Final Report: Morphological and Chemical Characteristics of the Submicron Atmospheric Aerosol: Implication for StandardsEPA Grant Number: R826232
Title: Morphological and Chemical Characteristics of the Submicron Atmospheric Aerosol: Implication for Standards
Investigators: Friedlander, Sheldon
Institution: University of California - Los Angeles
EPA Project Officer: Shapiro, Paul
Project Period: January 2, 1998 through January 31, 2001
Project Amount: $345,247
RFA: Ambient Air Quality (1997) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Current plans to set a revised particulate ambient air quality standard are based largely on epidemiological data that show an association between adverse health effects and aerosols. Efforts to set a scientifically defensible standard have been hampered by the failure to identify the specific active agents, chemical and/or physical, to which the health effects can be attributed. It has been hypothesized that the ultrafine component of the atmospheric aerosol (dp < 0.1 (m)) contributes significantly to adverse health effects. Much of the atmospheric black carbon also passes through the ultrafine particle range. Our project provided the first systematic study of the morphological characteristics of atmospheric ultrafine particles.
The principal objectives of this research project were to: (1) establish the prevalence and physical properties of the two morphological modes (ultrafine agglomerates (dp < 0.1 (m)) and accumulation mode (0.1 to 1.0 (m) micro-droplets); (2) characterize the fractal properties for the ultrafine particle range; (3) integrate morphological concepts into conventional approaches to aerosol characterization (size and average chemical composition distribution) to produce a new synthesis for characterizing ambient ultrafine aerosols; and (4) apply the results to the dynamics of the atmospheric aerosol, for example, to the scavenging of ultrafine particles by the accumulation mode. (This component of the program was investigated as part of an undergraduate student project but a detailed investigation has not yet been made.) A subsidiary objective included a conceptual evaluation of possible methods of monitoring oxidants in the atmospheric aerosol.
The morphological studies by electron microscopy, including measurements of fractal properties of atmospheric aggregates, were very successful. However, the parallel energy dispersive x-ray fluorescence Energy Dispersive Spectroscopy (EDS)/Transmission Electron Microscope (TEM) studies, originally planned for chemical characterization, proved to be significantly more difficult than anticipated; the amount of material needed for the morphological measurements was extremely small, but the small size of the samples made the EDS/TEM measurements correspondingly difficult.
For these reasons, we expanded that portion of the work dealing with morphology in several ways:
· Joint measurements were made with the group of Professor C. Sioutas at the University of South Carolina to determine whether the Aerosol Concentrator caused irreversible changes in the morphological properties of ultrafine fractal-like particles.
· While participating in an Environmental Protection Agency (EPA) meeting at Research Triangle Park in September 2000, Mr. Cheng Xiong, an advanced graduate student in charge of our morphological studies, arranged to collect aerosol samples with the help of the group directed by Dr. Robert Stevens of the EPA. These samples were analyzed and their fractal characteristics were compared with those in Los Angeles.
· Additional measurements also were made at a rural site in California.
The ultrafine size range (dp < 0.1 (m)) of the atmospheric aerosol is composed of both primary and secondary particulate matter. The primary component, emitted directly from sources, often includes aggregates of much smaller (10 to 50 nm) particles. (Note that the term "primary" in this context differs from its use to designate the individual particles that compose aerosol aggregate structures.) The secondary component is composed of particulate matter formed in the atmosphere, including sulfuric acid generated by gas phase reactions and organic compounds of low volatility. Particles that form by gas to particle conversion tend to evaporate when placed in the electron microscope, the principal observational method used in the project. Our research focused on the aggregate structures present in the ultrafine range, which can be conveniently studied by electron microscopy. Ultrafine solid particles in the locations where our measurements were made include soot from incomplete combustion (e.g., diesel emissions), and machine shop sources such as welding fumes. The aggregate structures resulted from collisions among the primary particles generated initially. Much of the aggregation probably took place at the source but, as discussed below, there is some evidence that it continued in the atmosphere.
Such particles currently are of great public health interest. Epidemiological studies have shown an association between adverse health effects and aerosols, although the responsible agents, chemical and/or physical, are not known; evidence for the health effects of ultrafine particles has been given by Oberdörster (2001); Oberdörster, et al. (1992); Pagano, et al. (1996); and Peters, et al. (1997). Information on the morphology of the ultrafine atmospheric particles may help biological scientists take into account the physical characteristics of the particles in analyzing their health effects. Ultrafine particles also play an important role in the atmospheric sciences. For example, they may serve as condensation nuclei; both their atmospheric residence time and ability to serve as nuclei depend on their structure. The aggregates may be composed of carbon particles which play an important part in the radiation balance (Toon and Pollack, 1980; Schwartz and Andreae, 1996).
