Environmental Science: An Explorer's Guide

By Forrest M. Mims III

What interests you most about the environment? Are you concerned about water pollution? Air quality? Energy production? Forest fires? Space exploration? Your interests and questions matter.

Illustrated with more than 800 photographs, charts, and graphics, this practical guide allows you to start with your curiosity and follow your q

• Language: English
• Publisher: Influence Press
• Release date: Aug 30, 2019
• ISBN: 9781645426998

Forrest M. Mims III

Forrest M. Mims III is the bestselling author of Getting Started in Electronics and Engineer's Notebook, plus more than 1,500 columns and articles for Nature, Science, Scientific American, MAKE Magazine and many others. Mims is a Rolex Award laureate who has been assigned major scientific studies and projects by NOAA, the EPA, and NASA. Discover magazine named Mims one of the "50 Best Brains in Science."

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AIR (AER)

Air is essential to life as we know it. Air is primarily composed of nitrogen, oxygen, and very small quantities of argon, water vapor, and carbon dioxide. However, air is rarely pure, for soil, plants, animals, bodies of water, volcanoes, and human activity all add gases and particles to the air we inhale. While air is primarily found in the atmosphere, it is also present in water, plants, and soil.

Air forms Earth’s atmosphere, the thin layer of gases that blankets the planet, captured in this stunning photograph by the Expedition 28 crew of the International Space Station (ISS) (see Fig. 1).

Figure 1

The photo was made on July 31, 2011, when the ISS was 391 km (211 nautical miles) above the southern Indian Ocean. The moon appears to be resting on top of the uppermost atmosphere, which is 100 km (62 miles) above the ocean. The undulating dark band across the bottom of the photo is the tops of clouds. The orange band over the dark clouds is the top of the troposphere, the region of the atmosphere where weather occurs. The thin, dark band that caps the orange troposphere is a permanent layer of microscopic particles known as aerosols.

In this unit, we will look at these atmospheric structures and their composition. We will also examine the gases and particles that drift in and out of the atmosphere. We will see how the atmosphere and its contents are measured, both up close and from a distance. And we’ll close with a look at day-to-day weather and the long-term climate that arises from atmospheric processes. We shall do all this under the umbrella of meteorology, the scientific study of the atmosphere and its phenomena.

The Atmosphere

The atmosphere is the ocean of air in which we live. Though consisting primarily of air, the atmosphere also includes various other gases, particles, and clouds, all of which blanket the entire earth. It’s a surprisingly thin blanket, and it thins very rapidly with altitude. While the earth has a diameter of 12,756 km (7,926 miles), the densest portion of the atmosphere is only about 50 km (31 miles) thick. The thickness of the atmosphere is generally agreed to be about 100 kilometers (62.1 miles), the highest elevation that can support the flight of aircraft. The temperature of the atmosphere and the density of its air fall rapidly with altitude.

The thinness of the atmosphere with respect to the size of Earth is illustrated in this image of our planet from the DSCOVR satellite, which orbits the sun between the sun and Earth (see Fig. 2). DSCOVR is nearly 1.6 million kilometers (1 million miles) from Earth, and its EPIC camera looks at Earth as it rotates about its axis. In this image, which shows the Indian Ocean, Earth is 1,521 pixels wide when viewed in an image program. The diameter of Earth is 12,756 kilometers (7,926 miles). This means that the thickness of the atmosphere over Africa at 100 km is only 12 pixels in this image (thin red bar indicated by red arrow) or only 0.008 the diameter of Earth.

Figure 2

Though the atmosphere is ultra-thin when compared to the size of Earth, it hosts life as we know it. It also protects us from intense solar ultraviolet radiation, cosmic radiation, and the tons of micrometeoroids that bombard our planet every day.

The atmosphere contains the oxygen that is essential to life. It provides a blanket of ozone high overhead that protects us from excessive levels of ultraviolet radiation from the sun. It regulates our comfort when we are outdoors. And it provides a universal byway for everything that flies, including gnats, butterflies, bats, condors, quadcopters, and passenger jets. The atmosphere also provides a temporary reservoir for the water vapor that condenses into clouds, rain, and snow. All weather events come to us via the atmosphere, including the showers that nourish the Earth’s plants and animals, as well as catastrophic tornadoes and hurricanes. The water droplets, ice crystals, and particles in the atmosphere cause such wonderfully colorful phenomena as rainbows, coronas, and twilight glows.

