Uptake of Gas-Phase Ammonia. 1. Uptake by Aqueous Surfaces as a Function of pH

Dust emission reduction enhanced gas-to-particle conversion of ammonia in the North China Plain

Ammonium salt is an important component of particulate matter with aerodynamic diameter less than 2.5 µm (PM_2.5) and has significant impacts on air quality, climate, and natural ecosystems. However, a fundamental understanding of the conversion kinetics from ammonia to ammonium in unique environments of high aerosol loading is lacking. Here, we report the uptake coefficient of ammonia (γ_NH3) on ambient PM_2.5 varying from 2.2 × 10^-4 to 6.0 × 10^-4 in the North China Plain. It is significantly lower than those on the model particles under simple conditions reported in the literature. The probability-weighted γ_NH3 increases obviously, which is well explained by the annual decrease in aerosol pH due to the significant decline in alkali and alkali earth metal contents from the emission source of dust. Our results elaborate on the complex interactions between primary emissions and the secondary formation of aerosols and the important role of dust in atmospheric chemistry.

Experimental evidence that halogen bonding catalyzes the heterogeneous chlorination of alkenes in submicron liquid droplets

A key challenge in predicting the multiphase chemistry of aerosols and droplets is connecting reaction probabilities, observed in an experiment, with the kinetics of individual elementary steps that control the chemistry that occurs across a gas/liquid interface. Here we report evidence that oxygenated molecules accelerate the heterogeneous reaction rate of chlorine gas with an alkene (squalene, Sqe) in submicron droplets. The effective reaction probability for Sqe is sensitive to both the aerosol composition and gas phase environment. In binary aerosol mixtures with 2-decyl-1-tetradecanol, linoleic acid and oleic acid, Sqe reacts 12-23× more rapidly than in a pure aerosol. In contrast, the reactivity of Sqe is diminished by 3× when mixed with an alkane. Additionally, small oxygenated molecules in the gas phase (water, ethanol, acetone, and acetic acid) accelerate (up to 10×) the heterogeneous chlorination rate of Sqe. The overall reaction mechanism is not altered by the presence of these aerosol and gas phase additives, suggesting instead that they act as catalysts. Since the largest rate acceleration occurs in the presence of oxygenated molecules, we conclude that halogen bonding enhances reactivity by slowing the desorption kinetics of Cl2 at the interface, in a way that is analogous to decreasing temperature. These results highlight the importance of relatively weak interactions in controlling the speed of multiphase reactions important for atmospheric and indoor environments.

Photochemical Oxidation of Water-Soluble Organic Carbon (WSOC) on Mineral Dust and Enhanced Organic Ammonium Formation

Water-soluble organic carbon (WSOC), which is closely related to biogenic emissions, is of great importance in the atmosphere for its ubiquitous existence and rich abundance. Levoglucosan, a typical WSOC, is usually considered to be stable and thus used as a tracer of biomass burning. However, we found that levoglucosan can be photo-oxidized on mineral dust, with formic acid, oxalic acid, glyoxylic acid, 2,3-dioxopropanoic acid, dicarbonic acid, performic acid, mesoxalaldehyde, 2-hydroxymalonaldehyde, carbonic formic anhydride, and 1,3-dioxolane-2,4-dione detected as main products. Further, we observed the heterogeneous uptake of NH₃ promoted by the carboxylic acids stemming from the photocatalytic oxidation (PCO) of levoglucosan. The mineral-dust-initiated PCO of levoglucosan and enhanced heterogeneous uptake of NH₃, which are highly influenced by irradiation and moisture conditions, were for the first time revealed. The reaction mechanisms and pathways were studied in detail by diffuse reflection infrared Fourier transform spectroscopy (DRIFTS), high-pressure photon ionization time-of-flight mass spectrometry (HPPI-ToF-MS) and flow reactor systems. Diverse WSOC constituents were studied as well, and the reactivity toward NH₃ is related to the number of hydroxyl groups of the WSOC molecules. This work reveals a new precursor of secondary organic aerosols and provides experimental evidence of the existence of organic ammonium salts in atmospheric particles.

