## Polar Ozone Holes

Chlorofluorocarbons have disrupted stratospheric ozone generation, resulting in a thinning of the ozone layer at the poles.

### Learning Objectives

Recall that chlorofluorocarbons produce halogen radicals that catalytically destroy ozone.

### Key Takeaways

#### Key Points

• In the stratosphere, ozone is cyclically destroyed and re-formed.
• Atomic chlorine and bromine from chlorofluorocarbons and bromofluorocarbons can catalytically disrupt the cycle of ozone formation in the stratosphere, destroying many more ozone molecules then are re-made.
• The thinning of the ozone layer has resulted in increased levels of UV radiation at the Earth’s surface, which can cause skin cancer and cataracts.

#### Key Terms

• stratosphere: the region of the uppermost atmosphere where ozone is found
• Chlorofluorocarbons: compounds that were commonly used prior to the 1980s for a number of industrial applications, including refrigerants and cleaning solutions; they can cause holes in the ozone layer
• ozone: a molecule found in the upper regions of the Earth’s atmosphere; important for blocking UV light from reaching Earth; chemical symbol O3

### The Hole in the Ozone Layer

There is a constant cycle of ozone formation and destruction in the stratospheric layer of the atmosphere. Ultraviolet light splits oxygen gas (O2) to form monatomic oxygen (O) that can react with additional oxygen gas molecules to form ozone (O3). The ozone produced can then go on to react with monatomic oxygen and re-form oxygen gas.

Chlorofluorocarbons (CFCs) are compounds that were commonly used prior to the 1980s for a number of industrial applications, including refrigerants and cleaning solutions. CFCs released at the Earth’s surface migrated to the stratosphere, where they interrupted the cycle of ozone generation. Atomic chlorine radicals formed from CFCs (and bromine radicals from bromofluorocarbons, or BFCs) act as catalysts to destroy ozone. The whole process, using CFCl3 as the CFC, is outlined below:

$\text{CFCl}_3+\text{UV} \text{light}\rightarrow \text{CFCl}_2+\text{Cl}\cdot$

Ultraviolet light breaks a C-Cl bond, resulting in a highly-reactive chlorine radical (Cl$\cdot$). The chlorine radical can then react with ozone, as shown below:

$\text{Cl}\cdot + \ O_3\rightarrow \text{ClO}+\text{O}_2$

The chlorine breaks an oxygen off from ozone, producing diatomic oxygen. The resulting ClO can also destroy another ozone molecule to form more diatomic oxygen:

$\text{ClO}+\text{O}_3\rightarrow \text{Cl} \cdot + \ 2 \ \text{O}_2$

Having destroyed two ozone molecules, the chlorine radical is produced once more and can destroy more ozone molecules.

CFCs have caused a gradual decrease in ozone levels throughout the stratosphere. However, air currents have resulted in increased accumulation at the poles, leading to pronounced thinning in these regions during the spring. This thinning is commonly referred to as a hole in the ozone layer, and it allows for harmful ultraviolet radiation to reach the Earth.

Increased UV radiation has been linked to a number of negative health effects, including skin cancer and cataracts. The role of CFCs in the destruction of the ozone layer was determined in 1974, and a number of bans on CFCs were passed in the years that followed; these bans led to a decrease in the size of the holes in the ozone layer.

Ozone depletion: From September 21-30, 2006. The average area of the ozone hole was the largest ever observed, at 10.6 million square miles. There is the least ozone at the blue and purple areas, and the most at the green, yellow, and red areas.

## Volcanoes

Volcanoes can cause environmental damage due to the hot lava, ash, and gases that are released during an eruption.

### Learning Objectives

Recall that volcanoes release toxic gases, including sulfur dioxide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride.

### Key Takeaways

#### Key Points

• Volcanoes are vents on the Earth’s surface that release lava, ash, and a variety of gases including sulfur dioxide and carbon dioxide.
• The carbon dioxide released in a volcanic eruption can contribute to the greenhouse effect, while sulfur dioxide is a component in acid rain.
• Sulfur dioxide is also converted into sulfuric acid, which can reflect solar radiation and cool the atmosphere.
• Volcanoes are considered active if they have erupted in the last 10,000 years; volcanic activity is often preceded by earthquakes or the release of new gases from the volcano.

