Crystallization pathways of sulfate-nitrate-ammonium aerosol particles

Crystallization experiments are conducted for aerosol particles composed of aqueous mixtures of (NH(4))(2)SO(4)(aq) and NH(4)NO(3)(aq), (NH(4))(2)SO(4)(aq) and NH(4)HSO(4)(aq), and NH(4)NO(3)(aq) and NH(4)HSO(4)(aq). Depending on the aqueous composition, crystals of (NH(4))(2)SO(4)(s), (NH(4))(3)H(SO(4))(2)(s), NH(4)HSO(4)(s), NH(4)NO(3)(s), 2NH(4)NO(3) x (NH(4))(2)SO(4)(s), and 3NH(4)NO(3) x (NH(4))(2)SO(4)(s) are formed. Although particles of NH(4)NO(3)(aq) and NH(4)HSO(4)(aq) do not crystallize even at 1% relative humidity, additions of 0.05 mol fraction SO(4)(2-)(aq) or NO(3)(-)(aq) ions promote crystallization, respectively. 2NH(4)NO(3) x (NH(4))(2)SO(4)(s) and (NH(4))(3)H(SO(4))(2)(s) appear to serve as good heterogeneous nuclei for NH(4)NO(3)(s) and NH(4)HSO(4)(s), respectively. 2NH(4)NO(3) x (NH(4))(2)SO(4)(s) crystallizes over a greater range of aqueous compositions than 3NH(4)NO(3) x (NH(4))(2)SO(4)(s). An infrared aerosol spectrum is provided for each solid based upon a linear decomposition analysis of the recorded spectra. Small nonzero residuals occur in the analysis because aerosol spectra depend on particle morphology, which changes slightly across the range of compositions studied. In addition, several of the mixed compositions crystallize with residual aqueous water of up to 5% particle mass. We attribute this water content to enclosed water pockets. The results provide further insights into the nonlinear crystallization pathways of sulfate-nitrate-ammonium aerosol particles.     

Desorption electrospray ionization of aerosol particles

We have applied desorption electrospray ionization to aerosol particles. Ions were formed from aerosols by merging suspended dry particles with an electrospray of solvent in a modified ion trap mass spectrometer. Dry aerosol particles were generated using a fluidized bed powder disperser and directed toward the inlet of the mass spectrometer. A nanospray source was used to create a spray of solvent droplets directed at the inlet and at a right angle with respect to the aerosol. Ions generated by the interaction of the particles and electrospray were transferred into the ion trap mass spectrometer. Using this method, pure samples of caffeine and erythromycin A were analyzed. In addition, commonly available food and drug powders including instant cocoa powder, artificial sweetener and ibuprofen were analyzed.     

Uptake of phenol on aerosol particles

We present a study of the interaction between a phenol molecule and an aerosol particle. The aerosol particle is represented by a cluster of 128 water molecules. Using a classical approach, we present interaction energy surfaces for different relative distances and for three orientations of phenol relative to the particle. From the energy surfaces we find the reaction pathways with the largest interaction between the molecule and the particle. We use a quantum mechanics/molecular mechanics (QM/MM) method to calculate a potential energy curve for each reaction path. Coupled cluster methods are used for the part of the system described by quantum mechanics, while the part described by molecular mechanics is represented by a polarizable force field. We compare results obtained from the classical approach with the QM/MM results. Furthermore, we use the QM/MM results to calculate mass accommodation coefficients using a quantum-statistical (QM-ST) model and show how the mass accommodation coefficient depends on the relative orientation of phenol with respect to the aerosol particle.     

Vibrational spectroscopic investigations of ammonia gas sensing mechanism in polypyrrole nanostructures

This paper reports the application of Raman and Fourier transform infrared (FTIR) spectroscopy techniques for the investigation of molecular restructuring of polypyrrole (PPy) nanostructures in ammonia environment. Different types of PPy nanostructures such as nanofibers, nanorods, and nanoparticles were prepared in the presence of different surfactants such as cetyltrimethyl ammonium bromide (CTAB), methyl orange, sodium dodecyl sulfate, and Triton X-100, respectively. The prepared nanostructures were characterized for structural, morphological, and the gas sensing properties. The gas sensing reponse towards ammonia is estimated from change in the surface resistance of the sample. PPy nanofibers prepared in the presence of CTAB have a diameter of ∼63 nm and the gas sensing response of ∼18%, whereas, PPy nanoparticles prepared in the presence of Triton X-100 have a diameter of ∼94 nm and the lowest gas sensing response (6.5%) at 100 ppm level of ammonia. The mechanism of gas sensing has been investigated through vibrational (Raman and FTIR) spectroscopy techniques performed in the presence of analyte (ammonia) gas. The charge compensation via proton transfer process in ammonia environment is found to be main cause for the gas sensing response in the PPy nanostructures.     