Investigation on condensable particulate matter emission characteristics in wet ammonia-based desulfurization system

Particulate matter emissions from ammonia-based wet flue gas desulfurization (Ammonia-WFGD) systems are composed of a filterable particulate matter and a condensable particulate matter (CPM) portion. However, the CPM part has been ignored for a long time, which results in an underestimation of the aerosol problems caused by Ammonia-WFGD systems. In our research, the characteristics of the CPM that emits from an Ammonia-WFGD system are investigated experimentally for the first time, with the US Environmental Protection Agency Method 202 employed as the primary measurement. The influences of some essential desulfurizing parameters are evaluated based on the experimental data. The results show that CPM contributes about 68.8% to the total particulate matter emission. CPM consists mainly of ammonium sulfates/sulfites, with the organic part accounting for less than 4%. CPM is mostly in the submicron fraction, about 71.1% of which originates from the NH₃-H₂O-SO₂ reactions. The appropriate adjustments for the parameters of the flue gas and the desulfurizing solution can inhibit CPM formation to different extents. This indicates that the parameter optimizations are promising in solving CPM emission problems in Ammonia-WFGD systems, in which the pH adjustment alone can abate CPM emission by around 49%. The opposite variations of the parameters need attention because they can cause tremendous CPM emission increase.

Uptake of Gaseous Alkylamides by Suspended Sulfuric Acid Particles: Formation of Ammonium/Aminium Salts

Amides represent an important class of nitrogen-containing compounds in the atmosphere that can in theory interact with atmospheric acidic particles and contribute to secondary aerosol formation. In this study, uptake coefficients (γ) of six alkylamides (C₁ to C₃) by suspended sulfuric acid particles were measured using an aerosol flow tube coupled to a high resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS). At 293 K and < 3% relative humidity (RH), the measured uptake coefficients for six alkylamides were in the range of (4.8-23) × 10^-2. A negative dependence upon RH was observed for both N-methylformamide and N,N-dimethylformamide, likely due to decreased mass accommodation coefficients (α) at lower acidities. A negative temperature dependence was observed for N,N-dimethylformamide under < 3% RH, also consistent with the mass accommodation-controlled uptake processes. Chemical analysis of reacted sulfuric acid particles indicates that alkylamides hydrolyzed in the presence of water molecules to form ammonium or aminium. Our results suggest that multiphase uptake of amides will contribute to growth of atmospheric acidic particles and alter their chemical composition.

Evaporation of freely suspended single droplets: experimental, theoretical and computational simulations

Evaporation is ubiquitous in nature. This process influences the climate, the formation of clouds, transpiration in plants, the survival of arctic organisms, the efficiency of car engines, the structure of dried materials and many other phenomena. Recent experiments discovered two novel mechanisms accompanying evaporation: temperature discontinuity at the liquid-vapour interface during evaporation and equilibration of pressures in the whole system during evaporation. None of these effects has been predicted previously by existing theories despite the fact that after 130 years of investigation the theory of evaporation was believed to be mature. These two effects call for reanalysis of existing experimental data and such is the goal of this review. In this article we analyse the experimental and the computational simulation data on the droplet evaporation of several different systems: water into its own vapour, water into the air, diethylene glycol into nitrogen and argon into its own vapour. We show that the temperature discontinuity at the liquid-vapour interface discovered by Fang and Ward (1999 Phys. Rev. E 59 417-28) is a rule rather than an exception. We show in computer simulations for a single-component system (argon) that this discontinuity is due to the constraint of momentum/pressure equilibrium during evaporation. For high vapour pressure the temperature is continuous across the liquid-vapour interface, while for small vapour pressures the temperature is discontinuous. The temperature jump at the interface is inversely proportional to the vapour density close to the interface. We have also found that all analysed data are described by the following equation: da/dt = P(1)/(a + P(2)), where a is the radius of the evaporating droplet, t is time and P(1) and P(2) are two parameters. P(1) = -λΔT/(q(eff)ρ(L)), where λ is the thermal conductivity coefficient in the vapour at the interface, ΔT is the temperature difference between the liquid droplet and the vapour far from the interface, q(eff) is the enthalpy of evaporation per unit mass and ρ(L) is the liquid density. The P(2) parameter is the kinetic correction proportional to the evaporation coefficient. P(2) = 0 only in the absence of temperature discontinuity at the interface. We discuss various models and problems in the determination of the evaporation coefficient and discuss evaporation scenarios in the case of single- and multi-component systems.

Depression of ammonia uptake to sulfuric acid aerosols by competing uptake of ambient organic gases

The neutralization of acidic aerosols by ammonia has been studied through experiments which combine ambient air with laboratory generated sulfuric acid aerosol. Results indicated that acidic aerosol mixed with organic free air and ammonia was neutralized on a time scale<1 min, consistent with expectations. However, in the presence of ambient organic gases and ammonia, the rate of aerosol neutralization is significantly reduced. This reduction in ammonia uptake was concurrent with an increase in the amount of particle phase organics. A steady state in the NH4+/SO4(2-) in the presence of organic gases was established on time scales of 10 min to several hours, corresponding to NH3 uptake coefficients in the range of 4×10(-3)-2×10(-4). The degree to which neutralization was slowed was dependent upon the initial ammonia concentration and the organic mass added to the aerosols. These results suggest that inorganic equilibrium thermodynamic models may overestimate the rate of ammonia uptake and that ambient particles may remain acidic for longer than previously expected.

