Pollutant Glossary
"Acid rain," the term for any kind of precipitation that is unusually acidic, has come to be recognized as a major environmental problem. Acid rain, snow, or dew occurs when emissions of carbon dioxide, sulfur dioxide, and nitrogen oxides react with water molecules in the atmosphere to produce acids. In Canada and Scandinavia, acidic rainfall has killed fish populations and other marine life in many small lakes.
California has considerably lower emissions of sulfur oxides than other parts of the world, so the primary source of acid rainfall here is nitric acid from automobile emissions. Measurements in California have shown periods of acidic rain in the initial stages of storms, but so far no significant impact on vulnerable mountain lakes has been observed.
This is an odorless, invisible gas that in high concentrations can affect human health. Carbon monoxide is especially dangerous indoors when ventilation is inadequate.
Almost 70 percent of the Bay Area's carbon monoxide comes from motor vehicles. A substantial amount also comes from burning wood in fireplaces and woodstoves. State and federal controls on new cars and voluntary efforts to reduce woodburning have been implemented to prevent carbon monoxide from reaching harmful levels. The Bay Area has not exceeded the national or state standard for carbon monoxide for several years and is now formally recognized as an attainment area for CO.
A colorless gas with a strong "rotten egg" odor, hydrogen sulfide can be smelled at very low concentrations. It discolors paints and tarnishes many metals. This gas is produced largely at sewage treatment plants and at oil refineries as a by-product of refining crude oil. Concentrations of H2S are limited by Air District regulations.
Air is composed of about 80 percent nitrogen. When we burn things at high temperatures, some of the nitrogen in the air burns as well – combining with oxygen to form compounds such as nitric oxide (NO) and nitrogen dioxide (NO2). These mixtures are known as oxides of nitrogen and are involved in photochemical reactions that produce ozone. (See section on ozone below.)
At concentrations experienced in the Bay Area, nitrogen dioxide appears as a brown haze. On days with otherwise good visibility, the coloration effects will be noticeable. At higher concentrations, nitrogen dioxide may damage sensitive crops such as beans and tomatoes and cause pulmonary changes in laboratory animals. The U.S. Environmental Protection Agency, California Air Resources Board, and the Air District have all adopted measures to curtail emissions of nitrogen oxides. The Air District directly controls power plants, boilers, stationary turbines, and stationary engines that are sources of these pollutants. It indirectly addresses vehicular sources of NOx by working to change people's driving habits.
Organic gases, or hydrocarbons, are released in burning fuels or organic waste materials. These compounds are the result of incomplete combustion, and range in complexity from methane, a simple organic gas, to much more complex molecules containing carbon, hydrogen, and oxygen in varying proportions. Organic compounds are also emitted by consumer products such as aerosol sprays and by evaporation of paints, inks, solvents, and gasoline. Some organic compounds are not emitted directly, but are products of other compounds that are oxidized by the atmosphere.
Organic compounds are significant air pollutants because they react with oxides of nitrogen in the presence of sunlight to produce photochemical smog, or ozone. The Air District has adopted more than fifty rules to directly control organic compounds from operations such as petroleum production, refining and marketing; various coating operations; and semiconductor manufacturing.
In addition to their role in creating smog, some organic compounds such as benzene, 1,3-butadiene, formaldehyde, and acrolein are toxic. Health risks posed by these compounds include cancer risks; chronic, non-cancer risks, such as diseases of the lungs, liver, and kidneys; and acute risks, such as eye and respiratory irritations. The Air District has initiated the Community Air Risk Evaluation, or CARE, program to identify locations with high levels of toxic emissions and sensitive populations, and is using this information to reduce emissions in those areas.
Dust, mist, ash, smoke, and fumes are some of the liquid or solid particles found in the atmosphere. In many parts of the world, natural particles like dust and pollens are the principal source of air pollution, while in industrialized regions, most particulate matter comes from human activity. Some types of particulate matter are more toxic than others.
Smoke – composed of carbon and other products of incomplete combustion – is the most obvious form of particulate pollution. Wood burning, open fires, incinerators, petroleum refininery flares, and fuel burning in vehicles and aircraft all produce these highly visible particulates.
Industrial processes such as those used in refining crude oil and manufacturing chemicals also contribute to particulate formation. Liquid aerosols and solid particles form photochemically in the atmosphere when sunlight reacts with waste gases. When metals are melted, the heated material emits fumes that may condense to form metallic oxides in the atmosphere. Industrial dust is formed by grinding or pulverizing materials, as in cement production. Earth-moving operations, especially farming and construction, also cause large amounts of dust to enter into the air.
Studies of exposed workers have shown that particles from diesel combustion engines are highly carcinogenic, prompting tighter regulation of diesel-powered trucks, ships, trains, and construction equipment. High concentrations of diesel particulate matter have been found in communities near freeways and ports. Government agencies including the Air District are taking regulatory steps and using grant programs to reduce particulate exposure in these communities.
The Air District also passed its Wood Burning Rule to reduce pollution from the 1.4 million fireplaces and wood stoves, which collectively account for about one-third of the wintertime fine particle pollution.
Photochemical ozone—commonly referred to as "smog"—results from a chemical reaction in the atmosphere between sunlight and ozone precursors (reactive organic gases and oxides of nitrogen). Various factors affect this process, including the quantity of gases present, the volume of air available for dilution, the temperature, and the intensity of the ultraviolet light. Ozone is most likely to form in the summer and early fall on warm, windless, sunny days. The major effects of photochemical ozone or smog are aggravation of respiratory diseases, eye irritation, visibility reduction and vegetation damage.
Motor vehicles are the greatest contributor to ozone in the Bay Area, accounting for more than 50 percent of the reactive organic gases and nitrogen oxides in the region. California's motor vehicle emissions control program, together with the Air District's regulatory controls, have reduced exceedances of the national ozone standard from a high of 65 days in 1969 to just four days in 2011.
For additional information, see A Day in the life of Ozone.
Heating and burning "fossil fuels" such as coal and oil release the sulfur present in these materials. In areas where large quantities of fossil fuels are used, sulfur oxides can be a major air pollution problem. The most common kind of sulfur oxide is sulfur dioxide (SO2). This substance often further oxidizes to form sulfur trioxide (SO3), which in the presence of moisture can form sulfuric acid mist (H2SO4). These contaminants can damage vegetation and affect the health of both humans and animals.
In the past, sulfur oxides were a problem in the Bay Area, especially in the vicinity of the large oil refineries and chemical plants in Contra Costa County. The Air District has been controlling emissions from these sources since 1961, however, and no state or federal excesses of sulfur compound emissions have been recorded at Air District monitoring stations since 1976.
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