Coated-wall Flow Tube Research Articles

Uptake of gas-phase nitric acid to ice at low partial pressures: evidence for unsaturated surface coverage

The adsorption of gas-phase nitric acid onto water-ice surfaces at temperatures between 200 and 239 K has been studied over short time scales using a coated-wall flow tube coupled to a chemical ionization mass spectrometer. The nitric acid partial pressures used were between 10(-8) hPa and 10(-6) hPa, making this the first systematic study under partial pressure conditions present in the upper troposphere. Whereas previous findings using this technique have shown that the surface coverages are saturated at 2 to 3 x 10(14) molecules cm(-2) (referenced to the geometric surface area of the ice film) when partial pressures are larger than about 10(-7) hPa, the principal finding from this study is that the surface coverages are in the unsaturated regime at lower partial pressures. A conventional Langmuir adsorption isotherm describes the uptake in a quantitative manner while dissociative Langmuir isotherms that have been used in the past to model this process do not. The unsaturated surface coverages are strongly temperature dependent, in agreement with a number of field measurements of the nitric acid (or NOy) component of cirrus cloud particles. These laboratory results match those in the field better than do those measured at significantly higher partial pressures but, nevertheless, they still indicate somewhat greater uptake, particularly at higher temperatures.

Reversible Gas Adsorption in Coated Wall Flow Tube Reactors. Model Simulations for Langmuir Kinetics

Abstract A kinetic model has been developed to simulate reversible gas adsorption in coated wall flow tube reactors (CWFTs). The motivation of this work is to provide the theoretical framework for modelling studies in support of the results obtained from CWFT studies on the adsorption and desorption behaviour of atmospheric trace gases on ice surfaces at temperatures relevant to the upper troposphere/lower stratosphere (190–230 K). The model consists of an axial sequence of individual flow tube sections of equal volumes in which the gas phase is homogeneously mixed and interactions with the coated wall occur by adsoption and desorption exclusively. The adsorption rate is assumed to be kinetically controlled and not to be transport limited. Moreover, chemical reactions are not considered. Simulations have been performed for the temporal behaviour of the gas phase concentration at the exit of the flow tube as a function of laboratory time for typical operation procedures of CWFTs with moveable injectors i.e. (i) instantaneous injection of the gas, (ii) instantaneous termination of the gas flow and (iii) movements of the injector with constant velocities. It is found that the temporal profiles show complex behaviours due to the overlap of adsorption and desorption upon successive exposure of the gas to different length of the ice surface. The validity of the model is demonstrated for the adsorption of acetone on an ice surface at 200 K as a case study.

Experimental and Theoretical Adsorption Study of Ethanol on Ice Surfaces

Adsorption studies of ethanol on ice surfaces were performed by combining experimental and theoretical approaches. The experiments were conducted with use of a coated wall flow tube coupled to a mass spectrometric detector. The surface coverage increases with decreasing temperature and with increasing ethanol concentrations. The obtained experimental surface coverages were fitted according to the B.E.T. theory to determine the enthalpy of adsorption ΔHads and the monolayer capacity NM: ΔHads = −57 ± 8 kJ mol-1, NM = (2.8 ± 0.8) × 1014 molecule cm-2. The adsorption characteristics of ethanol on a proton disordered Ih(0001) ice surface were also jointly studied by performing classical molecular dynamics simulations. More specifically, the configurations of the molecules in their adsorption sites and the corresponding adsorption energies have been studied as a function of temperature and coverage. In the simulations, the saturation coverage is around NM = 3.2 × 1014 molecule cm-2, and corresponds to an adsorption energy equal to −56.6 kJ mol-1, in good agreement with the experimental value. The results are discussed and compared with previous determinations for alcohols.

