乙苯光氧化产生二次有机气溶胶的化学成分及反应机理分析

在自制的烟雾腔内,研究羟基自由基(OH·)启动的乙苯的光氧化反应和一系列后续反应,产生了二次有机气溶胶. 采用空气动力学直径粒谱分析仪分析了气溶胶粒子的尺寸分布;并用自制的气溶胶飞行时间质谱仪快速、实时地测量了单个二次有机气溶胶粒子的分子组分. 初步探讨了这些组分的可能反应机理.     

Photochemical aging of secondary organic aerosol particles generated from the oxidation of d-limonene

Secondary organic aerosol (SOA) particles are generated by reacting d-limonene vapor and ozone in a Teflon reaction chamber. The reaction is carried out in either dry or humid air in darkness. The resulting SOA particles are collected on glass fiber filters, and their photochemical properties are probed using a combination of UV photodissociation action spectroscopy and absorption spectroscopy techniques. Photolysis of limonene SOA in the tropospheric actinic region (lambda > 295 nm) readily produces formic acid and formaldehyde as gas-phase products. The UV wavelength dependence of the photolysis product yield suggests that the primary absorbers in SOA particles are organic peroxides. The relative humidity maintained during SOA particle growth is found to have little effect on the UV wavelength dependence of the photolysis product yield. The data suggest that direct photodissociation processes may play an important role in photochemical processing of atmospheric SOA particles.     

Kinetics, products, and mechanisms of secondary organic aerosol formation

Secondary organic aerosol (SOA) is formed in the atmosphere when volatile organic compounds (VOCs) emitted from anthropogenic and biogenic sources are oxidized by reactions with OH radicals, O(3), NO(3) radicals, or Cl atoms to form less volatile products that subsequently partition into aerosol particles. Once in particles, these organic compounds can undergo heterogenous/multiphase reactions to form more highly oxidized or oligomeric products. SOA comprises a large fraction of atmospheric aerosol mass and can have significant effects on atmospheric chemistry, visibility, human health, and climate. Previous articles have reviewed the kinetics, products, and mechanisms of atmospheric VOC reactions and the general chemistry and physics involved in SOA formation. In this article we present a detailed review of VOC and heterogeneous/multiphase chemistry as they apply to SOA formation, with a focus on the effects of VOC molecular structure on the kinetics of initial reactions with the major atmospheric oxidants, the subsequent reactions of alkyl, alkyl peroxy, and alkoxy radical intermediates, and the composition of the resulting products. Structural features of reactants and products discussed include compound carbon number; linear, branched, and cyclic configurations; the presence of C[double bond, length as m-dash]C bonds and aromatic rings; and functional groups such as carbonyl, hydroxyl, ester, hydroxperoxy, carboxyl, peroxycarboxyl, nitrate, and peroxynitrate. The intention of this review is to provide atmospheric chemists with sufficient information to understand the dominant pathways by which the major classes of atmospheric VOCs react to form SOA products, and the further reactions of these products in particles. This will allow reasonable predictions to be made, based on molecular structure, about the kinetics, products, and mechanisms of VOC and heterogeneous/multiphase reactions, including the effects of important variables such as VOC, oxidant, and NO(x) concentrations as well as temperature, humidity, and particle acidity. Such knowledge should be useful for interpreting the results of laboratory and field studies and for developing atmospheric chemistry models. A number of recommendations for future research are also presented.     

Secondary organic aerosol formation from low-NO(x) photooxidation of dodecane: evolution of multigeneration gas-phase chemistry and aerosol composition

