Tubular Flow Reactors With First-Order Kinetics

Uptake of Hg0(g) on TiO2, Al2O3, and Fe2O3 Nanoparticles: Importance in Atmospheric Chemical and Physical Processes

Mineral dust aerosols play an important role in tropospheric chemistry and aerosol-cloud interaction processes. Yet, their interactions with gaseous elemental mercury (Hg⁰_(g)) are not currently well understood. Using a coated-wall flow tube (CWFT) reactor, we measured the uptake of Hg⁰_(g) on some common components of mineral dust aerosols, including TiO₂, Al₂O₃, and Fe₂O₃, and the effects of irradiation (dark, visible and UV-A) and relative humidity (<2% to 60%) on the uptake kinetics. Under UV-A irradiation (320-400 nm) in dry air, we measured uptake coefficients (γ) equal to >1 × 10^-3 and (3 ± 1) × 10^-6 on TiO₂ and Al₂O₃, respectively. Under visible light irradiation (380-700 nm), Hg⁰_(g) uptake was only observed on TiO₂, with γ = (4 ± 3) × 10^-4. Raising the relative humidity inhibited the uptake on both TiO₂ and Al₂O₃, and the uptake coefficient at 60% RH for TiO₂ under UV-A irradiation was lower by ca. 3 orders of magnitude than dry conditions. Furthermore, we observed that water vapor induced the desorption of two distinct fractions from Hg-exposed surfaces via the displacement of weakly, physisorbed Hg and the photocatalyzed reduction of chemisorbed Hg. Based on the uptake coefficients from this report, we estimate that heterogeneous interactions with mineral dust may be significant under conditions with low relative humidity (<30%) and high dust loading masses. We herein discuss the implication of this study on understanding the life cycle analysis of atmospheric mercury in nature.

Heterogeneous Chemistry of CaCO3 Aerosols with HNO3 and HCl

Calcite (CaCO₃) aerosols often serve as an idealized proxy for calcium-rich mineral dust. Their use has also previously been proposed for stratospheric solar radiation management (SSRM). Little is known about the heterogeneous chemistry of calcite aerosols with trace gases HNO₃ and HCl and therefore their potential impact on stratospheric ozone (O₃). Here we report the results of an experimental study of the uptake of HNO₃ and HCl onto submicron CaCO₃ particles in two different flow reactors. Products and reaction kinetics were observed by impacting aerosolized CaCO₃ onto ZnSe windows, exposing them to the reagent gases at a wide range of concentrations, at 296 K and under dry conditions, and analyzing the particles before and after trace gas exposure using Fourier transform infrared spectroscopy (FTIR). A Ca(OH)(HCO₃) termination layer was detected in the form of a HCO₃^- peak in the FTIR spectra, indicating a hydrated surface even under dry conditions. The results demonstrate the reaction of HNO₃ with Ca(OH)(HCO₃) to produce Ca(NO₃)₂, water, and CO₂. HCl reacted with Ca(OH)(HCO₃) to produce CaCl₂ and also water and CO₂. The depletion of the Ca(OH)(HCO₃)/Ca(CO₃) signal due to reaction with HNO₃ or HCl followed pseudo-first-order kinetics. From the FTIR analysis, the reactive uptake coefficient for HNO₃ was determined to be in the range of 0.013 ≤ γ_HNO3 ≤ 0.14, and that for HCl was 0.0011 ≤ γ_HCl ≤ 0.012 within the reported uncertainty. The reaction of HCl with airborne CaCO₃ aerosols was also studied in an aerosol flow tube coupled with a quadrupole chemical ionization mass spectrometer (CIMS) under similar conditions to the FTIR study, and γ_HCl was determined to be 0.013 ± 0.001. Following previous modeling studies, these results suggest that the reactions of HCl and HNO₃ with calcite in the stratosphere could ameliorate the potential for stratospheric solar radiation management to lead to stratospheric ozone depletion.