#### Key Terms

• Volcano: A rupture on the crust of a planetary mass object, such as the Earth, which allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

Volcanoes form at locations on the Earth’s surface where tectonic plates converge or diverge. They release lava, gas, and ash, acting as vents for magma chambers miles below the surface. There are approximately 1,900 active volcanoes on the Earth’s surface.

### Volcanic Eruptions

A volcanic eruption, such as that of Mount Pinatubo in the Philippines, can cause severe environmental damage.

Volcanic eruption: The 1991 eruption of Mount Pinatubo in the Philippines.

Volcanic eruptions and lava can cause immediate danger to the surrounding area. The extremely hot lava can burn houses and forests very quickly. The ash can also make air travel difficult and be transferred over long distances by the wind, sometimes causing respiratory ailments such as asthma. Volcanoes can also cause more delayed damage through the gases and particulate matter that are released into the atmosphere.

### Contribution to Air Pollution

Volcanoes can contribute to air pollution through the release of toxic gases, which can include water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride, along with trace amounts of hydrogen, carbon monoxide, and hydrogen fluoride. Carbon dioxide is a greenhouse gas, and sulfur dioxide, hydrogen chloride, and hydrogen fluoride are all components of acid rain. Sulfur dioxide is converted into aerosolized sulfuric acid, which reflects solar radiation and leads to atmospheric cooling, agricultural decline, and occasionally famine. The release of these gases has also been linked to breathing problems and nausea in communities near volcanoes, such as the Hawaiian islands.

### Volcanic Activity

Volcanoes are considered active if they have had an explosion in the last 10,000 years. They have extremely long life spans and can appear to be inactive for many years before erupting with little warning. Often, eruptions are preceded by an earthquake or the release of new gases from the volcano. When either of these warnings happen, scientists monitor the volcano more carefully and may call for an evacuation of the surrounding towns.

Volcanoes can also have beneficial effects on the surrounding environment. Volcanic rocks, for example, add a variety of nutrients to the soil, which can improve agricultural yields.

## The Greenhouse Effect

The greenhouse effect is an elevation in surface temperatures due to atmospheric gases absorbing and re-radiating thermal energy.

### Learning Objectives

Recall the causes of global warming and how humans have aggravated the phenomenon.

### Key Takeaways

#### Key Points

• Solar radiation is absorbed at the Earth’s surface and emitted as thermal energy that can be re-absorbed by greenhouse gases.
• Greenhouse gases include water vapor, carbon dioxide, ozone, and methane; the major components of atmospheric gas — nitrogen, oxygen, and argon—are transparent to infrared radiation.
• The greenhouse effect insulates the Earth’s surface, making it hospitable to life; however, human activities have increased the concentration of these gases, causing global warming.

#### Key Terms

• greenhouse gases: gases such as water vapor, carbon dioxide, ozone, and methane that absorb and trap heat as it tries to escape from the Earth’s atmosphere
• global warming: a rapid increase in temperatures; caused by greenhouse gases trapping extra heat in the atmosphere; it may produce a number of environmental consequences
• greenhouse effect: an elevation in surface temperatures due to atmospheric gases absorbing and re-radiating thermal energy

While about 30 percent of the solar radiation directed at the Earth scatters at the outer atmosphere, the remainder is either absorbed by clouds and atmospheric gases or is transmitted to the Earth’s surface. The energy that reaches the Earth’s surface is absorbed and subsequently reflected back into the atmosphere in the form of infrared radiation, as shown in the diagram below. This thermal radiation from the surface has a much longer wavelength than the solar radiation that was initially absorbed.

The greenhouse effect: A summary of the heat transfer in the Earth’s atmosphere.

