Uptake measurements of ethanol on ice surfaces and on supercooled aqueous solutions doped with nitric acid between 213 and 243 K

Uptake of ethanol either on pure frozen ice surfaces or supercooled solutions doped with HNO3 (0.63 and 2.49 wt %) has been investigated using a coated wall flow tube coupled to a mass spectrometric detection. The experiments were conducted over the temperature range of 213-243 K. Uptake of ethanol on these surfaces was always found to be totally reversible whatever were the experimental conditions. The number of ethanol molecules adsorbed per surface unit was conventionally plotted as a function of ethanol concentration in the gas phase and subsequently analyzed using Langmuir's model. The amount of ethanol molecules taken up on nitric acid doped-ice surfaces was found to increase largely with increasing nitric acid concentrations. For example at 223 K, and for an ethanol gas-phase concentration of 1x10(13) molecules cm3, the number of adsorbed molecules are (in units of molecules cm-2): approximately 1.3x10(14) on pure ice; approximately 1.4x10(15) on ice doped with HNO3 0.63 wt %; approximately 7.5x10(15) on ice doped with HNO3, 2.49 wt %, i.e. 60 times larger than on pure ice. Since, according to the shape of the isotherms, the adsorption did not proceed beyond monolayer coverage, the enormous increase of ethanol uptake was explained by considering its dissolution in either a supercooled liquid layer (T<230 K) or a liquid solution (T>230 K). The formation of both was indeed favored by the presence of the HNO3. Our experimental results suggest that the amount of ethanol dissolved in such supercooled solutions follows Henry's law and that the Henry's law constants at low temperatures, i.e., 223-243 K, can be estimated by extrapolation from higher temperatures. Such supercooled solutions which exist in the troposphere either in deep convective clouds or in mixed clouds for temperature above 233 K, might be responsible for the scavenging of large amounts of soluble species, such as nitric and sulfuric acids, oxygenated VOCs including alcohols, carboxylic acids, and formaldehyde.     

The role of tropospheric ice surfaces in the elimination of the CFC substitute, trifluoroethanol

This work provides uptake results of CF(3)CH(2)OH on ice over the temperature range 203-223 K using a coated wall flow tube coupled to mass spectrometric detection. For experiments over pure ice, the adsorption was fully reversible and the data could be described in terms of the Langmuir isotherm for the range of concentrations and temperatures studied. For this temperature range, ΔH°(ads) = -46 ± 16 kJ mol(-1) was obtained (error is 2σ + 5%). For experiments on doped ice with nitric acid over the temperature range 203-223 K, the number of adsorbed molecules was slightly lower than over pure ice. At temperatures above 231 K, the extent of the reversible uptake of CF(3)CH(2)OH is enhanced in the presence of nitric acid due to coexistence of a liquid solution phase. Under such conditions the obtained solubility data follow Henry's law. Although pure ice and acid doped water surfaces do not permanently scavenge CF(3)CH(2)OH, the partitioning of CF(3)CH(2)OH between the gas phase and aqueous condensed phases may play a role as reservoirs or as a means of transport in the troposphere.     

Adsorption study of acetone on acid-doped ice surfaces between 203 and 233 K

Adsorption studies of acetone on pure ice surfaces obtained by water freezing or deposition or on frozen ice surfaces doped either with HNO3 or H2SO4 have been performed using a coated wall flow tube coupled to a mass spectrometric detection. The experiments were conducted over the temperature range 203-233 K and freezing solutions containing either H2SO4 (0.2 N) or HNO3 (0.2-3 N). Adsorption of acetone on these ice surfaces was always found to be totally reversible whatever were the experimental conditions. The number of acetone molecules adsorbed per ice surface unit N was conventionally plotted as a function of acetone concentration in the gas phase. For the same conditions, the amount of acetone molecules adsorbed on pure ice obtained by deposition are about 3-4 times higher than those measured on frozen ice films, H2SO4-doped ice surfaces lead to results comparable to those obtained on pure ice. On the contrary, N increases largely with increasing concentrations of nitric acid in ice surfaces, up to about 300 times under our experimental conditions and for temperatures ranging between 213 and 233 K. Finally, the results are discussed and used to reestimate the partitioning of acetone between the ice and gas phases in clouds of the upper troposphere.     

