Particle Size. PM is characterized by size, expressed as a particle's aerodynamic equivalent diameter in micrometers (or microns). Fine particles are typically defined as those less than 2.5 microns in diameter (PM2.5) and coarse mode particles are those greater than 2.5 microns, but less than 10 microns in diameter (PM10). Particle size is closely linked to relative health impacts, as well as source origination, and chemical composition.
Health Effects of PM. The adverse effects of PM on human health have been recognized for many decades. The major risks to human health include not only premature mortality from acute pollution episodes, but also aggravation of existing respiratory and cardiovascular disease, damage to lung tissue, impaired breathing and respiratory symptoms, and alterations to the body's physical and immune system defenses against inhaled particles. Populations at general risk include children, people of all ages with asthma, and elderly persons with illnesses such as bronchitis, emphysema, and pneumonia. PM causes adverse health effects by depositing in the lungs where it interferes with the respiratory process. The health risk from an inhaled dose of PM appears to depend on the size, composition, and concentration of the particulate.
PM in the Atmosphere. PM can result from direct emissions from various sources (primary PM) or it can be formed in the atmosphere by a combination of gaseous emissions (secondary PM).
Primary Particles. There are two major types of direct PM emissions:
Fugitive and Re-entrained Dust Particles. Fugitive dust includes wind blown particles and dust arising primarily from construction, agriculture, and natural landscapes. Material resuspended by vehicles traveling on unpaved and paved roads is called re-entrained dust and considerable quantities of re-entrained dust on road and pavement surfaces arise from vehicle tires tracking soil from unpaved roads, the erosion of the road surface itself, and the degradation of parts of the vehicle, especially the tires. The amount of re-entrained and fugitive dust is extremely difficult to predict or measure as it depends on factors such as the dust loading of the surface, the preceding dry period, wind speed and the volume of traffic.
Combustion-related Particles. Major sources of combustion-related particles include diesel motor vehicles, residential wood combustion, utility and commercial boilers. Metallurgical processes and many other industrial operations also lead to the emission of particles into the atmosphere. The mode of formation and chemical composition of the particles varies greatly from one source to another.
Chemical Characteristics of PM. PM10 and PM2.5 are composed of varying proportions of six broad chemical constituents:
Re-entrained and Fugitive Dust. Fugitive dust consists mainly of oxides of aluminum, silicon, calcium, titanium, iron, and other metal oxides. The precise combination of these minerals depends on the geology of the area and the presence of industrial processes such as steel making, smelting, mining, and cement production. Re-entrained dust includes material from vehicles and roads such as brake wear, tire wear, and pavement material. Re-entrained and fugitive dust is mostly in the coarse particle fraction.
Sulfates. Ammonium sulfate, ammonium bisulfate, and sulfuric acid are the most common forms of sulfate found in atmospheric particles, resulting from conversion of SOX gases to particles. These compounds are water-soluble and reside almost exclusively in the PM2.5 size fraction.
Nitrates. Ammonium nitrate is the most abundant nitrate compound, resulting from a reversible gas/particle equilibrium between ammonia gas (NH3), nitric acid gas, and particulate ammonium nitrate. Because this equilibrium is reversible, ammonium nitrate particles can easily evaporate in the atmosphere, or after they have been collected on a filter, owing to changes in temperature and relative humidity.
Sodium Chloride. Salt is found in suspended particles near seacoasts and after deicing materials are applied. In its raw form (e.g., deicing sand), salt is usually in the coarse particle fraction and classified as a geological material. After evaporating from a suspended water droplet (as in sea salt or when re-suspended from melting snow), it is abundant in the PM2.5 fraction. Sodium chloride is often neutralized by nitric or sulfuric acid and is classified as a sulfate or nitrate.
Organic and Elemental Carbon. Particulate organic carbon consists of hundreds, possibly thousands of separate compounds that contain carbon atoms. Because of this lack of molecular specificity, and owing to the semi-volatile nature of many carbon compounds, particulate "organic carbon" is operationally defined by the sampling and analysis method. Elemental carbon, often called "soot,"or carbon black, contains pure, graphitic carbon, and dark-colored, non-volatile organic materials such as tar, biogenics, and coke.
PM Regulatory Background. Regulation of PM in the United States has occurred in three broad phases, beginning in 1971:
Total Suspended Particulate Standard, 1971. The first NAAQS for PM was promulgated in 1971. The standard was expressed in terms of Total Suspended Particulate (TSP), and based on a reference method for measuring attainment that used a "high-volume" sampler, which collects PM up to a nominal size of 25-45 microns. The standard for TSP was 260 mg/m3, 24-hour average, not to be exceeded more than once per year, and a 75 mg/m3 annual geometric mean.
PM10 Standard, 1987. In 1987 EPA revised the PM NAAQS following nearly a decade of debate over evidence that smaller particulates pose a greater hazard to health. The new standard replaced TSP as the reference method with a new indicator that includes only particles with an aerodynamic equivalent of less than or equal to 10 microns (PM10). As with the TSP standard, a 24-hour average, and an annual mean were established.
PM2.5 Standard, 1997. In response to evidence that morbidity and mortality effects occur at levels well below the PM10 standard, in 1997, EPA added new PM2.5 standards, set at 15 micrograms per cubic meter (mg/m3) and 65 mg/m3, respectively, for the annual and 24-hour standards. In addition, the form of the 24-hour standard for PM10 was changed. EPA is beginning to collect data on PM2.5 concentrations. Beginning in 2002, based on three years of monitoring data, EPA will designate areas as nonattainment that do not meet the new PM2.5 standards.
The planned time frame for review and implementation of the PM2.5 NAAQS is shown in Figure A-1.