The majority of gases in the atmosphere, such as nitrogen, oxygen, and argon, cannot absorb this infrared radiation. Gases known as greenhouse gases, including water vapor, carbon dioxide, ozone, and methane, absorb and trap this heat as it tries to escape from the atmosphere. Greenhouse gases then re-radiate this energy back to Earth, elevating atmospheric temperatures even when the surface is not being directly irradiated by the sun. The cloud layer can also absorb infrared radiation and contribute further to the greenhouse effect.

Without this trapping effect, it is estimated that the surface of the Earth would be approximately 30 degrees cooler than current temperatures. The greenhouse effect modulates the temperature at the Earth’s surface and makes it hospitable to life.

Human activities have increased greenhouse gas concentrations n the atmosphere through the combustion of fossil fuels, release of methane from farms, industrial emissions, and deforestation. This increase in greenhouse gases is producing the phenomenon known as global warming, a rapid increase in atmospheric temperatures that may produce a number of environmental consequences, such as more extreme weather.

## Photochemical Smog

Photochemical smog forms when primary pollutants react with ultraviolet light to create a variety of toxic and reactive compounds.

### Learning Objectives

Recall what causes photochemical smog and why it is a problem for humans

### Key Takeaways

#### Key Points

• Photochemical smog is composed of primary and secondary pollutants.
• Primary pollutants, which include nitrogen oxides and volatile organic compounds, are introduced into the atmosphere via vehicular emissions and industrial processes.
• Secondary pollutants, like ozone, result from the reaction of primary pollutants with ultraviolet light.
• Photochemical smog is most common in sunny and dry cities, like Los Angeles.
• Smog has a variety of negative health impacts.

#### Key Terms

• volatile: evaporating or vaporizing readily under normal conditions; having a low boiling point
• Photochemical smog: a type of air pollution formed through solar radiation reacting with airborne pollutants, like nitrogen oxides and volatile organic compounds
• monatomic: a substance consisting of a single atom (not molecules of the element); examples include the noble gases and many metals

Photochemical smog is a major contributor to air pollution. The word “smog” was originally coined as a mixture of “smoke” and “fog” and was historically used to describe air pollution produced from the burning of coal, which released smoke and sulfur dioxide. Nineteenth and 20th century London was particularly well-known for this type of air pollution. The “Great Smog of 1952” was identified as the cause of over 4,000 deaths in London. While air pollution caused by burning coal has become less common, the combustion of fossil fuels continues to affect air quality.

Smog in London during the 19th and 20th centuries: Claude Monet made several trips to London between 1899 and 1901, during which he painted views of the Thames and Houses of Parliament which show the sun struggling to shine through London’s smog-laden atmosphere.

### What Causes Photochemical Smog?

The components of photochemical smog were established during the 1950s. This type of air pollution is formed through the reaction of solar radiation with airborne pollutants like nitrogen oxides and volatile organic compounds. These compounds, which are called primary pollutants, are often introduced into the atmosphere through automobile emissions and industrial processes. Ultraviolet light can split nitrogen dioxide into nitric oxide and monatomic oxygen; this monatomic oxygen can then react with oxygen gas to form ozone. Products like ozone, aldehydes, and peroxyacetyl nitrates are called secondary pollutants. The mixture of these primary and secondary pollutants forms photochemical smog.

Both the primary and secondary pollutants in photochemical smog are highly reactive. These oxidizing compounds have been linked to a variety of negative health outcomes; ozone, for example, is known to irritate the lungs. Smog is a particular health danger in some of the world’s sunniest and most populated cities, such as Los Angeles; Los Angeles is typically sunny, and the sun reacts with the chemicals produced by cars and other industrial processes. Smog can also affect areas of the country that are sunny less frequently, such as New York City. In fact, most major cities have problems with smog and air pollution.

Smog in New York City: Photochemical smog is composed of primary and secondary pollutants. Primary pollutants include nitrogen oxides and volatile organic compounds as a result of industrial processes, while secondary pollutants are created through the reaction of primary pollutants with ultraviolet light.