The fractal dimension often can be used for a (partial) quantitative description of the morphology of aggregates. A defining relationship can be written as follows:
Np = A(Rg/R o)Df (1)
where Np is the number of primary particles in the aggregate, A is the dimensionless pre-factor, Rg is the characteristic radius of the aggregate, which in our study is taken to be the radius of gyration. Ro is the average primary particle radius, and Df is the fractal dimension. The fractal dimension is useful in estimating agglomerate transport rates, light scattering, and chemical reactivity.
Values of Df have been determined for aggregates from various types of sources. Weber (1992) explored mechanisms by which particles form fractal aggregates under a variety of experimental conditions. His results agree well with aggregation models, giving a fractal dimension of about 1.9 for particles in the transition regime (20 nm < dp < 300 nm). Other researchers have found that fractal dimensions of soot agglomerates range from 1.5 to 2.2 using different measurement methods (Samson, et al., 1987; Nyeki and Colbeck, 1994). Skillas, et al. (1998), investigated diesel soot agglomerates under different engine loads and found 2.1 < Df < 2.9. Similar measures for spark ignition engines gave higher values, 2.2 < Df < 3.0.
Very little information is available on fractal-like atmospheric particles. Katrinak, et al. (1993) measured Df for 38 carbonaceous aggregates sampled from the atmosphere in Phoenix, Arizona. The particles, which were collected by impaction and analyzed by electron microscopy, were divided into three groups: (1) fractal-like aggregates with 1.35 < Df < 1.89, (2) possibly non-fractal particles with Df > 2, and (3) particles of mixed morphology. These investigators also observed aggregates that were coated with a layer of what they believed to be nitrates and sulfates. The type of impactor used was not described and the efficiency of particle collection as a function of particle size was not reported. Values of the prefactor A that appear in (1) also were not reported.
Measurements of Aggregate Characteristics
In our studies, particles were collected on the lowest stages (7 and 8) of a single-jet, eight-stage low-pressure Hering impactor. At urban sites, enough particles could be collected in a few minutes for morphological analysis. The impactor stages were fitted with TEM grids on which the particles were collected. Computer pictures taken of TEM grids from the impactor stages were analyzed to obtain the primary particle size and location in the aggregate. From these data, values of Dfand A were determined. Measured values of Df increased from near unity to above 2 as the number of primary particles composing the aggregate increased from 10 to 180, with a count mean prefactor (A) of 2.9. This increase may be due to further aggregation in the atmosphere, restructuring by condensation and evaporation in the atmosphere, and the rotation of branches of larger particles with longer residence times. Results for atmospheric particles were compared with literature data for laboratory generated soot and simulated aggregates. Although the basic structures are similar, the primary particles that make up atmospheric aggregates are more polydisperse than aggregates generated at a given source. Increased polydispersity probably results from aggregation in the atmosphere of aggregates from different sources.
Near our laboratory at University of California, Los Angeles (UCLA), where most of the measurements were made, aggregate concentrations were on the order of 400 particles/ml in the size range 50-120 nm. Concentrations in the same particle size range at a rural site outside Los Angeles were about 1 percent of the urban count. In one set of measurements, about 60 percent of the total number of particles in the size range 50-75 nm and 34 percent in the range 75-120 nm were aggregates. Aggregate surface areas based on the primary particles that compose the aggregates are much greater than values calculated from the nominal aerodynamic diameter.
Some accumulation mode particles were collected on stage 4 of the low pressure impactor. Electron microscope observations showed that these were frequently present as droplets which contained aggregates very similar to those present in the ultrafine particle size range. The aggregates probably were scavenged by Brownian diffusion to the accumulation mode microdroplets.
Application to Aerosol Concentrator Performance
Our group has collaborated with the aerosol concentrator group (C. Sioutas) of the EPA-funded Southern California Particle Research Center (headquartered at UCLA). In concentrating the ultrafine component (dp < 0.1 (m)) of the atmospheric aerosol, water vapor is condensed on the particles to grow them to the point where they can be separated in the concentrator by inertial mechanisms. Previously reported studies by other investigators indicate that evaporation tends to change the fractal properties of the particles. Using the methods developed in this project, we measured fractal dimensions of ultrafine particles before and after the concentrator; somewhat to our surprise, we found little effect of the condensation and evaporation process on fractal dimension, perhaps because condensation and evaporation take place rapidly (Kim, et al., 2001). This is potentially important in the use of the concentrator ultrafine fraction in animal exposure studies because fractal properties affect deposition in the lung and, perhaps, the behavior of deposited fractal structures.
Another objective of this research was a conceptual study of possible methods for measuring aerosol oxidants. The new concept of "aerosol oxidants" that we have introduced (Friedlander and Yeh, 1998) corresponds in a way to "aerosol acidity," an established parameter often used to characterize atmospheric aerosols. Our conceptual study included a review of methods of measuring atmospheric gas phase oxidants and the possibility of applying such methods to peroxides present. Based on our laboratory studies and discussions with our Chemistry faculty, we have initiated a cooperative research program with Dr. Susanne Hering of Aerosol Dynamics (Berkeley, CA). This involves development of a method of measuring the concentration of atmospheric aerosol oxidants. A preproposal was submitted and approved by the Health Effects Institute; we currently are participating in the preparation of a final proposal for the design of a suitable instrument.