Investigation of aerosol particles by atomic force microscopy

AFM has been applied for studying morphology and size distribution of nanometer-sized particles adsorbed on flat surfaces. In order to optimize imaging of these ultrafine particles different substrates were evaluated with respect to their roughness and stability under the influence of the sensing tip. Moreover, a method for calculating particle volumes from the three-dimensional AFM data is described. This greatly enhances the information content of AFM images, because a large number of particles in the raw data can be evaluated automatically in order to derive information on size distribution or surface coverage. This evaluation method has also been applied successfully to quantitatively describe changes on particles induced by different humidity of the surrounding atmosphere. Received: 15 July 1996 / Revised: 18 December 1996 / Accepted: 3 January 1997     

Characterisation of aerosol particles using combined thermoanalytical techniques

Four different artificial aerosol particles as well as one standard environmental aerosol particle (NIST SRM 1649a) were investigated. While the mass loss of the aerosol particles varies from 1.5 to 32% of the entire mass, only a small part of each volatile fraction consists of organic compounds. Due to the construction of the combined thermoanalytical system, only a small part of the evolved gases is used for the mass spectrometric investigation. This results in a restricted sensitivity which does not allow an identification of individual organic components. Additional investigations like thermal desorption GC-MS reveal up to 1600 individual components (SRM 1649a). This revised version was published online in July 2006 with corrections to the Cover Date.     

Methods for characterization of organic compounds in atmospheric aerosol particles

Atmospheric aerosol particles of primary or secondary, biogenic or anthropogenic origin are highly complex samples of changing composition in time and space. To assess their effects on climate or human health, the size-dependent chemical composition of these ubiquitous atmospheric constituents must be known. The development of novel analytical methods has enabled more detailed characterization of the organic composition of aerosols. This review gives an overview of the methods used in the chemical characterization of atmospheric aerosol particles, with a focus on mass-spectrometry techniques for organic compounds, either alone or in combination with chromatographic separation. Off-line, on-site, and on-line methods are covered, and the advantages and limitations of the different methods are discussed. The main emphasis is on methods used for detailed characterization of the composition of the organic compounds in aerosol particles. We address and summarize the current state of analytical methods used in aerosol research and discuss the importance of developing novel sampling strategies and analytical instrumentation.

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Graphical Abstract Challenges in the atmospheric aerosol analytics

     

Capillary electrophoretic separation of dicarboxylic acids in atmospheric aerosol particles

2,3-Pyrazinedicarboxylic acid (PZDA), 2,6-pyridinedicarboxylic acid (PDA) and 2,3-pyridinedicarboxylic acid (QUIN) solutions were studied as background electrolytes (BGEs) in the capillary electrophoretic analysis of dicarboxylic acids in aerosol particles with indirect UV detection. The BGEs were selected on the basis of similarity in structure with the analytes so that mobilities would be compatible. Optimised pH values for PZDA, PDA and QUIN solutions were 10.6, 11.0 and 10.2, respectively. Myristyltrimethylammonium hydroxide and myristyltrimethylammonium bromide were added to reverse the electroosmotic flow in the solutions in the direction of anode to enable fast anion detection. Separation was obtained for nine dicarboxylic acids (C₂–C₁₀) differing in the number of CH₂ groups in their skeleton. The electrophoretic mobilities were determined to lie in the range 3.0×10⁻⁴–7.0×10⁻⁴ cm² V⁻¹ s⁻¹. The relative standard deviations (RSDs) for the absolute migration times of the analytes were mostly less than 0.5% (n=6) in PZDA solution. In PDA solution the within-day and day-to-day RSD values for migration were less than 1% and between 2 and 4%, respectively. Peak heights and areas mostly deviated between 1 and 15% in both PZDA and PDA solutions. Detection limits ranged between 1 and 5 mg/l. Methods were applied to the analysis of dicarboxylic acids isolated from aerosol particles.     

