INHALABLE PARTICLES (SULFATES)

Airborne particles in the air that can be aspirated into the nose or mouth during normal breathing are known as "inhalable particles." Inhalability decreases gradually with increasing particle diameter, reaching a level of about 50 percent at 100 microns (µm). "Subfractions" of the inhalable fraction are the particles that can penetrate through the upper respiratory tract (head airways) and enter the thoracic airways (lower respiratory tract). The aerodynamic particle diameter for 50 percent penetration into the thorax is 10 µM, and air samplers that mimic this penetration are used to determine PM10 (particulate matter of 10 micrometers [microns] or less in diameter), or thoracic particulate matter standards. There is a smaller cut-size for 50 percent penetration through the conductive airways of the tracheobronchial tree that distributes the inhaled air to the gas-exchange airways in the lungs. By internationally agreed-upon convention among occupational health professionals, this fraction is known as "respirable particulate matter," and the cut-size is 4 µm. Size-selective sampler inlets for inhalable, thoracic, and respirable dust prevent the oversized particles from reaching the sampling filters used to determine the mass concentrations of the overall sample or specific chemical constituents within the sample.

For particulate matter in community air in the United States, the thoracic particulate matter (PM10) is, by regulatory specification, divided into fine particulate matter (that fraction penetrating through an inlet with a 50 percent cut-size at 2.5 µm [PM2.5], and the PM10 coarse fraction [PM10–2.5]). This particular size discrimination was a recognition that fine and coarse particles generally have distinct sources and formation mechanisms. Primary fine particles are formed from condensation of high-temperature vapors during combustion. Secondary fine particles are usually formed from gases in three ways: 1) nucleation (e.g., gas molecules coming together to form a new particle); (2) condensation of gases onto existing particles; and (3) by reaction of absorbed gases in liquid droplets. Particles formed from nucleation also coagulate to form relatively larger aggregate particles or droplets with diameters between 0.1 and 1.0 µm, and such particles normally do not grow into the coarse mode.

Particles also form as a result of the chemical reaction of gases in the atmosphere. Some examples include: (1) the conversion of sulfur dioxide (SO₂) to sulfuric acid droplets (H₂SO₄); (2) reactions of H₂SO₄ with ammonia (NH₃) to form ammonium bisulfate (NH₄HSO₄) and ammonium sulfate [(NH₄)₂SOM₄]; and (3) conversion of nitrogen dioxide (NO₂) to nitric acid vapor (HNO₃), which reacts further with NH₃ to form particulate ammonium nitrate (NH₄NO₃).

By contrast, most of the coarse fraction particles are emitted directly as particles and result from mechanical disruption such as crushing, grinding, evaporation of sprays, or suspensions of dust from construction and agricultural operations. Basically, most coarse particles are formed by breaking up bigger masses into smaller ones. Energy considerations normally limit coarse particle sizes to greater than 1.0 µm in diameter. Some combustion-generated mineral particles, such as fly ash, are also found in the coarse fraction. Biological material such as bacteria, pollen, and spores may also be found in the coarse mode. As a result of the fundamentally different chemical compositions and sources of fine and coarse fraction particles, the chemical composition of the sum of these two fractions, PM10, is more heterogeneous than either mode alone.

Fine accumulation–mode particles typically have longer atmospheric lifetimes (e.g., days to weeks) than coarse particles, and they tend to be more uniformly dispersed across an urban area or large geographic region, especially in the eastern United States. Larger particles generally deposit more rapidly than small particles, and as a result, total coarse-particle mass will be less uniform in concentration across a region than are fine particles.

In the United States, the Environmental Protection Agency (EPA) administrator promulgated revised PM NAAQS in July 1997 in recognition of the inadequate public health protection provided by enforcement of the 1987 NAAQS for PM10. For PM10, the annual average was retained without change, and the twenty-four-hour PM10 was effectively relaxed by permitting more exceedences each year. These PM10 standards were supplemented by the creation of new PM2.5 standards. Implementation of the new PM2.5 NAAQS will advance the degree of public health protection for ambient air particulate matter, especially in the eastern United States and in some large cities in the West where fine particles make up a major percentage of PM10.

Morton Lippmann

(see also: Airborne Particles; Ambient Air Quality [Air Pollution]; Clean Air Act; Environmental Protection Agency; Hazardous Air Pollutants; Smog [Air Pollution]; Sulfur-Containing Air Pollutants [Particulates]; Total Suspended Particles [TSP] )