Aerosol gas exchange system (AGES) for nanoparticle sampling at elevated temperatures: Modeling and experimental characterization

An aerosol gas exchange system (AGES) for nanoparticle sampling at elevated temperatures was developed, modeled, and further characterized with laboratory tests with respect to gas exchange efficiency and particle losses. The model describing the gas exchange was first verified with oxygen and later studied with several inert gases having molecular masses between 18 and 135 u. The exchange rate of the lightest compounds exceeds 90% efficiency at the flow rates used. In order to reach similarly high removal efficiencies for larger molecules, the residence time in the AGES has to be increased. The removal of sticky gases was studied with gaseous sulfuric acid. Results agreed with the model where the boundary condition is zero concentration on the wall. The AGES exhibits very limited particle losses (<5%) for mono-disperse 6 nm particles. Furthermore, diffusional losses for particles down to 1.2 nm were measured utilizing polydisperse aerosol. The experimental findings are in good agreement with the model derived. As both, gas exchange rate and particle losses, rely on the physical effect of diffusion, an optimization for enhanced gas exchange efficiency will come at the cost of increased diffusional particle losses. The presented model can be used as a tool to redesign and optimize the AGES for a desired application. With an application targeted design, particle dilution can be avoided, which can lead to improved results in many fields of aerosol measurement.

A study of the reactions of Al+ ions with O3, N2, O2, CO2 and H2O: influence on Al+ chemistry in planetary ionospheres

The reactions between Al+(31S) and O3, O2, N2, CO2 and H2O were studied using the pulsed laser ablation at 532 nm of an aluminium metal target in a fast flow tube, with mass spectrometric detection of Al+ and AlO+. The rate coefficient for the reaction of Al+ with O3 is k(293 K) = (1.4 ± 0.1) × 10-9 cm3 molecule-1 s-1; the reaction proceeds at the ion-dipole enhanced Langevin capture frequency with a predicted T-0.16 dependence. For the recombination reactions, electronic structure theory calculations were combined with Rice-Ramsperger-Kassel-Markus theory to extrapolate the measured rate coefficients to the temperature and pressure conditions of planetary ionospheres. The following low-pressure limiting rate coefficients were obtained for T = 120-400 K and He bath gas (in cm6 molecule-2 s-1, uncertainty ±σ at 180 K): log10(k, Al+ + N2) = -27.9739 + 0.05036 log10(T) - 0.60987(log10(T))2, σ = 12%; log10(k, Al+ + CO2) = -33.6387 + 7.0522 log10(T) - 2.1467(log10(T))2, σ =13%; log10(k, Al+ + H2O) = -24.7835 + 0.018833 log10(T) - 0.6436(log10(T))2, σ = 27%. The Al+ + O2 reaction was not observed, consistent with a D°(Al+-O2) bond strength of only 12 kJ mol-1. Two reactions of AlO+ were also studied: k(AlO+ + O3, 293 K) = (1.3 ± 0.6) × 10-9 cm3 molecule-1 s-1, with (63 ± 9)% forming Al+ as opposed to OAlO+; and k(AlO+ + H2O, 293 K) = (9 ± 4) × 10-10 cm3 molecule-1 s-1. The chemistry of Al+ in the ionospheres of Earth and Mars is then discussed.

ClNO2 Production from N2O5 Uptake on Saline Playa Dusts: New Insights into Potential Inland Sources of ClNO2

Nitryl chloride (ClNO₂), formed when dinitrogen pentoxide (N₂O₅) reacts with chloride-containing aerosol, photolyzes to produce chlorine radicals that facilitate the formation of tropospheric ozone. ClNO₂ has been measured in continental areas; however, the sources of particulate chloride required to form ClNO₂ in inland regions remain unclear. Dust emitted from saline playas (e.g., dried lakebeds) contains salts that can potentially form ClNO₂ in inland regions. Here, we present the first laboratory measurements demonstrating the production of ClNO₂ from playa dusts. N₂O₅ reactive uptake coefficients (γ_N2O5) ranged from ∼10^-3 to 10^-1 and ClNO₂ yields (φ_ClNO2) were >50% for all playas tested except one. In general, as the soluble ion fraction of playa dusts increases, γ_N2O5 decreases and φ_ClNO2 increases. We attribute this finding to a transition from aerosol surfaces dominated by silicates that react efficiently with N₂O₅ and produce little ClNO₂ to aerosols that behave like deliquesced chloride-containing salts that generate high yields of ClNO₂. Molecular bromine (Br₂) and nitryl bromide (BrNO₂) were also detected, highlighting that playas facilitate the heterogeneous production of brominated compounds. Our results suggest that parameterizations and models should be updated to include playas as an inland source of aerosol chloride capable of efficiently generating ClNO₂.