Uptake of SO2 on HOBr-ice surfaces

The uptake of SO2 on HOBr-treated ice surfaces has been studied using a flow reactor coupled with a differentially pumped quadrupole mass spectrometer at 190-240 K. The initial uptake coefficient was determined as a function of HOBr surface coverage, theta(HOBr), on the ice. The uptake coefficients increase as the HOBr coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOBr), where k(h) = 1.5 x 10(-19) molecules(-1) cm(-2) at 191 K and k(h) = 6.4 x 10(-21) molecules(-1) cm(-2) at 210 K and theta(HOBr) is in the range of 8 x 10(13) to 1.2 x 10(15) molecules cm(-2). The effects of temperature and film thickness on the uptake coefficients of SO2 by the HOBr-treated ice films were also studied. The activation energy E(a) of SO(2) on HOBr-ice surfaces is approximately -81 +/- 8 kJ/mol in the 190-215 K range. Kinetic results were interpreted in terms of the Eley-Rideal mechanism. This study suggests that the uptake of SO2 on ice/snow surfaces is enhanced by the presence of HOBr near the ice surface. The implication for atmospheric chemistry is that HOBr-ice surfaces may not provide a significant pathway to oxide S(IV) in the boundary layer due to both lower uptake coefficient and smaller HOBr surface coverage at T > 220 K.     

Uptake of NH(3) and NH(3) + HOBr reaction on ice surfaces at 190 K

The uptake of NH3 and the heterogeneous reaction of NH3 + HOBr --> products on ice surfaces at 190 K have been investigated in a flow reactor coupled with a differentially pumped quadrupole mass spectrometer. The uptake coefficient gammat for NH3 was determined to be (3.8 +/- 1.4) x 10(-4) on ice films at 189.8 K, for a partial pressure of NH3 in the range of 7.0 x 10(-7) to 3.8 x 10(-6) torr. The amount of NH3 uptake on the ice film was determined to be >2.9 x 10(15) molecules/cm(2), based on the total ice surface area at 189.2 K. The heterogeneous reaction of NH3 + HOBr on ice surfaces has been studied at 190 K. The reaction probability gammat was determined to be (5.3 +/- 2.2) x 10(-4) and was found to vary insignificantly as HOBr surface coverage changes from 2.1 x 10(13) to 2.1 x 10(14) molecules/cm(2). A reaction pathway is proposed on the basis of experimental observations.     

Heterogeneous reactions of SO2 with HOCl and HOBr on ice surfaces

The heterogeneous reactions of SO2 + HOX (X = Cl or Br) --> products on ice surfaces at low temperature have been investigated in a flow reactor coupled with a differentially pumped quadrupole mass spectrometer. Pseudo-first-order loss of SO2 over the ice surfaces has been measured under the conditions of concurrent HOX flow. The initial uptake coefficient of SO2 reaction with HOX has been determined as a function of HOX surface coverage, theta(HOX), on the ice. The initial uptake coefficients increase as the HOX coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOX), where k(h) is an overall rate constant of SO2 + HOCl, which was determined to be (2.3 +/- 0.6) x 10(-19) and (1.7 +/- 0.5) x 10(-19) molecules(-1) x cm2 at 190 and 210 K, and k(h) of SO2 + HOBr is (6.1 +/- 2.0) x 10(-18) molecules(-1) x cm2 at 190 K. theta( HOX) is in the range 8.1 x 10(13)-9.1 x 10(14) molecules x cm(-2). The kinetic results of the heterogeneous reaction of SO2 + HOX on ice surface are interpreted using the Eley-Rideal mechanism. The activation energy of the heterogeneous reaction of SO2 with HOCl on ice surface was determined to be about -37 +/- 10 kJ/mol in the 190-238 K range.     

Oxidation kinetics of hydrogenated amorphous carbon (a-CH(x)) overcoats for magnetic data storage media

The oxidation kinetics of a-CHx overcoats during exposure to oxygen and water vapor have been measured using X-ray photoemission spectroscopy (XPS) in an apparatus that allows oxidation and analysis of freshly deposited a-CHx overcoats without prior exposure of the overcoats to air. The uptake of oxygen on the surfaces of the a-CHx overcoats has been measured at O2 and H2O pressures in the range 10(-7)-10(-3) Torr at room temperature. The uptake of oxygen during O2 exposures on the order of 10(7) Langmuirs leads to saturation of the a-CHx overcoat surfaces at oxidation levels on the order of 20%. This indicates that the surfaces of a-CHx overcoats are relatively inert to oxidation in the sense that the dissociative sticking coefficient of O2 is approximately 10(-6). Oxygen uptake during exposure to H2O vapor is similar to the uptake during exposure to O2 gas. Although the surfaces of the a-CHx overcoats are quite inhomogeneous, it has been possible to model the uptake of oxygen on their surfaces using a fairly simple Langmuir-Hinshelwood mechanism. Interestingly, the saturation coverage of oxygen during exposure to air at atmospheric pressure is approximately 6%, significantly lower than that obtained during low-pressure exposure to O2 gas or H2O vapor.     

