Ultrafine size fractions

It has been found that the "unattached" fraction is an ultrafine particle aerosol with a size range of 0.5 to 3 nm. In order to initiate studies on the formation mechanism for these ultrafine particles, a series of experiments were made in the U.S. Bureau of Mines radon chamber. By introducing SO into the chamber, particles were produced with an ultrafine size distribution. It has been found that the particle formation mechanism is supressed by the presence of radical scavengers. These experiments suggest that radiolysis following the decay of Rn-222 gives rise to the observed aerosol and the properties of the resulting aerosol are dependent on the nature and the amount of reactive gas present. 

It is clear that, in terms of number, the overwhelming majority of freshly emitted particles from vehicular combustion reside in the ultrahne size range. The instability of many of these particles results in their relatively rapid transformation into a larger size fraction with an appreciable and readily-measured mass. How the variable time and spatial scales involved in these processes relate to human exposure to particles near this source was poorly understood until the first few years of the twenty-first century, when two separate but equally comprehensive studies were reported, based on measurements of the relationship between particle concentration and distance from roadways under various wind conditions (Sioutas et al. 2005). These studies, performed in Australia (Hitchins et al. 2000) and the USA (Zhu et al. 2002a, b), highlighted that ultrafine particle concentrations do not reach concentrations comparable to background until a distance of at least 300 m cross or downwind of major roads is reached. By comparison, these studies also noted that fine particle mass concentrations exhibited relatively small decay with distance from roadways. 

It has been found that the activity which is conventionally referred to as the "unattached" fraction is actually an ultrafine particle aerosol with a size range of 0.5 to 3 nm. The hydroxyl radical from water molecule radiolysis is a key element to the particle formation mechanism. By injecting different concentrations of S02 into the test chamber, a possible particle formation mechanism has been suggested as follows Oxidizable species such as S02 reacts promptly with hydroxyl radicals and form a condensed phase. These molecules coagulate and become ultrafine particles. 

The amount of particles determine the quantity of decay products that stay in the air (equilibrium fraction, F) and the fraction of activity associated with the "unattached or ultrafine mode of the size distribution (fDot) These decay products are certainly harmful at high concentrations but we cannot yet detect the effects at normal levels because the vast majority of lung cancer death are due to smoking. Models predict that potentially 9000 lung cancer deaths per year in the United States are due to indoor radon. Methods are currently available and new methods are being developed and tested for lowering the levels of radon in indoor air. 

In a similar study, Allen and co-workers (1996) determined the particle size distribution for 15 PAHs with molecular weights ranging from 178 (e.g., phenan-threne) to 300 (coronene) and associated with urban aerosols in Boston, Massachusetts. As for BaP in the winter (Venkataraman and Friedlander, 1994b), PAHs with MW >228 were primarily present in the fine aerosol fraction (Dp < 2 /Am). A study of 6-ring, MW 302 PAH at the same site showed bimodal distributions, with most of the mass in the 0.3- to 1.0-/zm particle size size range a smaller fraction was in the ultrafine mode particles (0.09-0.14 /xm) (Allen et al., 1998). For PAHs with MW 178—202, the compounds were approximately evenly distributed between the fine and coarse (D > 2 /am) fractions. Polycyclic aromatic hydrocarbons in size-segregated aerosols col-... 

Based on these characterizations, a model structure of 0.1 wt% Ni-loaded K4Nb60i7 was proposed as shown in Fig. 16.5. During the loading of the catalyst with nickel, most of the nickel enters the interlayer region I as Ni2+ by replacing K+ ions, leaving a very small fraction on the external surface. During reduction at 700°C, the Ni2+ cations are reduced to metallic nickel in the form of ultrafine particles of about 0.5 nm size. 

The particle size of barium sulfate may vary from a fraction of a micron to several microns or more (48). Ultrafine grain size by itself may give inferior visualization of the gastric mucosa but the particles can be more easily held in suspension. On the other hand, particles larger than 1 jam may offer better contrast, provided that they can be made to stay in suspension. An electron micrograph of barium sulfate particles of less than 1 jam in diameter shows that they are of irregular shape (53). The influence of microcrystalline shape on the coating property of the suspension is not well studied. 

