2000 Progress 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 Period Covered by this Report: January 2, 1999 through January 31, 2000
Project Amount: $345,247
RFA: Ambient Air Quality (1997) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
The objectives of this research are to: (1) characterize the portion of the atmospheric aerosol composed of fractal-like structures (agglomerates); these usually originate from high temperature sources such as diesel engines and welding; (2) integrate morphological concepts into conventional approaches to aerosol characterization (size distribution function); and (3) apply the results to the dynamics of the atmospheric aerosol especially the scavenging of ultrafine particles by the accumulation mode; a subsidiary objective is a conceptual evaluation of possible methods of detecting oxidants in the atmospheric aerosol.
Aerosol Aggregates in the Submicron and Ultrafine Ranges. The
ultrafine (dp < 0.1 µm) of the atmospheric aerosol is
composed of both primary and secondary particulate matter. The primary
component, emitted directly from sources often includes aggregates or 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
in the gas phase probably evaporate after a time when placed in the electron
microscope, which is the primary observational method used in our research which
focuses on the aggregate structures present in the ultrafine range, which can be
conveniently studied by electron microscopy. Sources of ultrafine solid
particles in the locations where our measurements were made include soot,
welding fume and diesel emissions which are often present in the form of
aggregates, formed from primary particles by collision processes.
Such particles may be of 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 Peters, et al. (1997), Oberdorster, et al. (1992), and Pagano, et al. (1996). Information on the morphology of the ultrafine atmospheric particles may help biological scientists take into account the physical characteristics of the particles. 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 a part in the radiation balance (Toon and Pollack, 1980; Schwartz and Andreae, 1996).
The fractal dimension can often be used for a (partial) quantitative description of the morphology of aggregates. A defining relationship can be written as follows:
where Df is the fractal dimension, Np is the number of primary particles in the aggregate, A is the dimensionless prefactor, Ro is the average primary particle radius, Rg is the characteristic radius of the aggregate which in our study is taken to be the radius of gyration. 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; Yeki 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 3 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 mostly 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) were also not reported, although they are needed to characterize fractal-like aerosols.
In our studies, particles were collected from the air outside our laboratory
on the lowest stages (7 and 8) of a single-jet, eight-stage low-pressure Hering
impactor for periods of a few minutes. The impactor stages were fitted with
transmission electron microscope (TEM) grids on which the particles were
collected. Using a suitable computer program, 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 the fractal dimension
(Df) and prefactor (A) were determined. The fractal
dimension 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 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.
The fraction of aggregates among the atmospheric particles corresponding to the size ranges of stages 7 and 8 was estimated by making simultaneous measurements using a differential mobility analyzer and condensation particle counter.
Application to Aerosol Concentrator Design. 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 mm) 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. Previously reported studies by other investigators indicate that evaporation tends to change the fractal properties of the particles. We measured fractal dimensions of ultrafine particles before and after the concentrator and found little effect of the condensation and evaporation process on fractal dimension, perhaps because condensation and evaporation take place rapidly. This is potentially important in the use of concentration fractions in animal exposure studies because fractal properties affect deposition in the lung and, perhaps, the behavior of deposited fractal structures.
Aerosol Oxidants. Another objective of this research was a conceptual study of possible methods for detecting aerosol oxidants. The new concept of "aerosol oxidants" that we have introduced corresponds in a way to "aerosol acidity" an established parameter often used to characterize atmospheric aerosols. Our conceptual study has started with a review methods of measuring atmospheric gas phase oxidants and the possibility of applying such methods to particulate matter without contamination by gas-phase particles. A collaborative study with the Department of Atmospheric Sciences also has been discussed.
Potential Practical Applications. A central issue in current air pollution research is to explain the associations that have been observed between adverse health effects and mass concentrations of submicron (PM2.5) in epidemiological studies. Active agents that have been proposed include ultrafine atmospheric particles (dp < 0.1 mm) that are in part agglomerates of nanoparticles which may redisperse in the lung (Seaton, et al., 1995; Warheit, et al., 1990; Oberdorster, 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 agglomerates which can be characterized by their fractal dimension and primary particle size. They also are present in accumulation mode particles. Our studies provide the first detailed measurements of the fractal parameters of ultrafine aggregates present in the atmosphere.
We also have initiated a conceptual study of possible methods of detecting
aerosol oxidants. This has been accompanied by discussions with a research group
in Atmospheric Chemistry at UCLA on bringing these concepts to practice. The
results of our research should have direct application in efforts to establish a
credible fine particle standard for atmospheric particulate matter.
Katrinak KA, Rezz P, Perkes PR, Buseck PR. Fractal geometry of carbonaceous aggregates from an urban aerosol. Environmental Science and Technology 1993;27:539-547.
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.
Oberdorster HG, Ferin J, Gelein R, Soderhom SC, Finkelstein J. Role of the alveolar macrophage in lung injury: studies with ultrafine particles. Environmental Health Perspective 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;30:3512-3516.
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;3:272-281.
Schwartz SE, Andreae MO. Uncertainty in climate change caused by aerosols. Science 1996;272:1121-1122.
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;29:411-419.
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 Institute. Labor Fur Radiochemie 1992;129.
A paper based on the results of the studies described above will be completed and submitted for publication. The results of similar measurements made during a visit by Mr. Xiong to the U.S. Environmental Protection Agency facility in North Carolina also will be analyzed. If time permits, an additional set of measurements will be made in a rural area in California. Based on these results, another paper will be written comparing the results for Los Angeles, North Carolina, and a rural site.
These papers with suitable appendices that amplify the results reported in the papers will be incorporated in the Ph.D. thesis of Mr. Cheng Xiong. Studies will continue on development of a technique for the detection of aerosol oxidant. These are of two types: (1) conceptual studies related to the detection of strong oxidants in individual aerosol particles, a program based in our group; and (2) collaborative studies with Atmospheric Sciences (Professor S. Paulson) on bulk (as opposed to single particle) aerosol samples. We also will continue to exchange information with the group at Rutgers (B. Turpin and D. Laskin), which has been studying the exposure or animals to ammonium sulfate aerosols containing hydrogen peroxide.
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|
||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.||