APCC - Types of Air Pollution and their Sources (Lesson)
Types of Air Pollution and their Sources
People have euphemisms for smog; sometimes it's fog, sometimes it's haze. It's hard to know sometimes whether the air is full of something natural, like water vapor, or something man-made, like ozone. But in cities like this, the air is often being marred by air pollution.
Air Quality
Pollutants include materials that are naturally occurring but are added to the atmosphere so that they are there in larger quantities than normal. Pollutants may also be human-made compounds that have never before been found in the atmosphere. Pollutants dirty the air, change natural processes in the atmosphere, and harm living things.
Problems with Air Quality
Air pollution started to be a problem when early people burned wood for heat and cooking fires in enclosed spaces such as caves and small tents or houses. But the problems became more widespread as fossil fuels such as coal began to be burned during the Industrial Revolution.
Smog
Air pollution became a crisis in 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.
In London in December 1952, the "Big Smog" killed 4,000 people over five days. Many thousands more likely died of health complications from the event in the next several months.
Photochemical Smog
Photochemical smog, a different type of air pollution, first became a problem in Southern California after World War II. The abundance of cars and sunshine provided the perfect setting for a chemical reaction between some of the molecules in auto exhaust or oil refinery emissions and sunshine (see image).
Photochemical smog consists of more than 100 compounds, most importantly ozone.
Smog over Los Angeles as viewed from the Hollywood Hills
The Clean Air Act
The terrible events in Pennsylvania and London, plus the recognition of the hazards of photochemical smog, led to the passage of the Clean Air Act in 1970 in the United States. The act now regulates 189 pollutants. The six most important pollutants regulated by the Act 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.
What is the result of the Clean Air Act? In short, the air in the United States is much cleaner. Visibility is better and people are no longer incapacitated by industrial smog. However, despite the Act, industry, power plants, and vehicles put 160 million tons of pollutants into the air each year. Some of this smog is invisible and some contribute to the orange or blue haze that affects many cities.
Regional Air Quality
Air quality in a region is not just affected by the number of pollutants released into the atmosphere in that location but by other geographical and atmospheric factors. Winds can move pollutants into or out of a region and a mountain range can trap pollutants on its leeward side. Inversions commonly trap pollutants within a cool air mass. 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.
The table below lists the smoggiest cities in 2011: eight of the 10 are in California. Why do you think California cities are among those with the worst air pollution?
The state has the right conditions for collecting pollutants including mountain ranges that trap smoggy air, arid and sometimes windless conditions, agriculture, industry, and lots and lots of cars.
Smoggiest U.S. Cities, 2011
|
Rank |
City, State |
|
1 |
Los Angeles, California |
|
2 |
Bakersfield, California |
|
3 |
Visalia-Porterville, California |
|
4 |
Fresno, California |
|
5 |
Sacremento, California |
|
6 |
Hanford, California |
|
7 |
San Diego, California |
|
8 |
Houston, Texas |
|
9 |
Merced, California |
|
10 |
Charlotte, North Carolina |
In the picture above, the smoke is stuck between two layers of air. The bottom layer is denser than the top layer, so there is no mixing between the two layers. In winter, an (atmospheric) inversion traps all of the pollutants that are emitted into the air over a region.
Types of Air Pollution
The two types of air pollutants are primary pollutants, which enter the atmosphere directly, and secondary pollutants, which form from a chemical reaction.
1. Primary Pollutants
Some primary pollutants are natural, such as volcanic ash. Dust is natural but exacerbated by human activities; for example, when the ground is torn up for agriculture or development. Most primary pollutants are the result of human activities, direct emissions from vehicles, and smokestacks. Primary pollutants include:
- Carbon oxides include carbon monoxide (CO) and carbon dioxide (CO2) (see Figure below). Both are colorless, odorless gases. CO is toxic to both plants and animals. CO and CO2 are both greenhouse gases.
High CO2 levels are found in major metropolitan areas and along the major interstate highways.
- 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 (NO2) are greenhouse gases. Nitrogen oxides contribute to acid rain.
- Sulfur oxides include sulfur dioxide (SO2) and sulfur trioxide (SO3). These form when sulfur from burning coal reaches the air. Sulfur oxides are components of acid rain.
- Particulates are solid particles, such as ash, dust, and fecal matter. They are commonly formed from the combustion of fossil fuels and can produce smog. Particulates can contribute to asthma, heart disease, and some types of cancers.
Particulates from a brush fire give the sky a strange glow in Arizona.
- Lead was once widely used in automobile fuels, paint, and pipes. This heavy metal can cause brain damage or blood poisoning.
- Volatile organic compounds (VOCs) are mostly hydrocarbons. Important VOCs include methane (a naturally occurring greenhouse gas that is increasing because of 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).
2. Secondary Pollutants
Any city can have photochemical smog, but it is most common in sunny, dry locations. A rise in the number of vehicles in cities worldwide has increased photochemical smog. Nitrogen oxides, ozone, and several other compounds are some of the components of this type of air pollution.
Photochemical smog forms when car exhaust is exposed to sunlight. Nitrogen oxide is created by gas combustion in cars and then into the air. In the presence of sunshine, the NO2 splits and releases an oxygen ion (O). The O then combines with an oxygen molecule (O2) to form ozone (O3). This reaction can also go in reverse: Nitric oxide (NO) removes an oxygen atom from ozone to make it O2. The direction the reaction goes depends on how much NO2 and NO there is. If NO2 is three times more abundant than NO, ozone will be produced. If nitric oxide levels are high, ozone will not be created.
