2802
J . Phys. Chem. 1993,97, 2802-2803
D. R. Hanson' and A. R. Ravishankara' Aeronomy Laboratory, National Oceanic and Atmospheric AdministrationlERL, R/E/AL2, 325 Broadway, Boulder, Colorado 80303, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309
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Received: January 6, I993
Keyser, Leu, and Moore' (KLM) argue that the ice films used in our recent studies2 are very porous and the true uptake coefficients are much smaller than those measured. They base this assertion on their recent elegant theoretical modeling and surface characterization studies. They claim that the measured invariance of the uptake coefficient with substrate thickness observed by us is not a stringent test for porosity. Further, they use some of our data in their model and show that they are consistent with a highly porous substrate when assumptions made in the models are different from those we made. The bottom line of their argument is that the substrates used in laboratory studies need to be characterized, and they imply that the uptake coefficients measured by us are not applicable to the real atmosphere. We agree with KLM that it is necessary to characterize the ice surfaces used in laboratory studies of heterogeneous reactions where the ice surface is a surrogate for polar stratospheric clouds (PSC). We also agree that there are probably pores in our ice surfaces and that they may affect our measured uptake rates. However, we disagree with them about the magnitude of the contribution of pores to our measured uptake coefficients, and we believe that our measured values are applicable to the atmosphere. We suggest that the ice surface can also be characterized from observations such as those in our studies, rather than those of KLM. Moreover, we suggest that the conclusions of KLM, which are based on their theoretical treatment of their observations, are not unambiguous. Whereas KLM do not use the same ice samples to measure the physical parameters used for the characterization and to measure the uptake coefficients, we have performed diagnostics on the ice surface upon which the heterogeneous chemistry studies were performed. In this paper, we report results of the measurements carried out to obtain information on the ice surfaces used in our laboratory and comment specifically on the points raised by KLM. We conclude that the large porosity effect predicted by KLM does not appear to be real or at least they do not apply to our ices. We characterize the composition of the upper most layer of the ice surface from the measured vapor-phase concentrations of the constituents. The information about the physical characteristics of the surface, which is the point of contention between KLM and us, is based on the observed surface coverage and the uptake coefficients as a function of the substrate thickness. W e argue that if the surface area that is important for the reactions we studied changes with thickness, the number of molecules taken up by the surface must also change. In addition, since the number of molecules taken up correspond to that expected for a smooth, nonporous surface, the assumed surface area must be realistic. Lastly, our arguments are based on physical simplicity and correlation of various observations made during o u r measurements. These points will be made below. Thedetermination of the reaction probabilities, y,as a function of ice thickness, were performed as before2 for C10N02 and N205 on pure ice and on HN03-coated ice. The latter surface we believe to be ice with a NAT "monolayer" on its surface.3 These data are shown in Figures 1 and 2 as plots of y versus ice 0022-3654/93/2091-2802~04.00/0
... ClONO,
Response to "Comment on Porosities of Ice Films Used To Simulate Stratospheric Cloud Surfaces"
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Figure 1. Plots of measured uptake coefficients, y, for CION02 as a functionofsubstrate thickness. Thediamonds arevalues fromourprevious paper.2 The open symbols are for uptake on pure ice surfaces and the closed symbols are for uptake on ice with a monolayer of NAT (see text). The filled square is for a substrate compsed entirely of NAT. The value of the uptake coefficient measured on bare cold glass in the presence of water vapor is also shown in the figure. Notice tht even for uptake on a N A T surface, which is very small, the dependence on thickness is very small. The quoted thickness can be converted to a geometrical thickness by knowing the density of the substrate. If the specific density is 1, a 0.1 mg c m 2 surface would be 1 Fm thick.
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Figure 2. Plots of measured uptake coefficients, y,for N2Os as a function of substrate thickness. The diamonds are reanalyzed values from our previous paper.2 The open symbols are for uptake on pure ice surfaces, and the closed symbols are for uptake on ice with a monolayer of NAT (see text). The value of the uptake coefficient measured on bare cold glass in the presence of water vapor is also shown in the figure. Notice that even for uptake on a NATsurface, which is very low, the dependence on thickness is very small.
thickness, given in mass per unit area. Along with these new measurements, we have reanalyzed our previous data for N2O5, and they are also plotted in Figure 2. This more careful reanalysis of our previous data was performed because KLM extracted parameters from only one set of the published data for their model calculation and we want to provide the most accurate values possible for such calculations. This reanalysis consisted of averaging all the previously measured, but not reported, data which improved the standard error of the mean. The corrected data all fall within a factor of 2 of the originally reported data and are within the uncertainty of those measurements. Finally, for the C10N02 H 2 0reaction, the number of molecules reacted per unit surface area to transform the ice surface into a NAT monolayer was also determined for each surface, and the results are shown in Figure 3 as open diamonds and circles. Also shown in Figure 3 is the number of HCI molecules adsorbed per unit area as a function of ice thickness, as was previously performed.
