Uptake Mechanism of Atmospheric Hydrogen Chloride Gas in Ice

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The uptake mechanism of atmospheric hydrogen chloride gas in ice crystals via hydrochloric acid droplets Ken Nagashima, Gen Sazaki, Tetsuya Hama, Ken-ichiro Murata, and Yoshinori Furukawa Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00531 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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Crystal Growth & Design

The uptake mechanism of atmospheric hydrogen chloride gas in ice crystals via hydrochloric acid droplets Ken Nagashima,* Gen Sazaki, Tetsuya Hama, Ken-ichiro Murata, and Yoshinori Furukawa Institute of Low Temperature Science, Hokkaido University, N19-W8, Kita-ku, Sapporo 0600819, Japan

*

To whom correspondence should be addressed. E-mail: [email protected]

KEYWORDS: ice crystals, uptake mechanisms, hydrogen chloride gas, advanced optical microscopy, VLS growth

ACS Paragon Plus Environment

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ABSTRACT

Surfaces of ice have attracted considerable attention as “reaction sites” where atmospheric gases cause various chemical reactions in nature. Hence, revealing the uptake mechanism of atmospheric gases on/in ice remains an experimental challenge. Here we show the direct observation of ice crystal surfaces by advanced optical microscopy in the presence of hydrogen chloride (HCl) gas, which triggers a series of chemical reactions that cause ozone depletion. We found that the HCl gas induced the appearance of droplets of HCl solution on ice crystal surfaces. Under supersaturated water vapor pressure, the HCl droplets were quickly embedded in the ice crystals during the growth of the ice. In contrast, under undersaturated conditions, the embedded HCl droplets reappeared on the ice crystal surfaces during the evaporation of the ice. We estimated that the mole fraction of HCl incorporated into the ice as the HCl droplets (0.19% at -15°C) was ten-times larger than the solubility of HCl gas in an ice crystal (0.017%). This picture of the uptake of HCl gas in ice is quite different from the conventional speculation in which HCl gas is confined to ice surfaces.


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1. INTRODUTION Ice has a great influence on the global environment, due to its abundance on the earth.1 During the last couple of decades, “surfaces of ice” have been the focus of attention as “reaction sites” of various atmospheric gasses in nature.2–4 In particular, the interaction between ice surfaces and atmospheric acidic gases is thought to cause the various environmental issues: e.g. the formation of acid snow/rain in clouds,2 the generation of poisonous gases on snow,4,5 and the development of ozone depletion in the stratosphere.6 Among atmospheric acidic gases, hydrogen chloride (HCl) gas has been studied most intensively, since HCl gas on ice surfaces triggers a series of heterogeneous chemical reactions, and chloride radicals resultantly formed under UV irradiation destroy stratospheric ozone layers in spring and induce ozone depletion.6 To promote such a series of chemical reactions on ice surfaces of stratospheric clouds,7 a certain amount of HCl gas first needs to exist on/in ice crystals. Hence, many studies measured the amount of the uptake of HCl gas on/in ice by mass spectroscopy. However, the reported amounts of the HCl uptake showed considerable variations from 1012 to 1015 molecules/cm2,2 demonstrating that the uptake mechanism of HCl gas on/in ice is still unclear. The contribution of bulk ice crystals to the HCl uptake is expected to be less important than that of ice surfaces,8 because of the small solubility of HCl gas in ice9 and slow diffusion of chloride ions in ice.9 Several reports suggested that HCl gas induces the surface disordering of ice and HCl gas is stored in such disordered surface layers.10 The considerable variations in the HCl uptake amount on/in ice could partly result from a lack of spatial and temporal resolution and variations in ice samples, indicating that the technique used to investigate this topic needs to have high spatial and temporal resolution. We and Olympus Engineering Co., Ltd. have developed one such technique namely, laser confocal


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microscopy combined with differential interference contrast microscopy (LCM-DIM),11 which can directly visualize the 0.37-nm-thick elementary steps on ice crystal surfaces.12 Accordingly, we have observed ice crystal surfaces in the presence of HCl gas by LCM-DIM, and found that HCl gas induces the appearance of liquid droplets on ice basal faces.13 In this study, we directly visualized uptake processes of HCl gas on/in ice crystals by LCM-DIM, and then discovered that HCl gas is stored in ice crystals as droplets of an HCl aqueous solution.

