The physicochemical processes of the formation of atmospheric aerosols

The current state of the physical chemistry of atmospheric aerosols is described. Most attention is given to the recent achievements in the theory of particle formation and subsequent growth.     

On dry granular bed filtration of aerosols induced by irradiation

An analytical solution of the aerosol filtration process and determination of pressure drop on the granular bed have been presented. Results of simulated calculations and measurements show a good agreement. The elaborated analytical procedure applied for the postirradiation aerosol filtration enables an adjustment of granular filter parameters in certain conditions of the filtration process.     

Photolytic processing of secondary organic aerosols dissolved in cloud droplets

The effect of UV irradiation on the molecular composition of aqueous extracts of secondary organic aerosol (SOA) was investigated. SOA was prepared by the dark reaction of ozone and d-limonene at 0.05-1 ppm precursor concentrations and collected with a particle-into-liquid sampler (PILS). The PILS extracts were photolyzed by 300-400 nm radiation for up to 24 h. Water-soluble SOA constituents were analyzed using high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) at different stages of photolysis for all SOA precursor concentrations. Exposure to UV radiation increased the average O/C ratio and decreased the average double bond equivalent (DBE) of the dissolved SOA compounds. Oligomeric compounds were significantly decreased by photolysis relative to the monomeric compounds. Direct pH measurements showed that acidic compounds increased in abundance upon photolysis. Methanol reactivity analysis revealed significant photodissociation of molecules containing carbonyl groups and the formation of carboxylic acids. Aldehydes, such as limononaldehyde, were almost completely removed. The removal of carbonyls was further confirmed by the UV/Vis absorption spectroscopy of the SOA extracts where the absorbance in the carbonyl n→π* band decreased significantly upon photolysis. The effective quantum yield (the number of carbonyls destroyed per photon absorbed) was estimated as ～0.03. The total concentration of peroxides did not change significantly during photolysis as quantified with an iodometric test. Although organic peroxides were photolyzed, the likely end products of photolysis were smaller peroxides, including hydrogen peroxide, resulting in a no net change in the peroxide content. Photolysis of dry limonene SOA deposited on substrates was investigated in a separate set of experiments. The observed effects on the average O/C and DBE were similar to the aqueous photolysis, but the extent of chemical change was smaller in dry SOA. Our results suggest that biogenic SOA dissolved in cloud and fog droplets will undergo significant photolytic processing on a time scale of hours to days. This type of photolytic processing may account for the discrepancy between the higher values of O/C measured in the field experiments relative to the laboratory measurements on SOA in smog chambers. In addition, the direct photolysis of oligomeric compounds may be responsible for the scarcity of their observation in the field.     

Simulation of electrical-conduction and filtration processes in porous media

Results of simulation of electrical-conduction and filtration processes in porous media have been reported. For better visualization and simplification of the analysis of the results obtained, simple model distributions of pore sizes with the patterns of a triangle, rectangle, semiellipse, etc., have been used. The porous medium has been simulated in terms of a capillary lattice model, which represents a three dimensional cubic lattice of capillaries the sizes and number of which are selected according to a preset pore size distribution. The data obtained reflect the general regularities of the influence of the pore structure on the transfer processes, show the interrelation between the parameters characterizing these processes, and illustrate the potential of the capillary-lattice model for describing the properties of porous media.     

Identification of secondary organic aerosols based on aerosol mass spectrometry

Secondary organic aerosol (SOA) is one of the major components of aerosols in the atmosphere and has not been well understood so far. Due to the complex chemical composition of organic aerosols, the identification of SOA has been a hotspot and difficult issue in the field of aerosol study. This study attempts to quantitatively identify SOA in winter of Shenzhen based on positive matrix factorization (PMF) analysis. Major sources were resolved and SOA was identified subsequently according to the characteristic ion fragments measured by highly time-resolved aerosol mass spectrometer measurement. It showed that in the winter of Shenzhen the average SOA concentration was 9.41 ± 6.33 μg/m³, accounting for 39.9 ± 21.8% of the total organic mass. Compared with primary organic aerosol (POA), the SOA concentrations had no large variation, suggestive of characteristics of regional secondary pollutants. The ratio of SOA/BC had pronounced diurnal variation, similar to that of O _x (O₃+NO₂), indicating SOA formation was significantly controlled by activity of photochemistry in the atmosphere. The most effective period for SOA formation was from 9 am ～3 pm since the SOA/BC ratio increased by 122% during this period. This study provides a new technical method and a new idea for SOA investigation.     