Dissociative Recombination of FeO(+) with Electrons: Implications for Plasma Layers in the Ionosphere

The dissociative recombination (DR) of FeO(+) ions with electrons has been studied in a flowing afterglow reactor. FeO(+) was generated by the pulsed laser ablation of a solid Fe target, and then entrained in an Ar(+) ion/electron plasma where the absolute electron density was measured using a Langmuir probe. A kinetic model describing gas-phase chemistry and diffusion to the reactor walls was fitted to the experimental data, yielding a DR rate coefficient at 298 K of k(FeO(+) + e(-)) = (5.5 ± 1.0) × 10(-7) cm(3) molecule(-1) s(-1), where the quoted uncertainty is at the 2σ level. Fe(+) ions in the lower thermosphere are oxidized by O3 to FeO(+), and this DR reaction is shown to provide a more important route for neutralizing Fe(+) below 110 km than the radiative/dielectronic recombination of Fe(+) with electrons. The experimental system was first validated by measuring two other DR reaction rate coefficients: k(O2(+) + e(-)) = (2.0 ± 0.4) × 10(-7) and k(N2O(+) + e(-)) = (3.3 ± 0.8) × 10(-7) cm(3) molecule(-1) s(-1), which are in good agreement with the recent literature.

Measurements of the HO2 uptake coefficients onto single component organic aerosols

Measurements of HO2 uptake coefficients (γ) were made onto a variety of organic aerosols derived from glutaric acid, glyoxal, malonic acid, stearic acid, oleic acid, squalene, monoethanol amine sulfate, monomethyl amine sulfate, and two sources of humic acid, for an initial HO2 concentration of 1 × 10(9) molecules cm(-3), room temperature and at atmospheric pressure. Values in the range of γ < 0.004 to γ = 0.008 ± 0.004 were measured for all of the aerosols apart from the aerosols from the two sources of humic acid. For humic acid aerosols, uptake coefficients in the range of γ = 0.007 ± 0.002 to γ = 0.09 ± 0.03 were measured. Elevated concentrations of copper (16 ± 1 and 380 ± 20 ppb) and iron (600 ± 30 and 51 000 ± 3000 ppb) ions were measured in the humic acid atomizer solutions compared to the other organics that can explain the higher uptake values measured. A strong dependence upon relative humidity was also observed for uptake onto humic acid, with larger uptake coefficients seen at higher humidities. Possible hypotheses for the humidity dependence include the changing liquid water content of the aerosol, a change in the mass accommodation coefficient or in the Henry's law constant.

Reactive uptake of an isoprene-derived epoxydiol to submicron aerosol particles

The reactive uptake of isoprene-derived epoxydiols (IEPOX) is thought to be a significant source of atmospheric secondary organic aerosol (SOA). However, the IEPOX reaction probability (γIEPOX) and its dependence upon particle composition remain poorly constrained. We report measurements of γIEPOX for trans-β-IEPOX, the predominant IEPOX isomer, on submicron particles as a function of composition, acidity, and relative humidity (RH). Particle acidity had the strongest effect. γIEPOX is more than 500 times greater on ammonium bisulfate (γ ∼ 0.05) than on ammonium sulfate (γ ≤ 1 × 10(-4)). We could accurately predict γIEPOX using an acid-catalyzed, epoxide ring-opening mechanism and a high Henry's law coefficient (1.7 × 10(8) M/atm). Suppression of γIEPOX was observed on particles containing both ammonium bisulfate and poly(ethylene glycol) (PEG-300), likely due to diffusion and solubility limitations within a PEG-300 coating, suggesting that IEPOX uptake could be self-limiting. Using the measured uptake kinetics, the predicted atmospheric lifetime of IEPOX is a few hours in the presence of highly acidic particles (pH < 0) but is greater than 25 h on less acidic particles (pH > 3). This work highlights the importance of aerosol acidity for accurately predicting the fate of IEPOX and anthropogenically influenced biogenic SOA formation.

