The interaction ice – HCl
5.1 First approach to ice – HCl ambient pressure X-ray spectroscopy experiments
5.1.1 Background and Preparation
As demonstrated in Chapter 1, the interaction between ice and HCl is of impor-tance for atmospheric chemistry for more than one reason. HCl is a reactive
halogen species which can facilitate catalytic ozone depletion in the atmosphere (e.g. Platt and Hönninger (2003)). Furthermore, partitioning of HCl to ice
in-fluences the oxidation capacity of the atmosphere. Reason for that is that the rate coefficients of Cl atoms for hydrocarbon oxidation are about103 times higher than those of OH. Hereby Cl atoms are produced for example by reaction of HCl with OH under relatively low humidities preventing wet deposition. The higher rate coefficients of Cl indicate that already low amounts of chlorine compounds have the ability to influence the degradation of volatile organic compounds, thus hydrocarbon oxidation capacity in the atmosphere (e.g. Monks (2005)). In addi-tion to these processes, the interacaddi-tion between ice and HCl is also of exemplary importance for atmospheric chemistry. By using the highly acidic HCl, we expand the range of acidities of trace gases used for direct ice – trace gas interaction ex-periments, thus enable more in depth investigation of the effect of the acidity.
Under environmentally relevant conditions, the investigation of the interaction between ice and HCl is challenging. Until now, the interaction between ice and HCl was never directly examined. One reason for that is the low HCl/H2O pres-sure ratios required for experiments performed under environmentally relevant conditions. HCl partial pressures (p(HCl)) prevalent in the environment are in maximum10−9 mbar therefore a stable dosing of the resulting HCl/H2O pressure ratios may be difficult. For example, the stickiness of HCl, leading to adsorption of HCl to the experimental set-up instead of the ice, is a major challenge. Depending on the experimental set-up, this stickiness may result in unrealistic equilibration times. In addition, the interaction between ice and HCl under environmentally relevant conditions results in very low HCl concentrations on the ice. This makes a direct investigation of the interaction between ice and HCl difficult.
I extensively tested and characterized the newly developed NAPP set-up presented in Chapter 2.2. As a result of these investigations, together with the gained knowl-edge, I am able to conclude that the HCl-ice interaction studies are feasible. Low p(HCl) is achieved by admitting dilute HCl to the in situ experimental cell using a high precision leak valve connected to Teflon tubing.
Indeed, using the NAPP set-up I was able to directly analyze the interaction be-tween HCl and ice. Already after about one hour of equilibration time, I detected HCl on the ice sample. Unfortunately, the actual HCl concentration in the gas phase, thus partial pressure of HCl in the in situ experimental cell, could not be determined during the performed experiments. Mass spectrometric analysis was not possible due to technical difficulties. Using the mixing ratio of HCl in N2 we can roughly estimate the upper limit of HCl pressure in the in situ experimental
cell by measuring the absolute pressure change. As discussed, for example, in chapter 4, the determination of the actual HCl pressure at the sample spot was accompanied with difficulties. In addition, the absolute pressure changes for the low dosing experiments of HCl, was below or close to the detection limit of changes in the pressure reading. Thus the investigation of the HCl partial pressure was only possible in a really limited way.
5.1.2 1st results
-Figure 5.1: Cl 2p PE spectra of the interaction ice – HCl normalised to the respective O1s PE spectrum. Purple colour indicates measurements for which no induced DI was observed. The spectra were acquired at SIM X11MA using a photon energy of ~600 eV. The ice temperature was ~250 K.
As visible in the Cl 2p PE spectra displayed in Figure 5.1, the interaction of HCl with ice leads to several chloride features on the ice surface. The Cl 2p PE spectrum of HCl interacting with ice consists of two doublets. Those two doublets can be interpreted as one representing molecular HCl and one dissociated HCl (e.g. Parent and Laffon (2005)). For the lower p(HCl) dosing, most of the HCl is present as molecular HCl on the ice surface. For the highest dosing, dissociated HCl is clearly the dominant species. Due to the low amount of Cl molecules on the ice, the acquisition time of the spectra was between 0.5 for the two upper spectra and 0.75-1 hour for the lower ones.
At the highest p(HCl), also a clear change of the DI can be observed as displayed in Figure 5.2. The O K-edge NEXAFS indicates the formation of a liquid (like) layer at the ice surface. More precisely, a strengthening of those features in the spectrum that represent water in liquid water, or in aqueous solution could be observed. However, whether actually a liquid layer emerges, the DI actually thick-ens, or the H2O molecules simply reorder in a way similar to an aqueous solution, remains open. For a more detailed discussion about the use and interpretation of O K-edge NEXAFS analysis of changes in the hydrogen-bonding network of the ice surface see Chapter 4.1.4. In general, I interpret the observation in a way that the increased mobility of the H2O molecules in the enhanced DI seems to facilitate dissociation. However, since chloride ions may also compete with ice for H2O molecules to establish a sufficient hydration shell, it might also be that more H2O molecules aggregate in structures as in an aqueous HCl solution.
Directly analyzing the interaction between HCl and ice, we discovered a hitherto not observed interplay between concentration of HCl, dissociation and change of the DI. One can only observe a change of the molecular structure of the ice surface layer and dominant dissociation close to the phase transition. For lower p(HCl), no effect on the natural DI can be observed and molecular HCl is the dominant species.
The detection of molecular HCl on the ice surface is a major finding which contra-dicts previous results and assumptions. For example, Th. Huthwelker postulated that molecular HCl is only present on ice surfaces in case of carbon contamination of the ice (personal communication). However, for the lowest p(HCl), thus the most intense molecular HCl signal, I observed only minor carbon contamination.
For higher p(HCl)s, thus less intense molecular HCl signals, the carbon contamina-tion was more pronounced but still magnitudes lower than the HCl concentracontamina-tion.
These observations support the presence of molecular HCl on the ice. A more in depth discussion on the observation of molecular HCl can be found in Chapter 5.2.
As with many X-ray studies, there is a possibility of beam damage during mea-surements. In our case, a decrease of the chlorine signal intensity with increasing exposure to the beam was observed. We attribute this to beam damage, more precisely radiolysis and oxidation of chloride, followed by desorption of atomic or molecular chlorine to the gas phase. Such effect was most pronounced on the signal of dissociated HCl, which decreased tremendously with increasing exposure. How-ever, the general picture of the observed change of the molecular HCl-to-dissociated HCl-ratio does not change due to beam damage, since the exposure of the sample
Normalised electron yield [a.u.]
560 555
550 545
540 535
530
Photon energy [eV]
Clean Ice
O K-edge
Figure 5.2: Auger yield O K-edge NEXAFS measurements of the interaction between HCl and ice. Purple colour indicates measurements for which no induced DI was observed.
The spectra were acquired at SIM X11MA.
to the beam are similar for the upper spectra presented in Figure 5.2.
However, due to the observed beam damage, and the fact that low HCl partial pressures were below the detection limit of the MS, no closer and quantitative analyses were conducted for the case of low p(HCl). In the following sections, we describe further analyses of higher p(HCl) cases, inducing an enhanced DI.