Course: BIS 105 Environmental Biology

Unit 12 Learning Module

Atmospheric Science (5/7)

Air Quality

Air pollution problems began centuries ago when fossil fuels began to be burned for heat and power. The problem grew into a crisis in the developed nations in the mid-20th century. Coal smoke and auto exhaust combined to create toxic smog that in some places caused lung damage and sometimes death. In Donora, Pennsylvania in October 1948, 20 people died and 4,000 became ill when coal smoke was trapped by an inversion. Even worse, in London in December 1952, the "Big Smoke" killed 4,000 people over five days, and it is likely that thousands more died of health complications from the event in the next several months. This type of pollution is known as industrial smog.

Noon in Donora, Pennsylvania.

A different type of air pollution became a problem in Southern California after World War II. Although there was no coal smoke, cars and abundant sunshine produced photochemical smog. This smog is the result of a chemical reaction between some of the molecules in auto exhaust or oil refinery emissions, and sunshine. Photochemical smog consists of more than 100 compounds, most importantly ozone.

Smog over Los Angeles as viewed from the Hollywood Hills.

In the United States, these events led to the passage of the Clean Air Act in 1970. The act now regulates 189 pollutants. The six most important pollutants (known as criteria pollutants) are ozone, particulate matter, sulfur dioxide, nitrogen dioxide, carbon monoxide, and the heavy metal lead. Other important regulated pollutants include benzene, perchloroethylene, methylene chloride, dioxin, asbestos, toluene, and metals such as cadmium, mercury, chromium, and lead compounds.

Besides human-caused emissions, air quality is affected by environmental factors. A mountain range can trap pollutants on its leeward side. Winds can move pollutants into or out of a region. Pollutants can become trapped in an air mass as a temperature inversion traps cool air beneath warm air. If the inversion lasts long enough, pollution can reach dangerous levels. Pollutants remain over a region until they are transported out of the area by wind, diluted by air blown in from another region, transformed into other compounds, or carried to the ground when mixed with rain or snow.

As a result of the Clean Air Act, air in the United States is much cleaner. Visibility is better and people are no longer incapacitated by industrial smog. Still, in the United States, industry, power plants and vehicles put 160 million tons of pollutants into the air each year. Some of this smog is invisible and some contributes to the orange or blue haze that affects many cities.

Rank | Metropolitan Area | Population | Smog Days in 2010 | Red-Alert Days
1 | Riverside-San Bernardino, CA | 3,254,821 | 110 | 24
2 | Visalia-Tulare-Porterville, CA | 368,021 | 78 | 6
3 | Bakersfield, CA | 661,645 | 69 | 10
3 | Los Angeles-Long Beach, CA | 9,519,338 | 69 | 3
5 | Fresno, CA | 922,516 | 42 | 6
6 | Baltimore, MD | 2,552,994 | 33 | 6
6 | Washington, DC-MD-VA-WV | 4,923,153 | 33 | 3
8 | Philadelphia, PA-NJ | 5,100,931 | 29 | 3
9 | Houston, TX | 4,177,646 | 27 | 1
10 | Atlanta, GA | 4,112,198 | 23 | 1
10 | Sacramento, CA | 1,628,197 | 23 | 4

The above table lists the smoggiest cities in 2010: six of the 10 are in California. The state has the right conditions for collecting pollutants including mountain ranges that trap smoggy air, arid and sometimes windless conditions, and lots and lots of cars.

Types of Air Pollution

Most air pollutants enter the atmosphere directly; these are primary pollutants. Primary pollutants include toxic gases, particulates, compounds that react with water vapor to form acids, heavy metals, ozone, and greenhouse gases. Secondary pollutants become pollutants only after undergoing a chemical reaction. Ozone is one of the major secondary pollutants. It is created by a chemical reaction that takes place in exhaust and in the presence of sunlight.

Primary Pollutants

Some primary pollutants are natural, such as dust and volcanic ash, but most are caused by human activities. Primary pollutants are direct emissions from vehicles and smokestacks. Some of the most harmful pollutants that go directly into the atmosphere from human activities include:

- Carbon oxides include carbon monoxide (CO) and carbon dioxide (CO_2). Both are colorless, odorless gases. CO is toxic to both plants and animals. CO and CO_2 are both greenhouse gases.