Multiphase chemical kinetics of NO3 radicals reacting with organic aerosol components from biomass burning

Multiphase reactions with nitrate radicals are among the most important chemical aging processes of organic aerosol particles in the atmosphere especially at nighttime. Reactive uptake of NO(3) by organic compounds has been observed in a number of studies, but the pathways of mass transport and chemical reaction remained unclear. Here we apply kinetic flux models to experimental NO(3) exposure studies. The model accounts for gas phase diffusion within a cylindrical flow tube, reversible adsorption of NO(3), surface-bulk exchange, bulk diffusion, and chemical reactions from the gas-condensed phase interface to the bulk. We resolve the relative contributions of surface and bulk reactions to the uptake of NO(3) by levoglucosan and abietic acid, which serve as surrogates and molecular markers of biomass burning aerosol (BBA). Applying the kinetic flux model, we provide the first estimate of the diffusion coefficient of NO(3) in amorphous solid organic matrices (10(-8)-10(-7) cm(2) s(-1)) and show that molecular markers are well-conserved in the bulk of solid BBA particles but undergo rapid degradation upon deliquescence/liquefaction at high relative humidity, indicating that the observed concentrations and subsequent apportionment of the biomass burning source could be significantly underestimated.

Reactive uptake kinetics of NO3 on multicomponent and multiphase organic mixtures containing unsaturated and saturated organics

We investigated the reactive uptake of NO(3) (an important night-time oxidant in the atmosphere) on binary mixtures containing an unsaturated organic (methyl oleate) and saturated molecules (diethyl sebacate, dioctyl sebacate, and squalane) which we call matrix molecules. These studies were carried out to better understand the reactivity of unsaturated organics in multicomponent and multiphase atmospheric particles. For liquid binary mixtures the reactivity of methyl oleate depended on the matrix molecule. Assuming a bulk reaction, H(matrix)√(D(matrix)k(oleate)) varied by a factor of 2.7, and assuming a surface reaction H(matrix)(S)K(matrix)(S)k(oleate)(S) varied by a factor of 3.6, where H(matrix)√(D(matrix)k(oleate) and H(matrix)(S)K(matrix)(S)k(oleate)(S) are constants extracted from the data using the resistor model. For solid-liquid mixtures, the reactive uptake coefficient depended on exposure time: the uptake decreased by a factor of 10 after exposure to NO(3) for approximately 90 min. By assuming either a bulk or surface reaction, the atmospheric lifetime of methyl oleate in different matrices was estimated for moderately polluted atmospheric conditions. For all liquid mixtures, the lifetime was in the order of a few minutes (with an upper limit of 35 min). These lifetimes can be used as lower limits to the lifetimes in semi-solid mixtures. Our studies emphasize that the lifetime of unsaturated organics (similar to methyl oleate) is likely short if the particle matrix is in a liquid state.

Uptake of gaseous hydrogen peroxide by submicrometer titanium dioxide aerosol as a function of relative humidity

Hydrogen peroxide (H(2)O(2)) is an important atmospheric oxidant that can serve as a sensitive indicator for HO(x) (OH + HO(2)) chemistry. We report the first direct experimental determination of the uptake coefficient for the heterogeneous reaction of gas-phase hydrogen peroxide (H(2)O(2)) with titanium dioxide (TiO(2)), an important component of atmospheric mineral dust aerosol particles. The kinetics of H(2)O(2) uptake on TiO(2) surfaces were investigated using an entrained aerosol flow tube (AFT) coupled with a chemical ionization mass spectrometer (CIMS). Uptake coefficients (gamma(H(2)O(2))) were measured as a function of relative humidity (RH) and ranged from 1.53 x 10(-3) at 15% RH to 5.04 x 10(-4) at 70% RH. The observed negative correlation of RH with gamma(H(2)O(2)) suggests that gaseous water competes with gaseous H(2)O(2) for adsorption sites on the TiO(2) surface. These results imply that water vapor plays a major role in the heterogeneous loss of H(2)O(2) to submicrometer TiO(2) aerosol. The results are compared with related experimental observations and assessed in terms of their potential impact on atmospheric modeling studies of mineral dust and its effect on the heterogeneous chemistry in the atmosphere.