Uptake measurements of acetic acid on ice and nitric acid-doped thin ice films over upper troposphere/lower stratosphere temperatures

The adsorption of gaseous acetic acid (CH(3)C(O)OH) on thin ice films and on ice doped with nitric acid (1.96 and 7.69 wt %) was investigated over upper troposphere and lower stratosphere (UT/LS) temperatures (198-208 K), and at low gas concentrations. Experiments were performed in a Knudsen flow reactor coupled to a quadrupole mass spectrometer. The initial uptake coefficients, γ(0), on thin ice films or HNO(3)-doped ice films were measured at low surface coverage. In all cases, γ(0) showed an inverse temperature dependence, and for pure thin ice films, it was given by the expression γ(0)(T) = (4.73 ± 1.13) × 10(-17) exp[(6496 ± 1798)/T]; the quoted errors are the 2σ precision of the linear fit, and the estimated systematic uncertainties are included in the pre-exponential factor. The inverse temperature dependence suggests that the adsorption process occurs via the formation of an intermediate precursor state. Uptakes were well represented by the Langmuir adsorption model, and the saturation surface coverage, N(max), on pure thin ice films was (2.11 ± 0.16) × 10(14) molecules cm(-2), independent of temperature in the range 198-206 K. Light nitration (1.96 and 7.69 wt %) of ice films resulted in more efficient CH(3)C(O)OH uptakes and larger N(max) values that may be attributed to in-bulk diffusion or change in nature of the gas-ice surface interaction. Finally, it was estimated that the rate of adsorption of acetic acid on high-density cirrus clouds in the UT/LS is fast, and this is reflected in the short atmospheric lifetimes (2-8 min) of acetic acid; however, the extent of this uptake is minor resulting in at most a 5% removal of acetic acid in UT/LS cirrus clouds.     

Heterogeneous glyoxal oxidation: a potential source of secondary organic aerosol

Laboratory studies are described that suggest reactive uptake of glyoxal on particulate containing HNO(3) could contribute to the formation of secondary organic aerosol (SOA) in the upper troposphere (UT). Using a Knudsen cell flow reactor, glyoxal is observed to react on supercooled H(2)O/HNO(3) surfaces to form condensed-phase glyoxylic acid. This product was verified by derivatization and GC-MS analysis. The reactive uptake coefficient, γ, of glyoxal varies only slightly with the pressure of nitric acid, from γ = 0.5 to 3.0 × 10(-3) for nitric acid pressures between 10(-8) and 10(-6) Torr. The data do not show any dependence on temperature (181-201 K) or pressure of glyoxal (10(-7) to 10(-5) Torr). Using the determined reactive uptake kinetics in a simple model shows that glyoxal uptake to supercooled H(2)O/HNO(3) may account for 4-53% of the total organic mass fraction of aerosol in the UT.     

Effect of relative humidity on OH uptake by surfaces of atmospheric importance

An experimental study of the dependence of the OH uptake coefficient gamma OH over a relative humidity of 0-48% was carried out at 100 Torr and room temperature, using a differential bead-filled flow tube coupled to a high-pressure chemical ionization mass spectrometer. Various organic (paraffin wax, pyrene, glutaric acid, and soot) and inorganic (NaCl, KCl, MgCl2, CaCl2, Na2SO4, and sea salt) surfaces served as proxies for tropospheric aerosols. A virtual cylindrical flow tube approximation and a surface coating dilution technique were successfully employed in the study, which included measurements of high radical uptake with an initial probability of near unity. For inorganic salts, the effect of water vapor, enhancement or inhibition of gamma OH, was found to be dependent on the blocking of anions and changes in surface pH. Although OH uptake by NaCl, the major component of sea-salt aerosols, is weakly dependent on water vapor, it is enhanced by a factor of approximately 2 for MgCl2 and determines the net relative humidity dependence of the radical uptake on sea salt, which is enhanced by a factor of approximately 4. For the organic surfaces studied, the enhancement effect of a factor 4 was also observed only for a hydrophilic organic surface, namely, glutaric acid. Results of the uptake studies suggest that the effect of relative humidity is important and should be accounted for in atmospheric modeling of tropospheric aerosol chemistry.     

