Antarctic Ozone Depletion Chemistry: Reactions of N2O5 with H2O and HCl on Ice Surfaces

Formulation of the cosmic ray-driven electron-induced reaction mechanism for quantitative understanding of global ozone depletion

This paper formulates the cosmic ray-driven electron-induced reaction as a universal mechanism to provide a quantitative understanding of global ozone depletion. Based on a proposed electrostatic bonding mechanism for charge-induced adsorption of molecules on surfaces and on the measured dissociative electron transfer (DET) cross sections of ozone-depleting substances (ODSs) adsorbed on ice, an analytical equation is derived to give atmospheric chlorine atom concentration: [Formula: see text] where Φe is the prehydrated electron (epre-) flux produced by cosmic ray ionization on atmospheric particle surfaces, [Formula: see text] is the surface coverage of an ODS, and ki is the ODS's effective DET coefficient that is the product of the DET cross section, the lifetimes of surface-trapped epre- and Cl-, and the particle surface area density. With concentrations of ODSs as the sole variable, our calculated results of time-series ozone depletion rates in global regions in the 1960s, 1980s, and 2000s show generally good agreement with observations, particularly with ground-based ozonesonde data and satellite-measured data over Antarctica and with satellite data in a narrow altitude band at 13 to 20 km of the tropics. Good agreements with satellite data in the Arctic and midlatitudes are also found. A previously unreported effect of denitrification on ozone loss is found and expressed quantitatively. But this equation overestimates tropospheric ozone loss at northern midlatitudes and the Arctic, likely due to increased ozone production by the halogen chemistry in polluted regions. The results render confidence in applying the equation to achieve a quantitative understanding of global ozone depletion.

Interfacial Vibrational Dynamics of Ice Ih and Liquid Water

Insights into energy flow dynamics at ice surfaces are essential for understanding chemical dynamics relevant to atmospheric and geographical sciences. Here, employing ultrafast surface-specific spectroscopy, we report the interfacial vibrational dynamics of ice I_h. A comparison to liquid water surfaces reveals accelerated vibrational energy relaxation and dissipation at the ice surface for hydrogen-bonded OH groups. In contrast, free-OH groups sticking into the vapor phase exhibit substantially slower vibrational dynamics on ice. The acceleration and deceleration of vibrational dynamics of these different OH groups at the ice surface are attributed to enhanced intermolecular coupling and reduced rotational mobility, respectively. Our results highlight the unique properties of free-OH groups on ice, putatively linked to the high catalytic activities of ice surfaces.

Tailoring electric field standing waves in reflection-absorption infrared spectroscopy to enhance absorbance from adsorbates on ice surfaces

The spectroscopic detection of molecules adsorbed onto ice surfaces at coverages similar to those encountered under typical environmental conditions requires high surface selectivity and sensitivity that few techniques can afford. An experimental methodology allowing a significant enhancement in the absorbance from adsorbed molecules is demonstrated herein. It exploits Electric Field Standing Wave (EFSW) effects intrinsic to grazing incidence Reflection-Absorption Infrared (RAIR) spectroscopy, where film thickness dependent optical interferences occur between the multiple reflections of the IR beam at the film-vacuum and the substrate-film interfaces. In this case study, CH₄ is used as a probe molecule and is deposited on a 20 ML coverage dense amorphous solid water film adsorbed onto solid Ar underlayers of various thicknesses. We observe that, at thicknesses where destructive interferences coincide with the absorption features from the CH stretching and HCH bending vibrational modes of methane, their intensity increases by a factor ranging from 10 to 25. Simulations of the RAIR spectra of the composite stratified films using a classical optics model reproduce the Ar underlayer coverage dependent enhancements of the absorbance features from CH₄ adsorbed onto the ice surface. They also reveal that the enhancements occur when the square modulus of the total electric field at the film's surface reaches its minimum value. Exploiting the EFSW effect allows the limit of detection to be reduced to a coverage of (0.2 ± 0.2) ML CH₄, which opens up interesting perspectives for spectroscopic studies of heterogeneous atmospheric chemistry at coverages that are more representative of those found in the natural environment.