Uptake of gas-phase nitrous acid by pH-controlled aqueous solution studied by a wetted wall flow tube

Uptake kinetics of gas phase nitrous acid (HONO) by a pH-controlled aqueous solution was investigated by using a wetted wall flow tube. The gas phase concentration of HONO after exposure to the aqueous solution was measured selectively by the chemical ionization mass spectrometer in a high sensitive manner. The uptake rate of the gaseous HONO was found to depend on the pH of the solution. For the uptake by neutral and alkaline solutions, the gas phase concentration was observed to decay exponentially, suggesting that the uptake was fully limited by the gas phase diffusion. On the other hand, the uptake by the acidic solution was found to be determined by both the gas phase diffusion and the liquid phase processes such as physical absorption and reversible acid dissociation reaction. The decay was analyzed by the rate equations using the time dependent uptake coefficient involving the saturation of the liquid surface. While the uptake processes by the solution at pH = 2-3 were well described by those calculated using the physical and chemical parameters reported for the bulk, the uptake rates by the solution at 4 < pH < 7 deviate from the calculated ones. The present result can suggest that the pH at the liquid surface is lower than that in the bulk liquid, which is responsible for the additional resistance of mass transfer from the gas to the liquid phase.

An asymptotic theory for dispersion of reactive contaminants in parallel flow

The classic problem, first treated by Taylor [18], of the dispersion of inert soluble matter in fluid flow continues to attract attention from researchers describing the approach to the asymptotic state [5, 17]. The present article considers some of the complications caused when the solute is chemically active, Dispersing chemically active solutes occur in diverse fields such as chromatography, chemical engineering and environmental fluid mechanics.

The asymptotic large-time analysis of Chatwin [5] is re-worked to handle the case of reactive solutes dispersing in parallel flow. Matching between moderate and large-time solutions requires consideration of the integral moments of the reactive contaminant could, and the Aris method of moments is therefore invoked and modified for reaction effects. The results are applied in detail to the outstanding paractical example—the chemical flow reactor (a device used to measure reaction rates for chemical reactions taking place between fluids). For this case, the paper provides a practical alternative to recent variable diffusion coefficient studies [6, 7, 15], and presents further results for the concentration distribution and for the limiting behaviour under weak and vigorous recactions at the boundary of the flow.

Reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of polycyclic aromatic hydrocarbon surfaces

We investigated the reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of solid polycyclic aromatic hydrocarbons (PAHs) using a coated wall flow tube reactor coupled to a chemical ionization mass spectrometer. The PAH surfaces studied were the 4-ring systems pyrene, benz[a]anthracene, and fluoranthene. Reaction of NO3 radicals with all three PAHs was observed to be very fast with the reactive uptake coefficient, gamma, ranging from 0.059 (+0.11/-0.049) for benz[a]anthracene at 273 K to 0.79 (+0.21/-0.67) for pyrene at room temperature. In contrast to the NO3 reactions, reactions of the different PAHs with the other gas-phase species (N2O5, NO2, HNO3, and O3) were at or below the detection limit (gamma <or= 6.6 x 10(-5)) in all cases, illustrating that these reactions are at best slow. For NO3 we also investigated the time dependence of the reactive uptake to determine if the surface-bound PAH molecules were active participants in the reaction (i.e., reactants). Reaction of NO3 on all three PAH surfaces slowed down at 263 K after long NO3 exposure times, suggesting that the PAH molecules were reactants. Additionally, NO2 and HNO3 were identified as major gas-phase products. Our results show that under certain atmospheric conditions, NO3 radicals can be a more important sink for PAHs than NO2, HNO3, N2O5, or O3 and impact tropospheric lifetimes of surface-bound PAHs.

Uptake of NH3 and NH3 + 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 Chemistry of Organic Acids on Soot Surfaces

We have investigated the heterogeneous interaction between a number of carboxylic acids and soot generated from different fuel sources and formation mechanisms. A low-pressure fast flow reactor in conjunction with ion drift-chemical ionization mass spectrometry detection was employed to study uptake of monocarboxylic (benzoic, oleic, and steric) and dicarboxylic (glutaric, maleic, oxalic, and phthalic) acids on deposited soot surfaces formed by combustion of methane, propane, and kerosene. Most acids exhibited irreversible uptake on the soot surfaces and the uptake coefficient was measured in the range of 9 x 10(-4) to 1 x 10(-1) estimated based on the geometric surface areas. Brunauer, Emmett, and Teller surface areas of the deposited soot surfaces were measured and the soot bulk and surface chemical compositions were evaluated with Fourier transform infrared spectroscopy and attenuated total reflection spectroscopy. Plausible uptake mechanisms were discussed on the basis of the measured soot physiochemical properties by comparing the mono and dicarboxylic acids.

