Humidity-dependent surface tension measurements of individual inorganic and organic submicrometre liquid particles

Surface tension, an important property of liquids, is easily measured for bulk samples. However, for droplets smaller than one micron in size, there are currently no reported measurements. In this study, atomic force microscopy (AFM) and force spectroscopy have been utilized to measure surface tension of individual submicron sized droplets at ambient pressure and controlled relative humidity (RH). Since the surface tension of atmospheric aerosols is a key factor in understanding aerosol climate effects, three atmospherically relevant systems (NaCl, malonic and glutaric acids) were studied. Single particle AFM measurements were successfully implemented in measuring the surface tension of deliquesced particles on the order of 200 to 500 nm in diameter. Deliquesced particles continuously uptake water at high RH, which changes the concentration and surface tension of the droplets. Therefore, surface tension as a function of RH was measured. AFM based surface tension measurements are close to predicted values based on bulk measurements and activities of these three chemical systems. Non-ideal behaviour in concentrated organic acid droplets is thought to be important and the reason for differences observed between bulk solution predictions and AFM data. Consequently, these measurements are crucial in order to improve atmospheric climate models as direct measurements hitherto have been previously inaccessible due to instrument limitations.     

On-line analysis of ambient air aerosols using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy is developed for the detection of aerosols in ambient air, including quantitative mass concentration measurements and size/composition measurements of individual aerosol particles. Data are reported for ambient air aerosols containing aluminum, calcium, magnesium and sodium for a 6-week sampling period spanning the Fourth of July holiday period. Measured mass concentrations for these four elements ranged from 1.7 parts per trillion (by mass) to 1.7 parts per billion. Ambient air concentrations of magnesium and aluminum revealed significant increases during the holiday period, which are concluded to arise from the discharge of fireworks in the lower atmosphere. Real-time conditional data analysis yielded increases in analyte spectral intensity approaching 3 orders of magnitude. Analysis of single particles yielded composition-based aerosol size distributions, with measured aerosol diameters ranging from 100 nm to 2 μm. The absolute mass detection limits for single particle analysis exceeded sub-femtogram values for calcium-containing particles, and was on the order of 2–3 femtograms for magnesium and sodium-based particles. Overall, LIBS-based analysis of ambient air aerosols is a promising technique for the challenging issues associated with the real-time collection and analysis of ambient air particulate matter data.     

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.     

A theoretical study of atmospheric propagation of laser and return light for stand-off laser induced breakdown spectroscopy purposes

A theoretical study of atmospheric extinction mechanisms of optical radiation (molecular/aerosol scattering and absorption) has been carried out in order to assess their influences on stand-off laser-induced breakdown spectroscopy (LIBS) measurements. The atmospheric extinction of laser radiation at wavelengths commonly used in laser-induced breakdown spectroscopy (1064 nm and 532 nm) and of the laser-induced breakdown spectroscopy plasma emission beyond 250 nm is small compared to the attenuation with range due to the inverse square law. The fundamental problem with light propagation through the atmosphere is that the atmospheric transmittance does not remain constant within the whole spectral interval, and that this variation results in a change in the spectral distribution of the light received by the detector. Knowledge of atmospheric transmittance would allow for compensation of this effect.     

气溶胶粒子通过填充柱的保留时间分布测定

采用亚微米单分散聚苯乙烯球形硬气溶胶粒子和脉冲进样技术,测定了气溶胶粒子通过无规则石英砂填充柱的保留时间分布,从保留时间分布曲线得到了气溶胶粒子在填充柱中的平均保留时间和穿透率.研究了平均保留时间和穿透率与流体流速、填充柱的长度、填料粒度和气溶胶粒子大小之间的关系.研究发现,流速越大,保留时间分布曲线越尖锐,流速越小,保留时间分布曲线越平坦;气溶胶粒子的穿透率随着柱长的增加而降低,随流速、气溶胶粒子粒径和石英砂颗粒大小的减小而减小;平均保留时间随柱长增加而增大,随流速增大而减小,随气溶胶粒子粒径减小而减小,而与石英砂颗粒大小几乎无关.     