Perhaps John Muir described the atmosphere best:

This grand show is eternal. It is always sunrise somewhere; the dew is never all dried at once; a shower is forever falling; vapor ever rising. Eternal sunrise, eternal sunset, eternal dawn and gloaming, on seas and continents and islands, each in its turn, as the round earth rolls. (John Muir, John of the Mountains: The Unpublished Journals of John Muir. New York: Houghton, Mifflin, 1938, p. 438.)

This glorious view of sunrise from Hawaii’s high-altitude Mauna Loa Observatory certainly affirms John Muir’s grand show (see Fig. 3).

Figure 3

Structure of the Atmosphere

The atmosphere is divided into distinct layers that are nicely illustrated in this National Oceanic and Atmospheric Administration (NOAA) poster.

While the boundary between the atmosphere and space is generally considered to be the Karman Line (100 km or 62 miles), atmospheric phenomena can occur much higher (see Fig. 4). Molecules of air can even slow the orbits of Earth satellites. But down here at the surface, we’re much more concerned about daily weather, nearly all of which occurs within the troposphere, the lowest layer in NOAA’s chart. Let’s look first at the troposphere and then the layers above it with the help of a second NOAA chart (see Fig. 5).

Figure 4

Figure 5

Boundary Layer

The boundary layer, which is not shown in the nearby chart, is the region of air closest to the surface where the air is warmed by the ground below and where the movement of air is altered by trees, hills, mountains, and even buildings. The boundary layer extends up to about 1 kilometer (3,000 feet) from the surface.

The boundary layer usually contains more dust than the air above. The dust created by this tractor near Carlsbad, New Mexico, will stay in the boundary layer before descending back to the ground unless a vigorous updraft carries it much higher (see Fig. 6).

Figure 6

Troposphere

The boundary layer is the bottom of the troposphere, which is the layer of air from the surface to 8 to 14.5 kilometers (5 to 9 miles). The troposphere is thickest near the equator and thinnest near the poles. Most of the atmosphere’s water vapor is found in the lower troposphere, and nearly all weather occurs there. The top of this post-sunset thunderstorm extends all the way to the top of the troposphere.

Figure 7

Tropopause

The boundary between the troposphere and the next layer above is called the tropopause. As noted above, its height varies with latitude. While weather ordinarily stays below the tropopause, the tops of major thunderstorms can punch through the tropopause and enter the lower stratosphere.

Stratosphere

The layer of air between the tropopause and 50 km (31 miles) above Earth’s surface is the stratosphere, a region that is much drier than the troposphere. If all the water vapor in a column through the stratosphere could be brought down to the surface, it would form a layer of liquid water only around a millimeter thin. Most of the ozone layer is found within the stratosphere, with the peak ozone concentration being from 20 to 25 km (12.4 to 15.5 miles) above the surface. Giant volcano eruptions can inject thousands of tons of sulfur dioxide and water vapor into the lower stratosphere. The sulfur dioxide reacts with water vapor to form a layer of microscopic droplets of sulfuric acid that remains suspended in the lower stratosphere for up to several years. These droplets and small particles of dust are collectively known as aerosols. A permanent layer of aerosols in the lower stratosphere is replenished by volcanoes and other sources.

Sunlight scattered by aerosols from powerful volcanic eruptions makes the lower stratosphere visible, especially during twilight. The scattered sunlight dramatically brightens the sky and prolongs twilight glows. This is nicely illustrated in this sunset photograph following the eruption of Kasatochi, an uninhabited volcanic island in Alaska’s Aleutian chain. On August 7, 2008, Kasatochi erupted and sent a plume of volcanic ash and gases into the lower stratosphere. On September 3, the Kasatochi plume drifted over Texas, where it produced the brilliant twilight glow shown here (see Fig. 8).

The inset photo of Kasatochi was made by Ross Clement from an Alaska Airlines plane, three years prior to the eruption.

Figure 8

Mesosphere

The mesosphere, or middle atmospheric layer, extends 35 km (22 miles) above the top of the stratosphere. While the air in the mesosphere is very thin, it is sufficiently dense to heat meteors passing through it to incandescence. While you cannot see the mesosphere, you can see evidence of its presence when you see a meteor flashing across the night sky. The one shown here was photographed by a camera aboard the International Space Station.