Characterization of porosity in vapor-deposited amorphous solid water from methane adsorption

We have characterized the porosity of vapor-deposited amorphous solid water (ice) films deposited at 30-40 K using several complementary techniques such as quartz crystal microgravimetry, UV-visible interferometry, and infrared reflectance spectrometry in tandem with methane adsorption. The results, inferred from the gas adsorption isotherms, reveal the existence of microporosity in all vapor-deposited films condensed from both diffuse and collimated water vapor sources. Films deposited from a diffuse source show a step in the isotherms and much less adsorption at low pressures than films deposited from a collimated source with the difference increasing with film thickness. Ice films deposited from a collimated vapor source at 77 degrees incidence are mesoporous, in addition to having micropores. Remarkably, mesoporosity is retained upon warming to temperatures as high as 140 K where the ice crystallized. The binding energy distribution for methane adsorption in the micropores of ice films deposited from a collimated source peaks at approximately 0.083 eV for deposition at normal incidence and at approximately 0.077 eV for deposition at >45 degrees incidence. For microporous ice, the intensity of the infrared bands due to methane molecules on dangling OH bonds on pore surfaces increases linearly with methane uptake, up to saturation adsorption. This shows that the multilayer condensation of methane does not occur inside the micropores. Rather, filling of the core volume results from coating the pore walls with the first layer of methane, indicating pore widths below a few molecular diameters. For ice deposited at 77 degrees incidence, the increase in intensity of the dangling bond absorptions modified by methane adsorption departs from linearity at large uptakes.     

Electrical processes upon the evaporation and condensation of water and ice

A mathematical model of electrical processes that take place upon the evaporation of water and sublimation of ice, as well as the condensation growth of water and ice phases from vapor, is proposed. The transfer of the main charge carriers, such as (i) protons and hydroxide ions (in ice, water, and vapor and (ii) orientational defects (in ice and water) is taken into account. Upon the evaporation of water and the sublimation of ice, the first carriers are accumulated before the phase front and cause positive charges in the surface of the water and ice, while the second carriers are depleted (their concentration becomes lower than the thermodynamic value) and impart a negative charge to water and ice. The contribution of protons and hydroxide ions dominates at a low rate of evaporation. In the condensation of vapor and relevant growth of water and ice phases, the polarity of surface charge is opposite to that observed upon evaporation. The values of interfacial current and signs of phase charges upon sublimation, evaporation, and condensation that are predicted in the model comply with experimental data.     

Kinetics of ClONO(2) reactive uptake on ice surfaces at temperatures of the upper troposphere

The reactive uptake kinetics of ClONO(2) on pure and doped water-ice surfaces have been studied using a coated wall flow tube reactor coupled to an electron impact mass spectrometer. Experiments have been conducted on frozen film ice surfaces in the temperature range 208-228 K with P((ClONO)(2)) < or = 10(-6) Torr. The uptake coefficient (gamma) of ClONO(2) on pure ice was time dependent with a maximum value of gamma(max) approximately 0.1. On HNO(3)-doped ice at 218 K the gamma(max) was 0.02. HOCl formation was detected in both experiments. On HCl-doped ice, uptake was gas-phase diffusion limited (gamma > 0.1) and gas-phase Cl(2) was formed. The uptake of HCl on ice continuously doped with HNO(3) was reversible such that there was no net uptake of HCl once the equilibrium surface coverage was established. The data were well described by a single site 2-species competitive Langmuir adsorption isotherm. The surface coverage of HCl on HNO(3)-doped ice was an order of magnitude lower than on bare ice for a given temperature and P(HCl). ClONO(2) uptake on this HCl/HNO(3)-doped ice was studied as a function of P(HCl). gamma(max) was no longer gas-phase diffusion limited and was found to be linearly dependent on the surface concentration of HCl. Under conditions of low HCl surface concentration, hydrolysis of ClONO(2) and reaction with HCl were competing such that both Cl(2) and HOCl were formed. A numerical model was used to simulate the experimental results and to aid in the parametrization of ClONO(2) reactivity on cirrus ice clouds in the upper troposphere.     