- Nitrogen oxides are produced when nitrogen and oxygen from the atmosphere come together at high temperatures. This occurs in hot exhaust gas from vehicles, power plants or factories. Nitrogen oxide (NO) and nitrogen dioxide (NO_2) are greenhouse gases. Nitrogen oxides contribute to acid rain.

- Sulfur oxides include sulfur dioxide (SO_2) and sulfur trioxide (SO_3). These form when sulfur from burning coal reaches the air. Sulfur oxides are components of acid rain.

- Particulate matter is solid or liquid particles that are small enough to be suspended in the atmosphere, such as ash, dust and fecal matter. They are commonly formed from combustion of fossil fuels, and can produce smog. In addition, particulate matter can contribute to asthma, heart disease, and some types of cancers. The EPA identifies particulate matter based on the size; PM_10 are particles less than 10 microns, and PM_2.5 are less than 2.5 microns. Road dust is the largest source of PM_10 and most PM_2.5 is the result of fires.

- Lead was once widely used in automobile fuels, paint, and pipes. This heavy metal causes can cause brain damage or blood poisoning.

- Volatile organic compounds (VOCs) are mostly hydrocarbons, compounds made of hydrogen and carbon. Important VOCs include methane (a naturally occurring greenhouse gas that is increasing due to human activities), chlorofluorocarbons (human-made compounds that are being phased out because of their effect on the ozone layer), and dioxin (a byproduct of chemical production that serves no useful purpose, but is harmful to humans and other organisms).

Photochemical Smog

Any city can have photochemical smog, but it is most common in arid locations. A rise in the number of vehicles in cities worldwide has increased photochemical smog. This smog forms when car exhaust is exposed to sunlight. Nitrogen oxides are created in car combustion chambers. If there is sunshine, the NO_2 splits and releases an oxygen atom (O). The oxygen ion then combines with an oxygen molecule (O_2) to form ozone (O_3). This reaction can also go in reverse: Nitric oxide (NO) removes an oxygen atom from ozone to make it O_2. The direction the reaction proceeds depends on how much NO_2 and NO there is. If NO_2 is three times more abundant than NO, ozone will be produced. If nitrous oxide levels are high, ozone will not be created.

Ozone is an acrid-smelling, whitish gas. Warm, dry cities surrounded by mountains, such as Los Angeles, Phoenix, and Denver, are especially prone to photochemical smog. Photochemical smog peaks at midday on the hottest days of summer. Other compounds in addition to ozone are found in photochemical smog. Ozone is also a greenhouse gas.

Number of People Living in Counties with Air Quality Concentrations Above the Level of the National Ambient Air Quality Standards (NAAQS) in 2010

Indoor Air Pollution

Air quality indoors is also a problem and there can be different chemicals that are of concern. Oftentimes the concentrations of these pollutants are greater indoors than outdoors. In addition, a typical American spends over 90% of their time indoors. The World Health Organization estimates 2.4 million deaths occur worldwide each year due to air pollution, with 1.5 million of these deaths attributed to indoor air pollution.

Indoor air quality concerns differ in developed and developing nations. In developing nations, cooking and heating often rely on wood, animal dung, or crop wastes as energy sources. These homes frequently have little to no ventilation, resulting in high levels of carbon monoxide and soot. Levels of particulates are commonly found to be 20 times greater than U.S. EPA standards.

In developed countries, emissions from combustion are typically better ventilated and use cleaner burning fuels such as natural gas. The top indoor air pollution risk in developed nations is from cigarette smoke. Even those who don't smoke are at risk due to secondhand smoke. A 2006 report by the Surgeon General estimated 50,000 people die each year in the U.S. due to secondhand smoke. This is why many cities and states have banned smoking indoors in public places.

The second leading cause of lung cancer in developed nations is from radon exposure. Radon is a naturally occurring radioactive gas that is potentially found in soil and water. Because it is colorless and odorless, it can be difficult to know if a given building may have a radon problem. Radon tests from 118,447 homes across Illinois found more than 41% had radon levels above the recommended level for taking action. Fortunately, testing is relatively easy and cheap (Visit http://www.radon.illinois.gov/MeasurementLabsPriceList.asp for a list of vendors).