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.     

基质与分析物的摩尔比和激光能量对分析物离子信号的影响

A method of aerosol introduction for matrix-assisted laser desorption/ionization (MALDI) is described. The aerosol particles containing matrix and analyte enter directly into the aerosol time-of-flight mass spectrometer (ATOFMS) at atmospheric pressure. The scattered light signals from the aerosol particles are collected by a photomultiplier tube (PMT) and are passed on to an external electronic timing circuit, which determines particle size and is used to trigger a 266 nm pulsed Nd:YAG laser. The aerosol MALDI mass spectra and aerodynamic diameter of single particles can be obtained in real-time. Compared with other methods of liquid sample introduction, this method realizes detection of single particles and, more importantly, the sample consumption is lower. The effects of matrix-to-analyte ratio and laser pulse energy on analyte ion yield are examined. The optimal matrix-to-analyte ratio and laser energy are 50-110:1 and 200-400 μJ respectively.     

Direct on-strip analysis of size- and time-resolved aerosol impactor samples using laser induced fluorescence spectra excited at 263 and 351 nm

We report a novel atmospheric aerosol characterization technique, in which dual wavelength UV laser induced fluorescence (LIF) spectrometry marries an eight-stage rotating drum impactor (RDI), namely UV-LIF-RDI, to achieve size- and time-resolved analysis of aerosol particles on-strip. The UV-LIF-RDI technique measured LIF spectra via direct laser beam illumination onto the particles that were impacted on a RDI strip with a spatial resolution of 1.2 mm, equivalent to an averaged time resolution in the aerosol sampling of 3.6 h. Excited by a 263 nm or 351 nm laser, more than 2000 LIF spectra within a 3-week aerosol collection time period were obtained from the eight individual RDI strips that collected particles in eight different sizes ranging from 0.09 to 10 μm in Djibouti. Based on the known fluorescence database from atmospheric aerosols in the US, the LIF spectra obtained from the Djibouti aerosol samples were found to be dominated by fluorescence clusters 2, 5, and 8 (peaked at 330, 370, and 475 nm) when excited at 263 nm and by fluorescence clusters 1, 2, 5, and 6 (peaked at 390 and 460 nm) when excited at 351 nm. Size- and time-dependent variations of the fluorescence spectra revealed some size and time evolution behavior of organic and biological aerosols from the atmosphere in Djibouti. Moreover, this analytical technique could locate the possible sources and chemical compositions contributing to these fluorescence clusters. Advantages, limitations, and future developments of this new aerosol analysis technique are also discussed.     

Thermal oxidation of 6 nm aerosolized silicon nanoparticles: size and surface chemistry changes

The earliest stages of thermal oxidation of 6 nm diameter silicon nanoparticles by molecular oxygen are examined using a tandem differential mobility analysis (TDMA) apparatus, Fourier-transform infrared (FTIR) spectroscopy, time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and X-ray photoelectron spectroscopy (XPS). Particles are synthesized in and then extracted from a nonthermal RF plasma operating at approximately 20 Torr into the atmospheric pressure TDMA apparatus. The TDMA apparatus was used to measure oxidation-induced size changes over a broad range of temperature settings and N2-O2 carrier gas composition. Surface chemistry changes are evaluated in situ with an FTIR spectrometer and a hybrid flow-through cell, and ex situ with ToF-SIMS and XPS. Particle size measurements show that, at temperatures less than approximately 500 degrees C, particles shrink regardless of the carrier gas oxygen concentration, while FTIR and ToF-SIMS spectra demonstrate a loss of hydrogen from the particles and minimal oxide formation. At higher temperatures, FTIR and XPS spectra indicate that an oxide forms which tends toward, but does not fully reach, stoichiometric SiO2 with increasing temperature. Between 500 and 800 degrees C, size measurements show a small increase in particle diameter with increasing carrier gas oxygen content and temperature. Above 800 degrees C, particle growth rapidly reaches a plateau while FTIR and XPS spectra change little. ToF-SIMS signals associated with O-Si species also show an increase in intensity at 800 degrees C.     

Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting

Laser-induced breakdown spectroscopy of particles ejected by optical catapulting is discussed for the first time. For this purpose, materials deposited on a substrate were ejected and transported from the surface in the form of a solid aerosol by optical catapulting using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at 1064 nm. A Q-switched Nd:YAG laser at 532 nm was used for chemical characterization of the particles by laser-induced breakdown spectroscopy. Both lasers were synchronized in order to perform suitable spectral detection. The optical catapulting was optimized and evaluated using aluminum silicate particles, nickel spheres, and quartz and stainless steel particles. Experimental parameters such as the interpulse delay time, the sampling distance, the laser fluence, the sampling rate and the particle size have been studied. A correlation between these parameters and the particle size is reported and discussed.     

On the use of wavelet filtering and correlation techniques in atmospheric condensed phase spectroscopy

The application of wavelet filtering and analysis in spectroscopy is discussed in relation to the analysis of complex atmospheric spectra, where contributions from condensed phase particles and gas phase molecules are present in the form of broad-band features and narrow lines, respectively. The broad-band contribution can be extracted as the 'smooth signal' component of the wavelet transform, with a large reduction in the size of the corresponding data files. This procedure is applied to an investigation of the H2SO4 aerosol content of a series of atmospheric spectra measured in the ATMOS missions. The sulfate content of the smooth signal is analysed by means of correlation techniques, using a set of laboratory reference spectra of varying sulfuric acid concentration and temperature. Correlation density maps and correlation curves are used to select the most appropriate spectral zones for sulfate analysis and to assess the sulfate aerosol content in the atmosphere subsequent to the eruption of the Mount Pinatubo volcano.     

Speciation of copper and zinc in size-fractionated atmospheric particulate matter using total reflection mode X-ray absorption near-edge structure spectrometry

The health effects of aerosol depend on the size distribution and the chemical composition of the particles. Heavy metals of anthropogenic origin are bound to the fine aerosol fraction (PM_2.5). The composition and speciation of aerosol particles can be variable in time, due to the time-dependence of anthropogenic sources as well as meteorological conditions. Synchrotron-radiation total reflection X-ray fluorescence (SR-TXRF) provides very high sensitivity for characterization of atmospheric particulate matter. X-ray absorption near-edge structure (XANES) spectrometry in conjunction with TXRF detection can deliver speciation information on heavy metals in aerosol particles collected directly on the reflector surface. The suitability of TXRF-XANES for copper and zinc speciation in size-fractionated atmospheric particulate matter from a short sampling period is presented. For high size resolution analysis, atmospheric aerosol particles were collected at different urban and rural locations using a 7-stage May cascade impactor having adapted for sampling on Si wafers. The thin stripe geometry formed by the particulate matter deposited on the May-impactor plates is ideally suited to SR-TXRF. Capabilities of the combination of the May-impactor sampling and TXRF-XANES measurements at HASYLAB Beamline L to Cu and Zn speciation in size-fractionated atmospheric particulate matter are demonstrated. Information on Cu and Zn speciation could be performed for elemental concentrations as low as 140 pg/m³. The Cu and Zn speciation in the different size fraction was found to be very distinctive for samples of different origin. Zn and Cu chemical state typical for soils was detected only in the largest particles studied (2–4 μm fraction). The fine particles, however, contained the metals of interest in the sulfate and nitrate forms.     