Figure 9

Thermosphere

The thermosphere is thicker (taller) than any of the layers below it. It extends about 513 km (319 miles) over the top of the mesosphere. The International Space Station (ISS) and many other satellites orbit Earth from within the thermosphere. Aside from its thickness, the most interesting feature of the thermosphere is its temperature. NASA explains:

Thermo means heat, and the temperature in this layer can reach up to 4,500 degrees Fahrenheit. If you were to hang out in the thermosphere, though, you would be very cold because there aren’t enough gas molecules to transfer the heat to you. This also means there aren’t enough molecules for sound waves to travel through.

While the thermosphere contains little air, enough is present to give rise to the aurora borealis (northern lights) and aurora australis (southern lights) in the night sky, especially when solar activity is high. The ISS sometimes passes through auroras, and this photograph of the aurora borealis (see Fig. 10) was made from the ISS. The full image is available online, as is a description of the image on NASA’s Earth Observatory webpage.

Figure 10

Exosphere

The outermost and thickest layer of the atmosphere is the exosphere, which is about 10,000 km (6,200 miles) thick. NASA explains:

The exosphere has gases like hydrogen and helium, but they are very spread out. There is a lot of empty space in between. There is no air to breathe, and it’s very cold.

Ionosphere

The ionosphere is not considered a distinctive layer of the atmosphere. Instead, it is a multi-layered region of the thermosphere between 80 and about 600 km (50 and 373 miles), characterized by free electrons that are separated from molecules of air by extreme ultraviolet radiation and X-rays emitted by the sun. These electrons play a crucial role in long-distance communications, for they reflect radio waves. This enables radio signals to be bounced to distant receivers that are not in a direct line with the radio transmitter.

The ionosphere is of special interest to the Space Weather and Prediction Center (SWPC) of the National Oceanic and Atmospheric Administration (NOAA). The SWPC explains how the ionosphere is strongly influenced by solar flares and the 24-hour cycle of day and night:

The atmospheric atoms and molecules are impacted by the high energy EUV and X-ray photons from the sun. The amount of energy (photon flux) at EUV and X-ray wavelengths varies by nearly a factor of ten over the 11-year solar cycle. The density of the ionosphere changes accordingly. Due to spectral variability of the solar radiation and the density of various constituents in the atmosphere, there are layers are created within the ionosphere, called the D, E, and F-layers. Other solar phenomena, such as flares, changes in the solar wind, and geomagnetic storms, also affect the charging of the ionosphere. Since the largest amount of ionization is caused by solar irradiance, the night-side of the earth and the pole pointed away from the sun (depending on the season) have much less ionization than the day-side of the earth and the pole pointing towards the sun. [edited for sense and consistency]

Composition of the Atmosphere

The atmosphere mainly consists of three gases: nitrogen (78%), oxygen (20%), and argon (1%). It also includes highly variable amounts of water vapor and various gases in such small quantities they are called trace gases. These trace gases include carbon dioxide, carbon monoxide, nitrogen compounds, volatile organic compounds, and a host of others. The National Weather Service (NWS) has summarized all but one of the most common atmospheric ingredients in this chart:

Figure 11

Local conditions can change these numbers, sometimes quite rapidly. For example, forest and brush fires can dramatically increase carbon monoxide. Sulfur dioxide can be greatly increased by plumes from volcanoes and coal-burning power plants. Other gases can be increased by air pollution from vehicles and factories. Missing from this chart is water vapor. It’s not included because it’s so variable, ranging from near zero percent in deserts and mountains to as much as four percent on damp days in tropical regions. Here’s an NWS chart that shows how water vapor changes the fraction of the three most common atmospheric gases:

Figure 12

The atmosphere is more than gases, for it also includes a wide variety of tiny particles known as particulate matter or aerosols. Dust, smoke, smog, clouds, and volcanic ash are aerosols. Then there are the larger objects, including spider silk, pollen, spores, insect and plant fragments, and even tiny bits of manufactured fabrics. The microscopic water droplets that form clouds are aerosols. Because each of the atmosphere’s ingredients serves significant purposes, let’s explore them in more detail.