Adsorption studies of acetone and 2,3-butanedione on ice surfaces between 193 and 223 K

Adsorption studies of acetone and of 2,3-butanedione on ice surfaces were performed using a new vertical coated wall flow tube coupled to a mass spectrometric detection. Adsorption of acetone on ice was found to be totally reversible for ice temperatures ranging from 193 to 223 K and for gas phase acetone concentrations varying between 5.4 × 1010 and 6.4 × 1013 molecules cm−3. Adsorption of 2,3-butanedione was also reversible at 213 and 223 K but partially irreversible at 193 and 203 K when its concentrations were larger than 1 × 1013 molecules cm−3. It was shown that, at 203 K, the surface coverage increases when the ice surface contains large and dense cracks but is independent of the presence of cracks at 223 K. The surface coverage also increases with decreasing temperature and with increasing acetone or 2,3-butanedione concentrations. The obtained experimental surface coverages were fitted according to the Langmuir and BET theories in order to determine the enthalpy of adsorption ΔHads and the monolayer capacity NM. The following values of NM were derived (in units of 1014 molecule cm−2): NM = 1.3 ± 0.2 for acetone and NM = 1.2 ± 0.5 for 2,3-butanedione. The corresponding enthalpies of adsorption are (in kJ mol−1): −49 ± 7 for acetone and −59 ± 8 for 2,3-butanedione. The results are discussed and compared with previous determinations for acetone. Finally, the obtained results are used to estimate the partitioning of acetone between the ice and gas phases in clouds of the upper troposphere.

Heterogeneous reaction of ozone with liquid unsaturated fatty acids: detailed kinetics and gas-phase product studies

Detailed kinetics and product yield studies have been performed for the heterogeneous reaction between gas-phase ozone and three liquid fatty acids using a coated-wall flow tube and chemical ionization mass spectrometry. Gas-surface reaction probabilities for ozone loss of (8.0 ± 1.0) × 10−4, (1.3 ± 0.1) × 10−3, and (1.8 ± 0.2) × 10−3 have been measured at room temperature (298 K) for oleic acid, linoleic acid and linolenic acid, respectively. The temperature dependence of the uptake coefficients was found to be small and positive. Comparison of these results to the kinetics of the equivalent gas-phase reactions implies that there is a definite enhancement in the rate for the heterogeneous process due to entropic factors, i.e. due to collisional trapping of ozone in the surface layers of the liquid, and a possible effect on the activation energy of the reaction. For linoleic acid, the reaction probability was found to be independent of relative humidity (up to 55%), to ±10%, at 263 K. Volatile reaction products were observed using proton-transfer-reaction mass spectrometry. Nonanal was observed with a 0.50 (±0.10) yield for the reaction with oleic acid, whereas hexanal and nonenal were observed for linoleic acid with 0.25 (±0.05) and 0.29 (±0.05) yields, respectively. These results indicate that the primary ozonide formed initially in the reaction can decompose via two equal probability pathways and that a secondary ozonide is not formed in high yield in the aldehydic channel. These reactions represent a source of oxygenates to the atmosphere and will modify the hygroscopic properties of aerosols.

Adsorption of Gas-Phase Nitric Acid ton-Hexane Soot: Thermodynamics and Mechanism

The adsorption of gas-phase nitric acid on n-hexane soot was measured as a function of temperature, relative humidity (RH), and nitric acid partial pressure in a coated-wall flow tube coupled to an electron-impact mass spectrometer. The specific surface area (SSA) of the soot, determined from the BET isotherm of Kr at 77 K for each sample, was 88−372 times larger than the geometric surface area. The SSA increased linearly with soot mass for thin samples but saturated at high mass. For the most part, the nitric acid adsorption was reversible in the submonolayer regime, and no indication of HONO formation was observed. The uptake increased with decreasing temperature and, for surface coverages between 1012 and 1013 molecules cm-2, the average enthalpy of adsorption was −55.8 ± 7.7 kJ mol-1. A Langmuir−Freundlich model with the heterogeneity parameter equal to 0.5 closely describes the uptake data, implying either that the surface sites are energetically heterogeneous or that nitric acid is dissociating on the surface. The nitric acid adsorption isotherms were independent of relative humidity up to 80% RH at 243 K. Exposure to ppm levels of ozone for about 1 h had no effect on the adsorption. The atmospheric implications of this work are discussed.

