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Module 6: Air Pollutants and Control Techniques - Particulate Matter - Characteristics
Characteristics of Particles
Particle Size and PM Regulations
Practice Problems
Objectives
- List five characteristics of particulate matter that are especially important in air pollution control.
- Describe the general differences between total particulate matter, PM10, and PM2.5.
This lesson focuses on total particulate matter, PM10 particulate matter, and PM2.5 particulate matter.
The most important characteristic of particulate matter (PM) is the particle size. This property has the greatest impact on the behavior of particulate matter in control equipment, the atmosphere, and the respiratory tract. Particles of importance in air pollution control span a broad size range from extremely small (0.01 micrometer) to more than 1,000 micrometers. As a frame of reference, a human hair has a diameter of approximately 50 micrometers.
A particle size is usually expressed in terms of its aerodynamic diameter instead of its actual or physical diameter. For more information about the aerodynamic diameter, see the lesson called Aerodynamic Diameter in Module 3.
The chemical composition of the particulate matter is also important. Absorption and heterogeneous nucleation of vapor phase pollutants onto existing particles can create toxic particulate matter. Other characteristics besides size and chemical composition should be considered when selecting an appropriate particulate control device for a gas stream. Other important characteristics of particulate matter in gas streams include stickiness, resistivity, and explosiveness.
Anthropogenic sources of particulate matter are shown in Figure 1. Major sources include fugitive dust, agricultural processes, industrial processes, coal-burning electric power plants, residential fuel combustion, and highway vehicles.
The next section describes how and why the U.S. EPA, in its research efforts and regulations, has focused on progressively smaller and smaller particles.
Particle Size and PM Regulations
Particulate matter of all sizes is regulated as total filterable particulate matter. This was the first category of air pollutants that was subject to air pollution control regulations. The reason for this early attention was the tendency for the large-diameter particulate matter to settle on houses, deposit and discolor urban buildings, and reduce visibility.
Less than forty years ago, the total filterable particulate problem in many metropolitan areas was measured using "dustfall" buckets. These tests indicated the deposition of total particulate in units of tons per square mile per month!
In 1971, the U.S. Environmental Protection Agency (U.S. EPA) established the National Ambient Air Quality Standard (NAAQS) for total filterable particulate matter to reduce community exposure to this pollutant. Particulate control regulations developed by State and local agencies in response to the Particulate NAAQS have been instrumental in reducing particulate matter emissions since 1971. Communities no longer have the severe conditions that are apparent in Figure 2.
During the 1970s and early 1980s, research studies indicated that the severity of adverse health effects of particles depended, in part, on their size. Particulate matter that was less than or equal to 10 micrometers (PM10) was more closely associated with adverse health effects than the larger sized particulate matter. This phenomenon is due to the ability of smaller particles to more effectively penetrate through the upper respiratory tract, which serves as the body's first line of defense. The relationship between particle size and health effects is also due to the difference in chemical composition of small particles and their ability to remain airborne for long time periods.
In order to focus attention on particles having the greatest impact on health, the U.S. EPA revised their ambient air quality standards with respect to particulate matter in 1988. The NAAQS for total filterable particulate matter was replaced with a NAAQS for PM10.
PM10 limitations are now being applied to stack emissions from stationary sources so that the ambient levels of PM10 do not exceed the NAAQS ambient air concentration limits.
In the mid-1980s, research began to focus increasingly on the possible adverse health effects associated with particles that are less than 2.5 micrometers in diameter (PM2.5). The composition of these particles is considerably different from larger particulate matter (even particles in the size range from 2.5 to 10 micrometers). Most particles less than 2.5 micrometers are formed by the following mechanisms:
- Heterogeneous nucleation of vapor phase material
- Homogeneous nucleation of vapor phase material
- Atmospheric reactions of gases to form vapors that homogeneously
and heterogeneously nucleate to form particles
These formation mechanisms combine to generate a set of particles that are different from larger sized particulate matter in both chemical composition and health effects. Due to the special characteristics of these small particles, EPA in July of 1997 promulgated a new NAAQS for particulate matter less than 2.5 micrometers. As of the writing of these modules (December 1999) the NAAQS standard for PM2.5 has been remanded by a court and is under litigation. Air quality standards are presented in these modules as they were promulgated by the EPA, with no presumptions made regarding the outcome of the pending litigation.
The U.S. EPA developed the different sets of particulate matter, illustrated in Figure 3, as part of its research into the health effects of small particulate matter.
The U.S. EPA has classified particles into four size categories shown in Figure 3 (ultrafine, fine, coarse, and supercoarse). For more information about the characteristics of particles and U.S. EPA regulated categories for particulate matter, see Module 3, which is dedicated to these topics.
Practice Problems
Particulate Matter - Characteristics
- Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.