Mass distribution, particulate measurement

Airborne particulate matter, which includes dust, dirt, soot, smoke, and liquid droplets emitted into the air, is small enough to be suspended in the atmosphere. Airborne particulate matter may be a complex mixture of organic and inorganic substances. They can be characterized by their physical attributes, which influence their transport and deposition, and their chemical composition, which influences their effect on health. The physical attributes of airborne particulates include mass concentration and size distribution. Ambient levels of mass concentration are measured in micrograms per cubic meter (mg/m ) size attributes are usually measured in aerodynamic diameter. Particulate matter (PM) exceeding 2.5 microns (/i) in aerodynamic diameter is generally defined as coarse particles, while particles smaller than 2.5 mm (PMj,) are called fine particles. 

Eldering, A., and R. M. Glasgow, Short-Term Particulate Matter Mass and Aerosol-Size Distribution Measurements Transient Pollution Episodes and Bimodal Aerosol-Mass Distributions, Atmos. Environ., 32, 2017-2024 (1998). 

Optical measurements of airborne combustion aerosols have been carried out for a number of years, usually with light scattering techniques. However, due to the particle size dependence of light scattering and the variable particle size distributions of smokes, it is extremely difficult to relate light scattering properties to particulate mass concentrations. The measurement of light absorption by particles can be directly related to particle mass if two conditions are met ... 

Figure 2 Representative example of a mass distribution of ambient particulate matter as a function of particle diameter. Mass distribution per particle size interval is shown as Amass/A(logDa) (in Rgni ) plotted against particle size (ZJa) in micrometers. Tbe figure also shows the range of aerosol sizes included in various methods of aerosol measurement wide range aerosol classifiers (WRAC), total suspended particulate (TSP) samplers, PMjo and PM25 samplers (source Lippman and Schlesinger, 2000) (reproduced by permission of Annual Reviews from Annual Review of Public Health 2000, 21, 309-333).

It is based on the principle that particles, suspended at low concentration in an electrolyte solution, are detected upon their passage through a small orifice in an insulating wall by the modulation of an electrical field existing within the orifice. This electrical modulation is sensed as a voltage pulse for each particle, the height of which is proportional to the volume of the particle. This is very convenient since a measurement yields directly a number distribution of equivalent volume diameters. This distribution is, in turn, easily converted to a volume-based distribution of equivalent volume diameters. Since the volume of a particle is directly related to its mass through the density, this distribution is identical to a mass distribution and, thus, most relevant to characterize a lot of particulate material. Therefore, in our opinion standardisation and... 

The three major characteristics of particulate pollutants in the ambient atmosphere are total mass concentration, size distribution, and chemical composition. In the United States, the PM q concentration, particulate matter with an aerodynamic diameter <10 /nm, is the quantity measured for an air quality standard to protect human health from effects caused by inhalation of suspended particulate matter. As shown in Chapter 7, the size distribution of particulate pollutants is very important in understanding... 

Particulate filtration removes suspended liquid or solid materials whose size, shape, and mass allow them to remain airborne at the air velocity conditions present. Filters are available in a range of efficiencies, with higher efficiency indicating removal of a greater proportion of particles and of smaller particles. Moving to medium efficiency pleated filters is advisable to improve lAQ and increase protection for equipment. However, the higher the efficiency of the filter, the more it will increase the pressure drop within the air distribution system and reduce total airflow (unless other adjustments are made to compensate). It is important to select an appropriate filter for the specific application and to make sure that the HVAC system will continue to perform as designed. Filters are rated by different standards which measure different aspects of performance. 

All of the above particulate investigations were based on mini-radiocarbon measurement techniques, with sample masses typically in the range of 5-10 mg-carbon. This constituted a major advantage, because it was practicable to select special samples (given region, source impact, sediment depth) and to further subject such samples to physical (size) or chemical separation before 14C measurement. This type of "serial selectivity" provides maximum information content about the samples and in fact it is essential when information is sought for the sources or atmospheric distributions of pure chemical species, such as methane or elemental carbon. 

Most of the studies on size-resolved aerosol mass concentrations in areas with different levels of pollution show that particulate matter typically exhibit a bimodal distribution, with most of their mass being found in the submicron size range (dae < 1pm) and an additional minor mode in the coarse fraction (1 < dae < 10 pm) (Maenhaut et al., 2002 Smolfk et al., 2003 Wang et al., 2003 Gajananda et al., 2005 Samara and Voutsa, 2005). However, with instrumentation allowing more precise measurements, the aerosol mass size distribution was found to be multimodal with the preponderance of a fine mode (dae < 0.2 pm) and an accumulation mode (dae 0.5pm), with a minor coarse mode at d 3-4pm (Raes et al., 2000 Pillai and Moorthy, 2001 Berner et al., 2004). Traditionally, atmospheric researchers classify airborne particles into three size classes coarse (2.5 < c/ ie < 10pm), fine... 

Aerosols present a special case in that the investigator needs to measure the mass concentration of the chemical, the chemical composition as a function of particulate size, and the particle-size distribution of the aerosol. No continuous sampling instruments are available to measure both particle-size and chemical concentration. Particle detection can be accomplished using both forward- and back-scatter detectors. A typical back-scatter allows for non-invasive determinations over a range from 6 to 10 000 mg m . In the test, the aerosol is drawn through an orifice and articles impact on a surface positioned between a source and a counter. 

Very few, if any, practical particulate systems are mono-sized. Most show a distribution of sizes and, depending on the quantity measured, the distribution can be by number, surface or mass. Conversion from one type of distribution to another is theoretically possible but it assumes a constant shape factor throughout the distribution which often is not true and such conversion is in error. The conversions are therefore to be avoided whenever possible by choosing a measurement method which measures the desired type of distribution directly. Except for a few specialized applications like rating of filter media, the most relevant types in powder handling are usually the mass or the surface distributions. 

Fig. 3-13. Left Vertical distribution of carbonyl sulfide and sulfur dioxide in the stratosphere. [From data of Maroulis et al. (1977), Sandalls and Penkett (1977), Torres el al. (1980), Mankin et al. (1979), Inn et al. (1979, 1981) for COS, and from Jaeschke et al. (1976), Maroulis et al. (1980), Georgii and Meixner (1980), Inn et al. (1981) for S02.] Curves represent calculations of Turco et al. (1980, 1981a) for an assumed cutoff of COS photodissociation of 312 nm. Right Vertical distribution of gaseous and particulate sulfuric acid. Solid squares and circles are from mass spectrometric measurements of Arijs et al. (1982) and Viggiano and Arnold (1983), respectively. Open circles with error bars (one standard deviation) are from filter collections of Lazrus and Gandrud (1977). The range given by the thin lines indicates the seasonal variability of particulate sulfate. The solid line indicates the vapor pressure of H2S04 over a 75% H2S04/25% H20 mixture.

The applications of isokinetic sampling cover but are not limited to the sampling of aerosols such as flu gas in chimney, soots (unbumed carbons) from diesel engine exhaust, dusts suspended in the atmosphere, and fumes from various sprayers measurements of particulate mass fluxes in pneumatic transport pipelines and other particulate pipe flows solid fuel (also some liquid fuels) distributions in furnaces, engines, and other types of combustors and calibrations of instruments for the measurements of particle mass concentrations. Isokinetic sampling can also be applied to flows with liquid droplets. In this case, the droplet sample is usually collected by an immiscible liquid (Koo et al., 1992 Zhang and Ishii, 1995). 

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