Direct Measurement and Theoretical Calculation of the Rate Coefficient for Cl + CH3 in the Range from T = 202−298 K

The rate coefficient has been measured under pseudo-first-order conditions for the Cl+CH3 association reaction at T=202, 250, and 298 K and P=0.3-2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3 were generated rapidly and simultaneously by reaction of F with HCl and CH4, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F2 in He. The decay of CH3 was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3 concentration ([Cl]0/[CH3]0=9-67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T=202 K, the rate coefficient increases from 8.4x10(-12) at P=0.30 Torr He to 1.8x10(-11) at P=2.00 Torr He, both in units of cm3 molecule-1 s-1. A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin-orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k2=6.0x10(-11) cm3 molecule-1 s-1, independent of temperature in the range from 200 to 300 K.

Heterogeneous reactions of HOI, ICl and IBr on sea salt and sea salt proxies

The heterogeneous chemistry of HOI, ICl and IBr on sea salt and sea salt proxies has been studied at 274 K using two experimental approaches: a wetted wall flow tube coupled to an electron impact mass spectrometer (WWFT-MS) and an aerosol flow tube (AFT) coupled to a differential mobility analyser (DMA) and a chemical ionisation mass spectrometer (CIMS). Uptake of all three title molecules into bulk aqueous halide salt films was rapid and controlled by gas phase diffusion. Uptake of HOI gave rise to gas-phase ICl and IBr, with the latter being the predominant product whenever Br(-) was present. Only partial release of IBr was observed due to high solubility of dihalogens in the film. ICl uptake gave the same yield of IBr as HOI uptake. Uptake of ICl on NaBr aerosol was accommodation limited with alpha = 0.018 +/- 0.004 and gas phase IBr product has a yield of 0.6 +/- 0.3. The results show that HOI can act as a catalyst for activation of bromine from sea-salt aerosols in the marine boundary layer, via the reactions: HOI(aq) + Cl + H--> ICl(aq) + H(2)O(l) and ICl(aq) + Br--> IBr(aq) + Cl.

Uptake of HNO3 on Hexane and Aviation Kerosene Soots

The uptake of HNO(3) on aviation kerosene (TC-1) soot was measured as a function of temperature (253-295 K) and the partial pressure of HNO(3), and the uptake of HNO(3) on hexane soot was studied at 295 K and over a limited partial pressure of HNO(3). The HNO(3) uptake was mostly reversible and did not release measurable amounts of gas-phase products such as HONO, NO(3), NO(2) or N(2)O(5). The heat of adsorption of HNO(3) on soot was dependent on the surface coverage. The isosteric heats of adsorption, Delta(0)H(isosteric), were determined as a function of coverage. Delta(0)H(isosteric) values were in the range -16 to -13 kcal mol(-1). The heats of adsorption decrease with increasing coverage. The adsorption data were fit to Freundlich and to Langmuir-Freundlich isotherms. The heterogeneity parameter values were close to 0.5, which suggested that a HNO(3) molecule can occupy two sites on the surface with or without being dissociated and that the soot surface could be nonuniform. Surface FTIR studies on the interaction of soot with HNO(3) did not reveal formation of any minor product such as organic nitrate or nitro compound on the soot surface. Using our measured coverage, we calculate that the partitioning of gas-phase nitric acid to black carbon aerosol is not a significant loss process of HNO(3) in the atmosphere.

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.

Hydrogen chloride-induced surface disordering on ice

Characterizing the interaction of hydrogen chloride (HCl) with polar stratospheric cloud ice particles is essential for understanding the processes responsible for ozone depletion. We studied the interaction of gas-phase HCl with ice between 243 and 186 K by using (i) ellipsometry to monitor the ice surface and (ii) coated-wall flow tube experiments, both with chemical ionization mass spectrometry detection of the gas phase. We show that trace amounts of HCl induce formation of a disordered region, or quasi-liquid layer, at the ice surface at stratospheric temperatures. We also show that surface disordering enhances the chlorine activation reaction of HCl with chlorine nitrate (ClONO(2)) and also enhances acetic acid (CH(3)COOH) adsorption. These results impact our understanding of the chemistry and physics of ice particles in the atmosphere.