Coefficients of evaporation and gas phase diffusion of low-volatility organic solvents in nitrogen from interferometric study of evaporating droplets

Evaporation of motionless, levitating droplets of pure, low-volatility liquids was studied with interferometric methods. Experiments were conducted on charged droplets in the electrodynamic trap in nitrogen at atmospheric pressure at 298 K. Mono-, di-, tri-, and tetra(ethylene glycols) and 1,3-dimethyl-2-imidazolidinone were studied. The influence of minute impurities (<0.1%) upon the process of droplet evaporation was observed and discussed. The gas phase diffusion and evaporation coefficients were found from droplet radii evolution under the assumption of known vapor pressure. Diffusion coefficients were compared with independent measurements and calculations (in air). Good agreement was found for mono- and di(ethylene glycols), and for 1,3-dimethyl-2-imidazolidinone, which confirmed the used vapor pressure values. The value of equilibrium vapor pressure for tri(ethylene glycol) was proposed to be 0.044 +/- 0.008 Pa. The evaporation coefficient was found to increase from 0.035 to 0.16 versus the molecular mass of the compound.

Heterogeneous chemistry of alkylamines with sulfuric acid: implications for atmospheric formation of alkylaminium sulfates

The heterogeneous interaction of alkylamines with sulfuric acid has been investigated to assess the role of amines in aerosol growth through the formation of alkylaminium sulfates. The kinetic experiments were conducted in a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The measurements of heterogeneous uptake of methylamine, dimethylamine, and trimethylamine were performed in the acidity range of 59-82 wt % H(2)SO(4) and between 243 and 283 K. Irreversible reactive uptakes were observed for all three alkylamines, with comparable uptake coefficients (gamma) in the range of 2.0 x 10(-2) to 4.4 x 10(-2). The measured gamma value was slightly higher in more concentrated sulfuric acid and at lower temperatures. The results imply that the heterogeneous reactions of alkylamines contribute effectively to the growth of atmospheric acidic particles and, hence, secondary organic aerosol formation.

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.

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.

Collisions and reactions of gaseous propanol with molten NaOH∕KOH

Molecular beam scattering experiments are used to investigate collisions of a protic molecule, deuterated 1-propanol (PrOD), with an extremely basic solvent, the 5149 mol % NaOH/KOH eutectic mixture. This powerful deprotonating medium readily absorbs PrOD from the gas phase. Nearly all PrOD molecules that thermalize at the surface of the melt enter the liquid and dissolve for long times, most likely residing as PrO- after deprotonation by OH-. The PrO- solvation time is controlled by dissolved H2O, which reprotonates the anion and liberates D --> H exchanged PrOH. We find no evidence for decomposition of the alcohol; at the 463 K temperature of the experiments, the hydroxide solution appears to store propanol reversibly.

A polarizable continuum model for molecules at diffuse interfaces

In this work we illustrate an extension of the polarizable continuum model to describe solvation effects on molecules at the interface between two fluid phases (liquid/liquid, liquid/vapor). This extension goes beyond the naive picture of the interface as a plane dividing two distinct dielectrics, commonly employed in continuum models. The main feature of the model is the use of a diffuse interface with an electric permittivity depending on the position. This characteristic clearly allows the study of simple interfaces as well as more complex membrane or multilayer structures. Moreover the smooth variation of the permittivity in the diffuse interface, in contrast to the sharp boundary between two regions, overcomes the numerical divergences due to charges placed at the boundary. The implementation of the model relies on the integral equation formalism, which allows one to calculate the reaction field acting on a molecule immersed in a dielectric environment once the proper Green's function is known. In the present case, such a Green's function is obtained numerically, allowing a large flexibility in the choice of the dielectric permittivity profile. The applications have been selected with the aim of illustrating the capabilities of the model; its present limitations are also discussed.

Liquid-vapor interfacial properties of water-ammonia mixtures: Dependence on ammonia concentration

The equilibrium and dynamical properties of the liquid-vapor interfaces of water-ammonia mixtures are investigated by means of molecular-dynamics simulations. Altogether, we have simulated seven different systems of different concentration of ammonia. The inhomogeneous density, anisotropic orientational profiles, surface tension, and the pattern of hydrogen bonding are calculated for both water and ammonia molecules in order to characterize the location, width, thermodynamic aspects, and microscopic structure of the liquid-vapor interfaces of each of the water-ammonia systems. The dynamical aspects of the interfaces are investigated in terms of the anisotropic diffusion and dipole orientational relaxation of water and ammonia molecules. The properties of the interfaces are compared with those of the corresponding bulk phases. The present theoretical results are also compared with experimental findings wherever available.