Uptake and reaction of HOBr on frozen and dry NaCl/NaBr surfaces between 253 and 233 K

Abstract. The uptake and reaction of HOBr with frozen salt surfaces of variable NaCl / NaBr composition and temperature were investigated with a coated wall flow tube reactor coupled to a mass spectrometer for gas-phase analysis. HOBr is efficiently taken up onto the frozen surfaces at temperatures between 253 and 233 K where it reacts to form the di-halogens BrCl and Br2, which are subsequently released into the gas-phase. The uptake coefficient for HOBr reacting with a frozen, mixed salt surface of similar composition to sea-spray was <approx> 10-2. The relative concentration of BrCl and Br2 released to the gas-phase was found to be strongly dependent on the ratio of Cl- to Br - in the solution prior to freezing / drying. For a mixed salt surface of similar composition to sea-spray the major product at low conversion of surface reactants (i.e. Br - and Cl-) was Br2. Variation of the pH of the NaCl / NaBr solution used to prepare the frozen surfaces was found to have no significant influence on the results. The observations are explained in terms of initial formation of BrCl in a surface reaction of HOBr with Cl-, and conversion of BrCl to Br2 via reaction of surface Br -. Experiments on the uptake and reaction of BrCl with frozen NaCl / NaBr solutions served to confirm this hypothesis. The kinetics and products of the interactions of BrCl, Br2 and Cl2 with frozen salt surfaces were also investigated, and lower limits to the uptake coefficients of > 0.034, >0.025 and >0.028 respectively, were obtained. The uptake and reaction of HOBr on dry salt surfaces was also investigated and the results closely resemble those obtained for frozen surfaces. During the course of this study the gas diffusion coefficients of HOBr in He and H2O were also measured as (273 ± 1) Torr cm2 s-1 and (51 ± 1) Torr cm2 s-1, respectively, at 255 K. The implications of these results for modelling the chemistry of the Arctic boundary layer in springtime are discussed.

Adsorption to Ice of n-Alcohols (Ethanol to 1-Hexanol), Acetic Acid, and Hexanal

The gas-to-ice uptakes of a variety of small organic molecules (C2 to C6 n-alcohols, acetic acid and hexanal) have been measured in a coated-wall flow tube coupled to an electron-impact mass spectrometer. The ice films are prepared by freezing liquid water and the temperature range covered is between 213 and 245 K. With the exceptions of 1-hexanol and high partial pressures of 1-pentanol, the surface coverages at 228 K are adequately described by a standard Langmuir adsorption isotherm model consistent with a saturated surface coverage of 2 to 3 × 1014 molecules/cm2, where the area is the geometric surface area of the film. For similar partial pressures, 1-hexanol exhibits very much larger uptakes than the other species, an indication that multilayer adsorption is occurring. The equilibrium constant for the partitioning between the gas-phase organic and its adsorbed form has been measured from the linear dependence of the surface coverage on pressure at low partial pressures. The temperature dependence of...

Oxidation of SO<sub>2</sub> by H<sub>2</sub>O<sub>2</sub> on ice surfaces at 228 K: a sink for SO<sub>2</sub> in ice clouds

Abstract. The heterogeneous reaction SO2 + H2O2 H2SO4 on ice at 228 K has been studied in a low temperature coated-wall flow tube. With H2O2 in excess of SO2, the loss of SO2 on an ice surface is time dependent with the reaction most efficient on a freshly exposed surface. The deactivation of the surface arises because the protons formed in the reaction inhibit the dissociation of adsorbed SO2. This lowers the surface concentrations of HSO3-, a participant in the rate-determining step of the oxidation mechanism. For a fixed SO2 partial pressure of 1.4 x 10-4 Pa, the reaction probabilities for SO2 loss on a freshly exposed surface scale linearly with H2O2 partial pressures between 2.7 x 10-3 and 2.7 x 10-2 Pa because the H2O2 surface coverage is unsaturated in this regime. Conversely, the reaction probabilities decrease as the partial pressure of SO2 is raised from 2.7 x 10-5 to 1.3 x 10-3 Pa, for a fixed H2O2 partial pressure of 8.7 x 10-3 Pa. This is expected if the rate determining step for the mechanism involves HSO3- rather than SO2. It may also arise to some degree if there is competition between gas phase SO2 and H2O2 for adsorption sites. The reaction is sufficiently fast that the lifetime of SO2 within ice clouds could be controlled by this heterogeneous reaction and not by the gas-phase reaction with OH.