Nitrogen (N2)

Nitrogen comprises about 78% of the gaseous atmosphere, where it is available to play active roles in the growth of plants and animals through the process known as the nitrogen cycle. Living organisms require nitrogen to form proteins, but most organisms cannot directly make use of the atmosphere’s nitrogen. Instead, they depend on compounds formed by nitrogen with other elements in a process known as fixing. Some nitrogen compounds are formed from lightning as it passes through the atmosphere. Others are formed by bacteria found in water, soil, and the roots of plants known as legumes. The latter include clover, peanuts, beans, peas, alfalfa, and bluebonnets. The Environmental Protection Agency (EPA) has provided this diagram of the nitrogen cycle.

Figure 13

ENVIRONMENTAL IMPACTS

Positive

Nitrogen compounds have many applications in medicine and agriculture. Nitrogen-based fertilizers have transformed life for billions of people who would otherwise be malnourished.

The fixing of nitrogen into compounds that nourish plants and animals is not a one-way process. Animal waste and decaying plants release nitrogen compounds into the soil, where it is exploited by lower forms of life. Eventually, fixed nitrogen compounds break apart and release pure nitrogen back into the atmosphere from where it came.

Negative

People have significantly altered the nitrogen cycle, mainly by using nitrogen-based fertilizers. While these fertilizers have greatly increased the availability of nutritious food for billions of people, nitrogen-based fertilizers have a significant impact on the natural environment. For example, nitrogen-based fertilizers that are washed into bodies of water and oceans can create undesirable plumes of algae, some of which are known as red tides. Cement factories and the internal-combustion engines that propel cars and trucks release compounds of nitrogen and oxygen that play a key role in the production of ground-level ozone. Nitrogen compounds are used to produce a wide variety of pharmaceuticals and even explosives. Ammonium nitrate fertilizers can double as the explosive ingredient of powerful bombs. Patrick Di Justo described some unfortunate consequences of the explosive nature of ammonium nitrate in The Fertilizer Bomb (The New Yorker, April 18, 2013):

The explosion of the West Fertilizer Company plant in the town of West, Texas, has devastated the small community of twenty-eight hundred, killing at least five people and injuring more than a hundred and fifty. At least fifty to sixty homes have been damaged.

This is not the first fertilizer explosion to devastate an entire community: in 1947, Texas City, Texas, was the site of what has been called the worst industrial accident in U.S. history, in which a ship carrying twenty-three hundred tons of ammonium nitrate fertilizer caught fire and exploded, setting off a chain reaction that killed nearly six hundred people. More recently, Timothy McVeigh used a bomb primarily composed of fertilizer to destroy Oklahoma City’s Alfred P. Murrah Federal Building in 1995.

Scott Fields has addressed the major problems created by excessive production of nitrogen, as well as ways to reduce production of nitrogen compounds, in "Global Nitrogen: Cycling out of Control" (Environmental Health Perspectives, 2004, 112(10): A556--A563).

Oxygen (O2)

Oxygen comprises around 20 percent of the atmosphere. While oxygen is a single atom, it most commonly exists as a molecule of two oxygen atoms. Oxygen reacts readily with most chemical elements to form oxides and other compounds. Free oxygen and oxides of oxygen make oxygen the most common element on Earth’s surface. Oxygen is dissolved in water, and water itself is a compound of a single oxygen atom and two atoms of hydrogen. Iron oxide, which is commonly known as rust, is a compound of oxygen and iron. Some compounds of oxygen release considerable heat when they are formed. The classic example is the explosive combination of liquid oxygen with liquid hydrogen that propels space missiles while leaving behind only a plume of harmless water vapor. The space shuttle’s three main engines were propelled by liquid oxygen and liquid hydrogen. Occasionally, a visitor to Hawaii’s high-altitude Mauna Loa Observatory becomes short of breath or even faints due to the reduced concentration of oxygen in the air. These oxygen cylinders provide quick relief (see Fig. 14).

Figure 14

Products like Boost (see Fig. 15) and O2 Naked Air provide a convenient, portable source of oxygen for hikers, mountain climbers, athletes, and people with a respiratory illness.