Potential Practical Applications
A central issue in current air pollution research is to explain the associations observed between adverse health effects and mass concentrations of submicron (PM2.5) in epidemiological studies. Active agents that were proposed include ultrafine atmospheric particles (dp < 0.1 (m)) that are, in part, agglomerates of nanoparticles which may redisperse in the lung (Seaton, et al., 1995; Warheit, et al., 1990; Oberdörster, et al, 1992). These hypotheses are closely linked to the morphological properties of the submicron aerosol. The ultrafine particles are likely to be present in the form of aggregates, which may be freely suspended in the atmosphere. They also are present in accumulation mode microdroplets. Our studies provide the first detailed measurements of the fractal parameters of ultrafine aggregates present in the atmosphere. Our data will permit better estimates of atmospheric aggregate transport and deposition in the lung. This will help efforts to establish adverse health effects due to atmospheric aggregates by providing biological scientists with information on size and surface area.
Data on the size distribution, fractal structure, and primary particle size for agglomerates in the ultrafine range will permit improved calculations of the atmospheric aerosol dynamics for this size range. According to current theory, the residence time distribution for this size range depends primarily on attachment of these particles to accumulation mode particles. The ultrafine particles also may serve as condensation sites for condensable, hygroscopic vapors. The fractal structure of these particles affects particle transport properties and rates of heterogeneous condensation. Finally, atmospheric aerosol black carbon plays a significant role in the absorption of solar radiation by the atmospheric aerosol. Our data will permit more accurate estimates of the contributions of soot agglomerates to optical extinction.
Friedlander S, Yeh EK. The submicron atmospheric aerosol as a carrier of reactive chemical species: case of peroxides. Applied Occupational and Environmental Hygiene 1998;13(6):416-420.
Katrinak KA, Rezz P, Perkes PR, Buseck PR. Fractal geometry of carbonaceous aggregates from an urban aerosol. Environmental Science and Technology 1993;27(3):539-547.
Kim S, Jaques PA, Chang M, Barone T, Xiong C, Friedlander SK, Sioutas C. Versatile Aerosol Concentration Enrichment System (VACES) for simultaneous in vivo and in vitro evaluation of toxic effects of ultrafine, fine and coarse ambient particles. Part II: Field evaluation. Journal of Aerosol Science 2001;32(11):1299-1314.
Nyeki S, Colbeck I. The measurement of fractal dimension of individual in situ soot agglomerates using a modified cell technique. Journal of Aerosol Science 1994;25:75-90.
Oberdörster G. Pulmonary effects of inhaled ultrafine particles. International Archives of Occupational and Environmental Health 2001;74(1):1-8.
Oberdörster HG, Ferin J, Gelein R, Soderhom SC, Finkelstein J. Role of the alveolar macrophage in lung injury: studies with ultrafine particles. Environmental Health Perspectives 1992;97:193-199.
Pagano P, Zaiacomo T, Scarcella E, Bruni S, Calamosca M. Mutagenic activity of total and particle-sized fractions of urban particulate matter. Environmental Science and Technology 1996;30(12):3512-3516.
Peters A, Dockery DW, Heinrich J, Wichmann HE. Short-term effects of particulate air pollution on respiratory morbidity in asthmatic children. European Respiratory Journal 1997;10:872-879.
Samson RJ, Mulholland GW, Gentry JW. Structural analysis of soot agglomerates. Langmuir 1987;3(2):272-281.
Schwartz SE, Andreae MO. Uncertainty in climate change caused by aerosols. Science 1996;272(5):1121-1122.
Seaton A, Macnee W, Donaldson K, Godden D. Particulate air pollution and acute health effects. Lancet 1995;345:176-178.
Skillas G, Kunxel S, Burtscher H, Baltensperger U, Siegmann K. High fractal-like dimension of diesel soot agglomerates. Journal of Aerosol Science 1998;29(4):411-419.
Toon OB, Pollack JB. Atmospheric aerosols and climate. American Scientist 1980;68:268-278.
Warheit DB, Seidel WC, Carakostas MC, Hartsky MA. Attenuation of perfluoropolymer fume pulmonary toxicity: effect of filters, combustion method, and aerosol age. Experimental and Molecular Pathology 1990;52:309-329.
Weber AP. Characterization of the geometrical properties of agglomerated aerosol particles. Paul Scherrer Institut Labor Fur Radiochemie 1992; 129.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
|Other project views:||All 17 publications||3 publications in selected types||All 2 journal articles|
||Xiong C, Friedlander SK. Morphological properties of atmospheric aerosol aggregates. Proceedings of the National Academy of Sciences of the United States of America 2001;98(21):11851-11856.||