2. EXPERIMENTAL SECTION An observation chamber (Fig. S1 in the Supporting Information) had upper and lower Cu plates, whose temperatures were separately controlled by using Peltier elements. At the center of the upper Cu plate, a cleaved AgI crystal was attached as a substrate for the heterogeneous nucleation of ice crystals (sample ice crystals). On the lower Cu plate, a large amount of ice crystals (source ice crystals) were grown by supplying water vapor to the inside of the chamber by nitrogen gas bubbled through water. Then after nitrogen gas including 0.1% HCl gas (partial pressure of HCl gas, PHCl = 100 Pa) was injected into the chamber, the sample ice Ih crystals for the observation were grown heteroepitaxially on the AgI crystal at -15°C. Note that no HCl hydrate crystal was formed despite high PHCl condition,14 because a temperature range adopted in this study (-15 ~ -5°C) was significantly higher than the freezing temperature of the HCl hydrate crystals (Fig. S2). Further detailed information of the sample ice preparation is presented in our previous work.13 By separately changing the temperatures of the sample and source ice crystals, we adjusted the growth temperature of the sample ice crystals (T) and the partial pressure of water vapor (PH2O) in the chamber independently. Since the volume of the source ice crystals was several orders of


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magnitude larger than that of the sample ice crystals, the PH2O in the chamber was determined by the temperature of the source ice crystals. The degree of supersaturation is expressed as σ = (PH2O – Pe)/ Pe, where Pe is the equilibrium vapor pressure of the sample ice crystals.15,16 Note that the equilibrium condition (σ = 0) could be precisely determined by observing the lateral movement of individual elementary steps and the growth and sublimation of crystal edges by LCM-DIM.17,18

3. RESULTS AND DISCUSSION HCl droplets on ice crystal surfaces. When we grew ice single crystals in a nitrogen environment including 0.1% HCl gas, there were many hemispherical objects on the ice surfaces. Fig. 1A1 shows a typical example of the hemispherical objects that appeared on a basal face of an ice crystal under equilibrium PH2O. In Fig. 1, the differential interference contrast was adjusted as if the ice crystal surface were illuminated by a light beam slanted from the upper-left to the lower-right direction, as explained in Fig. S3. Hence, the upper-left and lower-right halves of the hemispherical objects appeared white and black, respectively. As reported in our previous paper,13 from the temporal changes in the shape of the hemispherical objects during their coalescence and splitting, we could judge that these objects were liquid droplets induced by the presence of HCl gas, because of their significant fluidity. The HCl gas-induced droplets (hereafter HCl droplets) always existed on ice crystal surfaces under our experimental conditions: T (-15 ~ -5°C) and PH2O (100 ~ 600 Pa).


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Fig. 1.

Embedding of HCl droplets in ice basal faces. Figures 1A-C show the behavior of the HCl droplets on an ice basal face under supersaturated and undersaturated conditions at T = -10°C. We first gradually increased the PH2O from undersaturated to supersaturated via equilibrium conditions (schematically shown in Fig. 1D). Then, as Figs. 1A2-4 demonstrate, the ice basal face started to grow preferentially from the surface of each droplet: we will show experimental evidence for this preferential growth later in Fig. 4. As a result, bunched steps (marked by the


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black arrows), in which many elementary steps were bundled, appeared from the rims of the HCl droplets, and grew in the lateral direction producing a cylindrical shape. In Fig. 1A, because of inhomogeneity of PH2O in the growth chamber, the growth of the ice basal face always started in the right external area of the field of view, then steps grew laterally to the left. Finally, the cylindrical macrosteps coalesced with each other and the ice basal face gradually became flat, leaving a few droplets and many holes (Figs. 1A5-6). The holes were observed (Fig. 1A6) at the positions where the HCl droplets were originally located (Fig. 1A1): this will be further discussed with regard to Fig. 2. We next gradually decreased the PH2O to undersaturated condition (schematically shown in Fig. 1D). As Figs. B1-2 present, the ice basal face evaporated preferentially from the surroundings of the holes, consequently valleys with flat bottoms appeared around the holes. While the flatbottomed surfaces spread in the lateral direction, the HCl droplets appeared again on the ice basal face (Fig. 1B3). The locations of the reappeared droplets were the same as those where the HCl droplets originally existed (Fig. 1A1). The video of the embedding and reappearance processes is available in Video S1 in the Supporting Information. When the ice basal face experienced undersaturated conditions, the HCl droplets always reappeared (Fig. 1C1). We could confirm the same embedding and reappearance even after the ice crystal was grown for more than 40 min. (Figs. 1C). From these results, we conclude that the HCl droplets are embedded in ice crystals without freezing. In our previous paper,13 we pointed out the possibility that the droplets we observed were not bulk pure water but thermodynamically stable HCl solution, because droplets could stably exist on an ice basal face under undersaturated condition (PH2O = 100 Pa and σ = -0.6) at T = -15°C for 1 h without the solidification or evaporation. With respect to this issue, the in-situ