Impact of gel layer formation on colloid retention in membrane filtration processes

Colloidal particles in the feed streams of membrane filtration processes control membrane fouling rate in many instances. In this study, the non-gelling colloidal Na-alginate and the gelling colloidal Ca-alginate are employed to investigate the significance of gel layer formation in membrane filtration processes in terms of contribution to membrane fouling and supplementary impurity removal. The results show that contribution of colloidal particles to membrane fouling depends on the gelling propensity of the colloids and the operational mode (constant pressure or constant flux) implemented. A small dose of either Na-alginate or Ca-alginate was found to greatly increase membrane fouling rate during constant pressure filtration. Both the resistance to removal by application of shear and the lower susceptibility of the concentration polarization layer to shear resulted in more severe fouling during constant flux filtration in the presence of Ca-alginate assemblages than in the presence of Na-alginate. Apparent channel sizes of the Ca-alginate gel layer were calculated from the material properties of the fouling layer. Incomplete catalase retention highlighted the likely heterogeneity in size of liquid transport pathways. Adsorption also contributed to the trapping of colloidal particles according to the retention behaviour of BSA by the Ca-alginate gel layer. Gel layer formation propensity should be seriously considered for the operation of membrane filtration processes. Two simple methods based on (i) a permeability recovery experiment and (ii) comparison of dead-end filtration behaviour with and without shear application are proposed for evaluation of the gelling propensity of colloidal dispersions.     

Simulation of electrical-conduction and filtration processes in porous media with two-phase saturation

Results of simulating electrical-conduction and filtration processes in porous media with two-phase saturation have been presented. A three-dimensional cubic lattice of capillaries has been used as a model porous medium. The sizes and amount of capillaries in the model are selected according to a preset pore-size distribution. The results obtained enable one to establish the general regularities of the influence of the pore structure on transfer processes at two-phase saturation, analyze the dependence of the electrical-conduction and filtration processes on the features of porous-medium saturation, and illustrate the potential of the capillary-lattice model for describing the properties of porous media with two-phase saturation.     

Surface analysis of the size-fractioned urban aerosols by secondary ion mass spectrometry (ToF-SIMS)

The secondary ion mass spectrometry method (ToF-SIMS) has been applied to the analysis of some mineral and organic species on the surface layers of airborne urban particulates. Particulate matter was collected on aluminum sheets of the 8-stage Anderson type cascade impactor from three different sampling sites in the city of Lodz, Poland, which were different in terms of aerosol source apportionment. The obtained results show that vehicle combustion products occur mainly in fine fraction with aerodynamic diameters below 1μm. The main contribution to the mineral components comes from direct and traffic related soil resuspension.      

Nature of capillary condensation and evaporation processes in ordered porous materials

We report on a detailed experimental study of capillary condensation-evaporation processes of N(2) in ordered mesoporous SBA-15 silica. We have carried out measurements of boundary hysteresis loops, reversal curves, and subloops in order to test whether this material behaves as an assembly of independent cylindrical pores open at both ends. With these data, we come to the conclusion that, whereas the boundary hysteresis loop has the classical shape of type H1 associated with condensation-evaporation in cylindrical pores open at both ends, the capillary evaporation does not take place at equilibrium as it is generally assumed. Moreover, the pores do not desorb independently of one another.     

Nature and philosophy of thermal processes in minerals and inorganic materials

Solid-state thermal processes have their own specificity, distinguishing them from the processes in gases and liquids. This specificity includes, among others, their limited reversibility and the limited or modified role of chemical affinity as the main driving force indicating the direction of real solid-state processes. The consequency is the formation of metastable compounds or phases during heating, as intermediate steps towards the state of thermodynamic equilibrium. They are a results of the particular properties of the rigid internal structure of minerals and materials as the thermal reaction medium. The peculiarities of thermal reactions of solids are presented on the example of those of oxides (silicates and borates) with polymeric structures. The significance of crystal-chemical factors for the prediction or explanation of the course of high-temperature processes, as complementing the thermodynamic factors, is discussed.Dedicated to Dr. Robert Mackenzie on the occasion of his 75th birthdayThis work was supported by grant P040703406 from the National Committee of Scientific Research of Poland.     