Uptake of small oxygenated organic molecules onto ammonium nitrate under upper tropospheric conditions

The uptake of formic (C1), propanoic (C3), butanoic (C4), and pentanoic (C5) acids onto ammonium nitrate (AN) has been investigated as a function of temperature and relative humidity using a Knudsen cell flow reactor coupled with FTIR-reflection absorption spectroscopy (FTIR-RAS). The uptake of acetone and methanol onto AN was also briefly studied. Initial uptake coefficients (gamma) were determined over the temperature range 200-240 K. Formic, propanoic, and butanoic acids exhibited efficient but temperature-dependent uptake on AN, with larger uptake coefficients observed at lower temperatures. Pentanoic acid was not taken up by AN under any of the conditions studied. Uptake of acetone and methanol onto AN was observed, but in insignificant amounts under atmospherically relevant conditions. Infrared spectra revealed that propanoic and butanoic acids ionized on the surface, despite the fact that the AN films were effloresced. Formic acid reacted with the AN film to produce ammonium formate and ionized nitric acid. Adding small amounts of water vapor (4% RH) to the chamber resulted in dramatically increased gamma values for all of the acids. Furthermore, the IR spectra showed the formation of a liquid layer when propanoic and butanoic acids adsorbed on the surface at RH = 20% and greater. Liquid water features were not observed at a similar relative humidity in the absence of the acids. These results show that small organic acids can be efficiently scavenged by AN and lead to enhanced water uptake under upper tropospheric conditions.     

(H2O)10 and (H2O)12 on a virtual metal surface: the growth of ice

(H2O)10 and (H2O)12 are used to investigate the growth of ice on metal surfaces with hexagonal symmetry. The model of the virtual metal surface was used to separate the electronic structure of the metal from that of the water cluster while maintaining the geometric constraints imposed by the metal surface on the water cluster. To complement the ab initio calculations on the water cluster, an additional multicenter analysis was done to analyze the hydrogen bonds within the clusters. These calculations suggested that the water bilayer structure adjacent to the virtual metal surface effectively shields the growing ice crystal from the metal surface.     

Theoretical study of the dissociation of nitric acid at a model aqueous surface

The issue of acid dissociation of nitric acid at an aqueous surface is relevant in various portions of the atmosphere in connection with ozone depletion. This proton-transfer reaction is studied here via electronic structure calculations at the HF/SBK+(d) level of theory on the HNO(3) x (H(2)O)(3) model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters with an increasing degree of solvation of the nitrate group. Free energy estimates for all the cases examined favor undissociated, molecular nitric acid over the 0-300 K temperature range, including that relevant for the upper troposphere, where it is connected to the issue of the mechanism of nitric acid uptake by water ice aerosols. The presence of molecular HNO(3) at 300 K at the surface is further supported by vibrational band assignments in good agreement with a very recent surface-sensitive vibrational spectroscopy study of diluted HNO(3)/H(2)O solutions.     

Reactive uptake of nitric acid onto sodium chloride aerosols across a wide range of relative humidities

Reactive uptake coefficients for nitric acid onto size-selected (d(ve) = 102 and 233 nm) sodium chloride aerosols are determined for relative humidities (RH) between 85% and 10%. Both pure sodium chloride and sodium chloride mixed with magnesium chloride (X(Mg/Na) = 0.114, typical of sea salt) are studied. The aerosol is equilibrated with a carrier gas stream at the desired RH and then mixed with nitric acid vapor at a concentration of 60 ppb in a laminar flow tube reactor. At the end of the reactor, the particle composition is determined in real time with a laser ablation single particle mass spectrometer. For relative humidities above the efflorescence relative humidity (ERH), the particles exist as liquid droplets and the uptake coefficient ranges from 0.05 at 85% RH to >0.1 near the ERH. The droplet sizes, relative humidity and composition dependencies, are readily predicted by thermodynamics. For relative humidities below the ERH, the particles are nominally "solid" and uptake depends on the amount of surface adsorbed water (SAW). The addition of magnesium chloride to the particle phase (0.114 mole ratio of magnesium to sodium) facilitates uptake by increasing the amount of SAW. In the presence of magnesium chloride, the uptake coefficient remains high (>0.1) down to 10% RH, suggesting that the displacement of chloride by nitrate in fine sea salt particles is efficient over the entire range of conditions in the ambient marine environment. In the marine boundary layer, displacement of chloride by nitrate in fine sea salt particles should be nearly complete within a few hours (faster in polluted areas)-a time scale much shorter than the particle residence time in the atmosphere.     