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observation shown in Fig. 1 provides much stronger evidence than our previous paper. As demonstrated in Figs. 1C, the droplets inside the ice did not freeze for 40 min. If the droplets were pure water, it would be impossible for the droplets to survive without freezing inside the ice crystals and to then reappear from the ice crystals. From this result, we can conclude that the droplets are an HCl aqueous solution. Next, we consider the question - how can we estimate the HCl concentration, CHCl, inside the HCl droplets? Since the HCl droplets in the ice did not freeze, the freezing point of the HCl droplets (TF) was lower than, or the same as, the ice temperature (T): TF ≤ T. From the phase diagram of an HCl aqueous solution (Fig. S2), we know that the CHCl whose TF corresponds to our experimental temperature range (T = -15 ~ -5°C) is 10 ~ 5 wt%. Because the solubility of 100 Pa HCl gas (initially filled in the chamber) in water is higher than the 28 wt% (Fig. S2),19 the CHCl in the HCl droplets could easily reach 5 ~ 10 wt% and prevent freezing. When TF > T of the embedded HCl droplets (Fig. S2A), the ice crystal grows, leading to an increase in CHCl and a decrease in TF. In contrast, when TF < T of the embedded HCl droplets (Fig. S2C), the ice crystal around the embedded HCl droplets melts, resulting in a decrease in CHCl and an increase in TF. As a result, CHCl in the embedded HCl droplets (and even in the HCl droplets located on the ice surface) can automatically become the value whose TF = T (Fig. S2B). We now will further explain more detailed features (Fig. 2) that were observed during the embedding and reappearance of the HCl droplets on the ice basal face. We found two distinctive characteristics of the ice crystals grown just above the embedded HCl droplets. The first is the slower growth of the ice surfaces just above the embedded droplets than the growth of other ice surfaces. Consequently, many hollows (marked by the black arrows in Figs. 2A and B) were formed just above the embedded droplets (Figs. 2A4 and B4). Probably the


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larger amount of strain and/or defects in the ice crystals just above the embedded HCl droplets is responsible for the slower growth, increasing the chemical potential of the ice above the droplets (making the ice more unstable) and hence decreasing the driving force for the growth. The size of the hollows became gradually smaller (Figs. 2A5 and B5), and the ice surfaces finally became flat. As the HCl droplets were gradually embedded more deeply in the ice crystal, the growth rate of the ice surface was recovered. The second is the small HCl droplets (marked by the white arrows in Fig. 2A) newly nucleated on the ice basal faces just above the embedded HCl droplets (Figs. 2A3 and B3): they were only recognized as small protrusions. After such protrusions were embedded in the ice surface, second protrusions appeared again on the ice surface (Figs. 2A6 and B6). Such appearance and embedding of the small HCl droplets was repeated several times. When we gradually decreased the PH2O from supersaturated to undersaturated conditions, the holes and the small HCl droplets (marked by the black and white arrows in Fig. 2C, respectively) reappeared several times (Figs. 2C1-3). Hence, we can conclude that the TF of the small HCl droplets was also equal to T, as explained in Fig. S2. After such events, the relatively large embedded HCl droplets reappeared (Fig. 2C4-6). The nucleation of the small HCl droplets might be also due to the larger amount of strain and/or defects in the ice crystals just above the embedded HCl droplets (due to the more unstable ice), although at present the mechanisms are unclear.


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Fig. 2.