## Indoor Pollution: Radon

Radon gas, the result of radium’s radioactive decay, can severely compromise indoor air quality.

### Learning Objectives

Recall the process by which radon is produced, why it is dangerous to humans, and how to prevent radon poisoning

### Key Takeaways

#### Key Points

• Radon is a colorless, odorless gas, the primary source of indoor air pollution.
• Radon sinks in air because it has a high density; it is therefore often found in the basements of homes, particularly in areas where with a lot of shale and boulders in the soil.
• Radon results from the radioactive decay of radium in the soil, and it further decays to produce radioactive daughters including polonium and lead.
• Radon gas, along with decay products that can attach to dust and airborne particles, enters the lungs and decays, producing alpha and beta radiation that damages DNA and causes lung cancer.

#### Key Terms

• alpha: a large, positive particle; the same size and charge as a helium nucleus; the result of a nuclear reaction or nuclear decay
• beta particle: an energetic electron or positron produced as the result of a nuclear reaction or nuclear decay

A variety of contaminants can affect the quality of indoor air and the health of the people who inhabit that space. One serious source of indoor air pollution is radon. Radon is a dense, colorless, odorless noble gas that occurs naturally in the soil as the product of the radioactive decay of radium; it is a decay product of uranium and thorium, which occur naturally in the Earth’s crust.

Radon gas: A gold tube filled with radon gas.

### Sources of Radon

Radon has a variety of sources, including uranium, and contains rocks like granite, shale, phosphate rock, and pitchblende. Radon can escape from these sources and migrate into the surrounding air and water supplies. It can be found in well water, natural gas sources, and building materials. Radon sources are found throughout the United States, in houses, schools, and businesses that have been constructed on top of radon-rich soil. Due to its heavy density, radon typically floats downward and is often found in the basements of buildings.

### Health Effects

Radon is a major source of indoor air pollution and is the cause of tens of thousands of deaths annually in the United States and Europe. Radon decays to form daughters, or decay products, which include radioactive polonium, lead, and bismuth. Radon is a gas, but these decay products are solids that can attach to dust and enter the lungs. Radon and its daughters continue to decay in the lungs, releasing alpha and beta particles that can damage cellular DNA and result in lung cancer. Radon and its daughters are the leading cause of lung cancer in non-smokers.

### Detection and Prevention

Radon levels can be tested through a number of available assays, and contamination can be addressed by sealing basements and cellars to prevent the exchange of gas with the surrounding soil or by increasing ventilation. Many states require radon testing before selling a house.

## Indoor Pollution: CO and CO2

Carbon monoxide and carbon dioxide are products of combustion reactions; in large amounts, carbon monoxide can cause suffocation.

### Learning Objectives

Recall how and why carbon monoxide poisoning occurs and why it can be fatal.

### Key Takeaways

#### Key Points

• Combustion reactions can produce both carbon monoxide and carbon dioxide, depending on the availability of oxygen.
• Inhaled carbon monoxide competes with oxygen to bind to hemoglobin, resulting in oxygen deprivation to the organs, which can lead to death.
• Carbon dioxide is the product of human metabolic activity and is more benign than carbon monoxide; it has few negative health effects.
• Carbon monoxide poisoning can be prevented by installing carbon monoxide detectors that can detect unsafe levels of the toxic gas.

#### Key Terms

• hemoglobin: the protein in red blood cells that transports oxygen from the lungs to the rest of the body; carbon monoxide can bind to it instead of oxygen

### Carbon Monoxide v. Carbon Dioxide

Carbon monoxide (CO) and carbon dioxide (CO2) are both colorless, odorless gases.

Carbon monoxide: The space-filling model of carbon monoxide.

Carbon dioxide: The space-filling model of carbon monoxide.