Light Element Analysis of Individual Microparticles Using Thin-Window EPMA

 The determination of the concentration of light elements, such as carbon, nitrogen and oxygen, in e.g. atmospheric aerosol particles is important to study the chemical behaviour of atmospheric pollution. The knowledge of low-Z element concentrations gives us information on the speciation of nutrients (species having nutritional value for plants) and toxic heavy metals in the particles. The capability of the conventional energy-dispersive EPMA is strongly limited for the analysis of low-Z elements, mainly because the Be window in the EDX detector hinders the detection of characteristic X-rays of light elements such as C, N, O and Na. WDS is suitable for analysis of light elements, but the measurement of beam sensitive microparticles requires the minimisation of the beam current and the measurement time. A semi-quantitative analytical method based on EPMA using an ultra-thin window EDX detector was developed. It was found that the matrix and geometric effects that are important for low-energy X-rays can be reliably evaluated by Monte Carlo calculations. Therefore, the quantification part of the method contains reverse Monte Carlo calculation done by iterative simulations. The method was standardised and tested by measurements on single particles with known chemical compositions. Beam-sensitive particles such as ammonium-sulphate and ammonium-nitrate were analysed using a liquid nitrogen cooled sample stage. The shape and size of the particles, which are important for the simulations, were determined using a high-magnification secondary electron image. Individual marine aerosol particles collected over the North Sea by a nine-stage Berner cascade impactor were analysed using this new method. Preliminary results on five samples and 4500 particles show that the method can be used to study the modification of sea-salt particles in the troposphere.     

Physicochemical properties of nitrate aerosols: implications for the atmosphere

As aerosols, such as sea salt and mineral dust, are transported through the atmosphere they undergo heterogeneous reactions with nitrogen oxides to form nitrate salts. The nitrate salt can have quite different physicochemical properties than the original aerosol, resulting in an aerosol that will markedly differ in its climate impact, heterogeneous chemistry, and photoactivity. In this Feature Article, we will review some aspects of the importance of aqueous nitrate aerosols as well as describe a new multi-analysis aerosol reactor system (MAARS) that is used to measure the physicochemical properties of these atmospherically relevant aerosols. Here we show measurements of the hygroscopic properties, cloud condensation nuclei activity, and FTIR extinction of nitrate salt aerosol. In particular, we have measured the hygroscopic growth of 100 nm size-selected nitrate particles including NaNO3, Ca(NO3)2, Mg(NO3)2, and a 1:1 mixture of Ca(NO3)2 and Mg(NO3)2 as a function of relative humidity (RH) at 298 K. Using K?hler theory, we have quantified the water content of these particles with increasing RH. FTIR extinction measurements of the full size distribution of each of the nitrate aerosols are analyzed to yield information about the local solvation environment of the nitrate ions and the long-wavelength light scattering of the particles at different RH. Furthermore, we have measured and compared the cloud condensation nuclei (CCN) activity of CaCO3, a large component of mineral dust aerosol, and Ca(NO3)2, a product of atmospherically aged CaCO3 through reaction with nitrogen oxides, at supersaturations from 0.1% to 0.9%. These quantitative physicochemical data are needed if we are to better understand the chemistry as well as the climate effects of atmospheric aerosols as they are entrained, transported, reacted, and aged in the atmosphere. Our studies here focus on aqueous nitrate salts, the products of the reaction of nitrogen oxides with sea salt and mineral dust aerosol.     

Novel synthesis of stable polypyrrole nanospheres using ozone

In this study, a novel and exceedingly simple method for the aqueous synthesis of stable, unagglomerated polypyrrole nanospheres was investigated. The method is template- and surfactant-free and uses only pyrrole monomer, water, and ozone. When the monomer concentration, exposure time to ozone, and temperature were varied, it was determined that the temperature was the critical factor controlling the particle size through particle size measurements via dynamic light scattering and transmission electron microscopy (TEM). From the particle size measurements, a particle size distribution with a number-weighted mean diameter of 73 nm and a standard deviation of 18 nm was achieved. The particles were also investigated using ζ-potential measurements, ultraviolet-visible (UV-vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis in an effort to determine the identity of the nanoparticles as well as the mechanism by which the nanoparticles are formed and stabilized.     