Ultrafine particles have been defined as those, which are smaller than 0.1 pm. Another classification is into submicrometre particles, which are smaller than 1 pm, and supermicrometre particles, which are larger than 1 pm. The terminology that has been used in the wording of the ambient air quality standards, and also for characterisation of indoor and outdoor particle mass concentrations, includes PM2.5 and PM fractions and the total suspended particulate (TSP). PM2.5 (fine particles) and PM, are the mass concentrations of particles with aerodynamic diameters smaller than 2.5 and 10 pm, respectively (more precisely the definitions specify the inlet cutoffs for which 50% efficiency is obtained for these sizes). TSP is the mass concentration of all particles suspended in the air. There have been references made in the literature to PMj or PMq 1 fractions, which imply mass concentrations of particles smaller than 1 and 0.1 pm, respectively. These terms should be used with caution, as particles below 1 pm, and even more those below 0.1 pm, are more commonly measured in terms of their number rather than their mass concentrations, and therefore these terms could be misleading. 

This chapter relates aerosol and hydrodynamic particle size to in vivo deposition, phagocytosis, and release of cytokines. We are interested in comparing these properties in ultrafine particles (nanoparticles <100nm) with those in the accumulation fraction (100 to 600 nm), the intermodal fraction (600 to 2500 nm) of fine particles and the coarse firaction (2500 to 10,000 nm). We expect the most interesting comparison to be between the minimally aggregated ultrafine particles (nanoparticles) and moderately aggregated accumulation fraction of the fine particles. 

Particles that can be inhaled, those less than 10 p,m (10,000 nm PMIO) can be separated by the high volume cascade impactor (HVCI) into four fractions. When the HVCI was used to collect organic urban aerosols presumably from transportation, combustion, and the Earth s crust, the breathable particulate matter (PM) was divided into PM 10 to 2.5 p.m (coarse aerosols, which are mechanically produced), and the PM 2.5 to 1 p.m (intermodal) fiaclion, which is expected to have particles that contain properties of both coarse (larger) and fine (smaller) aerosols. In addition it separates the PM 1 to 0.2 p,m (1000 to 200 nm accumulation) fraction (just larger than nano- or ultrafine particles with properties similar to those particles) and PM 0.2 (particles <200 run diameter in air) firaction. The cutoff size (200 nm) was chosen for convenience. 

Metal Surface Composition To determine which metal bound to surfaces of road dust, soil dust was collected that contained larger particles (400 to 3000 nm diameter in air) and that compared closely to amorphous silica dust (6, 7). This dust was thought to be composed of both particles ground from the Earth s emst and particles generated by transportation vehicles that drive on the roads from which road dust was collected. Dust was collected near a road in four fractions. <56 nm, <100nm, both ultrafine (nano) particle aerosols, a fraction <2500 nm (a fine aerosol), and one < 10,000 nm (a coarse aerosol). Different elements were enriched on the surface of different-sized particles (6, 7). 

Ultrafine particles are particles with real diameters less than or equal to 100 nm (0.1 p,m) in diameter. These particles arc randomly buffeted by gas molecules and deposit by diffusion in the alveoli. Accumulation fractions are the first sizes beyond ultrafine particles (100 to 500 nm), are deposited mostly by diffusion, and have only minimal settling velocity. Both particles with real physical size are small enough to go into the deep Itmg to deposit in the alveoli. 

Inorganic solids are often made as powders. The average particle size can be varied over many orders of magnitude (Figure 8.6). Ultrafine nanosized particles are best made from the gas phase or if they are oxides, from colloids. The physical properties of the particles, such as density and magnetism, are used to separate the larger particles from the fine fractions. A hydrocyclone, shown in Figure 8.7, is a typical device used for this purpose. 

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