The extended photooxidation of and secondary organic aerosol (SOA) formation from dodecane (C(12)H(26)) under low-NO(x) conditions, such that RO(2) + HO(2) chemistry dominates the fate of the peroxy radicals, is studied in the Caltech Environmental Chamber based on simultaneous gas and particle-phase measurements. A mechanism simulation indicates that greater than 67% of the initial carbon ends up as fourth and higher generation products after 10 h of reaction, and simulated trends for seven species are supported by gas-phase measurements. A characteristic set of hydroperoxide gas-phase products are formed under these low-NO(x) conditions. Production of semivolatile hydroperoxide species within three generations of chemistry is consistent with observed initial aerosol growth. Continued gas-phase oxidation of these semivolatile species produces multifunctional low volatility compounds. This study elucidates the complex evolution of the gas-phase photooxidation chemistry and subsequent SOA formation through a novel approach comparing molecular level information from a chemical ionization mass spectrometer (CIMS) and high m/z ion fragments from an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Combination of these techniques reveals that particle-phase chemistry leading to peroxyhemiacetal formation is the likely mechanism by which these species are incorporated in the particle phase. The current findings are relevant toward understanding atmospheric SOA formation and aging from the "unresolved complex mixture," comprising, in part, long-chain alkanes.     

Real-time analysis of secondary organic aerosol particles formed from cyclohexene ozonolysis using a laser-ionization single-particle aerosol mass spectrometer.

A real-time analysis of secondary organic aerosol (SOA) particles formed from cyclohexene ozonolysis in a smog chamber was performed using a laser-ionization single-particle aerosol mass spectrometer (LISPA-MS). The instrument obtains both size and chemical compositions of individual aerosol particles with a high time-resolution (approximately 2 s at the maximum). Both positive and negative-ion mass spectra are obtained. Standard particles generated from dicarboxylic acid solutions using an atomizer were also analyzed. For both standard and SOA particles, the negative-ion mass spectra provided information about the molecular weights of the organic compounds in the particles, since the intense ions in the negative-ion mass spectra are mainly attributable to the molecular-related ions [M-H]-. It was demonstrated that the real-time single-particle analysis of SOA particles by the LISPA-MS technique can reveal the formation and transformation processes of SOA particle in smog chambers.     

RAMAN STUDIES OF AEROSOL CHEMICAL REACTIONS

Since Professor Matijevité and his colleagues published pioneering work on aerosol chemical reactions, based on experiments with monodisperse aerosol generators and laminar flow reactors, there has been considerable progress in the chemical characterization of aerosol particles and the study of their chemical reactions. This paper surveys recent developments and new research on the application of Raman spectroscopy to gas/liquid and gas/solid aerosol reactions. Of particular interest are applications of the vibrating orifice aerosol generator and electrodynamic and optical levitators coupled to Raman spectrometers to explore aerosol chemistry. The systems examined include the production of polymeric microsphcrcs, the generation of metal oxide particles from alkoxide droplets, SQ2/sorbent particle reactions used for demilitarization of stick gases, chemical characterization of particle arrays, and reactions following collisions of dissimilar particles. The complications associated with the interpretation of Raman data introduced by morphology-dependent resonances in the elastically scattered light are also examined.     

Photo-oxidation of Isoprene with Organic Seed: Estimates of Aerosol Size Distributions Evolution and Formation Rates

ondary organic aerosol (SOA) formation from OH-initiated photo-oxidation of isoprene in the presence of organic seed aerosol. The dependence of the size distributions of SOA on both the level of pre-existing particles generated in situ from the photo-oxidation of trace hydrocarbons of indoor atmosphere and the concentration of precursor, has been investi-gated. It was shown that in the presence of high-level seed aerosol and low-level isoprene (typical urban atmospheric conditions), particle growth due to condensation of secondary organic products on pre-existing particles dominated; while in the presence of low-level seed aerosol and comparatively high-level isoprene (typical atmospheric conditions in rural re-gion), bimodal structures appeared in the size distributions of SOA, which corresponded to new particle formation resulting from homogeneous nucleation and particle growth due to condensation of secondary organic products on the per-existing particles respectively. The effects of concentrations of organic seed particles on SOA were also investigated. The particle size distributions evolutions as well as the corresponding formation rates of new particles indifferent conditions were also estimated.     