Figure 15

ENVIRONMENTAL IMPACTS

Positive

Through the process of photosynthesis in plants, sunlight stimulates the production of carbohydrates (starch) from water and carbon dioxide. Oxygen is the waste product of this process. Thus, the same plants that provide shade, building materials, and nutrition for animals also provide life-giving oxygen. Aircraft pilots and mountain climbers are well acquainted with the side effects of breathing air with less oxygen than at lower altitudes. Physician Andrew T. Taylor has described the symptoms of what can occur in hikers, climbers, and pilots when the air they breathe has significantly reduced oxygen in "High-Altitude Illnesses: Physiology, Risk Factors, Prevention, and Treatment" (Rambam Maimonides Medical Journal, 2011, 2(1): e0022). Some people can tolerate reduced oxygen much better than others. Those who become ill may experience severe headaches and might even faint. While some medications may provide limited relief, by far the best remedy for altitude sickness is to return to a lower elevation as quickly as possible.

Negative

Oxygen in the atmosphere readily oxidizes (reacts with) a variety of metals and materials. Rust is iron oxide, the orange compound produced when oxygen reacts with iron in the presence of water. Orange and red soils indicate the presence of natural iron that has been oxidized. Structures like bridges and buildings can be seriously damaged by rust. Pure oxygen is highly flammable, which is why it is used together with a fuel to propel liquid-fueled rockets.

Argon (Ar)

Most argon in the atmosphere is a byproduct of the natural radioactive decomposition of potassium 40 in rocks and minerals. Argon is only about one percent of the atmosphere, and it plays no known significant roles in nature.

Figure 16

ENVIRONMENTAL IMPACTS

Positive

Chemists and engineers have found many uses for argon, for it is one of the few noble gases that react poorly or not at all with other elements. Therefore, argon is commonly used during welding, because it provides a non-reactive blanket that protects what is being welded from reacting with oxygen in the air. This protective property is why high-temperature furnaces lined with graphite are filled with argon, which prevents the hot graphite from burning if air is present. The US Declaration of Independence is preserved in a transparent case filled with argon. Argon is also used in a variety of glow lamps and in lasers that emit powerful blue and green beams. Neon signs is a generic term for glowing glass tubes filled with a gas that glows in various colors when stimulated by a high voltage. Neon glows red, and argon glows blue.

Negative

Because argon does not react with other elements, it has few health hazards. Like all other gases, it is distributed under pressure in steel cylinders that must be treated with care.

Water Vapor (H2O)

Water vapor is the gaseous phase of liquid water. While water vapor is typically only around a few percent of the atmosphere, it varies considerably from nearly zero percent to as much as four percent. It’s much denser in the lower atmosphere and especially the boundary layer. At 3,400 meters (11,200 feet) above sea level at Hawaii’s Mauna Loa Observatory, it’s typically less than half a percent of the atmosphere.

The stratosphere is much drier than the troposphere, which is where most weather takes place. If all the water vapor in the stratosphere could be condensed to liquid water, it would form a layer only around a millimeter thin, about the thickness of a US dime. This highly variable distribution is very different from that of nitrogen, oxygen, argon, and most atmospheric trace gases, which are more uniformly distributed through the atmosphere.

Water vapor has a far greater impact on the atmosphere and the earth itself than suggested by its relatively low abundance, for water vapor controls both weather and climate. This is obvious just outside the original building of Hawaii’s Mauna Loa Observatory, where these words are being typed, for a wall of white clouds is rapidly rising up the mountain and will soon envelope the station in a chilly, foggy mist. When the humidity is high, water vapor in warm air will condense into the liquid droplets that form clouds. When air containing abundant water vapor is suddenly cooled by a cold surface, like an ice-filled beverage or a cool glass window, it will condense and merge into drops like those shown here.

Figure 17

ENVIRONMENTAL IMPACTS

Positive

Water vapor plays the key role in weather, which we will cover later. Water vapor is also key to the hydrologic cycle that is covered in the WATER unit. Water is essential for the growth of plants and the survival of animals. Life as we know it would not exist without water vapor.

Negative

Water vapor provides precipitation, too much of which can saturate farm fields and cause flooding.

Carbon Dioxide (CO2)

While carbon dioxide is only around 0.04 percent of the atmosphere, it plays a crucial role in the growth of plants and animals everywhere on Earth, including you, me, and this flower that was photographed in Anchorage, Alaska.

Figure 18

Through the sunlight-activation process known as photosynthesis, the leaves of plants transform carbon dioxide absorbed from the air and water collected by the roots into the food and energy required for plants to grow. Farmers often pump carbon dioxide into greenhouses to enhance the growth of the plants inside. Like water vapor, carbon dioxide is a greenhouse gas, for it traps infrared radiation emitted by the earth and warms the atmosphere.

