Characteristics of Ultrafine Particles
Transport and Fate in the Environment
Prevention or Control of Exposures
Human Health Effects of Ultrafine Particles
Absorption and Distribution
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CHARACTERIZATION OF ULTRAFINE PARTICLES
A literature review was conducted of the pertinent technical literature to describe the characterization of ultrafine particles. Ultrafine Particles are characterized by their morphology (in terms of diameter) as well as their composition. The composition and morphology often reflect the various sources of the air emissions. The following is a description of the various physical and chemical properties of ultrafine particles and their related emission sources.
In an effort to better understand particulate matter (commonly referred to as dust), it is important to understand some of the features that classify and distinguish the pollutant. Atmospheric particles are described by their morphology and composition. The categories of particulate matter with a diameter less the 10 micrometers but greater than 2.5 micrometers are known as Particulate Matter 10 micron fraction (PM10), particulate matter with a diameter less than 2.5 but greater than 0.1 micrometers is known as Particulate Matter 2.5 micron fraction (PM 2.5), and particles with a diameter of less than .1 micrometers are considered as the ultrafine particle fraction (UFP) (Ibald-Mulli et al., 2002). The word diameter is widely used to describe particle size, but dust particles are rarely spherical with a single diameter. Honey and McQuitty (1976), refer to eight distinct measures of particle size. Probably the most useful is aerodynamic diameter, the diameter of a hypothetical sphere of unit density that has the same settling speed as the particle in question. Additionally, as UFPs collide, they can form chain-like structures that modify the symmetry of the particle as well as increase size (Aggeli et al., 2001). This is commonly related to the primary fraction, generally occurring at emission, as secondary formation UFRs tend to be hygroscopic and are probably spherical (Aggeli et al., 2001). However, other particle analyzers that work by light scattering refer to projected diameter, the diameter of a circle with the projected area as the particle in question.
Figure #1 Relative Size of Airborne Particulate
Environmental Protection Agency/Queensland Parks and Wildlife Service
The size distinction is important as the particle size reflects in part, the penetration potential into the respiratory tract. The terms inspirable and inhalable are also sometimes referred used to refer to all particles that can enter the nose and mouth with breathing (Takai et al., 1998). Particles less than 5 _m are generally referred to as respirable. Particles smaller than 0.5 _m normally remains suspended and are expelled when exhaling. Particles larger than 5 _m is considered less important and damaging since many are trapped in the upper respiratory tract, nose and windpipe. Ultrafine particles are more insidious as they are capable of deep penetration and deposition into the lung cavity (Daigle et al., 2003).
UFPs are composed of both primary and secondary particulate matter (Xiong and Friedlander, 2001). The primary fraction is generally between 50 and 100 nm which is typically the dominating concentration (Wahlin et al., 2001). This fraction of UFP, emitted directly from the emission source, often includes aggregates of smaller particles (Xiong and Friedlander, 2001). The primary fraction is generally believed to be the product of diesel engines and automobiles which are thought to initially have been emitted at around the 50nm diameter size (the so called nucleation mode) and later coagulate into the larger fraction of the ultrafine mode.
The secondary component is composed of particulate matter formed in the atmosphere, including sulfuric acid and sulfates, and organic reaction products of low volatility (Xiong and Friedlander, 2001). This size fraction is generally between 100 and 200 nm which is partially distinguishable from other directly anthropogenic sources. These changes involve photoreactions of oxides of nitrogen (NOx), and sulfur dioxide (SO2). Both of which are products of combustion. There is also a component of secondary particle chemistry that result in production of ammonium sulphates, nitrates, and chlorides, but these materials are thought to have less toxicological significance (Donaldson et al., 2003).
Figure # 2. TEM-image (mag. 100.000-times) of gas soot with a particle size of 13nm and a specific surface of 460m2/g (Rudolph, 2000)
The composition of UFPs is variable. An important source diagnostic (and likely less expensive to measure) is the relationship between the elemental composition of UFPs as they relate to their emission source. Crustal minerals contribute some fraction to the amount of ultrafine particle load in the environment, which can be identified through the presence of Si, Al, Ti and Ca (typically low concentrations) (Cyrys et al., 2003). There appears to be a strong correlation with UFPs and NO, NO2, CO, Zn, and Cu, which appear to reflect motor vehicle traffic (Cyrys et al., 2003). Much of the elemental carbon in the atmosphere is emitted in the form of aggregates, primarily as a function of the emission characteristics (i.e. diesel engine combustion) (Aggeli et al., 2001).
UFPs are generated from a variety of emission sources including processes related to the work place. Motor vehicle emissions are the primary contributors to ultrafine particles with respect to urban areas and highly used traffic routes, and may be the primary source of UFP emissions to the environment (Junker et al., 2000; Molnar,Wahlin et al., 2001; 2002; Zhu et al., 2002;Palmgren et al., 2003). For example, in California, emission factors for elemental carbon are between one and two-orders of magnitude higher for heavy duty diesel trucks than for light duty vehicles while for particle-bound polycyclic aromatic hydrocarbons (PAHs), emission factors are found to be an order of magnitude higher for heavy diesel vehicles than for light duty vehicles (Miguel et al., 1998).
An investigation into the nature of UFPs in the former East Germany indicated that changes in combustion technology resulted in greater emissions of UFPs (Pitz et al., 2001). The investigation collected ambient air quality data on particulate matter and correlated the findings to the transition in combustion technology as the former East Germany modernized their coal-fired industry and power generation as well as automobiles and home heating. The findings of this six year investigation indicate that the overall mass concentration of fine particles decreased, the UFP concentrations increased. It is likely that the optimized combustion processes lead to the rise in the very small UFP number concentration (<0.03 µm) both by direct emissions and by the diminished scavenging effect of coagulation (Pitz et al., 2001).