Chemical Composition and Reaction Mechanisms for Secondary Organic Aerosol from Photooxidation of Toluene

A laboratory study was carried out to investigate the secondary organic aerosol (SOA) products from photooxidation of the aromatic hydrocarbon toluene. The experiments were conducted by irradiating toluene/CH₃ONO/NO/air mixtures in a home‐made smog chamber. The aerosol time‐of‐flight mass spectrometer (ATOFMS) was used to measure the size and the chemical composition of individual secondary organic aerosol particles in real‐time. According to a large number of single aerosol diameters and mass spectra, we obtained the size distribution and chemical composition of SOA statistically. Expeperimental results showed that aerosol created by toluene photooxidation is predominantly in the form of fine particles, which have diameters less than 2.5 μm (i.e. PM2.5), and the predominant components of aerosol are furane, methyl glyoxylic acid, phenol, benzaldehyde, benzyl alcohol, cresol, 3‐hydroxy‐2,4‐dioxo‐pentanal, methyl nitrophenol, and 5‐hydroxy‐4,6‐dioxo‐2‐heptenal. The possible reaction mechanisms leading to these products were also discussed.     

Kinetics of N(2)O(5) hydrolysis on secondary organic aerosol and mixed ammonium bisulfate-secondary organic aerosol particles

The kinetics of the hydrolysis reaction of N(2)O(5) on secondary organic aerosol (SOA) produced through the ozonolysis of α-pinene and on mixed ammonium bisulfate-SOA particles was investigated using an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. We report room temperature uptake coefficients, γ, on ammonium bisulfate and SOA particles at 50% relative humidity of 1.5 × 10(-2) ± 1.5 × 10(-3) and 1.5 × 10(-4) ± 2 × 10(-5), respectively. For the mixed ammonium bisulfate-SOA particles, γ decreased from 2.6 × 10(-3) ± 4 × 10(-4) to 3.0 × 10(-4) ± 3 × 10(-5) as the SOA mass fraction increased from 9 to 79, indicating a strong suppression in γ with the addition of organic material. There is an order-of-magnitude reduction in the uptake coefficient with the smallest amount of SOA material present and smaller additional reductions with increasing aerosol organic content. This newly coated organic layer may either decrease the mass accommodation coefficient of N(2)O(5) onto the particle or hinder the dissolution and diffusion of N(2)O(5) into the remainder of the aerosol after it has been accommodated onto the surface. The former corresponds to a surface effect and the latter to bulk processes. The low value of the uptake coefficient on pure SOA particles will likely make N(2)O(5) hydrolysis insignificant on such an aerosol, but atmospheric chemistry models need to account for the role that organics may play in suppressing the kinetics of this reaction on mixed organic-inorganic particles.     

Role of quantum chemical calculations in molecular biophysics with a historical perspective

We discuss how the basic principles of quantum chemistry and quantum mechanics can be and have been applied to a variety of problems in molecular biophysics. First, the historical development of quantum concepts in biophysics is discussed. Next, we describe a series of interesting applications of quantum chemical methods for studying biologically active molecules, molecular structures, and some of the important processes which play a role in living organisms. We discuss the application of quantum chemistry to such processes as energy storage and transformation, and the transmission of genetic information. Quantum chemical approaches are essential to comprehend and understand the molecular nature of these processes. To conclude our work, we present a short discussion of the perspectives of quantum chemical methods in modern biophysics, the field of experimental and theoretical chiral vibrational and electronic spectroscopy.     

Data processing in on-line laser mass spectrometry of inorganic, organic, or biological airborne particles

A general solution for data processing of large numbers of micrometer- or submicrometer-particle mass spectra in aerosol analysis is described. The method is based on immediate evaluation of bipolar laser desorption ionization mass spectra acquired in an on-line (impact-free) time-of-flight instrument. The goal of the procedure is a characterization of the particle population under investigation in terms of chemical composition of particle classes, particle distributions, size distributions, and time courses, rather than an investigation of each individual particle. After automatic peak analysis of each newly acquired bipolar mass spectrum, the mass spectral information is statistically evaluated by a fuzzy clustering algorithm, providing for an immediate attribution of the particle to predefined particle classes. The particle distributions over these classes can be monitored as a function of time and particle size range. Definition of the particle classes as used for on-line evaluation is performed in an earlier step, either by manual approach, or by selection from a particle class database, or, as in most cases, by fuzzy clustering of a set of particle mass spectra from the population (the aerosol) under investigation. Definition of the particle classes is depending only on the distinguishability of the spectra patterns of different particles. It is not necessary for the clustering approach to fully “understand” the mass spectra. The range of possible applications of the method is therefore very broad. Particles dominated by inorganic components, as typically observed in aerosol chemistry for example, can be investigated the same way as organic particles (e.g., from smoke or automobile exhaust) or even biological particles such as bacteria, yeast, or pollen. The data processing method has been successfully applied in several fields of stationary applications and will be employed in mobile instruments for large scale field studies in atmospheric chemistry, engine combustion research, and the characterization of house dust.     