Hydroxyl Groups on the Graphene Surfaces Facilitate Ice Nucleation

Although it is crucial to the formation of cirrus clouds and consequently the climate on Earth, the unambiguous effect of carbonaceous materials (CM) on ice nucleation remains to be unveiled as the chemical variation on the surface of CM is always complicated by the change in morphology. Here, we separately investigate the effects of the surface chemistry and morphology of CM on ice nucleation by studying ice nucleation on highly oriented pyrolytic graphite surfaces treated with different types of plasmas. We discover unambiguously that increasing the density of hydroxyl groups leads to an increased activity of ice nucleation on the surface of graphene, while no observable effects are found when carboxylic groups are introduced. Analysis based on the classical nucleation theory reveals that the increase in the density of hydroxyl groups on the graphene surface results in an increased binding energy between the ice nucleus and the graphene surface, which consequently facilitates the formation of the critical ice nucleus.

Multifunctional Photo- and Magnetoresponsive Graphene Oxide-Fe3O4 Nanocomposite-Alginate Hydrogel Platform for Ice Recrystallization Inhibition

Tuning ice recrystallization (IR) has attracted tremendous interest in fundamental research and a variety of practical applications, including food and pharmaceutical engineering, fabrication of anti-icing coating and porous materials, and cryopreservation of biological cells and tissues. Although great efforts have been devoted to modulation of IR for better microstructure control of various materials, it still remains a challenge, especially in cryopreservation, where insufficient suppression of IR during warming is fatal to the cells. Herein, we report an all-in-one platform, combining the external physical fields and the functional materials for both active and passive suppression of IR, where the photo- and magnetothermal dual-modal heating of GO-Fe₃O₄ nanocomposites (NCs) can be used to suppress IR with both enhanced global warming and microscale thermal disturbance. Moreover, the materials alginate hydrogels and GO-Fe₃O₄ NCs can act as IR inhibitors for further suppression of the IR effect. As a typical application, we show that this GO-Fe₃O₄ nanocomposite-alginate hydrogel platform can successfully enable low-cryoprotectant, high-quality vitrification of stem cell-laden hydrogels. We believe that the versatile ice recrystallization inhibition platform will have a profound influence on cryopreservation and tremendously facilitate stem cell-based medicine to meet its ever-increasing demand in clinical settings.

Ion-specific ice recrystallization provides a facile approach for the fabrication of porous materials

Ice recrystallization is of great importance to both fundamental research and practical applications, however understanding and controlling ice recrystallization processes remains challenging. Here, we report the discovery of an ion-specific effect on ice recrystallization. By simply changing the initial type and concentration of ions in an aqueous solution, the size of ice grains after recrystallization can be tuned from 27.4±4.1 to 277.5±30.9 μm. Molecular dynamics simulations show that the ability of the ion to be incorporated into the ice phase plays a key role in the ultimate size of the ice grains after recrystallization. Moreover, by using recrystallized ice crystals as templates, 2D and 3D porous networks with tuneable pore sizes could be prepared from various materials, for example, NaBr, collagen, quantum dots, silver and polystyrene colloids. These porous materials are suitable for a wide range of applications, for example, in organic electronics, catalysis and bioengineering.

Mechanism for formation of atmospheric Cl atom precursors in the reaction of dinitrogen oxides with HCl/Cl− on aqueous films

Formation of atmospheric chlorine atom precursors ClNO₂ and ClNO in the reaction of HCl with oxides of nitrogen on a water film: left – formation of N–Cl bond as N–O bond breaks; right – concurrent changes in Mulliken charges.

The effect of freezing on reactions with environmental impact

The knowledge that the freezing process can accelerate certain chemical reactions has been available since the 1960s, particularly in relation to the food industry. However, investigations into such effects on environmentally relevant reactions have only been carried out since the late 1980s. Some 20 years later, the field has matured and scientists have conducted research into various important processes such as the oxidation of nitrite ions to nitrates, sulfites to sulfates, and elemental mercury to inorganic mercury. Field observations mainly carried out in the polar regions have driven this work. For example, researchers have found that both ozone and mercury are removed from the troposphere completely (and almost instantaneously) at the time of Arctic polar sunrise. The monitoring activities suggested that both the phenomena were caused by involvement of bromine (and possibly iodine) chemistry. Scientists investigating the production of interhalide products (bromine and iodine producing interhalides) in frozen aqueous solutions have found that these reactions result in both rate accelerations and unexpected products. Furthermore, these scientists did this research with environmentally relevant concentrations of reagents, thereby suggesting that these reactions could occur in the polar regions. The conversion of elemental mercury to more oxidized forms has also shown that the acceleration of reactions can occur when environmentally relevant concentrations of Hg(0) and oxidants are frozen together in aqueous solutions. These observations, coupled with previous investigations into the effect of freezing on environmental reactions, lead us to conclude that this type of chemistry could potentially play a significant role in the chemical processing of a wide variety of inorganic components in polar regions. More recently, researchers have recognized the implications of these complementary field and laboratory findings toward human health and climate change. In this Account, we focus on the chemical and physical mechanisms that may promote novel chemistry and rate accelerations when water-ice is present. Future prospects will likely concentrate, once again, on the low-temperature chemistry of organic compounds, such as the humic acids, which are known cryospheric contaminants. Furthermore, data on the kinetics and thermodynamics of all types of reaction promoted by the freezing process would provide much assistance in determining their implications to environmental computer models.