Evaporation of Water through Butanol Films at the Surface of Supercooled Sulfuric Acid

The evaporation of water was monitored from 60, 64, and 68 wt % D(2)SO(4) at 213 K containing 0-0.18 M 1-butanol. Measurements were performed in vacuum using a mass spectrometer to record the velocities and relative fluxes of the desorbing D(2)O. In addition, the surface activity of butanol in the acid was characterized by hyperthermal argon atom scattering in conjunction with surface tension and butanol evaporation measurements. The segregated butyl species reach surface concentrations of approximately 4 x 10(14) cm(-2) (approximately 80% surface coverage) at 0.18 M bulk concentration. We find that the butyl films do not impede the evaporation of D(2)O from the acid to within the 5% uncertainty of the measurements. This result implies that small, soluble surfactants such as butanol form porous films that will not alter the growth or shrinkage of supercooled sulfuric acid droplets in the atmosphere.

Uptake of Organic Gas Phase Species by 1-Methylnaphthalene

Organic compounds are a significant component of tropospheric aerosols. In the present study, 1-methylnaphthalene was selected as a surrogate for aromatic hydrocarbons (PAHs) found in tropospheric aerosols. Mass accommodation coefficients (alpha) on 1-methylnaphthalene were determined as a function of temperature (267 K to 298 K) for gas-phase m-xylene, ethylbenzene, butylbenzene, alpha-pinene, gamma-terpinene, p-cymene, and 2-methyl-2-hexanol. The gas uptake studies were performed with droplets maintained under liquid-vapor equilibrium conditions using a droplet train flow reactor. The mass accommodation coefficients for all of the molecules studied in these experiments exhibit negative temperature dependence. The upper and lower values of alpha at 267 and 298 K respectively are as follows: for m-xylene 0.44 +/- 0.05 and 0.26 +/- 0.03; for ethylbenzene 0.37 +/- 0.03 and 0.22 +/- 0.04; for butylbenzene 0.47 +/- 0.06 and 0.31 +/- 0.04; for alpha-pinene 0.47 +/- 0.07 and 0.10 +/- 0.05; for gamma-terpinene 0.37 +/- 0.04 and 0.12 +/- 0.06; for p-cymene 0.74 +/- 0.05 and 0.36 +/- 0.07; for 2-methyl-2-hexanol 0.44 +/- 0.06 and 0.29 +/- 0.06. The uptake measurements also yielded values for the product HD(l)(1/2) for most of the molecules studied (H = Henry's law constant, D(l) = liquid-phase diffusion coefficient). Using calculated values of D(l), the Henry's law constants (H) for these molecules were obtained as a function of temperature. The H values at 298 K in units 10(3) M atm(-1) are as follows: for m-xylene (0.48 +/- 0.05); for ethylbenzene (0.50 +/- 0.08); for butylbenzene (3.99 +/- 0.93); for alpha-pinene (0.53 +/- 0.07); for p-cymene (0.23 +/- 0.07); for 2-methyl-2-hexanol (1.85 +/- 0.29).

MOLECULAR BEAM STUDIES OF GAS-LIQUID INTERFACES

Molecular beam scattering experiments provide a way to disentangle the elementary steps involved in energy transfer and chemical reactions between gases and liquids. After surveying the history and recent progress in this field, we review studies of the kinematics of gas-liquid collisions and proton exchange of HCl, DCl, and HBr with supercooled sulfuric acid and liquid glycerol. These experiments help to clarify the role of the surface region in controlling trapping and interfacial- and bulk-phase reactions.

Aqueous Solution/Air Interfaces Probed with Sum Frequency Generation Spectroscopy

An important issue for developing a molecular-level mechanism of heterogeneous interactions at the aqueous interface is determining changes in the interface with changes in the bulk composition. Development of the nonlinear spectroscopy, sum frequency generation (SFG) provides a technique to probe these changes. Several molecular and ionic solutes have been used to investigate changes in the structure of the aqueous interface. Molecular solutes include glycerol and ammonia. Ionic and associated ion complexes include sulfuric acid as well as alkali sulfate and bisulfate salts. Molecular solutes and associated ion complexes penetrate to the top monolayer of the aqueous-air interface displacing water from the interface. Specifically, the conjectured ammonia-water complex is observed with ammonia tilted, on average, 25-38° from the normal. Ionic solutes generate a double layer in the interfacial region due to the differential distribution of anions and cations near the interface. The strength of the double layer is dependent on ion size and charge. Due to the extreme size of the proton, the strongest field is generated by acidic solutes. As the ionic solute concentration increases, associated ion pairs form and these penetrate to the top monolayer. These results have wide implications because the aqueous interface is ubiquitous in atmospheric and biological systems.