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Module 3: Characteristics of Particles - Particle Size Categories
EPA Particle Size TerminologyRegulated Particulate Matter Categories
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Total Suspended Particulate Matter
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PM10
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PM2.5
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Particles Less Than 0.1 Micrometer
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Condensable Particulate Matter
Objectives
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Identify the size ranges for the different particle size
categories.
- Describe the characteristics and behavior of particles with respect to their size.
Since the range of particle sizes of concern for air emission evaluation is quite broad it is beneficial to divide this range into smaller categories. Defining different size categories is useful since particles of different sizes behave differently in the atmosphere and the respiratory system.
The EPA has defined four terms for categorizing particles of different sizes. Table 1 displays the EPA terminology along with the corresponding particle sizes.
Figure 1 provides a visual comparison of the size
of a fine particle (1.0 
), coarse particle (10 ), and a supercoarse particle (100 ). There is a substantial difference in size between these
particles, all of which are considered moderate-to-large in air
pollution control.
Regulated Particulate Matter Categories
In addition to the terminology provided in Table 1 the EPA also categorizes particles as follows:
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Total Suspended Particulate Matter (TSP)
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PM10
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PM2.5
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Particles less than 0.1
- Condensable Particulate Matter
These particle categories are important because particulate matter is regulated and tested for under these categories. The National Ambient Air Quality Standard for PM2.5 was remanded by a District of Columbia court in May of 1999 and is under litigation as of the writing of these modules (December 1999). 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.
Figure 2 displays a typical size distribution of atmospheric particulate matter that combines the two classification schemes discussed above.
Total Suspended Particulate Matter
Particles ranging in size from 0.1 micrometer to about 30 micrometer in diameter are referred to as total suspended particulate matter (TSP). TSP includes a broad range of particle sizes including fine, coarse, and supercoarse particles.
The U.S. EPA defines PM10 as particulate matter with a diameter of 10 micrometers collected with 50% efficiency by a PM10 sampling collection device. However, for convenience in these modules, the term PM10 will be used to include all particles having an aerodynamic diameter of less than or equal to 10 micrometers.
PM10 is regulated as a specific type of "pollutant" because this size range is considered respirable. In other words, particles less than approximately 10 micrometers can penetrate into the lower respiratory tract. The particle size range between 0.1 and 10 micrometers is especially important in air pollution studies. A major fraction of the particulate matter generated in some industrial sources is in this size range. PM10 is discussed in more detail in Module 6.
As with PM10, EPA defines PM2.5 as particulate matter with a diameter of 2.5 micrometers collected with 50% efficiency by a PM2.5 sampling collection device. However, for convenience in these modules, the term PM2.5 will be used to include all particles having an aerodynamic diameter of less than or equal to 2.5 micrometers.
EPA chose 2.5 micrometers as the partition between fine and coarse particulate matter. Particles less than approximately 2.5 micrometers are regulated as PM2.5. Air emission testing and air pollution control methods for PM2.5 particles are different than those for coarse and supercoarse particles.
PM2.5 particles settle quite slowly in the atmosphere relative to coarse and supercoarse particles. Normal weather patterns can keep PM2.5 particles airborne for several hours to several days and enable these particles to cover hundreds of miles. PM2.5 particles can cause health problems due to their potentially long airborne retention time and the inability of the human respiratory system to defend itself against particles of this size.
In addition, the chemical makeup of PM2.5 particles is quite different than for coarse and supercoarse particles. EPA data indicate that PM2.5 particles are composed primarily of sulfates, nitrates, organic compounds, and ammonium compounds. The EPA also determined that PM2.5 particles often contain acidic materials, metals and other contaminants believed to be associated with adverse health effects.
Particles less than 1 micrometer in diameter are termed submicrometer particles and can be the most difficult size to collect. Particles in the range of 0.2 to 0.5 micrometers are common in many types of combustion, waste incineration, and metallurgical sources. Particles in the range of 0.1 to 1.0 micrometers are important because they can represent a significant fraction of the particulate emissions from some types of industrial sources and because they are relatively hard to collect.
Particles Less Than 0.1 Micrometer
Particles can be much smaller than 0.1 micrometer. In fact, particles composed of as little as 20 to 50 molecules clustered together can exist in a stable form. Some industrial processes such as combustion and metallurgical sources generate particles in the range of 0.01 to 0.1 micrometer. These sizes are approaching the size of individual gas molecules, which are in the range of 0.0002 to 0.001 micrometer. However, particles in the size range of 0.01 to 0.1 micrometer tend to agglomerate rapidly to yield particles in the greater than 0.1 micrometer range. Accordingly, very little of the particulate matter entering an air pollution control device or leaving the stack remains in the very small size range of 0.01 to 0.1 micrometer.
Condensable Particulate Matter
Particulate matter that forms from condensing gases or vapors is referred to as condensable particulate matter. Condensable particulate matter forms by chemical reactions as well as by physical phenomena.
Condensable particulate matter is usually formed from material that is not particulate matter at stack conditions but which condenses and/or reacts upon cooling and dilution in the ambient air to form particulate matter. The formation of condensable particulate matter occurs within a few seconds after discharge from the stack.
From a health standpoint, condensable particulate matter is important because it is almost entirely contained in the PM2.5 classification.
Practice Problems
Particle Size Categories
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.