flow reactor is about 290 cmz, The temperature of the reactor was regulated by a refrigerated rhethanol circulator (Haake, Model FK2) and measured by a pair of thermocoupleslocated at the end section and in the middle section. During the experiment the temperature was maintained within 0.5 K. The pressure inside the reactor was monitored by a high-precision pressure meter (MKS Instruments, Model 390 HS, 10 Torr full scale), which was located about 2 cm from the flow reactor at the downstream end, as shown in Figure 1. The ice film was prepared as follows: Helium carrier gas was bubbled through a water reservoir which was kept in a constanttemperature circulator, normally at 293 K, and the helium gas saturated with the water vapor was admitted to the inlet of the sliding Pyrex injector. During the period of deposition the sliding injector was slowly pulled out at a constant speed and a homogeneous ice film was coated on the inner surface of the reactor, which was held at temperatures of 188 or 193 K. After the ice film was prepared, the injector was kept at the upstream end in order to prevent warming of the substrate. A detailed discussion of warming by the injector has been given previou~1y.I~ The amount of ice substrate deposited was calculated from the water vapor pressure, the mass flow rate of the helium-water mixture (which was measured by a Hasting mass flow meter), and the deposition time. Some of the substrates were transferred to a U-tube at 77 K and weighed on an analytical balance. The results from these two methods are in good agreement. In a typical experiment the mass of the ice substrate deposited in the flow reactor was about 25 mg. Average film thickness was calculated by using the measured geometric area and weight of
the deposit with a value of 0.63 g/cm3 for the bulk density of vapor-deposited water ice;z0the results ranged from 0.5 to 16
m. HCl-He mixtures were prepared by mixing Matheson semiconductor-purity HCl (99.995%)and Matheson-purity helium (99.9999%)in a glass manifold which was previously evacuated to 10-6 Torr. Flow rates of the mixtures were monitored by a Hasting mass flowmeter. At first the He-HCl mixture was admitted to the flow reactor through an inlet located at the downstream end;this bypassed the ice film and allowed the vacuum lines to be conditionedwith HCl. At the start of a typical uptake measurement, the flow was redirected through another inlet at theupstreamend of the ice film. Note that HCl was not admitted to the flow reactor through thesliding injector, in order to prevent its adsorption on the inner surface of the injector. Uptake measurements of HCl in HN03-HzO ices were performed in a similar manner, except that a nitric acid solution rather than water was used in the bubbler. After the substrate was transferred from the reactor to a U-tube at liquid nitrogen temperature, bulk HNOs-HzO ice compositions were determined by standard acid-base titration. The bulk density of vapordeposited HN03-H20 ice films was found to be 0.82 g/cm3 in a separate investigation,z0and this value was used to calculate the average film thickness. We constructed a differentially pumped quadrupole mass spectrometer capable of monitoring HCl partial pressures in the flow reactor as low as 5 X 1 V Torr (1.6 X 109 molecules/cm3 at 298 K). Such high sensitivity was achieved by properly designing the sampling pinhole in the first vacuum chamber and by using ultra-high-vacuum techniques with pulse-counting electronics. The first vacuum chamber was evacuated by a 1500L/s turbomolecular pump (Balzers, Model TPU-1500). The vacuum chamber in which the mass spectrometer was installed wasevacuatedby a 500-L/s turbomolecular pump (Bahrs, Model TPU-510). In order to achieve a better vacuum, another turbomolecular pump (60 L/s, Balzers, Model TPU-60) was operated in series with the 500-L/s turbomolecular pump. Two high-capacity rotary pumps (Sargent Welch, Model 1375 and