Comparison of the accuracy of aerosol refractive index measurements from single particle and ensemble techniques

The ability of two techniques, aerosol cavity ring down spectroscopy (A-CRDS) and optical tweezers, to retrieve the refractive index of atmospherically relevant aerosol was compared through analysis of supersaturated sodium nitrate at a range of relative humidities. Accumulation mode particles in the diameter range 300-600 nm were probed using A-CRDS, with optical tweezer measurements performed on coarse mode particles several micrometers in diameter. A correction for doubly charged particles was applied in the A-CRDS measurements. Both techniques were found to retrieve refractive indices in good agreement with previously published results from Tang and Munkelwitz, with a precision of ±0.0012 for the optical tweezers and ±0.02 for the A-CRDS technique. The coarse mode optical tweezer measurements agreed most closely with refractive index predictions made using a mass-weighted linear mixing rule. The uncertainty in the refractive index retrieved by the A-CRDS technique prevented discrimination between predictions using both mass-weighted and volume-weighted linear mixing rules. No efflorescence or kinetic limitations on water transport between the particle and the gas phase were observed at relative humidities down to 14%. The magnitude of the uncertainty in refractive index retrieved using the A-CRDS technique reflects the challenges in determining particle optical properties in the accumulation mode, where the extinction efficiency varies steeply with particle size.     

Exploring the complexity of aerosol particle properties and processes using single particle techniques

The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.     

Influence of physical properties and chemical composition of sample on formation of aerosol particles generated by nanosecond laser ablation at 213 nm

The influence of sample properties and composition on the size and concentration of aerosol particles generated by nanosecond Nd:YAG laser ablation at 213 nm was investigated for three sets of different materials, each containing five specimens with a similar matrix (Co-cemented carbides with a variable content of W and Co, steel samples with minor differences in elemental content and silica glasses with various colors). The concentration of ablated particles (particle number concentration, PNC) was measured in two size ranges (10–250 nm and 0.25–17 µm) using an optical aerosol spectrometer. The shapes and volumes of the ablation craters were obtained by Scanning Electron Microscopy (SEM) and by an optical profilometer, respectively. Additionally, the structure of the laser-generated particles was studied after their collection on a filter using SEM.     

Possibilities and limitations of the photoelectric aerosol sensor array applied to heavy metal aerosols

Summary The technique of substance-selective aerosol charging by photoelectron emission was applied to several ultrafine heavy metal aerosols. To enhance the selectivity of the Photoelectric Aerosol Sensor (PAS), an array operating with four different wavelengths (185 nm, 214 nm, 229 nm, 254 nm) was used. The photoelectric activity at each wave-length and the linearity of the PAS signal was investigated for monodisperse and quasi-polydisperse systems. The aerosol mass concentration was determined by correlation with atomic absorption spectroscopy measurements of filter samples. Pattern recognition was applied to differentiate between four different aerosol systems and to identify common interferences.

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Möglichkeiten und Grenzen des photoelektrischen Aerosolsensors für die Analyse von Schwermetallaerosolen

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Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday     

Surface chemistry of aerosolized silicon nanoparticles: evolution and desorption of hydrogen from 6-nm diameter particles

The surface chemistry of pristine, 6-nm silicon nanoparticles has been investigated. The particles were produced in an RF plasma and studied using a tandem differential mobility analysis apparatus, Fourier transform infrared spectroscopy (FTIR), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and transmission electron microscopy. Particles were extracted from the plasma, which operates at approximately 20 Torr, into an atmospheric pressure aerosol flow tube, and then through a variable-temperature furnace that could be adjusted between room temperature and 1200 degrees C. DMA measurements show that freshly generated silicon particles shrink with heating, with particle diameters decreasing by approximately 0.25 nm between 350 and 400 degrees C. FTIR results indicate that freshly generated particles are primarily covered with SiH2 groups and smaller amounts of SiH and SiH3. Spectra recorded as a function of heating temperature indicate that the amount of surface hydrogen, as measured by the intensity of modes associated with SiH, SiH2, and SiH3, decreases with heating. ToF-SIMS measurements also suggest that hydrogen desorbs from the particles surfaces over the same temperature range that the particles shrink.