Frequent nucleation of HCl droplets on ice high-index faces. In addition to the ice basal faces, we also observed high-index faces, which were often formed under relatively highly supersaturated condition. Fig. 3A shows an ice single crystal grown under T = -15°C, PH2O = 400 Pa, and σ = 1.4. The upper and lower halves of the ice crystal show basal and high-index faces, respectively: Fig. 3B presents the schematic cross section of the ice crystal. There were many HCl droplets on the high-index face. Although the HCl droplets were embedded in the highindex face as time elapsed, many HCl droplets were always observed on the surface, demonstrating that the nucleation of the HCl droplets occurred simultaneously on the high-index face (Figs. 3C-E and Video S2). The nucleation rate of the HCl droplets on the ice high-index


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face was remarkably higher than that on the basal faces. In addition, on the basal faces the nucleation of the HCl droplets occurred mainly above the embedded droplets (Fig. 2), whereas on the high-index faces the nucleation was spatially random (Figs. 3C-E). These observations indicate that the amount of incorporation of HCl into ice is significantly increased when a highindex ice face is exposed to air. Since the high-index face has a larger surface free energy than the basal face (the high-index face is more unstable than the basal face), we suppose that the nucleation of the HCl droplets on the high-index face and that on the embedded HCl droplets can be understood using the same principles, although the mechanisms are as yet unclear.

Fig. 3.

In order to estimate the amount of HCl incorporated into an ice crystal, we measured the volume of the HCl droplets utilizing interference fringes that appeared on an HCl droplet. Fig. 4 shows a relatively large HCl droplet on an ice basal face at -15°C during the gradual increase in PH2O from undersaturated to supersaturated conditions. Although images of our advanced optical


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microscopy mainly exhibited the differential interference contrast (e.g. upper-left area of the HCl droplet in Fig. 4A1), HCl droplets larger than ~10 µm in diameter also showed interference fringes formed by the interference of two light beams reflected at the outer surface of the HCl droplet and the ice-HCl droplet interface. The lower-right area of the HCl droplet in Fig. 4A1 shows typical interference fringes that correspond to contour lines of the droplet surface: an enlarged image of the area marked by the white rectangle in Fig. 4A1 is shown in Fig. 4B. From the interference fringes and refractive index of the HCl droplets (=1.36 at -15°C), we could determine the height profile of the HCl droplet (Fig. 4B): the refractive index20 was calculated from the HCl concentration (10 wt%) in the HCl droplet whose TF = T (Fig. S2). From the fitting curve shown in Fig. 4B, we determined the contact angle (~10°) of the HCl droplet on the ice basal face. Once we obtained the contact angle, we could calculate the height and volume of the HCl droplets: e.g. the HCl droplet of 2 µm in diameter shown in Fig. 3 had a height and volume of 90 nm and 0.14 µm3, respectively.


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Fig. 4.

Next, we measured the nucleation rate of the HCl droplets in the lower half area of Figs. 3C-E on the high-index face. From the nucleation rate, the vertical growth rate, CHCl and the volume of the HCl droplets, we can calculate the mole fraction of HCl (XHCl) incorporated into the ice crystal as the HCl droplets. Because the elementary steps were not observed on the high-index face, we used the vertical growth rate of the basal face (~0.5 µm/s) as the vertical growth rate of the high-index face. The value of XHCl thus obtained was 0.19%. In contrast, if no HCl droplets are formed on ice surfaces (although we never see such a situation), the maximum amount of HCl incorporation is the solubility of HCl gas in ice (0.017%).9 In conclusion, the embedding of the HCl droplets in ice crystals can incorporate a ten-times larger amount of HCl than the solubility of HCl gas in ice.


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In the previous studies, the amounts of the HCl uptake ranged from 1012 to 1015 molecules/cm2.2 These amounts of HCl can be incorporated into ice crystals of 0.2 to 200 nm in thickness when XHCl = 0.19% utilizing the uptake mechanism found in this study. This comparison demonstrates that when the HCl droplets are incorporated into the bulk of ice crystals (not just on ice surfaces), a much larger amount of HCl can be potentially incorporated into ice crystals because of the high efficiency of the incorporation into the bulk ice. Note that we measured the amounts of the HCl uptake in single ice crystal. In the case of polycrystalline ice such as clouds, the uptake amounts might increase because there is a possibility that the grain boundaries might work as fast diffusion paths of Cl ions. The embedding mechanism of HCl droplets in ice. The most important point of the incorporation of the HCl droplets into ice is the preferential growth of ice crystals from the surface of the HCl droplets, as discussed in Figs. 1A2-3. The observation of the interference fringes on the HCl droplets (Fig. 4) supports this scenario. When PH2O was undersaturated (Fig. 4A1), we could clearly observe the interference fringes, indicating the existence of two light beams reflected at the outer surface of the HCl droplet and the ice-HCl droplet interface. However, after PH2O became supersaturated, the contrast of the interference fringes gradually became faint (Figs. 4A2-4), and finally disappeared (Figs. 4A5 and 6). Such changes in the contrast of the interference fringes indicate that the reflection at the ice-HCl droplet interface was gradually weakened by the freezing of the droplet surface, strongly suggesting the formation of ice crystals on the surface of the HCl droplet. In Fig. 2, the bright circular contrasts shown in the lower-right of the HCl droplets (Fig. 2A1) disappeared as the ice crystal grew (Figs. 2A2 and 3), corresponding to the disappearance in the interference contrast demonstrated in Fig. 4. Most likely, the growth of the ice started from the droplet-ice-vapor interfaces (contact lines), and then