Uptake of HCl in Water Ice and Nitric Acid Ice Films Model 1397) were also used in conjunction with the turbomolecular pumps to evacuate the vacuum chamber. The ultimate vacuum was better than 2 X 10-*0Torrafter thevacuum chamber was baked at 473 K for 60 h. The quadrupole mass spectrometer, supplied by Extrel Corp., consisted of a cross-beam electron-impact ionizer, a set of quadrupole rods (9 in. long by 3 / 4 in. diameter), a high-gain electron multiplier, and a C-50 electronics module. The mass spectrometer can be operated from 1 to 250amu. A pulse counting technique was used for detection because of low signal intensity. The channeltron output of the QMS was sent to an amplifierdiscriminator (MIT Model T-50), and the TTL pulse output from the discriminatorwas registered in a counter (EG&G Model 994) in synchronization with the beam stopper that was located in the first vacuum chamber. The HC1 signal at 36 amu was calibrated by measuring a known amount of HCl concentration in the flow reactor. Typically, the S / N ratio is about 1 at an HC1 partial pressure of 5 X lo-* Torr for 30-s integration. Ice films formed from vapor deposition were also examined by environmental scanning electron microscopy (ESEM). In these experiments water vapor was condensed on an aluminum or a Pyrex plate that was maintained at a temperature between 160 and 215 K; total pressures were 2-4 Torr in a N2 carrier. Hexagonally shaped ice crystals, some of which were nearly perfect and some with defects, were observed in the photomicrographs. The size of the ice crystals was smaller than the thickness of the ice film. Moreover, grain size increased with the thickness. The total surface areas of the ice films were greater than the area of the aluminum or Pyrex plate. Detailed information is given in separate publications.20J Nitric acid-water ice films were previously investigated near 200 K by transmission infrared spectroscopyin our laboratoryZZ and elsewhere.23-2s Recently, we have constructed a reflectance infrared cell in which a temperature-controlledgold-coated copper plate was used to support ice films formed by vapor deposition.26 Experiments were performed in the temperature range 180-200 K with various compositions from pure water to pure nitric acid. At 54 wt % HN03, the data suggest that the film contains pure NAT under the experimental conditionsused for the HCl uptake measurement. The identificationwas based on the characteristic infrared absorption spectra of oxonium ions and nitrate ions. The substrate with less than 54 wt % may be comprised of substantial quantities of water ice, which was identified by its absorption features at 3240 cm-l (OH stretch) and 850 cm-l (HzO rotation), superposed on the NAT spectra (both CY and fl forms) at 1380 cm-1 (nitrate ions) and 3 100-3500 cm-1 (OH stretch in oxonium ions). Although the substrate comprises both water ice and NAT, the surface may be in equilibrium with vapor containing H20 and HN03 in the reactor. Because of the sublimation of water vapor from the substrate due to pumping or flushing by helium, addition of a trace of water vapor over the substrate was required in order to obtain and maintain the NAT surface. Details are given in the next section.
In.
Results
HCI Uptake in Water Ice Films. Typical HC1 uptake data for various ice film thicknesses are shown in Figure 2. The total amount of HCl adsorbed in the ice films was obtained by integratingthe HC1 flow rates and calibratedHCl signal intensities over time. The film thickness was varied from 0.50 to 15.7 Mm, and the HCl partial pressure was held at (2.1 f 0.1) X lo-' Torr. These results are summarized in Figure 3, which shows the HC1 uptake in units of surface density as a function of film thickness. The uptake increases by approximately a factor of 5 when the film thickness is varied from 0.5 to 15.7 pm. HCl uptake in ice at 193 and 188 K at a film thickness of 1.4 f 0.2 pm is shown in Figure 4 for HCl pressures ranging from about 7.3 X 10-8 to 5.6 X 10-6 Torr. The results indicate that
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7781
.-
10
1
c,
OB3
C
a
a-
4
6-
-a
4-
C
P
.
A
iij
1
uI
2 -
-
0 0
'
Average Thickness
0
A
0.50pm 3.16pm 7.54pm 15.7pm
4
I .
9
0
0-
0
30
60
120
90
150
Time (min) Figure 2. Time dependence plot of HCl uptake data at P(HC1) = (2.1 f 0.1) X lo-' Torr and T = 188 K. The thickness of the ice film is varied from 0.5 to 15.7 pm. Note that the rise time is much longer for thicker ice films.
1
Ice Film Thickness (pml
Figure 3. Plot of HCl uptake vs the ice film thickncss at P(HC1) = (2.1 f 0.1) X lo-' Torr and T 3: 188 K. The line through the data is the best fit obtained by using a layer model for the ice film. See text for details.