Resonance Raman intensity analysis of ClNO(2) dissolved in methanol

Halogens such as chlorine are converted from halides, including ClNO(2), to reactive radicals by UV solar radiation. These radicals can affect ozone production and destruction in the stratosphere. Recently, it became clear that halogen radicals can also play a significant role in the chemistry of the troposphere. The photochemistry of ClNO(2) has been the subject of several studies in the gas and solid state that demonstrated a clear phase-dependent reactivity. Here, we report our initial studies of nitryl chloride in solution. Resonance Raman (RR) spectra of ClNO(2) dissolved in methanol after excitation within the 1(1)A(1)-2(1)A(1) absorption band (D band) in the region 200-240 nm are presented. RR intensity along the NO symmetric stretch coordinate (v(1)) at 1291 cm(-1) is observed at all excitation wavelengths, whereas limited intensity corresponding to the transition of the N-Cl symmetric stretch (v(3)) was only observed at 199.8 nm, whereas no intensity corresponding to the O-N-O symmetric bend (v(2)) was observed. Depolarization ratios and absolute resonance Raman cross sections for v(1) were obtained at several excitation wavelengths spanning the D band. Depolarization ratios were found to deviate significantly from 1/3, consistent with more than a single dipole-allowed electronic transition contributing to the scattering. RR intensity analysis (RRIA) reveals that two closely spaced excited electronic states contribute to the scattering, which are dissociative along the Cl-N coordinate. In this study the role the solvent environment plays in ClNO(2) state energetics and excited structural evolution along fundamental coordinates is discussed.

Quantum state-resolved CO2 collisions at the gas-liquid interface: surface temperature-dependent scattering dynamics

Energy transfer dynamics at the gas-liquid interface are investigated as a function of surface temperature both by experimental studies of CO2 + perfluorinated polyether (PFPE) and by molecular dynamics simulations of CO2 + fluorinated self-assembled monolayers (F-SAMs). Using a normal incident molecular beam, the experimental studies probe scattered CO2 internal-state and translational distributions with high resolution infrared spectroscopy. At low incident energies [Einc = 1.6(1) kcal/mol], CO2 J-state populations and transverse Doppler velocity distributions are characteristic of the surface temperature (Trot approximately Ttrans approximately TS) over the range from 232 to 323 K. In contrast, the rotational and translational distributions at high incident energies [Einc = 10.6(8) kcal/mol] show evidence for both trapping-desorption (TD) and impulsive scattering (IS) events. Specifically, the populations are surprisingly well-characterized by a sum of Boltzmann distributions where the two components include one (TD) that equilibrates with the surface (TTD approximately TS) and a second (IS) that is much hotter than the surface temperature (TIS > TS). Support for the superthermal, yet Boltzmann, nature of the IS channel is provided by molecular dynamics (MD) simulations of CO2 + F-SAMs [Einc = 10.6 kcal/mol], which reveal two-temperature distributions, sticking probabilities, and angular distributions in near quantitative agreement with the experimental PFPE results. Finally, experiments as a function of surface temperature reveal an increase in both sticking probability and rotational/translational temperature of the IS component. Such a trend is consistent with increased surface roughness at higher surface temperature, which increases the overall probability of trapping, yet preferentially leads to impulsive scattering of more highly internally excited CO2 from the surface.