ENVIRONMENTAL IMPACTS

Positive

Look around and you will quickly see the vital role carbon dioxide plays in your life. Carbon dioxide is responsible for the algae in a pond, every tree in a forest, and everything made from wood, including pencils, paper, posters, photographs, and building materials. Carbon dioxide is also responsible for almost everything you eat.

Negative

Carbon dioxide in the atmosphere has risen significantly over the past century, primarily due to the burning of fossil fuels (coal, oil, and natural gas). While this added carbon dioxide has enhanced the growth of plants around the world, it has also contributed to a warming of the atmosphere.

Ozone (O3)

Most oxygen in the atmosphere consists of molecules of two oxygen atoms (O2). Free oxygen atoms (O) are also present, and they quickly bond with oxygen molecules to form the triatomic molecule of oxygen known as ozone (O3). Ultraviolet sunlight causes the production of ozone by splitting oxygen molecules into free oxygen atoms that join oxygen atoms. Most ozone is found in a veil around earth that peaks around 25 km (about 82,000 feet) above the surface. This is the ozone layer, which reduces the most intense ultraviolet rays in sunlight to a level acceptable to both plants and animals. Around 10 percent of ozone is found closer to the surface.

The atmosphere’s ozone is surprisingly thin considering the vital role it plays. If all the ozone in a vertical column through the atmosphere could be brought down to the surface, it would form a layer only about 3 millimeters (0.12 inch) thin, about the thickness of two US pennies. While ozone in outdoor air is primarily produced by sunlight and chemical reactions involving nitrogen compounds and volatile organic compounds, ozone is also produced by lightning. Electric motors that employ brushes and some kinds of copy machines can also produce ozone. The balloon shown here is headed toward the lower stratosphere over Hilo, Hawaii, where its instruments will measure the ozone concentration.

Figure 19

ENVIRONMENTAL IMPACTS

Positive

Life as we know it would be impossible without the ozone layer, which absorbs the most dangerous rays of the sun’s ultraviolet radiation. Ozone near the surface can also suppress microorganisms. Some water purification systems employ ozone to kill or suppress pathogenic (disease causing) microorganisms.

Negative

Ozone can irritate plant and animal tissue, including the respiratory systems of people. This can cause difficulties for people with asthma. Ozone can also cause the deterioration of rubber vehicle tires.

Volatile Organic Compounds

The air we breathe both indoors and outdoors includes various carbon-based vapors that have evaporated into the air. These gases are called volatile organic compounds (VOCs). In addition to carbon, VOCs may include oxygen, hydrogen, and nitrogen, as well as potentially toxic gases such as chlorine, fluorine, and bromine. Some VOCs have no detectable odor. Others have pleasant or highly offensive odors.

Some VOCs produce ozone in reactions involving nitrogen compounds and sunlight. Both plants and people produce VOCs. For example, VOCs are found in the exhaust from fossil-fuel-powered cars, trucks, ships, aircraft, and power plants. VOCs are also released from solvents, adhesives, paints, paint thinners, some cosmetics, motor fuel, dry cleaning fluid, some synthetic construction materials, and many other products. Even air fresheners may release VOCs. The pleasing aroma from bakeries is caused by VOCs. In combination with nitrogen oxide and sunlight, VOCs can initiate the production of ozone. For this reason, petrochemical plants, such as this one in New Mexico, are required to limit their emission of VOCs.

Figure 20

ENVIRONMENTAL IMPACTS

Positive

Many kinds of jobs, professions, and hobbies either rely on or produce VOCs. Some VOCs are produced by chemicals that have long played essential roles in modern life. The classic example is VOCs released from evaporating gasoline and diesel fuels.

Negative

Many VOCs have unexpected side effects. For example, the pleasant aroma of baking yeast-leavened bread is due to the ethanol released when the bread is being baked. Ethanol and other VOCs can lead to the formation of smog. While many VOCs are harmless, others can cause respiratory reactions and various illnesses, some very serious. Some of these health effects have been described by the National Institutes of Health:

The health effects of volatile organic compounds can vary greatly according to the compound, which can range from being highly toxic to having no known health effects. The health effects of volatile organic compounds will depend on the nature of the volatile organic compound, the level of exposure, and the length of exposure.