Low‐Molecular Weight and Oligomeric Components in Secondary Organic Aerosol from the Photooxidation of p‐Xylene

A laboratory study was performed to investigate the composition of secondary organic aerosol (SOA) products from photooxidation of the aromatic hydrocarbon p‐xylene. The experiments were conducted by irradiating p‐xylene/CH₃ONO/NO/air mixtures in a home‐made smog chamber. The aerosol time‐of‐flight mass spectrometer (ATOFMS) was used to measure the size and the chemical composition of individual secondary organic aerosol particles in real‐time. According to a large number of single aerosol diameters and mass spectra, the size distribution and chemical composition of SOA were determined statistically. Experimental results showed that aerosol created by p‐xylene photooxidation is predominantly in the form of fine particles, which have diameters less than 2.5 μm (i.e. PM2.5), and aromatic aldehyde, unsaturated dicarbonys, hydroxyl dicarbonys, and organic acid are major product components in the SOA after 2 hours photooxidation. After aging for more than 8 hours, about 10% of the particle mass consists of oligomers with a molecular mass up to 600 daltons. The possible reaction mechanisms leading to these products are also proposed.     

Atmospheric organic aerosol production by heterogeneous acid-catalyzed reactions.

Exploratory evidence from our laboratories shows that acidic surfaces on atmospheric aerosols lead to very real and potentially multifold increases in secondary organic aerosol (SOA) mass and build-up of stabilized nonvolatile organic matter as particles age. One possible explanation for these heterogeneous processes are the acid-catalyzed (e.g., H2SO4 and HNO3) reactions of atmospheric multifunctional organic species (e.g., multifunctional carbonyl compounds) that are accommodated onto the particle phase from the gas phase. Volatile organic hydrocarbons (VOCs) from biogenic sources (e.g., terpenoids) and anthropogenic sources (aromatics) are significant precursors for multifunctional organic species. The sulfur content of fossil fuels, which is released into the atmosphere as SO2, results in the formation of secondary inorganic acidic aerosols or indigenous acidic soot particles (e.g., diesel soot). The predominance of SOAs contributing to PM2.5 (particulate matter, that is, 2.5 microm or smaller than 2.5 microm), and the prevalence of sulfur in fossil fuels suggests that interactions between these sources could be considerable. This study outlines a systematic approach for exploring the fundamental chemistry of these particle-phase heterogeneous reactions. If acid-catalyzed heterogeneous reactions of SOA products are included in next-generation models, the predicted SOA formation will be much greater and have a much larger impact on climate-forcing effects than we now predict. The combined study of both organic and inorganic acids will also enable greater understanding of the adverse health effects in biological pulmonary organs exposed to particles.     

Phase, shape, and architecture of SF6 and SF6/CO2 aerosol particles: infrared spectra and modeling of vibrational excitons

Information on the phase, shape, and architecture of pure SF(6) and mixed SF(6)/CO(2) aerosol particles is extracted from experimental infrared spectra by comparison with predictions from quantum mechanical exciton calculations. The radius of the particles lies around 50 nm. The following extensions to our previous vibrational exciton model are included: (i) To account for the many degrees of freedom of degenerate vibrational bands of aerosol particles, we take a time-dependent approach to calculate infrared absorption spectra directly from the dipole autocorrelation function. (ii) In addition to the dipole-dipole interaction, dipole-induced dipole terms are included to account for the high polarizability of SF(6) and CO(2). We find SF(6) aerosol particles with a cubiclike shape directly after their formation and a change in the shape toward elongated particles with increasing time. Our microscopic model reveals that the cubic-to-monoclinic phase transition at 96 K found in the bulk cannot be observed with infrared spectroscopy because the two phases show almost identical spectra. Infrared spectra of two-component SF(6)/CO(2) particles with core-shell structure show characteristic split absorption bands for the shell. By contrast, homogeneously mixed SF(6)/CO(2) particles lead to broad infrared bands for both the core and the shell. The molecular origin of these various spectral features is uncovered by the analysis of the vibrational eigenfunctions.     