Heterogeneous reactions of gaseous HNO3 and NO2 on the clay minerals kaolinite and pyrophyllite

Airborne clay mineral particles have long atmospheric lifetimes due to their relatively small size. To assess their impact on trace atmospheric gases, we investigated heterogeneous reactions on prototype clay minerals. Diffuse reflectance infrared spectroscopy identified surface-adsorbed products formed from the uptake of gaseous nitric acid and nitrogen dioxide on kaolinite and pyrophyllite. For kaolinite, a 1:1 phyllosilicate, HNO3 molecularly adsorbed onto the octahedral aluminum hydroxide and tetrahedral silicon oxide surfaces. Also detected on the aluminum hydroxide surface were irreversibly adsorbed monodentate, bidentate, bridged, and water-coordinated nitrate species as well as surface-adsorbed water. Similar adsorbed products formed during the uptake of NO2 on kaolinite at relative humidity (RH) of 0%, and the reaction was second order with respect to reactive surface sites and 1.5 +/- 0.1 for NO2. Reactive uptake coefficients, calculated using Brunauer, Emmett, and Teller surface areas, increased from (8.0 +/- 0.2) x 10(-8) to (2.3 +/- 0.4) x 10(-7) for NO2 concentrations ranging from 0.56 x 10(13) to 8.8 x 10(13) molecules cm(-3). UV-visible spectroscopy detected gaseous HONO as a product for the reaction of NO2 on wet kaolinite. The uptake of HNO3 on pyrophyllite, a 2:1 phyllosilicate, resulted in stronger signal for nitric acid molecularly adsorbed on the silicon oxide surface compared to kaolinite. Monodentate, bridged, and water-coordinated nitrate species bound to aluminum sites also formed during this reaction indicating that reactive sites on edge facets are important for this system. The uptake of NO2 on pyrophyllite, gammaBET = (7 +/- 1) x 10(-9), was significantly lower than kaolinite because NO2 did not react with the dominant tetrahedral silicon oxide surface. These results highlight general trends regarding the reactivity of tetrahedral silicon oxide and octahedral aluminum hydroxide clay surfaces and indicate that the heterogeneous chemistry of clay aerosols varies with mineralogy and cannot be predicted by elemental analysis.     

A kinetic model for uptake of HNO3 and HCl on ice in a coated wall flow system

A simple model of gas flow and surface exchange with a single site Langmuir mechanism has been developed to describe effects of adsorption and desorption on trace gas concentrations at the outflow from a coated wall flow tube reactor. The model was tested by simulating experimental results for the uptake of HNO3 and HCl on ice films at temperatures and gas concentrations corresponding to the ice stability region in the upper troposphere. The experimental time-dependent uptake profiles were best fitted with an additional process involving diffusion of the adsorbed molecules into the ice film. The model allowed true surface coverages to be distinguished from total uptake including transfer to the bulk, leading to more accurate estimates of the Langmuir constant, Keq, for surface adsorption. A revised expression was obtained for the temperature dependence of the Keq=-(4.43 +/- 0.77)x 10(5)T+(10.72 +/- 1.75)x 10(7) hPa-1. Reasonable fits to the desorption profiles observed following cessation of exposure of the film to HNO3 or HCl were obtained at high surface coverage but at low coverage desorption was too slow. The analysis suggested that the ice surface was characterised by sites of different binding energy, some weakly bound sites from which the acid molecules desorbed rapidly, and some strong-binding sites which led to essentially irreversible uptake. Experiments involving competitive co-adsorption of HNO3 and HCl, conducted at relatively high equilibrium surface coverage, were well simulated by the model, as were those where the same surface was repeatedly exposed to gas phase acids.     

Spectroscopic studies of model polar stratospheric cloud films

Fourier transform infrared (FTIR) spectroscopy has been used to study nitric acid/ice films representative of type I polar stratospheric clouds (PSCs). These studies reveal that in addition to amorphous nitric acid/ice mixtures, there are three stable stoichiometric hydrates of nitric acid—nitric acid monohydrate (NAM), dihydrate (NAD) and trihydrate (NAT). Two distinct crystalline forms of the trihydrate were also observed. These two forms appear to differ in their concentration of crystalline defects, but not in their chemical composition. In addition to probing the composition of type I PSCs, we have also used FTIR spectroscopy to characterize laboratory surfaces on which measurements of heterogeneous reaction rates are performed. Our studies suggest that “water-rich NAT” is a two-phase system with separate ice and NAT crystalline regimes. Finally, we have used FTIR spectroscopy to determine the desorption kinetics for evaporation of model PSC films. Ice evaporation was found to follow zero-order desorption kinetics with a desorption barrier of 12±2 kcal/mol and a preexponential factor of 10^30.5±1.5 molec/cm²-s.