Uptake of Gas-Phase SO2 and H2O2 by Ice Surfaces: Dependence on Partial Pressure, Temperature, and Surface Acidity

The uptakes of gas-phase SO2 and H2O2 by ice surfaces have been investigated at temperatures from 213 to 238 K and from 10-7 to 10-4 Torr partial pressure. These experiments have been conducted in a low-temperature, coated-wall flow tube coupled to an electron-impact, quadrupole mass spectrometer which monitors changes in the SO2 and H2O2 partial pressure. The ice surfaces are formed by freezing liquid water. Unlike the uptakes of strong acids such as HNO3 and HCl, the SO2 and H2O2 uptakes are fully reversible on the time scale of the experiment and the surface coverages are roughly a thousandth of a monolayer at 10-6 Torr partial pressure and 228 K. The SO2 uptakes scale with the square root of the partial pressure of the SO2 gas, indicating that dissociation of the hydrated form of adsorbed SO2 is occurring on the surface. The H2O2 uptakes scale linearly with the H2O2 partial pressure, indicating that dissociation does not occur. The uptakes are driven by H-bond interactions in this case. Support for th...

Uptake and reaction of HOI and IONO2 on frozen and dry NaCl/NaBr surfaces and H2SO4

The uptake and reaction of HOI and IONO2 with dry and frozen NaCl/NaBr salt surfaces was investigated using a coated-wall flow tube reactor coupled to a mass spectrometer for gas-phase analysis. HOI and IONO2 react with both surfaces to form the di-halogens IBr and ICl, which are released into the gas phase. On mixed Cl−/Br− surfaces in which the concentration of Cl− greatly exceeds that of Br−, both HOI and IONO2 react to form ICl or IBr. ICl reacts further with Br− to form the observed gas-phase product, IBr. Formation and release of ICl is only observed once the Br− concentration has been chemically depleted. The sum of IBr + ICl in the gas phase was found to remain constant during the reaction, and to account for all of the HOI/IONO2 taken up, indicating no loss of iodine to the surface, and a catalytic role of iodine in the activation and release of bromine and chlorine from mixed salt surfaces. The uptake coefficient of HOI on a frozen, mixed salt surface of similar composition to sea-water at 243 K was >10−2. Similar results (γ>10−2) were obtained for the uptake of HOI onto dry, mixed salt surfaces of similar composition at 298 and 243 K. Lower limits for the accommodation coefficient of HOI on frozen salt (243 K), dry salt (253 K) and H2SO4 (253 K) of 0.05, 0.12 and 0.3, respectively were obtained. The implications of these results for tropospheric halogen chemistry are briefly discussed.

HOBr in Sulfuric Acid Solutions: Solubility and Reaction with HCl as a Function of Temperature and Concentration

A detailed study of the interaction of HOBr and HCl in cold sulfuric acid solutions has been performed using a coated-wall flow tube coupled to an electron-impact mass spectrometer. The liquid-phase bimolecular rate constants, measured over a temperature range from 213 to 238 K and in solutions from 59.7 to 70.1 wt % composition, show a strong positive dependence on both acid composition and temperature. The solubility of HOBr has also been measured in these solutions by analyzing its time-dependent uptake. Henry's Law constants (H) determined from the measured values of HD1/2 and the liquid-phase diffusion coefficient (D) are independent of acid composition over the above range of solution compositions. The values of H demonstrate a clear Clausius−Clapeyron temperature dependence, with a heat of solution of −9 ± 1 kcal/mol. When the atmospheric importance of these data is assessed, two conclusions are reached. In the stratosphere, under aerosol conditions observed soon after the Mt. Pinatubo volcanic eru...