Heterogeneous atmospheric aerosol chemistry: laboratory studies of chemistry on water droplets

Heterogeneous chemical reactions on aerosol particles play a pivotal role in atmospheric chemistry. In this review, the fundamental concepts underlying the chemical dynamics of liquid aerosol droplets are discussed, with particular emphasis on the properties of the aqueous-air interface and the reaction mechanisms of key chemical processes. Recent laboratory studies of heterogeneous chemistry on aqueous aerosol particles are reviewed, with techniques that probe the gas phase, liquid phase and the interface directly, discussed in turn.     

Spectroscopic evidence of keto-enol tautomerism in deliquesced malonic acid particles

Scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) and optical microscopy coupled with Fourier transform infrared spectroscopy (micro-FTIR) have been applied to observe hygroscopic growth and chemical changes in malonic acid particles deposited on substrates. The extent of the hygroscopic growth of particles has been quantified in terms of the corresponding water-to-solute ratios (WSR) based on STXM/NEXAFS and micro-FTIR data sets. WSR values derived separately from two applied methods displayed a remarkable agreement with previous data reported in the literature. Comparison of NEXAFS and FTIR spectra acquired at different relative humidity (RH) shows efficient keto-enol tautomerization of malonic acid, with the enol form dominating at higher RH. The keto-enol equilibrium constants were calculated using relevant peak intensities in the carbon and oxygen K-edge NEXAFS spectra as a function of RH. We report strong dependence of the equilibrium constant on RH, with measured values of 0.18 ± 0.03, 1.11 ± 0.14, and 2.33 ± 0.37 corresponding to 2, 50, and 90% RH, respectively. Enols are important intermediates in aldol condensation reactions pertaining to formation and atmospheric aging of secondary organic aerosol (SOA). The present knowledge assumes that constituents of atmospheric deliquesced particles undergo aqueous chemistry with kinetic and equilibrium constants analogous to reactions in bulk solutions, which would estimate absolute dominance of the keto form of carboxylic acids. For instance, the keto-enol equilibrium constant of malonic acid in diluted aqueous solution is <10(-4). Our results suggest that in deliquesced micrometer-size particles, carboxylic acids may exist in predominantly enol forms that need to be explicitly considered in atmospheric aerosol chemistry.     

Ozonolysis of mixed oleic acid/n-docosane particles: the roles of phase, morphology, and metastable states

The reaction kinetics of ozone with oleic acid (OA) in submicron particles containing n-docosane has been studied using aerosol CIMS (chemical ionization mass spectrometry) to monitor changes in particle composition. Internally mixed particles with X(OA) > 0.72 were found to exist as supercooled droplets when cooled to room temperature. Partial reaction of the oleic acid was seen to completely inhibit further reaction and was attributed to the formation of a metastable solid rotator phase of the n-docosane at the surface. This reaction-induced phase change is believed to prevent further reaction by slowing ozone diffusion into the particle. When these particles were cooled to 0 degrees C before reaction, they reacted to a further extent and did not demonstrate such an inhibition. This shift in reactivity upon cooling is attributed to the formation of the thermodynamically stable form of n-docosane, the triclinic solid. This transition was accompanied by an increase in the n-docosane density, which led to the development of "cracks" through which ozone can diffuse into the particle. The aerosol with X(OA) < 0.72 consisted of an external mixture of particles containing n-docosane in either the rotator or the triclinic solid phase because of the stochastic nature of the rotator --> triclinic transition. The reactivity of the oleic acid was seen to increase with increasing n-docosane content as a larger fraction of the particles underwent the rotator --> triclinic transition and therefore contained cracks at the surface. These findings demonstrate the importance of transient, metastable phases in determining particle morphology and how such morphological changes can influence rates of reactions in organic aerosols.     