Building an oxidation reactor in Taiwan: From volatile organic compounds to secondary organic aerosols

Large amounts of volatile organic compounds (VOCs) are emitted into the atmosphere from both human and natural sources. A significant portion of VOCs would be oxidized via their reactions with atmospheric oxidants like OH, NO₃, ozone, etc. The products of the oxidation reactions are often of low volatility and may condense to form secondary organic aerosols (SOA). To study the effect of VOC oxidation in aerosol formation, we are building an oxidation flow reactor system, which consists of (1) a 22-l aluminum chamber, (2) an ozone source with an ozone detector, (3) a UV-C (254 nm) lamp, (4) a photoionization detector to measure the effective VOC concentration, (5) various flow/concentration controlling apparatuses, and (6) a scanning mobility particle sizer to monitor the generated particles. Under the conditions of high UV and ozone levels, the oxidation process can be speeded up by orders of magnitude in this reactor. We hope to use this reactor: (i) to learn the “potential” mass of SOA that can be formed from a given VOC source like a traffic or industry site; (ii) to trace back the SOA source by utilizing the shortened reaction times; (iii) to learn the trends from VOC to SOA.     

Low-pressure membrane filtration of secondary effluent in water reuse: Pre-treatment for fouling reduction

Fouling in the low-pressure membrane filtration of secondary effluent for water reuse can be severe due to the complex nature of the components in the water. Pre-filtration, coagulation and anion exchange resin were investigated as pre-treatments for reducing fouling of microfiltration (MF) and ultrafiltration (UF) membranes in the treatment of activated sludge-lagoon effluent. The key fouling components were determined using several analytical techniques to detect differences in the organic components between the feed and permeate.Pre-filtration (1.5 μm) enhanced the permeate flux for MF by removing particulates, but had little effect for UF. Marked flux improvement was obtained by coagulation pre-treatment at 5 mg L⁻¹ Al³⁺ with internal membrane fouling being substantially alleviated. Anion exchange resin removed >50% of effluent organic matter but did not improve the flux or reduce irreversible membrane fouling. These results, together with detailed organic compositional analyses, showed that the very high-molecular weight organic materials (40–70 kDa) comprised of hydrophilic components such as soluble microbial products, and protein-like extracellular matter were the major cause of membrane fouling.     

FTIR–ATR in situ observation on the efflorescence and deliquescence processes of Mg(NO_3)_2 aerosols

The efflorescence and deliquescence processes of Mg(NO3)2 aerosol particles deposited on ZnSe substrate have been investigated through in situ Fourier transform infrared-attenuated total reflection (FTIR-ATR) technique at the molecular level. At relative humidity (RH) of ~3%, Mg(NO3)2 particles existed as amorphous states. The amorphous Mg(NO3)2 particles were transformed into crystalline Mg(NO3)2·nH2O (n ≤ 5) with slight increasing of RH. Thermodynamically stable Mg(NO3)2·6H2O crystals were gradually formed on the particle surface and started to be dissolved at the saturation point (~53% RH). At the same time, a continuous phase transition from Mg(NO3)2·nH2O (n≤5) to Mg(NO3)2·6H2O occurred on the particle surface. This led the solid particles to completely deliquesce at 76% RH, which was much higher than the saturation point of 53% RH. In the efflorescence process, Mg(NO3)2 droplets entered into the supersaturated region due to the gradual evaporation of water. Finally, amorphous particles were formed when RH decreased below 5%. In the FTIR-ATR spectra of the supersaturated Mg(NO3)2 droplets, the absorbance of the symmetric stretching vibration of NO3- (v1- NO-3) clearly became stronger. It resulted from the continuous formation of solvent share ion pairs (SIPs), and even the contact ion pairs (CIPs) between Mg2 and NO3.     

