Sulfate particles are sub-micron sized, sulfur-containing airborne particles. Most sulfate is a secondary pollutant, formed by the oxidation in the atmosphere of sulfur dioxide gas. Sulfur dioxide is emitted largely in fossil fuel combustion , particularly from power plants burning coal . A small fraction (generally well under 10%) of sulfur is emitted as primary sulfate at the combustion source. The use of coal-cleaning, scrubbers , and low sulfur coal have reduced sulfur dioxide emissions in the United States, and thus airborne sulfate concentrations have decreased.
In the atmosphere, sulfur dioxide (SO2) emissions are slowly transformed to sulfate (SO4) at a rate of 0.1–5% per hour, with the rate increased by higher temperatures, sunshine, and the presence of oxidants. Further reactions with water vapor may produce sulfuric acid (H2SO4), a corrosive acid which is injurious to ecosystems and humans, and also ammonium sulfate (NH4)2SO4, which is particularly effective at impairing visibility . The atmospheric residence time for sulfate particles is long, 2–10 days, which permits transport on a regional or continental scale across hundreds or thousands of miles. Because sulfate particles are relatively soluble, precipitation effectively washes them out, resulting in acid rain .Of the various sizes of aerosols, sulfate particles are in the accumulation mode, and typically there might be about 10,000 particles per 0.06 in3 (1 cm3) in an urban area.
In addition to problems of acid rain and visibility degradation, sulfate particles can cause a number of health problems. Community epidemiological studies report associations of annual and multi-year average concentrations of PM10 (particulates), PM2.5 (fine particulate ), and sulfates with health effects that include premature mortality , increased respiratory symptoms and illness (e.g., bronchitis and cough in children), and reduced lung function. The risks associated with long-term exposures, although highly uncertain, appear to be larger than those associated with short-term exposures. Other analyses have shown statistically significant associations between sulfate and other particulate air pollutants with total and cardiopulmonary mortality. Additionally, animal studies suggest that concentrations approaching ambient levels of ammonium sulfate and nitrate can cause morphometric changes that could lead to a decrease in compliance or a "stiffening" of the lung.
Because of the long time and distance scales required for transformation, and because sulfate particles are small, which allows them to remain airborne for several days, sulfate concentrations tend to be uniformly distributed across broad regions. In the eastern United States, sulfate particles constitute half or more of fine fraction particle concentrations (also called PM2.5. Thus, much of the sulfate in an urban area or airshed arises from distant or "regional" sources. If at high levels, this "background" component can pose a dilemma for air quality management, since emission reductions from local sources do not control the problem. Instead, trans-boundary agreements such as the Canada-United States Air Quality Agreement, which was signed March 13, 1991, have been used to monitor and control emissions from distant sources that cause most of the emissions.
Governmental regulations in the United States do not directly address sulfate; however, the National Ambient Air Quality Standards do set limits on particulate concentrations (PM10), and regulations on fine particulate (PM2.5) have been proposed. In addition to these ambient standards, regulations limit emissions of sulfur dioxide at sources (in the New Source Performance Standards), and the degradation of existing air quality levels (Prevention of Significant Deterioration). In Europe, deposition of acidic compounds, largely from atmospheric sulfate, is controlled by setting critical loads that depend on the capacity of a region to withstand acid inputs without lake and soil acidification .
[Stuart Batterman ]