I
'
'0
" "188K "'
V
193K
'
'
"""'
'
'
" 'I O
i
"%
8q
a
10.8
107
10 6
105
HCI Pressure (Torr)
Figure 4. Plot of HCl uptake vs partial HCl pressure at 188 and 193 K. Film thickness is 1.4 f 0.2 pm. The solid lines drawn in the figure are derived from a linear least-squares fit of the data. Note that the HCl uptake at 188 K is slightly greater than that at 193 K.
in general HCl uptake increases with HCl partial pressure. They also show that at 188 K there is a larger HCl uptake than that at 193 K. A set of experiments were also performed at 198 K.
Chu et al.
7782 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993
lo-' 10-6
i 1 ....
10-
I
-10
I
I
I
0
10
20
30
Time ( m i n )
Figure 5. Plot of the vapor pressures of HCI, HNO3, and H20 before, during,and after the uptake of HCI on a HNOo-HzO substratewith bulk compositionof47.1wt%HNO3. VaporpressuremeasurementsofHNO, and H20 over the substrateare consistent with a surface composition of NAT. Zero time indicatesthe beginning of HCI flow over the substrate. See text for details.
TABLE I: The HCl Uptake in HNOa-HzO Ice Films at 188 K' adsorption desorption net uptake P(H20): (molecules/ (molecules/ desorption: (molecules/ P(HNO3)b cm2) cm2) adsorption' cm2) 290 1.92 x 1014 2.10 x 1014 1.09 o 430 5.07 X l O I 4 1.94 X loL4 0.24 3.13 X loL4 7.74 X 1013 0.96 2.60 X 10'2 660 8.00 X 10" 0.65 1.74 X l O I 4 665 5.11 X l O I 4 3.37 X lo1' 2000 5.25 X 10'' 1.53 X 1014 0.29 3.72 X l O I 4 See text for further details. b Pressure ratio. c Peak ratio. The HCl uptake at 198 K is smaller than that a t 193 K. Unfortunately, the ice films evaporated rapidly during the experiment and accurate HCl uptake measurements were not feasible. A few experiments were also carried out at higher HCl partial pressures (greater than 1 X 10-5 Torr) at 193 K. The uptake of HC1 is significantly enhanced as compared with the data taken at lower HCl pressures (lO-8-lW Torr). We attribute this effect to the formation of hexahydrate or trihydrate of HCl according to the phase diagram of the HCl-H20 system.l3 HCl Uptake in HNOJ-H~OIce Films. For these experiments the substrates were prepared by co-condensation of water and nitric acid vapors at 188 K. The HCl partial pressure was about 4.5 X Torr, and the film thickness was 1.6 pm. Acid-base analysis yielded 47.1 wt 7%for the bulk composition of the HN03H2O substrate. As discussed in the previous section, infrared analysis of this type of ice film showed that it may comprise a mixtureofwatericeandNAT. Duetopumpingor toentrainment by helium gas, water vapor may preferentially sublime from the surface of this substrate. Therefore, in these experiments a trace of water vapor was added to manipulate the surface composition. Formation of H20 ice was prevented by keeping the H2O partial pressure below itsvalueat the frost point. Typical data areshown in Figure 5. HC1 molecules were found to adsorb and desorb during the uptake experiment. Furthermore, vapor pressures of H20 and HN03 were found to change significantly. Data for HCl uptake in H N O r H 2 0 ice films are summarized in Table I. The P(H20)/P(HNO3) ratio was varied from 290 to 2000. The slope of a plot of log P(HN03) versus log P(H20) is about -3 as shown in Figure 6, a condition for the formation of NAT27v2* on the surface. The adsorption was found to be in the range of 8.0 X 1013 to 5.3 X 1014molecules/cm2. The isothermal HCl desorption was observed in all experiments.
-7
-6
-5
-4
-3
-2
-1
0
Log ,o PHz0 (Torr)
Figure 6. Vapor pressures of HzO and HNO3 during HCl uptake plotted in the phase diagram for the HNO3-HzO sy~tem.~~.2*
IV. Discussion HCl Uptake in HzO Ice Films. Bulk Diffusion. The amount of HCl that may dissolve in the bulk ice depends on the exposure time and the diffusion coefficient of HCl in H20 ice. Using a simple equation z = fi29-31 where z is the distance HCl can diffuse, t is the exposure time, and D is the diffusion coefficient, we may estimate z. Wolff et al." reported an apparent diffusion coefficient of
