Applications of Complementary Analytical Techniques to Study Chemical Composition and Properties of Atmospheric Particles
Aerosols are widely recognized as key elements in the atmospheric environment. Chemical and morphological data of individual particles are of crucial importance for understanding their formation, reactions, atmospheric history and aging. Microprobe analytical techniques have been extensively used in the past to characterize the size, morphology, phase and composition of particles collected in field and laboratory studies. These techniques coupled with appropriate time-resolved aerosol sampling are capable of generating time- resolved single-particle data, which then can be used to follow in detail the time evolution of specific types of aerosols. In this presentation we give a summary of recent research projects carried out in our laboratory that demonstrates how the use of complementary microprobe methods and other analytical techniques provides new insights into the atmospheric reactions of aerosols, their physical and chemical transformations, and how the obtained data is utilized to define future directions in laboratory and field studies of aerosols.
A New Method for Determining Nickel Speciation in Workplace Aerosols Using X-Ray Near- Edge Structure Spectroscopy
X-ray absorption near-edge structure (XANES) spectroscopy provides new interpretations of Ni speciation in sulfidic Ni processing, particularly with respect to soluble Ni in workplace aerosol samples collected at the Copper Cliff nickel smelter and refinery. Typically, Ni speciation is identified using a sequential extraction developed by the Ni industry for speciating workplace exposures. However, as this technique and its variants become more widely applied, there is evidence that the method may not always accurately estimate soluble and metallic Ni. Because of the importance of speciated exposure reconstruction to the determination of carcinogenicity classification of individual Ni compounds, we have begun to evaluate alternative speciation methodologies for Ni. Here we present the results of a study comparing Ni speciation results obtained using XANES spectroscopy and sequential extraction.
Optical Properties and Structure of Internally Mixed Aerosols Composed of Succinic Acid and Ammonium Sulfate Determined by Cavity Ring-Down Spectroscopy, Atomic Force Microscopy, and Raman Microscopy
The structure and composition of aerosol particles determines aerosol optical properties. The aerosol direct effect, which stems from the interaction of aerosols with radiation, is one of the largest uncertainties in calculations of anthropogenic contributions to radiative forcing. We have determined the refractive indices for internal mixtures of succinic acid and ammonium sulfate using Cavity Ring-Down Spectroscopy. At high organic weight fractions, we find that the refractive indices of the mixtures are larger than either of the pure components. Optical mixing rules for homogeneous particles and particles with small heterogeneities fail to fit the experimentally derived optical constants. To investigate whether more complex internal structures are present, we have used Atomic Force and Raman Microscopies to examine aerosol particles composed of succinic acid and ammonium sulfate as a function of the organic weight fraction. Atomic Force Microscopy is a surface sensitive and non-destructive technique for obtaining the topography of the aerosol surface in the nanometer size regime. Using substrates with hydrophilic and hydrophobic surfaces, the internal structure of the aerosols can be examined. From these studies, we observe core-shell structures for many organic weight fractions. Raman Microscopy is used to determine the chemical composition of aerosol particles in the micrometer size regime that are generated using several different atomization methods. At high organic weight fractions, we find that the aerosol particles have succinic acid shells. We will discuss the implications of such structures on the optical properties of aerosols.
Comparison of Organic Acid, Hydroxyl, and Sulfate Group Composition in Aerosol Measurement Field Campaigns from 2000-2008
Organic components account for a third or more of the mass of aerosol particles in the atmosphere. This study compares the chemical characteristics of the organic fraction of submicron particles from more than a dozen field measurement campaigns in and near North America, eastern Asia, western South America, and northern Europe. Each study spanned a period of 30 days or more in the years 2000-2008, typically showing very similar composition during each campaign despite varying atmospheric concentrations. In contrast, a comparison of the different campaigns to each other shows significant differences with location. Carboxylic acid group and non-acidic organic hydroxyl group (including alcohol and polyol groups) concentrations are weakly anti-correlated. The organic acid groups are more closely associated with recent fossil fuel combustion emissions, and the non-acidic organic hydroxyl groups are associated with both biomass burning and biogenic emissions. Organic sulfate groups have been measured most frequently in remote regions with low ammonium concentrations.