Benzene and formaldehyde are listed as human carcinogens in the Fourteenth Report on Carcinogens published by the National Toxicology Program; diesel exhaust particulates, perchloroethylene, and styrene are listed as reasonably anticipated to be human carcinogens. People at the highest risk of long-term exposure to these three volatile organic compounds are industrial workers who have prolonged exposure to the compounds in the workplace; cigarette smokers; and people who have prolonged exposure to emissions from heavy motor vehicle traffic.

Long-term exposure to volatile organic compounds can cause damage to the liver, kidneys, and central nervous system. Short-term exposure to volatile organic compounds can cause eye and respiratory tract irritation, headaches, dizziness, visual disorders, fatigue, loss of coordination, allergic skin reactions, nausea, and memory impairment.

Metal Vapors

Various metals and metallic compounds in soil can become airborne when the wind blows. Of more concern are vapors of mercury, lead, and other metals. Lead vapor is emitted by lead foundries. Lead dust is released when lead-based paint is sanded.

Lead

For many years, it was believed that air was first polluted by lead during the Industrial Revolution that began during the 1800s. However, careful analysis of the lead content in ancient ice cores from the Colle Gnifetti glacier in the Swiss-Italian Alps has revealed that lead pollution extends back at least 2,000 years. The results of the study are given in "Next Generation Ice Core Technology Reveals True Minimum Natural Levels of Lead (Pb) in the Atmosphere: Insights from the Black Death," by Alexander F. More, et al. (GeoHealth, May 31, 2017), which reported the following:

We show that, contrary to the conventional wisdom, low levels at or approaching natural background occurred only in a single four-year period in the ca. 2000 years documented in the new ice core, during the Black Death (ca. 1349--1353 C.E.), the most devastating pandemic in Eurasian history.

Atmospheric lead poses a serious hazard to those who inhale it, especially children. Significantly, this study shows that acceptable levels of lead in the atmosphere exceed the undetectable levels that occurred during the Black Death, when mining activities came to a halt.

Mercury

Atmospheric mercury also poses environmental hazards. Some industrial processes and the burning of coal in power plants release considerable mercury vapor into the atmosphere. According to the Global Mercury Assessment 2013, Coal burning emitted some 475 tonnes of mercury in 2010, most which is from power generation and industrial use. Even landfills release mercury, which is largely from discarded fluorescent lamps and tubes. This mercury can be emitted into the atmosphere or leached into ground water. Gold mining is a major source of environmental mercury. Thermometers that use mercury, such as the one shown here, have been largely replaced by ones that use safe liquids or by electronic temperature sensors.

Figure 21

ENVIRONMENTAL IMPACTS

Positive

None that are known.

Negative

Mercury and its vapor, along with lead dust and its vapor, are highly toxic and can pose a serious health hazard, especially to children. Mercury deposited in bodies of water may be eventually consumed by fish. Shanti Menon of the National Resources Defense Council has written about mercury’s health hazards in "Mercury Guide," which includes this summary of mercury-tainted fish:

King mackerel, marlin, orange roughy, shark, swordfish, tilefish, ahi tuna, and bigeye tuna all contain high levels of mercury. Women who are pregnant or nursing or who plan to become pregnant within a year should avoid eating these fish. So should children younger than six.

Other Atmospheric Gases

The atmosphere includes a range of additional gases from natural sources and human activity. These include nitrogen compounds from automobiles, cement production, coal-burning power plants, and fires. Sulfur compounds from various industrial sources, the burning of diesel fuel, and a wide range of natural sources can increase the natural acidity of rain.

Atmospheric Aerosols

The atmosphere is home to a surprisingly wide variety of birds, insects, bacteria, solid particles, and liquid droplets. The smallest particles are known as aerosols or particulate matter, and they include microscopic particles of smoke, smog, soil dust, and sea salt. The atmosphere often includes microscopic salt crystals. These occur when ocean waves inject droplets of salt water into the air, which evaporate and leave behind their salt content. Those shown here were collected on a glass microscope slide at a California beach by amateur scientist Jim Scanlon.