Other major sources of air pollution indoors are the building materials and furnishings. Many plastics, paints, and carpeting emit VOCs over time, particularly when new. The 'new car smell' is the result of VOCs evaporating from the carpeting, plastics, and foam cushioning and can be harmful to your health. Items such as photocopiers and fax machines release VOCs when they are used.

There can be biological hazards indoors as well. Mold and bacteria have become a greater problem when homes became more air-tight to reduce heating and cooling costs. While aesthetically unappealing, certain species can also be harmful and even deadly if inhaled. One example is known as Legionaires Disease, after it was first identified at an American Legion conference. The bacteria that causes this illness grows in warm, moist environments and can be found in hot tubs, air cooling towers, and hot water tanks.

Causes of Air Pollution

Most air pollutants come from burning fossil fuels or plant material. Some are the result of evaporation from human-made materials. Nearly half (49%) of air pollution comes from transportation, 28% from factories and power plants, and the remaining pollution from a variety of other sources.

Fossil Fuels

Fossil fuels are burned in most motor vehicles and power plants. They fuel manufacturing and other industries. Pure coal and petroleum can theoretically burn cleanly, emitting only carbon dioxide and water, which are both greenhouse gases. But most of the time, these fossil fuels do not completely burn, so these incomplete chemical reactions produce pollutants. In addition, few fossil fuels are pure and so other pollutants are usually released. These pollutants include carbon monoxide, nitrogen dioxide, sulfur dioxide and hydrocarbons.

A power plant and its emissions before emission control equipment was added.

In large car-dependent cities such as Los Angeles and Mexico City, 80% to 85% of air pollution is from motor vehicles. Auto emissions are the most common source of the reactants that form ozone. Carbon monoxide is toxic in enclosed spaces like tunnels. Nitrous oxides come from the exhaust from a vehicle or a factory. Lead was once put in gasoline to improve engine knock, but is now banned in the United States. Still, enormous quantities of lead are released into the air every year from other sources.

A few pollutants come primarily from power plants or industrial plants. They pour out of smokestacks that burn coal or oil. Sulfur dioxide (SO_2) is a major component of industrial air pollution. It is released whenever coal and petroleum are burned. SO_2 mixes with H_2O in the air to produce sulfuric acid (H_2SO_4). The heavy metal mercury is released when coal and some types of wastes are burned. Mercury is emitted as a gas, but as it cools, it becomes a droplet. Mercury droplets eventually fall to the ground. If they fall into sediments, bacteria convert them to the most dangerous form of mercury: methyl mercury. Highly toxic, methyl mercury is one of the metal's organic forms.

Biomass Burning

Fossil fuels are ancient plants and animals that have been converted into usable hydrocarbons. Burning plant and animal material directly also produces pollutants. Biomass is the total amount of living material found in an environment. The biomass of a rainforest is the amount of living material found in that rainforest.

The primary way biomass is burned is by slash-and-burn agriculture. The rainforest is slashed down and then the waste is burned to clear the land for farming. Biomass from other biomes, like savannah, is also burned to clear farmland. The pollutants are much the same as from burning fossil fuels: CO_2, carbon monoxide, methane, particulates, nitrous oxide, hydrocarbons, and organic and elemental carbon. Burning forests increase greenhouse gases in the atmosphere by releasing the CO_2 stored in the biomass and also by removing the forest so that it cannot store CO_2 in the future. As with all forms of air pollution, the smoke from biomass burning often spreads far and pollutants can plague neighboring states or countries.

Particulates result when anything is burned. About 40% of the particulates that enter the atmosphere above the United States are from industry and about 17% are from vehicles. Particulates also occur naturally from volcanic eruptions or windblown dust. Like other pollutants, they travel all around the world on atmospheric currents.

Evaporation

Volatile organic compounds (VOCs) enter the atmosphere by evaporation. VOCs evaporate from human-made substances, such as paint thinners, dry cleaning solvents, petroleum, wood preservatives, and other liquids. Naturally occurring VOCs evaporate off of pine and citrus trees. The atmosphere contains tens of thousands of different VOCs, nearly 100 of which are monitored. The most common is methane, a greenhouse gas. Methane occurs naturally, but human agriculture is increasing the amount of methane in the atmosphere.