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the ice film covered the droplet surface: this scenario is schematically shown in Fig. 5A. Several previous studies support this scenario, since they reported that the freezing of a water droplet proceeds more easily when an ice nucleus was in contact with a droplet surface than when the nucleus was immersed in the droplet.21–23

Fig. 5.

Finally, we shall consider the case in which the growth of the ice started from the ice-droplet interface. In this case, the nucleation of ice is not necessary. If the ice-droplet interface grew evenly in the vertical direction keeping the shape of the interface flat, as schematically shown in Fig. 5B, the grown ice crystal shows a columnar shape with the droplet on the tip: this is socalled the vapor-liquid-solid (VLS) growth mechanism.24 In this case, the HCl droplet is not incorporated into the ice during the growth. This discussion also strongly suggests that the preferential growth of the ice crystals from the surface of the HCl droplet (Fig. 5A) plays an important role in the incorporation of the HCl droplets into the ice crystal.


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4. CONCLUSIONS In this study, we carried out in situ observation of the HCl droplets on ice crystal surfaces by LCM-DIM. We found that the droplets of a HCl solution are embedded in ice crystals during their growth. In the conventional explanation of the HCl uptake process on ice, it has been thought that HCl gas is stored on ice crystal surfaces, as schematically shown in Fig. 6A, because of the small solubility of HCl gas in ice. However, the high spatial and temporal resolution of the in-situ LCM-DIM observations demonstrates that the embedding of the HCl droplets in the “bulk” of ice crystals allows a ten-times larger amount of incorporation of HCl than the solubility of HCl gas in ice (Fig. 6B). Although the ranges of the temperature and the pressure of HCl gas (T = -15 ~ -5°C and PHCl = 100 Pa) in this study were higher than those in the polar stratospheric clouds (T = -80 ~ -70°C and PHCl = 10-5 ~ 10-4 Pa),10 the insights obtained in this study may open a new avenue for the study of the uptake of atmospheric gases on ice.

Fig. 6.


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Figure Caption Figure 1. The embedding and reappearance of HCl droplets on an ice basal face at -10°C. (A) The embedding process at the supersaturation of σ = 0 (A1), 0.2 (A2) and 0.3 (A3) and (B) the reappearance processes at σ = -0.1 (B1), -0.15 (B2) and -0.35 (B3) were observed by LCM-DIM. (C) Reappearance of the droplets was confirmed even after a long embedding time (48 min.): σ =0 (C1 and C2) and -0.35 (C3). The hemispherical objects on the basal face (A1, B3, C1, and C3) are droplets of an HCl aqueous solution induced by atmospheric HCl gas. The black arrows show bunched steps, which grew from the surfaces of the droplets. (D) Time course of water vapor pressure (PH2O) during the processes of A-C. The area marked by the black dotted rectangle in C1-3 is further shown in Fig. 2. A video of the processes A and B is available as Video S1.

Figure 2. Magnified successive images during the embedding and reappearance processes of HCl droplets on an ice basal face at -10°C: (A) the embedding process; (B) schematic cross sections during the process A; (C) the reappearance process. The images (A) and (C) show the area marked by the black dotted rectangle in Fig. 1C and the time corresponds to the elapsed time of Fig. 1D. The white and black arrows show small HCl droplets and holes, respectively.

Figure 3. Frequent nucleation of HCl droplets on an ice high-index face. (A) An ice single crystal was grown under a relatively highly supersaturated condition (T = -15°C, PH2O = 400 Pa, and σ = 1.4): basal and high-index faces were developed. (B) A schematic cross section of the


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ice crystal shown in A. (C-E) Successive images of the nucleation and embedding processes of HCl droplets in the area marked by the white dot grid in (A). A video of the process C-E is available as Video S2.