The brown color of the air behind the Golden Gate Bridge is typical of California cities, because of nitrogen oxides.
Ozone is one of the major secondary pollutants. It is created by a chemical reaction that takes place when exhaust is in the presence of sunlight. The gas is acrid-smelling and whitish. 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. Ozone is also a greenhouse gas.
How come we don't see emissions like this too often anymore?
This photo of a power plant was taken before emission control equipment was added. Emissions are down since laws have been enacted to protect the air.
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.
1. Fossil Fuels
Fossil fuels are burned in most motor vehicles and power plants. These nonrenewable resources are the power for nearly all manufacturing and other industries. Pure coal and petroleum can burn cleanly and emit only carbon dioxide and water, but most of the time these fossil fuels do not burn completely and incomplete chemical reactions produce pollutants. Few sources of these fossil fuels are pure, so other pollutants are usually released. These pollutants include carbon monoxide, nitrogen dioxide, sulfur dioxide, and hydrocarbons.
In large car-dependent cities such as Los Angeles and Mexico City, 80% to 85% of air pollution is from motor vehicles. Ozone, carbon monoxide, and nitrous oxides come from vehicle exhaust. Auto exhaust, like the one in the picture to the right, means that the fuel is not burning efficiently.
Watch below to see the relative amounts of CO2 released by different fossil fuels.
A few pollutants come primarily from power plants or industrial plants that burn coal or oil. Sulfur dioxide (SO2) is a major component of industrial air pollution that is released whenever coal and petroleum are burned. SO2 mixes with H2O in the air to produce sulfuric acid (H2SO4).
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.
2. Biomass Burning
Fossil fuels are ancient plants and animals that have been converted into usable hydrocarbons. Burning plant and animal material directly also produce 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 for 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, such as the savannah, is also burned to clear farmland. The pollutants are much the same as burning fossil fuels: CO2, carbon monoxide, methane, particulates, nitrous oxide, hydrocarbons, and organic and elemental carbon. Burning forests increase greenhouse gases in the atmosphere by releasing the CO2 stored in the biomass and also by removing the forest so that it cannot store CO2 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.
3. 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 (see image). Methane occurs naturally, but human agriculture is increasing the amount of methane in the atmosphere.
Methane forms when organic material decomposes in an oxygen-poor environment. In the left image above, surface methane production is shown. Stratospheric methane concentrations in the right image above show that methane is carried up into the stratosphere by the upward flow of air in the tropics.
4. Mercury Pollution
Mercury is released into the atmosphere when coal is burned (see Figure). But breathing the mercury is not harmful. In the atmosphere, the mercury forms small droplets that are deposited in water or sediments.
Historic increases of mercury in the atmosphere as shown in the graph above:
1. blue is volcanic eruptions;
2. brown, purple, and pink are human-caused.
3. red shows the effect of industrialization on atmospheric mercury
Acid Rain
Acid rain is caused by sulfur and nitrogen oxides emanating from power plants or metal refineries. The smokestacks have been built tall so that pollutants don't sit over cities.
Tall smokestacks, like those to the right, allow the emissions to rise high into the atmosphere and travel up to 1,000 km (600 miles) downwind.
As they move, these pollutants combine with water vapor to form sulfuric and nitric acids. The acid droplets form acid fog, rain, snow, or they may be deposited dry. The most typical is acid rain (see diagram below).
Pollutants are deposited dry or in precipitation
pH and Acid Rain
Acid rainwater is more acidic than normal rainwater. Acidity is measured on the pH scale. Lower numbers are more acidic and higher numbers are less acidic (also called more alkaline). Natural rain is somewhat acidic, with a pH of 5.6; acid rain must have a pH of less than 5.0. A small change in pH represents a large change in acidity: rain with a pH of 4.6 is 10 times more acidic than normal rain (with a pH of 5.6). Rain with a pH of 3.6 is 100 times more acidic.
A pH scale goes from 0 to 14; numbers are shown with the pH of some common substances. A value of 7 is neutral. The most concentrated acids are at the low end of the scale and the most concentrated bases are at the high end.
Regions with a lot of coal-burning power plants have the most acidic rain. The acidity of average rainwater in the northeastern United States has fallen to between 4.0 and 4.6. Acid fog has an even lower pH with an average of around 3.4. One fog in Southern California in 1986 had a pH of 1.7, equal to toilet-bowl cleaner.
In arid climates, such as in Southern California, acids deposit on the ground dry. Acid precipitation ends up on the land surface and in water bodies. Some forest soils in the northeast are five to ten times more acidic than they were two or three decades ago. Acid droplets move down through acidic soils to lower the pH of streams and lakes even more. Acids strip the soil of metals and nutrients, which collect in streams and lakes. As a result, stripped soils may no longer provide the nutrients that native plants need.
Indoor Air Pollution
RESOURCES IN THIS MODULE ARE OPEN EDUCATIONAL RESOURCES (OER) OR CREATED BY GAVS UNLESS OTHERWISE NOTED. SOME IMAGES USED UNDER SUBSCRIPTION.
IMAGE OF GARGOYLE AT NOTRE DAME BY MIKE DAVIS, VIA FLICKR.