### Sources of Carbon Dioxide and Carbon Monoxide

Carbon dioxide and carbon monoxide are the products of combustion reactions, such as the burning of coal, wood, and natural gas, or the use of modern combustion engines (most frequently used in cars). When a hydrocarbon is burned in air, it produces carbon dioxide and water vapor. Carbon monoxide is often the product of incomplete combustion reactions. In the presence of adequate oxygen, combustion reactions will usually produce carbon dioxide. Most carbon dioxide found indoors is a result of human activity and exhalation. Carbon dioxide is also produced during cellular respiration, when humans turn glucose and oxygen into energy, water, and carbon dioxide.

### Health Effects

Carbon monoxide is highly toxic. In a number of countries, carbon monoxide is a major source of air poisoning. Carbon monoxide molecules compete with oxygen to bind to hemoglobin; as a result, not enough oxygen can bind to the hemoglobin. Furthermore, carbon monoxide molecules will not detach from hemoglobin, leaving them bound to the protein for long periods of time. When oxygen can no longer circulate in the bloodstream (because most or all of the body’s hemoglobin are bound to carbon monoxide), nausea and faintness result, followed by organ failure and potentially death.

Carbon dioxide has few negative health effects relative to carbon monoxide; it has been linked to drowsiness in crowded buildings and is thought to impede education in overcrowded schools.

### Prevention

Carbon monoxide is particularly dangerous because it is odorless; carbon monoxide sensors can be used to alert inhabitants to rising levels of this toxic gas, however. Bans on indoor smoking are also resulting in lower indoor levels of carbon monoxide. Proper air circulation is important to prevent the accumulation of this harmful gas. Ensuring the correct installation and ventilation of heaters, furnaces, and chimneys can also prevent the buildup of carbon monoxide. Older cars can also produce carbon monoxide. Leaving a car running in a closed garage can result in potentially lethal levels of carbon monoxide; more recently-built cars do not produce dangerous levels of carbon monoxide, however, even when in an inclosed space.

## Indoor Pollution: Formaldehyde

Formaldehyde, the simplest aldehyde, is a common component in building and household materials, and is highly toxic.

### Learning Objectives

Recall formaldehyde’s structure, why it is dangerous to humans, and what can be done to lessen exposure to it.

### Key Takeaways

#### Key Points

• Formaldehyde is a colorless, pungent gas that is highly irritating to the eyes, nose, and lungs.
• Formaldehyde, the simplest aldehyde, is used to manufacture of a number of household and building materials, particularly polymers.
• Indoor pollution levels improve through increased ventilation and the removal of furniture or building materials with high levels of formaldehyde.

#### Key Terms

• aldehyde: any of a large class of reactive organic compounds (R·CHO) with a carbonyl functional group attached to a hydrogen atom and any other organic molecule group (R)

### Formaldehyde

Formaldehyde is a colorless gas with a characteristic strong odor.

Formaldehyde: This is the chemical structure of formaldehyde.

Formaldehyde is the simplest aldehyde, meaning that the aldehyde group (CHO) is attached to one more carbon. Formaldehyde is used to manufacture building materials and household items. It is also produced during some combustion reactions, is found in exhaust gas, and is a precursor for synthesizing a number of organic compounds, particularly polymers.

Formaldehyde has a range of uses, including as a fabric stiffener in wrinkle-free clothing and as a component of insulation. It is commonly found in pressed wood products like plywood, particleboard, and furniture made from these materials. Formaldehyde is frequently used in adhesives and as a strengthening agent in paper products like facial tissues. It can also be used for its antibacterial and antifungal properties and is frequently employed as an embalming or preserving agent for biological samples.

### Health Effects

Formaldehyde can irritate the lungs, eyes, and nose, and exposure causes headaches and nausea; effects are particularly harmful on individuals with asthma, and childhood exposure has been linked to the development of asthma. Formaldehyde can also produce allergic reactions and has been classed as a probable carcinogen by the Environmental Protection Agency. At higher concentrations, it is highly toxic to humans.

### Prevention

Good ventilation and restricting the use of household and building materials with high levels of formaldehyde can effectively prevent the formaldehyde’s negative health impacts. The Department of Housing and Urban Development has been regulating the levels of formaldehyde in the pressed wood materials since 1985.