Enhanced protonation of cresol red in acidic aqueous solutions caused by freezing

The protonation degree of cresol red (CR) in frozen aqueous solutions at 253 or 77 K, containing various acids (HF, HCl, HNO3, H2SO4, and p-toluenesulfonic acid), sodium hydroxide, NaCl, or NH4Cl, was examined using UV/Vis absorption spectroscopy. CR, a weak organic diacid, has been selected as a model system to study the acid-base interactions at the grain boundaries of ice. The multivariate curve resolution alternating least-squares method was used to determine the number and abundances of chemical species responsible for the overlaying absorption visible spectra measured. The results showed that the extent of CR protonation, enhanced in the solid state by 2-4 orders of magnitude in contrast to the liquid solution, is principally connected to an increase in the local concentration of acids. It was found that this enhancement was not very sensitive to either the freezing rate or the type of acid used and that CR apparently established an acid-base equilibrium prior to solidification. In addition, the presence of inorganic salts, such as NaCl or NH4Cl, is reported to cause a more efficient deprotonation of CR in the former case and an enhanced protonation in the latter case, being well explained by the theory of Bronshteyn and Chernov. CR thus served as an acid-base indicator at the grain boundaries of ice samples. Structural changes in the CR molecule induced by lowering the temperature and a presence of the constraining ice environment were studied by the absorption and 1H NMR spectroscopies. Cryospheric and atmospheric implications concerning the influence of acids and bases on composition and reactivity of ice or snow contaminants were examined.

Measurement of Effective Knudsen Diffusion Coefficients for Powder Beds Used in Heterogeneous Uptake Experiments

The effective Knudsen diffusion coefficients for characteristic oxide powder beds used in heterogeneous uptake experiments have been measured using countercurrent diffusion and transient pressure drop techniques. Room-temperature thermal-velocity-normalized effective Knudsen diffusion coefficients are found to lie in the 0.15 to 0.35 microm range for magnesium silicate, aluminum oxide, and iron oxide powder beds. Measured values are compared with theoretical estimates and are consistent with low bed tortuosities (below 3) expected for media with open porosity above 0.5. The impact of uncertainties in effective diffusion coefficients on corrections of measured uptake coefficients is discussed. The value of careful uptake measurements in both the low and high sample mass limits is reinforced, as this allows uptake corrections independent of explicitly measured or estimated diffusion coefficient values. It is suggested that correction procedures requiring tortuosity values greater than 3 are suspect.

Ice (photo)chemistry. Ice as a medium for long-term (photo)chemical transformations--environmental implications

This review accounts for the current knowledge about the distribution, accumulation, and chemical/photochemical transformations of persistent, bioaccumulative, and toxic compounds (PBTs) in water ice, especially in the connection with polar regions and atmospheric cloud particles. (Photo)reactions on/in ice are discussed in terms of photochemistry, photobiology, paleochemistry, as well as astrophysics. Authors propose a model, in which a significant amount of some PBTs are generated by (photo)chemistry of primary pollutants in ice, which may subsequently be released to the environment. It is argued that ice photochemistry might play an important role in the chemical transformations in cold ecosystems and in the upper atmosphere, particularly now when the ozone layer is partially depleted.

Low-temperature fast atom bombardment mass spectra of frozen nitric acid-water solution

Positive ion low-temperature fast atom bombardment mass spectra of the frozen nitric acid-water system with an initial components ratio which provides preferential formation of the crystalline hydrate of the nitric acid trihydrate, HNO(3).3H(2)O, are reported. A complicated spectral pattern is created by a number of cluster sets which, on the basis of discussion, were attributed to (H(2)O)(n). NO(+) (n = 1-3), (H(2)O)(n).NO(2)(+) (n = 1, 2), (HNO(3))(m). (H(2)O)(n).H(+) (m = 1-5 and n variable) and (H(2)O)(n).H(+) (n = 1-9). Similarities between the size-dependent behavior of hydrate clusters of nitrogen oxides and nitric acid obtained with sputtering from the solid in the present low-temperature experiments and under gas-phase conditions (reported earlier in the literature) were revealed. A suggestion as to possibility of the yield of the cluster ions sputtered due to energetic particle collision with the frozen grains of water ice and nitric acid trihydrate, present in the atmosphere, to the total ionic population of the atmosphere is discussed. Copyright 1999 John Wiley & Sons, Ltd.

The Far-Infrared Spectrum of ClNO2 Studied by High-Resolution Fourier-Transform Spectroscopy

The far-infrared spectrum of nitryl chloride was studied using high-resolution Fourier-transform spectroscopy in the 300-525 cm-1 region. Vibrational band centers of fundamental, hot, and difference bands were determined. Furthermore, rotational and centrifugal distortion constants up to fourth order for the nu3 bands of 35ClNO2 and 37ClNO2 (centered at 370 and 364 cm-1, respectively) were obtained. The nu5 fundamental of ClNO2 (predicted around 410 cm-1) is very weak and overlapped by difference bands. Copyright 1998 Academic Press.