The PM-2.5 challenge

On July 17, 1997, the U.S. Environmental Protection Agency (EPA) published in the Federal Register a revision of the Clean Air Act Amendments that established a new regulated criteria pollutant - particulate matter having an aerodynamic diameter of 2.5 micrometers (µm) or less (PM-2.5). With the definition of the new pollutant, EPA began a new type of air quality sampling, which would, over the next five years, cost hundreds of millions to monitor and has the potential to cost billions of dollars to regulate.

What is PM-2.5?
PM is particulate matter, such as smoke and dust, suspended in the atmosphere. It can be solid or liquid, or a tarry composite like wood smoke. The "2.5" is related to the size of the particles, and is the equivalent aerodynamic diameter of the particle. This means that a particle is sized by how it behaves in its transport through the air, as if it were a perfect sphere. This allows one classification to apply to irregularly shaped particles.

The size of the particles is significant from several points of view. Large particles, 20 µm and larger, settle out of the atmosphere quickly and are not transported long distances. As size decreases, the rate of settling decreases. At around 2.5 µm, particles have very little tendency to settle and are supported by collisions with air molecules. Particles smaller than 2.5 µm can have very long lifetimes in the atmosphere.

Size is also important when particles are inhaled. Larger particles - greater than 10 µm - are removed in the nose and trachea. Smaller particles penetrate deep into the lungs, to the point where blood is oxygenated. Here, the particles have almost direct access to the bloodstream. This can create a serious health concern.

To put particle size into perspective, 2.5 µm particles are much too small to be seen with the naked eye. They are, however, the cause of haze in the atmosphere, when viewed over large areas. Pollen, by contrast, is much larger, from about 20 µm to 60 µm.

Why does EPA want to regulate PM-2.5?
Since 1987, EPA has regulated PM-10, particulate matter smaller than 10 µm. This was thought appropriate because these are the size particles that can enter the lungs. Specifications were created for sampling equipment, manufacturers built the instruments and a nationwide sampling network was established and maintained. The PM-10 standard requires annual average PM-10 particulate levels to be below 65 micrograms (µg) per cubic meter, and the peak readings on a one-day (24-hour) basis to be less than 150 µg per cubic meter.

Research in the past 10 years has shown a correlation between the levels of PM-2.5 and the rate of hospital admissions and deaths per day. This occurred even when the PM-10 levels were below levels considered a significant health risk. When this relationship was noticed in the academic press, the American Lung Association (ALA) filed suit, asking the courts to require EPA to review its current PM standards. The courts supported the ALAÕs claim, and ordered EPA to evaluate the evidence and decide whether new regulations would be beneficial.

At the end of an 18-month effort, EPA had determined the health hazards justified a new standard, which specified a new sampling system incorporating high-accuracy flow control, a precisely defined particle capture, a size separation and collection system, and mandated environmental controls to reduce sampling errors caused by the evaporation of volatile particles. The new samples are required to have the capability to monitor their operation and report on the quality of the sample acquired.

The particulate levels set in the new standard are 15 µg per cubic meter for the annual average and 65 µg per cubic meter for a one-day (24-hour) event. These levels were set by looking at the point in the health studies at which evidence indicated the population experienced minimal adverse health effects.

How is PM-2.5 measured?
As was done for the PM-10 and earlier total suspended particulate (TSP) standards, the allowable levels of PM-2.5 particulate are specified as a total mass per cubic meter of air. The implication is that the way to measure the variable is to collect the particulate mass in an atmospheric sample and weigh it. That is exactly the approach that EPA selected. In previous standards, manufacturers had considerable freedom in selecting how the sample was to be extracted from the atmosphere and how the larger particles were to be separated from the smaller measurable particles. This led to different instruments making different readings, even when located side by side.

Because the physics of aerosols is difficult to understand and the sources of these variations are hotly debated, the new standard did not offer any options related to the variability in the critical inlet and size selective elements of the standard for the primary instruments, the "federal reference monitors." In order to be accepted or "designated," EPA required these samplers to have their aerosol handling sections built according to EPA designs specifying shape, material surface finish and quality control methods. After an EPA-specified PTFE filter collected the sample, the strictness of specification was changed to control only the parameters of operation, such as flow rate, temperature and pressure measurement accuracy and temperature control of the acquired sample. Reference method samplers were built to accommodate either a single filter, allowing one sample per site visit, or multiple filters, changed under computer control by the instrument, allowing several samples to be taken between site visit s.

There is a provision in the EPA standard for "equivalent" method samplers, allowing for the incorporation of new technology into the process. These instruments are termed equivalent, because they must be shown to have the capability of making an equivalent measurement as judged by a set of strict statistical rules. However, as of June 1, EPA still has not approved any equivalent instruments.

Where does PM-2.5 come from?
Wind-blown dust and grinding and crushing operations do not generate significant quantities of very small particles. Most PM-2.5 is thought to be generated from human activities, or result from combustion. Some of PM-2.5's mass is created by the photochemical reactions that take place when sunlight interacts with water vapor, nitrogen oxides and hydrocarbons, generating heavier hydrocarbons that attach additional weight to the PM-2.5 and smaller particles.

One compound that exists in equilibrium between gas and solid phases is ammonium nitrate, which easily changes from a solid to its component gases, depending on the concentrations of water vapor, ammonia and nitric acid gas. Ammonium nitrate poses a problem for sampling systems because of the ease with which it makes the transition between solid and gaseous precursors, an equilibrium that is strongly influenced by temperature. Because of the long suspended life of these particles, it is likely there will be significant transport, due to atmospheric circulation.

What will be required to control PM-2.5?
EPA has installed a large number of the new PM-2.5 samplers in a nationwide network, and since Jan. 1, 1999, has been monitoring the level of PM-2.5 at several locations in each state. The data will define the pervasiveness of fine particulates. After three years of sampling, each state will be required to file an implementation plan describing how it will address any problems it has found.

Since PM-2.5 can result from combustion, emission of nitrogen oxides, ammonia, vehicle exhausts or other sources, the remedial actions could take many forms. It is unclear what the most effective control strategies will be. The only relatively certain conclusion is that it will involve the coordination of controls on many different emission sources.

For more information

Reference documents:

40 CFR 50

40 CFR 53

40 CFR 58

PM2.5 Quality Assurance Manual -

Andersen Instruments

This article originally appeared in the 06/01/1999 issue of Environmental Protection.

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