Interaction of HNO3 with water‐ice surfaces at temperatures of the free troposphere

The uptake of gas‐phase HNO3 by water‐ice films has been measured in a low‐temperature, coated‐wall flow tube under conditions where water‐ice, and not a hydrate of nitric acid, is thermodynamically stable. It is observed that there is uptake of HNO3 on the order of 1 → 3 ×1014 molecules per cm² of ice film on a short timescale, and a somewhat smaller uptake on a much longer timescale of 10's of minutes. The short timescale uptake is insensitive to the nitric acid partial pressure, varying by not more than a factor of two over a 20‐fold variation in the partial pressure of HNO3 from 1.3 × 10−7 to 3.1 × 10−6 torr. This implies that the ice surface is saturated with HNO3 at these partial pressures. Also, the uptake is somewhat dependent on temperature over the range 208 to 248 K, with the largest uptake at the lowest temperatures. The uptake is largely irreversible, with only 20 → 25% of the adsorbed HNO3 desorbing from the film when the exposure to gas‐phase HNO3 is stopped. These experiments suggest that adsorption to cirrus cloud surfaces may be an important scavenging process of HNO3 in the troposphere.

Heterogeneous Interactions of HBr and HOCl with Cold Sulfuric Acid Solutions: Implications for Arctic Boundary Layer Bromine Chemistry

The heterogeneous interactions of HBr and HOCl with cold sulfuric acid solutions have been measured with a low-temperature, coated-wall flow tube coupled to an electron-impact mass spectrometer. In particular, (i) the effective Henry's law constants (H*) for the solubility of HBr in sulfuric acid solutions (40.3−60.5 wt %) have been determined by measuring the HBr partial pressure as a function of temperature, (ii) the values of HD1/2 of HOCl, where D is the liquid-phase diffusion coefficient, have been measured by monitoring the time-dependent uptake of HOCl over sulfuric acid solutions (59.7−69.3 wt %), and (iii) when the steady-state decay of gas-phase HOCl is monitored in the presence of an excess of HBr, the liquid-phase rate constant for reaction between dissolved HOCl and HBr has been measured to be 2 × 106 M-1 s-1 in 69.3 wt % acid at 228 K. When coupled with field observations from the Arctic boundary layer, these experimental results suggest that (i) heterogeneous interactions involving HOCl could be important in maintaining ozone loss rates by acting as an efficient activation mechanism for HBr in the springtime and (ii) as a result of the extremely large Henry's law constants for HBr in dilute sulfuric acid, sulfate particles in the boundary layer may contain significant amounts of dissolved HBr.

Heterogeneous interactions of BrO and ClO: Evidence for BrO surface recombination and reaction with HSO3−/SO32−

The uptake coefficients (γ) for loss of BrO and ClO radicals on surfaces characteristic of participate material in the atmosphere have been measured using a coated‐wall flow tube coupled to a resonance fluorescence detector. Neither radical is lost efficiently at 213 K on sulfuric acid (60 to 70 wt%), pyrex or ice surfaces, with BrO uptake coefficients ranging from 5 × 10−4 to 1 × 10−3 on these surfaces and with ClO uptake coefficients smaller than 1×10−4. Based on the observation by mass spectrometry of Br2 formation coincident with BrO loss on ice surfaces, it is believed that radical self‐reaction is occurring on the surface. Finally, for BrO it was observed that the uptake coefficient on 23 wt% aqueous solutions of NaCl at 253 K was small (< 3 × 10−3) but that it could be significantly enhanced when sulfur(IV) species were added to the solution, either by exposure to gas phase SO2 or by the addition of Na2SO3. Aside from the BrO/S(IV) reactions which may promote condensed‐phase free‐radical chemistry, these findings imply that heterogeneous interactions of BrO and ClO in the atmosphere will be insignificant with respect to gas‐phase processes.