Infrared analysis of CO ice particles in the aerosol phase

Fourier transform infrared extinction spectra of a variety of CO ice aerosols, generated at low temperatures in a liquid helium cooled collisional-cooling cell, have been analyzed. Different operation modes of the cooling system were used for the generation of spherical and nonspherical CO nanoparticles at temperatures between 5 and 35 K and with diameters between 10 and 1000 nm. In contrast to the predominantly amorphous CO films described in the literature the presented CO particles are (poly)crystalline. A Mie inversion iterative scheme is presented and used to infer the optical constants of CO ice for the cases compact particles have been produced. The spectra of nonspherical CO aerosol particles are interpreted by modeling the extinction using the discrete dipole approximation procedure combined with the retrieved optical constants. A global positive matrix factorization scheme allows us to infer the dominant shapes in the observed particle distribution and can be used as a guide for further experiments. Near 25 K a pronounced shape evolution of smaller particles from spherical toward longish structures was observed at low buffer-gas pressure over 400 s.     

Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

Surface-sensitive vibrational spectroscopy is a common tool for measuring molecular organization and intermolecular interactions at interfaces. Peak intensity ratios are typically used to extract molecular information from one-dimensional spectra but vibrational coupling between surfactant molecules can manifest as signal depletion in one-dimensional spectra. Through a combination of experiment and theory, we demonstrate the emergence of vibrational exciton delocalization in infrared reflection–absorption spectra of soluble and insoluble surfactants at the air/water interface. Vibrational coupling causes a significant decrease in peak intensities corresponding to C–F vibrational modes of perfluorooctanoic acid molecules. Vibrational excitons also form between arachidic acid surfactants within a compressed monolayer, manifesting as signal reduction of C–H stretching modes. Ionic composition of the aqueous phase impacts surfactant intermolecular distance, thereby modulating vibrational coupling strength between surfactants. Our results serve as a cautionary tale against employing alkyl and fluoroalkyl vibrational peak intensities as proxies for concentration, although such analysis is ubiquitous in interface science.

Coupling between surfactant molecules at the air/water interface bleeds intensity into a diffuse background, such that single-wavelength vibrational intensity is effectively depleted at high surface coverage.     

In-situ investigation of aerosol particles by atomic force microscopy

In-situ imaging of aerosol particles deposited on mica in the TM-AFM liquid cell has been performed in order to study their dissolution behavior. The results show that the implementation of in-situ experiments is very useful for obtaining information on the physical and chemical behavior of individual particles. Experiments were carried out using ammonium sulfate and soot-like particles as test aerosols. Water soluble fractions can be easily distinguished from insoluble fractions. This can also be utilized to proof the existence of internally mixed particles. These model experiments are an important basis for further investigations on airborne particles involving other solvent systems and time resolved measurements.     

Phase transitions and surface morphology of surfactant-coated aerosol particles

Probe molecule spectroscopy and hygroscopic growth curves characterize the morphology of surfactant-coated aerosol particles as a function of relative humidity (RH). This study focuses on particles composed of either potassium iodide or sodium chloride and sodium dodecyl sulfate (SDS). At high RH, these mixed particles assume a reverse micelle type structure, and at low RH, they comprise a solid core of either KI or NaCl coated with SDS and water. The deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of the inorganic fraction of the mixed particles are very similar to those of the pure salts. The surface polarity and morphology sampled by the coumarin 314 probe molecule ranges from that of a water-organic interface to that of an ionic surface and depends strongly on the RH and the amount of SDS. When the SDS coverage of the droplet just prior to efflorescence reaches approximately one monolayer, a thin soap film persists on the surface to values of RH much lower than the ERH. Both the electronic spectroscopy and photoelectric charging efficiency show a separate efflorescence for this layer at RH < 5%. The spectroscopy further reveals that there is a hysteresis associated with this low RH phase transition for both KI and NaCl cores.     

Chemical composition and reaction analysis of single aerosol particles

In situ measurements of gas-liquid surface reactions of single aerosol microdroplets are presented. By means of optical levitation in combination with elastic (Mie) and inelastic (Raman) light scattering it is possible to get information on the chemistry of e.g. acid/base reactions as well as the physical behavior of single microparticles.     

