Particulate Matter PM

Particulate matter is the general term used for a mixture of solid particles and liquid droplets found in the air. Some particles are large or dark enough to be seen as soot or smoke, while others are so small they cannot be seen with the naked eye. These small particles, which come in a wide range of sizes, originate from many different stationary and mobile sources as well as natural sources [29]. Fine particles, those less than 2.5 ^m (PM2.5), result from fuel combustion from motor vehicles, power generation, industrial facilities, and residential fireplaces and woodstoves. Coarse particles, those larger than 2.5 ^m but classified as less than 10 ^m (PM10), are generally emitted from sources such as vehicles traveling on unpaved roads, materials handling, crushing and grinding operations, and windblown dust [29]. Some particles are emitted directly from their sources, such as smokestacks and cars. In other cases, gases such as SO2, NOZ, and VOCs react with other compounds in the air to form fine particles.

Coal generally contains from 5 to 20 weight percent (wt.%) mineral matter (i.e., ash content per a proximate analysis) [5]. During combustion, most of the minerals are transformed into dust-sized glassy particles and, along with some unaltered mineral grains and unburned carbon, are emitted from smokestacks. Particle composition and emission levels are complex functions of firing configuration, boiler operation, and coal properties [27]. In dry-bottom, pulverized coal-fired systems (various types of combustion systems are described in Chapter 5), combustion is very good, and the particles are largely composed of inorganic ash residue. In wet-bottom, pulverized coal-fired units and cyclone-fired boilers, the amount of fly ash is less than in dry-bottom units because some of the ash melts and is removed from the system as slag. Spreader stokers, which fire a mixture of fine and coarse coal, tend to have a significant quantity of unburned carbon in the fly ash. Overfed and underfed stokers emit considerably less particulate than pulverized coal-fired units or spreader stokers because combustion takes place on a relatively undisturbed bed [27]. Fly ash reinjection for increased consumption of unburned carbon or load changes can also affect particulate emissions.

Environmental Effects

Particulate matter is responsible for reduction in visibility. Visibility is principally affected by fine particles that are formed in the atmosphere from gas-phase reactions. Although these particles are not directly visible, carbon dioxide, water vapor, and ozone in increased concentrations change the absorption and transmission characteristics of the atmosphere [28]. Partic-ulate matter can cause damage to materials depending upon its chemical composition and physical state [28]. Particles will soil painted surfaces, clothing, and curtains merely by settling on them. Particulate matter can cause corrosive damage to metals either by intrinsic corrosiveness or by the action of corrosive chemicals absorbed or adsorbed by inert particles. Little is known of the effects of particulate matter in general on vegetation [28]. The combination of particulate matter and other pollutants such as sulfur dioxide may affect plant growth. Coarse particles, such as dust, may be deposited directly onto leaf surfaces and reduce gas exchange, increase leaf surface temperature, and decrease photosynthesis. Toxic particles containing elements such as arsenic or fluorine can fall onto agricultural soils or plants that are ingested by animals and thus can affect the animal's health.

Health Effects

Particulate matter alone or in combination with other pollutants constitutes a very serious health hazard. The pollutants enter the human body mainly via the respiratory system. Inhalable particulate matter includes both fine and coarse particles. These particles can accumulate in the respiratory system and are associated with numerous health effects [29]. Exposure to coarse particles is primarily associated with the aggravation of respiratory conditions such as asthma. Fine particles are most closely associated with such health effects as increased hospital admissions and emergency room visits for heart and lung disease, increased respiratory symptoms and disease, decreased lung function, and even premature death. Sensitive groups that appear to be at greatest risk include the elderly, individuals with cardiopulmonary disease such as asthma, and children [35].

As previously mentioned, because particulate matter has been linked with adverse health effects at levels currently observed in the United States, EPRI has initiated the epidemiological study ARIES and comprehensive air quality monitoring to identify many of the components that might be associated with a particular health endpoint [30]. This is a critical undertaking, as knowledge of the true causative agents allows for better protection of public health through regulation of those sources that produce harmful pollutants, especially in light of the fact that health effects drive the regulatory agenda for proposed multipollutant legislation and air quality standards (see Chapter 4 for a discussion of such legislation).

ARIES, initiated in 1998 in Atlanta, was designed to address the issue of air pollution components by coupling an extensive air quality monitoring effort with five health studies focused on distinct endpoints to cover a wide range of possible health effects: a daily mortality study conducted by Klemm Analysis Group; an emergency room visit study conducted by Emory University; a heart rate variability study conducted by Harvard University; a cardiac arrhythmic events study conducted by Emory University; and an unscheduled physician visit study conducted by Kaiser Permanente [30]. Atlanta is the first of six cities that will be studied, with Chicago currently in the planning stages to be followed (tentatively) by Houston, Texas; Pittsburgh, Pennsylvania; a northeast city; and a western city. The major findings from the Atlanta study include the following [30]:

• Current levels of air pollution in the United States appear to be causing health effects;

• Different pollutants appear to impact different health endpoints (e.g., for respiratory disease, the links are stronger for large particles and gases);

• Particulate matter components are important, but gases should not be overlooked;

• The most toxic fraction of fine particulate matter appears to be the carbon-containing fractions (organic and elemental carbon);

• There are no statistically significant associations between health impacts and sulfates and nitrates;

• Controlling the wrong pollutants will not yield health benefits.

It is generally recognized that coal-fired power plants can be important contributors to ambient fine particulate matter (PM2.5) mass concentrations and regional haze. In 1999, coal-fired power plants emitted 1.5% of the total primary PM2.5 in the United States [36]. In response to growing concerns over PM2.5 being emitted into the atmosphere from coal-fired power plants, the U.S. Department of Energy, National Energy Technology Laboratory (DOE-NETL) is pursuing a major research effort to provide the applied science necessary to quantitatively relate the emissions from energy production to ambient PM2.5 concentrations and composition at downwind receptors and to inform decision makers about management options applicable to coal-fired power generation to achieve the national standards for PM2.5 and regional haze. Special emphasis has been given to Pittsburgh, Pennsylvania, and the surrounding upper Ohio River valley region because, on many occasions throughout the year, this region is downwind of major coal-fired power plants and other industrial sources of air emissions and upwind of the Boston-Washington corridor, the largest regional complex of urban areas in the United States.

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