Figure 4. Interference fringes formed by the two light beams reflected at air-droplet and dropletice interfaces. (A) Successive images during the disappearance of the contrast of the interference fringes. The upper-left and lower-right areas of the droplet in A1 respectively show typical differential interference contrast and interference fringes. The temperature was kept constant at 15°C, and the water vapor pressure was changed from undersaturated (A1) to equilibrium (A2) and finally to supersaturated (A3-6) conditions. (B) An enlarged image of the area marked by the white rectangle in A1 and the height profile of the HCl droplet determined from the interference fringes. The solid curve shows the result of the curve fitting. The contact angle of the HCl droplet on the ice basal face is estimated to be ~10°.

Figure 5. The embedding mechanism of HCl droplets in ice. (A) The embedding mechanism found in this study. (B) The conventional VLS (vapor-liquid-solid) growth mechanism. Under supersaturated condition, the HCl concentration (CHCl) in the droplets decreases because of the condensation of water vapor (A1 and B1). Hence the freezing point (TF) of the droplets increases, and then a part of the droplets freezes. The values of CHCl and TF are shown when the temperature of ice is -10°C. When the growth of ice preferentially starts from the droplet-icevapor interfaces (contact lines) (A2), the ice films finally cover the droplet surfaces (A3). In contrast to A2, if the ice-droplet interfaces grew evenly (B2), the droplets were not embedded in


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ice by the VLS growth (B3). All our observations of the HCl droplets agreed with the embedding mechanism (A).

Figure 6. A comparison between the conventional explanation and the novel picture of the uptake processes of HCl in ice. (A) The conventional explanation: it has been thought that atmospheric HCl gas is adsorbed and stored on ice crystal surfaces, and that the contribution of bulk ice crystals to the HCl uptake is small because of the small solubility of HCl gas in ice (XHCl = 0.017%). (B) The novel picture found in this study: the HCl gas induces the formation of the droplets of an HCl aqueous solution, and the HCl droplets are embedded in the bulk of ice crystals. The uptake of HCl in the "bulk" of ice crystals can incorporate a ten-times larger amount of HCl (0.19%) than the solubility of HCl gas in ice (0.017%).


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ASSOCIATED CONTENT Supporting Information Observation chamber; freezing temperature of HCl aqueous solution; differential interference contrast (PDF) Video S1. Embedding and reappearance processes of HCl droplets (mpg) Video S2. Frequent nucleation of HCl droplets (mpg)

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Author Contributions K.N., G.S., and T.H. designed the research; K.N., G.S., K.M., and Y.F. performed research; K.N. analyzed data; and K.N. and G.S. wrote the paper. Funding Sources K.N. and G.S. are grateful for the partial support by JSPS KAKENHIs (K.N.: Grant No. 17K05604; G.S.: Grant Nos. 23246001 and 15H02016) and by ESPEC Foundation for Global Environment Research and Technology (Charitable Trust) (ESPEC Prize for the Encouragement of Environmental Studies) (K.N.). Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Y. Saito, S. Kobayashi and K. Ishihara (Olympus Corporation) for their technical support of LCM−DIM and S. Nakatsubo (Hokkaido University) for the development of the observation chamber. ABBREVIATIONS HCl, hydrogen chloride; LCM-DIM, laser confocal microscopy combined with differential interference contrast microscopy; VLS growth, vapor-liquid-solid growth.


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BRIEFS Atmospheric HCl gas induces the formation of droplets of an HCl aqueous solution on ice surfaces, and ice crystals can incorporate large amount of HCl component by embedding the droplets in ice crystals during their growth.


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For Table of Contents Use Only

The uptake mechanism of atmospheric hydrogen chloride gas in ice crystals via hydrochloric acid droplets Ken Nagashima,* Gen Sazaki, Tetsuya Hama, Ken-ichiro Murata, and Yoshinori Furukawa Institute of Low Temperature Science, Hokkaido University, N19-W8, Kita-ku, Sapporo 0600819, Japan *

To whom correspondence should be addressed. E-mail: [email protected]

Atmospheric HCl gas induces the formation of droplets of an HCl aqueous solution on ice surfaces (a), and HCl droplets are embedded in ice crystals during their growth (b-c). By this novel uptake mechanism, ice crystals can incorporate larger amount of HCl component than the solubility of HCl gas in ice.


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Uptake Mechanism of Atmospheric Hydrogen Chloride Gas in Ice

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