Chemical composition and reaction analysis of single aerosol particles

In situ measurements of gas-liquid surface reactions of single aerosol microdroplets are presented. By means of optical levitation in combination with elastic (Mie) and inelastic (Raman) light scattering it is possible to get information on the chemistry of e.g. acid/base reactions as well as the physical behavior of single microparticles.     

Inertial deposition of aerosol particles from laminar flows in fibrous filters

The deposition of aerosol particles onto filter fibers under the effect of inertial forces is studied in a wide range of Stokes numbers (St) at Reynolds numbers close to unity (Re ∼ 1). Coefficients η of the capture of inertial particles with finite sizes in model filters composed of parallel rows of identical parallel fibers located normal to the direction of a flow are determined based on the numerical solution of the Navier-Stokes and particle motion equations. It is shown that, at Re < 1 and a constant particle-to-fiber radius ratio, R = r _p/a, number St uniquely characterizes capture coefficients η for particles with different densities, while, at Re ≥ 1, the capture coefficient depends on both St and Re. At constant R and St values, the larger Re the higher the capture coefficient. The influence of the structure of the model filter on pressure drop Δp and η is investigated. A nonuniform arrangement of fibers in rows is shown to increase the Δp/U ratio at lower Re values and to make the η -St dependence more pronounced than that for systems of uniformly ordered fibers. The results of calculations agree with the experimental data.     

Preparation of titania-coated polystyrene particles in mixed solvents by ammonia catalysis

A mixed-solvent method was developed to coat polystyrene (PS) spheres with smooth, homogeneous shells of amorphous titania by ammonia catalysis. The TEM images showed that, in the presence of ammonia, the thickness of titania shells could be controlled in the range of 8-65 nm by varying the concentration of titanium tetrabutoxide (TBOT) in the ethanol/acetonitrile mixed solvents with an appropriate volume ratio. The diffusion-controlled mechanism of the mixed solvents and the catalysis mechanism of ammonia were investigated. After the calcination of core-shell particles for 2 h at 500 degrees C, spherical hollow titania shells could be obtained, and the surfaces of the particles remained quite smooth and homogeneous. The XRD analysis indicated that calcination promoted the transformation of amorphous titania into an anatase phase.     

Vibrational state selection of ammonia ions using resonant 2 + 1 multiphoton ionization

Photoelectron kinetic energy spectra are presented for the 2 + 1 multiphoton ionization of NH₃ via the vibronic levels of the B̃ and C̃′ Rydberg states of the neutral. The contribution from Δν = 0 state-selected ionization is greater than 80% through the C̃′ state and over 70% through the B̃ state. This allows for the production of large densities of NH₃⁺(ν) ions with a high degree of vibrational selectivity.     

Sampling and analysis of individual particles by aerosol mass spectrometry

Over the past decade, aerosol mass spectrometry has developed into a powerful method for characterizing individual particles in air. Recent advances in the design of inlets and mass spectrometers have extended the size range of particles that can be analyzed. In this tutorial, fundamental aspects of particle motion in sampling inlets are introduced. Basic experimental configurations for achieving a high analysis rate and the ability of laser ablation to provide chemical composition information are reviewed. An example of the use of this technology to study atmospheric phenomena is also presented. Significant opportunity exists for designing new experiments at the interface of aerosol mass spectrometry and conventional molecular mass spectrometry.     

Quantitative analysis of single aerosol particles using polycapillary X-ray optics

A laboratory micro X-ray fluorescence spectrometer based on polycapillary X-ray optics (PXRO) was used to carry out the quantitative X-ray fluorescence analysis of single aerosol particles with smaller size than that of focal spot of PXRO. The minimum detection limits measured with the thin-film reference standards were in the range from 13.3 to 0.7 ng cm^? 2 when the operating current and voltage were 70 mA and 35 kV, respectively. In order to reduce the effects of the inhomogeneous distributions of the X-ray intensity in the focal spot of the PXRO on the analysis results, the sensitivities were corrected by using a Gaussian function for the quantitative analysis of single aerosol particles. The accuracy of the analysis of single standard solution drops was on average 25% depending on the element and concentration. The precision of the analysis was better than 5%.