Glass transition and phase state of organic compounds: dependency on molecular properties and implications for secondary organic aerosols in the atmosphere

Recently, it has been proposed that organic aerosol particles in the atmosphere can exist in an amorphous semi-solid or solid (i.e. glassy) state. In this perspective, we analyse and discuss the formation and properties of amorphous semi-solids and glasses from organic liquids. Based on a systematic survey of a wide range of organic compounds, we present estimates for the glass forming properties of atmospheric secondary organic aerosol (SOA). In particular we investigate the dependence of the glass transition temperature T(g) upon various molecular properties such as the compounds' melting temperature, their molar mass, and their atomic oxygen-to-carbon ratios (O:C ratios). Also the effects of mixing different compounds and the effects of hygroscopic water uptake depending on ambient relative humidity are investigated. In addition to the effects of temperature, we suggest that molar mass and water content are much more important than the O:C ratio for characterizing whether an organic aerosol particle is in a liquid, semi-solid, or glassy state. Moreover, we show how the viscosity in liquid, semi-solid and glassy states affect the diffusivity of those molecules constituting the organic matrix as well as that of guest molecules such as water or oxidants, and we discuss the implications for atmospheric multi-phase processes. Finally, we assess the current state of knowledge and the level of scientific understanding, and we propose avenues for future studies to resolve existing uncertainties.     

The effect of humidity on the ozonolysis of unsaturated compounds in aerosol particles

Atmospheric aerosol particles are important in many atmospheric processes such as: light scattering, light absorption, and cloud formation. Oxidation reactions continuously change the chemical composition of aerosol particles, especially the organic mass component, which is often the dominant fraction. These ageing processes are poorly understood but are known to significantly affect the cloud formation potential of aerosol particles. In this study we investigate the effect of humidity and ozone on the chemical composition of two model organic aerosol systems: oleic acid and arachidonic acid. These two acids are also compared to maleic acid an aerosol system we have previously studied using the same techniques. The role of relative humidity in the oxidation scheme of the three carboxylic acids is very compound specific. Relative humidity was observed to have a major influence on the oxidation scheme of maleic acid and arachidonic acid, whereas no dependence was observed for the oxidation of oleic acid. In both, maleic acid and arachidonic acid, an evaporation of volatile oxidation products could only be observed when the particle was exposed to high relative humidities. The particle phase has a strong effect on the particle processing and the effect of water on the oxidation processes. Oleic acid is liquid under all conditions at room temperature (dry or elevated humidity, pure or oxidized particle). Thus ozone can easily diffuse into the bulk of the particle irrespective of the oxidation conditions. In addition, water does not influence the oxidation reactions of oleic acid particles, which is partly explained by the structure of oxidation intermediates. The low water solubility of oleic acid and its ozonolysis products limits the effect of water. This is very different for maleic and arachidonic acid, which change their phase from liquid to solid upon oxidation or upon changes in humidity. In a solid particle the reactions of ozone and water with the organic particle are restricted to the particle surface and hence different regimes of reactivity are dictated by particle phase. The potential relevance of these three model systems to mimic ambient atmospheric processes is discussed.     

Manipulation and characterization of aqueous sodium dodecyl sulfate/sodium chloride aerosol particles

Aerosol optical tweezers coupled with Raman spectroscopy can allow the detailed investigation of aerosol dynamics. We describe here measurements of the evolving size, composition, and phase of single aqueous aerosol droplets containing the surfactant sodium dodecyl sulfate and the inorganic salt sodium chloride. Not only can the evolving wet particle size be probed with nanometer accuracy, but we show that the transition to a metastable microgel particle can be followed, demonstrating that optical tweezers can be used to manipulate both spherical and non-spherical aerosol particles. Further, through the simultaneous manipulation and characterization of two aerosol droplets of different composition in two parallel optical traps, the phase behavior of a surfactant-doped particle and a surfactant-free droplet can be compared directly in situ. We also illustrate that the manipulation of two microgel particles can allow studies of the coagulation and interaction of two solid particles. Finally, we demonstrate that such parallel measurements can permit highly accurate comparative measurements of the evolving wet particle size of a surfactant-doped droplet with a surfactant-free droplet.