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0 1993 American Chemical Society
Comments
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 28803
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Figure 3. Plots of the number molecules per unit area (assuming the surface area to be the geometrical area) taken up to saturate the surface and form a monolayer as a function of thickness for CION01 (circles and diamonds) and HCI (filled squares). The lackofdependenceon thickness is consistent with the surface not being porous.
rate of -0.5 g h-I, and total geometrical surface area of the ice of -70 cm2. It is worth pointing out that our ice surface may be slightly heated (AT I 10 K) during deposition due to the heated injector and the latent heat release. In our earlier paper,2we suggested that one possible reason for the high value of surface area measured by KLM was that they used nonpolar, "small" gases such as N2, Ar, and He. Crystallographic studies of NAT crystals show a very large number of channels through which such molecules may diffuse.5 We did not, and do not now, claim that this is the explanation. It was offered as one possible reason. The theory developed by KLM to explain the nondependence of the uptake rate coefficient on thickness requires that the granule size increase with thickness as the void space also increases.These two effects together would indeed account for the previously observed lackof dependence of uptake coefficient on the thickness. The reason for the simple assumptions made in our earlier calculations2 was that the more elaborate calculations made by KLM are based on information that were either not published or published after our paper came out! However, the new measurements presented here on surfaces that are only -0.20.3 pm thick would not be consistent with the calculations of KLM, unless the granule size keeps decreasing even at this low thickness or some other process is invoked. In summary, it appears that there may be differences in the morphology of the ice surfaces used by us and KLM. The surface area derived from the surface characterization experiments of KLM are not consistent with those inferred from surface coverage measurements. The changes in uptake coefficient with the substrate thickness suggests that our surface is "smoother" than that suggested by KLM, but it is not inconsistent with a porous substrate. However, since the surface area inferred using the molecules whose reactive uptake is being measured is consistent with the assumption of a smooth surface, we believe that our measured uptake coefficients are applicable for atmospheric modeling.
The scatter in the ClON02 data is greater than that for HCl for two reasons: (1) an unmonitored HNO3 impurity in the CION02 and (2) a less accurate calibration for ClON02. Nonetheless, the data shown in Figures 1 and 2 do not reveal a large effect due to porosity down to a thickness of -0.2 pm (assuming a density of 1 g cm-3). There is additional evidence that indicates the porosity of our ices is small. In measurements of the uptake of H N 0 3 onto ice and frozen sulfuric acid surfaces? the number of molecules required to form a NAT monolayer was identical on both surfaces, a surface coverage of 5 X 1O t 4 molecules cm-2. This value is what is expected for a monolayer of NAT, assuming that the available ice surface area is the geometric area of the flow tube wall. In addition, the uptake and the surface coverage for a NAT monolayer were identical on the two, very differently prepared, surfaces. It is unlikely that the frozen sulfuric acid was highly porous because it was grown from a liquid solution that had the exact composition of the tetrahydrate, 57.7% (w/w) H2S04. The same observations were made on many such H2S04solid surfaces; it is unlikely that all such solids formed would have the same porosity and that this porosity would be the same as that for thin ice layers. Lastly, it is difficult to understand why an ice surface Acknowledgment. We thank Leon Keyser for helpful clarifications on their experimental conditions. This work was funded that is constantly "annealed" in the presence of water vapor, which is adsorbing and desorbing, would be so porous. by NOAA's Climate and Global Change program. It is clear that there are differences in the observations made References and Notes by KLM and us and it is not simply a case of differences in interpretation. Why is there such a difference between the (1) Keyser, L. F.; Leu, M.-T.; Moore, S.B. J. Phys. Chem., preceding surfaces studied by KLM and US? IS it real? If it is real, the paper in this issue. (2) Hanson, D. R.; Ravishankara, A. R. J. Phys. Chem. 1992,96,2682. answer may lie in deposition conditions, either intentional or ( 3 ) Hanson, D. R.; Ravishankara,A. R.J. Geophys. Res. 1991,96,5081. unnoticed. Our ice was deposited under the following condi(4) Hanson, D. R. Geophys. Res. Lett. 1992, 19, 2063. tions: flow tube temperature of 191 K, total He pressure of -0.5 (5) Turco, R. P.; Toon, 0. 8.;Hamill, P. J. Geophys. Res. 1989, 94, Torr, Hecarrier gas flowrateof --4STP~m~s-~,H~Odeposition16493.
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