Speciation of Micrometer-sized Particles Using Synchrotron X-ray Fluorescence, Absorption and Microdiffraction
Synchrotron-based X-ray microprobe analysis is well-suited to the characterization of aerosol particles chemically and mineralogically. It can provide quantitative information about major and trace element abundances with femtogram detection sensitivity by X-ray fluorescence. Multi-element mapping provides mass ratios for selected elements on thousands of particles, which can be used to infer mineralogy based on composition. Information on elemental speciation can be obtained utilizing x-ray absorption spectroscopy, and microbeam X-ray diffraction on selected particles can be used to unambiguously identify microcrystalline materials. For aerosol analysis particulate filters can be analysed directly with no additional preparation. Samples were collected in seven aerodynamically fractionated size ranges using a cascade impactor deployed at three abandoned gold mine tailings fields in Nova Scotia. Analysis was conducted at beamline X26A at the National Synchrotron Light Source. This study's focus was on As-bearing particles since our previous work had identified high concentrations of As in unconsolidated near-surface materials, including various weathering products of arsenopyrite. Results from the 1-2, 4-8 and 8-16 micrometer size fractions indicate a strong correlation between As and Fe, particularly in the smaller size fractions. There are no As-bearing particles that do not contain Fe and few Fe-bearing particles that do not contain As. Calculated As/Fe ratios suggest the particles are mostly scorodite (FeAsO4.2H2O) and hydrous Fe arsenate, consistent with our mineralogical studies of coarser near-surface material. The presence of nanocrystalline scorodite is confirmed by X-ray microdiffraction of selected As-rich particles. Microdiffraction also indicates the presence of amorphous As-Fe phases. MicroXANES analysis of selected As-rich particles on the filters indicates that most contain As(V), also consistent with mineralogical observations on near-surface tailings samples. Yukonite and other Ca-Fe arsenates are unlikely based on the lack of correlation between Ca and As. This is significant since our previous work has shown that yukonite- bearing samples are associated with higher gastric bioaccessibility than scorodite-rich samples. Windblown and vehicle-raised dust from the unvegetated portions of these mine tailings sites, some of which are publicly accessible and used for recreational activities, may be ingested or inhaled. Results from this study can be used to help assess the potential human health risks associated with exposure to As- bearing airborne particles.
Using aerosol mass spectrometry to study the roles of phase and morphology in model organic aerosol reactions
We present results from two model systems in which chemical speciation using aerosol mass spectrometry provides insight into the phase and morphology of the particles. In the first study, the reaction of oleic acid with ozone is seen to be influenced greatly when an unreactive normal alkane, n-docosane, is also present in the particles. Changes in the rate of this reaction as a function of n-docosane content and temperature history of the particles allows us to infer that the particles are inhomogeneous mixtures containing a shell of frozen n- docosane at the surface. In the second study, the ability to speciate oxidation products from Cl radical reactions with liquid and solid carboxylic acids offers insight into how particle phase can influence aging of organic aerosols in the atmosphere.
Microscopic Characterization of Carbonaceous Aerosol Aging in the Outflow from Mexico City
Organic species represent one of the largest components of urban aerosol and are thus expected to have a significant impact on human health, atmospheric visibility and global climate. In this work, a case study of organic aerosol aging in Mexico City is presented. Using microscopic techniques, we find that as the aerosol is sampled further from the city center the organic mass increases while the fraction of carbon-carbon double bonds decreases. Organic functional groups observed to increase include: carboxylic acids, alkyl groups and oxygen bonded alkyl groups. At the same time, a class of relatively homogeneously mixed organic material is seen to increase in both size and number with distance from the city. At the center of the city the most prevalent particle types were observed to have crystalline inorganic phases containing S, N, O and K that were coated with organic material. Based on single particle classifications, it is concluded that primary sources, including biomass burning are prevalent at the city center whereas aged secondary aerosol and mineral dust are enhanced in the rural sites. These observations demonstrate the unique behavior of physical mixing state and organic bonding with distance from a major anthropogenic aerosol source in a photochemically active environment.
From Engineered Nanoparticles to Mineral Dust: An Integrated Approach Toward Understanding Atmospheric and Environmental Impacts, Toxicity and Occupational Health Hazards of Metal-Containing Aerosols
One important issue in understanding the environmental and atmospheric fate, transport, toxicity and occupational health hazards of metal-containing aerosols is in the characterization of these aerosols. For example, will metal-containing aerosols be present in air as isolated particles or in the form of aggregates? will metal-containing aerosols dissolve in into soluble ionic components and under what conditions? In the studies discussed here, an integrated approach is used to address these questions and issues. The approach combines state-of-the-art characterization of the physicochemical properties of metal-containing aerosols, that include the range of engineered manufactured nanoparticles to mineral dust, with field, modeling and toxicity studies. This integrated approach is needed to better understand the atmospheric and environmental impacts, toxicity and occupational health hazards of metal-containing aerosols.