Particulate Matter

Particulate matter (PM) is the general term for microscopic solid or liquid phase (aerosol) particles suspended in air. PM exists in a variety of sizes, with diameters ranging from a few angstrom units to several hundred micrometers. Particles are either emitted directly from primary sources or are formed in the atmosphere by gas‐phase reactions (secondary aerosols).

Since particle size determines how deep into the lung a particle is inhaled, there are two NAAQS for PM, PM_2.5, and PM₁₀, where the subscripts indicate particle diameter in micrometers. Particles smaller than 2.5 μm, called “fine,” are composed largely of inorganic salts (primarily ammonium sulfate and nitrate), organic species, and trace metals. Fine PM can deposit deep in the lung where removal is difficult. Particles larger than 2.5 μm are called “coarse” particles and are composed largely of suspended dust. Coarse PM tends to deposit in the upper respiratory tract, where removal is more easily accomplished. In 1997, industrial processes accounted for 42.0% of the emission rate for traditionally inventoried PM₁₀. Non‐transportation fuel combustion and transportation sources accounted for 34.9 and 23.0%, respectively. As with the other criteria pollutants, PM₁₀ concentrations and emission rates have decreased modestly due to pollution control efforts.

Coarse particle inhalation frequently causes or exacerbates upper respiratory difficulties, including asthma. Fine particle inhalation can decrease lung functions and cause chronic bronchitis. Inhalation of specific toxic substances such as asbestos, coal mine dust, or textile fibers are now known to cause specific associated cancers (asbestosis, black lung cancer, and brown lung cancer, respectively).

An environmental effect of PM is limited visibility in many parts of the United States including some national parks. In addition, nitrogen‐ and sulfur‐containing particles deposited on land increase soil acidity and alter nutrient balances. When deposited in water bodies, the acidic particles alter the pH of the water and lead to death of aquatic organisms. PM deposition also causes soiling and corrosion of cultural monuments and buildings, especially those that are made of limestone. More than 100 areas in the United States are non‐attainment for PM₁₀.

Visibility Impairment

The Clean Air Act established a national visibility protection goal of preventing any further and remedying of any existing impairment of visibility in Class I federal areas in which impairment results from anthropogenic pollution. Both NO_x and SO_x contribute to regional haze and disturbance of the biochemical cycling of other nutrients and metals in eco‐systems (The New York Times, 17 February 2018b). Nitrogen deposited on the land contributes to the land becoming nitrogen‐saturated causing more available nitrogen to run off into nearby waters leading to increased acidification of both the soils and waters. Increased nitrate nitrogen removes calcium and magnesium from soil.

Sulfates and nitrates are secondary aerosols formed from conversion of SO₂ and NO_x to particulate form. The rates of conversion in the atmosphere depends upon a number factors, including weather conditions, relative humidity, and availability of chemicals like ammonia in the atmosphere. Both sulfate (SO₄) and nitrate (NO₃) aerosols are considered to be PM_2.5. In presence or in absence of humidity, these fine aerosols in atmosphere absorb and scatter lights that can cause poor visibility in the region (Figure 5.2).

SO₂, NO_x, and Acid Deposition

Sulfur dioxide (SO₂) is the most commonly encountered of the sulfur oxide (SO_x) gases and is formed upon combustion of sulfur‐containing solid and liquid fuels (primarily coal and oil). SO_x are generated by electric utilities, metal smelting, and other industrial processes. Nitrogen oxides (NO_x) are also produced in combustion reactions; however, the origin of most NO_x is the oxidation of nitrogen in the combustion air. After being emitted, SO_x and NO_x can be transported over long distances and are transformed in the atmosphere by gas phase and aqueous phase reactions to acid components (H₂SO₄ and HNO₃). The gas phase reactions produce microscopic aerosols of acid‐containing components, while aqueous phase reactions occur inside existing particles. The acid is deposited to the Earth’s surface as either dry deposition of aerosols during periods of no precipitation or wet deposition of acid‐containing rain or other precipitation. There are also natural emission sources for both sulfur and nitrogen‐containing compounds that contribute to acid deposition. When natural sources of sulfur and nitrogen acid rain precursors are considered, the “natural” background pH of rain is expected to be about 5.0. As a result of these considerations, “acid rain” is defined as having a pH less than 5.0. Figure 5.3 shows the major environmental cause and effect steps for acidification of surface water by acid rain.

In the United States, major sources of SO₂ emissions were non‐transportation fuel combustion (72.6%), industrial processes (18.7%), transportation (5.8%), and miscellaneous (2.9%) (see Table 4.6). Fifteen US urban areas are non‐attainment for SO₂. Emissions are expected to continue to decrease as a result of implementing the Acid Rain Program established in 1997 by EPA under Title IV of the Clean Air Act. The goal of this program is to decrease acid deposition significantly by controlling SO₂ and other emissions from utilities, smelters, and sulfuric acid manufacturing plants, and by reducing the average sulfur content of fuels for industrial, commercial, and residential boilers.

There are a number of health and environmental effects of SO₂, NO_x, and acid deposition (Figure 5.4a, b). SO₂ is absorbed readily into the moist tissue lining the upper respiratory system, leading to irritation and swelling of this tissue and airway constriction. Long‐term exposure to high concentrations can lead to lung disease and aggravate cardiovascular disease. Acid deposition on soil, plants, lakes, and rivers causes destruction of forest and plants, acidification of surface water, damage vegetation, monuments, etc., especially in regions of high SO₂ concentrations and low buffering and ion exchange capacity of soil and surface water. Acidification of water can harm fish populations, by exposure to heavy metals, such as aluminum which is leached from soil. Excessive exposure of plants to SO₂ decreases plant growth and yield and has been shown to decrease the number and variety of plant species in a region (McLaughlin and Taylor 1985; USEPA 1998a).

Air Toxics

Hazardous air pollutants (HAPs), or air toxics, are airborne pollutants that are known to have adverse human health effects, such as cancer. There are over 180 chemicals identified on the Clean Air Act list of HAPs given in Section C.2 (USEPA 1998c). Examples of air toxics include the heavy metals mercury and chromium, and organic chemicals such as benzene, hexane, perchloroethylene (perc), 1,3‐butadiene, dioxins, and PAHs.

The Clean Air Act defined a major source of HAPs as a stationary source that has the potential to emit 10 T/Y of any one HAP on the list or 25 T/Y of any combination of HAPs. Examples of major sources include chemical complexes and oil refineries. The Clean Air Act prescribes a very high level of pollution control technology for HAPs called maximum achievable control technology. Small area sources, such as dry cleaners, emit lower HAP tonnages but taken together are a significant source of HAPs. Emission reductions can be achieved by changes in work practices such as material substitution and other pollution prevention strategies.

HAPs affect human health via the typical inhalation or ingestion routes. HAPs can accumulate in the tissue of fish, and the concentration of the contaminant increases up the food chain to humans. Many of these persistent and bioaccumulative chemicals are known or suspected carcinogens.