Breakdown of colloid filtration theory: role of the secondary energy minimum and surface charge heterogeneities

The mechanisms and causes of deviation from the classical colloid filtration theory (CFT) in the presence of repulsive Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions were investigated. The deposition behavior of uniform polystyrene latex colloids in columns packed with spherical soda-lime glass beads was systematically examined over a broad range of physicochemical conditions, whereby both the fluid-phase effluent particle concentration and the profile of retained particles were measured. Experiments conducted with three different-sized particles in a simple (1:1) electrolyte solution reveal the controlling influence of secondary minimum deposition on the deviation from CFT. In a second series of experiments, sodium dodecyl sulfate (SDS) was added to the background electrolyte solution with the intent of masking near-neutrally charged regions of particle and collector surfaces. These results indicate that the addition of a small amount of anionic surfactant is sufficient to reduce the influence of certain surface charge inhomogeneities on the deviation from CFT. To verify the validity of CFT in the absence of surface charge heterogeneities, a third set of experiments was conducted using solutions of high pH to mask the influence of metal oxide impurities on glass bead surfaces. The results demonstrate that both secondary minimum deposition and surface charge heterogeneities contribute significantly to the deviation from CFT generally observed in colloid deposition studies. It is further shown that agreement with CFT is obtained even in the presence of an energy barrier (i.e., repulsive colloidal interactions), suggesting that it is not the general existence of repulsive conditions which causes deviation but rather the combined occurrence of "fast" and "slow" particle deposition.     

Investigation of the ionization processes occurring in liquid-assisted secondary ion mass spectrometry of derivatized monosaccharides

Several derivatized monosaccharides, the 2-deoxy-D -ribofuranoses, have been studied by liquid-assisted secondary ion mass spectrometry (LSIMS) in order to gain insight into the factors affecting ionization in FAB/LSIMS. Examination of the mass spectra for these compounds obtained in eight liquid matrices (diethanolamine, ethylene glycol, glycerol, 2-hydroxyethyl disulfide, 2-hydroxyphenethyl alcohol, 3-nitrobenzyl alcohol, sulfolane and thioglycerol) reveals that in all cases the anomalous [M – H]⁺ ion is the predominant species in the molecular ion region and that [M + Na]⁺ species are observed in the presence of Na⁺. The analysis of these compounds by chemical ionization with ammonia shows [M + H]⁺ as the major species while [M – H]⁺ is essentially absent. This indicates that the ionization processes occurring in the two techniques are not analogous. Thermodynamic considerations based on the gas-phase hydride ion affinities of the protonated matrices do not support a predominant gas-phase mechanism for the formation of [M – H]⁺ in LSIMS. However, it is possible using solvation energies to rationalize the formation of [M – H]⁺ in terms of condensed-phase ionization processes which take place either in the liquid matrix or in the dense selvedge region immediately above the surface where extensive solvation is present. Electrospray data obtained for one of the derivatized monosaccharides indicates that the [M – H]⁺ is not performed in the condensed phase in LSIMS and that it is the product of fast ion beam-induced processes. While the nature of the matrix is seen to have little effect on the intensities of [M – H]⁺ and [M + H]⁺ it is observed to be an important factor for the intensity of M⁺˙ for one of the monosaccharides. This effect can be related to the electron-scavenging properties of the matrices and reinforces the hypothesis that condensed phase processes are significant in ionization.     

Particle formation and surface processes on atmospheric aerosols: A review of applied quantum chemical calculations

Aerosols significantly influence atmospheric processes such as cloud nucleation, heterogeneous chemistry, and heavy-metal transport in the troposphere. The chemical and physical complexity of atmospheric aerosols results in large uncertainties in their climate and health effects. In this article, we review recent advances in scientific understanding of aerosol processes achieved by the application of quantum chemical calculations. In particular, we emphasize recent work in two areas: new particle formation and heterogeneous processes. Details in quantum chemical methods are provided, elaborating on computational models for prenucleation, secondary organic aerosol formation, and aerosol interface phenomena. Modeling of relative humidity effects, aerosol surfaces, and chemical kinetics of reaction pathways is discussed. Because of their relevance, quantum chemical calculations and field and laboratory experiments are compared. In addition to describing the atmospheric relevance of the computational models, this article also presents future challenges in quantum chemical calculations applied to aerosols.