Figure 22

The microdroplets of liquid water that form clouds and that merge together to form rain drops are also aerosols. Microdroplets form when water vapor condenses on tiny, solid aerosols or even bacteria floating in the air, or on microscopic salt crystals like the ones shown nearby. Aerosols that lead to the formation of water droplets are known as condensation nuclei. Microdroplets of liquid sulfuric acid form when sulfate particles that evolved from sulfuric dioxide gas merge with water vapor.

The International Cloud Atlas of the World Meteorological Organization includes photographs of most kinds of clouds formed by atmospheric aerosols. We next explore the vast family of atmospheric aerosols with the help of a comprehensive album of photographs, some rare examples of which have been submitted to the International Cloud Atlas.

Natural Water and Ice Clouds

Most clouds are masses of water-vapor droplets or ice crystals. Dust, smoke, and volcanoes also form clouds, and these are covered elsewhere in this unit.

Nearly all water-vapor clouds are formed in the troposphere between the surface and the tropopause---the boundary between the troposphere and the stratosphere. There are several major classes of clouds, each of which occurs within distinctive regions of the troposphere. There they play highly significant roles in weather, climate, and the circulation of water through the atmosphere, bodies of water, and the solid Earth itself. Noctilucent clouds are a special kind of cloud that forms in the mesosphere about 83 km (51.6 miles) above the surface. They are Earth’s highest clouds. Here we will examine images of all these clouds.

The National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASDA) have produced this Sky Watcher Chart, which identifies the main cloud types and includes the symbols that meteorologists use to identify them on charts (see Fig. 23). A more-recent, very high-resolution PDF version of this chart is available here.

Figure 23

This chart includes a second page with information about clouds, their formation, their altitude range, and how they were named (see Fig. 24).

Figure 24

This table explains the definitions of cloud cover used by the National Weather Service in its weather forecasts (additional definitions are sometimes used) (see Fig. 25).

Figure 25

Next, we will examine many variations of the clouds in the Sky Watcher Chart. You have probably seen some of these clouds, and you can begin building your own cloud-photograph collection using a camera. A basic cell phone camera is all you need, or you can use a dedicated camera for even better photographs.

Low-Level Clouds

Low-level clouds occur from the surface of the land or water to about 2 kilometers (6,500 feet).

Mist

When conditions are right, water vapor will form a mist in the air over bodies of water and moist surfaces. Here a damp fence (see Fig. 26) warmed by morning sunlight has created a visible mist that lasted for several minutes until the surface of the wood dried.

Figure 26

Fog

A layer of cloud that forms at or just above water or land is known as fog. A fog cloud may be a few meters (or a few yards) thick, or it may envelop tall buildings and blanket entire cities. Fog can be both beautiful and treacherous. The most dangerous fog is permeated with smoke, which so blocks one’s view of the landscape and roads that it has led to many major traffic accidents, some involving scores of cars and trucks. Following is a photo album of various rural fog events.

Here’s a layer of country fog (see Fig. 27).

Figure 27

Here’s a thicker layer of country fog (see Fig. 28).

Figure 28

Here’s a really thick country fog (see Fig. 29).

Figure 29

Here’s a morning fog in downtown Houston, Texas (see Fig. 30).

Figure 30

Here’s a night fog in New York City (see Fig. 31).

Figure 31

Hawaii’s Mauna Loa Observatory (MLO) is perched at 3,400 meters (11,200 feet) on the upper slope of Mauna Loa, the world’s largest volcano. When clouds rise to MLO, visibility is severely limited (see Fig. 32).

Figure 32

Fog sometimes hides the upper portion of the 40-meter (131-foot) air sampling tower at MLO (see Fig. 33).

Figure 33

While fog may appear very stable, the tiny droplets that form fog may be moving rapidly, as shown in this time-exposure photograph of fog in a flashlight beam (see Fig. 34).

Figure 34

Using a flash transforms the streaks in the fog photo into individual water droplets (see Fig. 35).

Figure 35

Cumulus Clouds

These puffy plumes of condensed water are the most common low-level clouds. While they begin very small, they can grow into huge thunderstorms that penetrate the lower stratosphere. Their name comes from the Latin term for heap or pile, for that’s what they resemble. In many regions during summer, cumulus clouds are scattered across the sky like giant cauliflowers or clusters of popcorn with flat bottoms. Staring at them will reveal faces, cartoon characters, animals, dragons, and many other imaginary objects. These clouds don’t just suddenly appear in the sky. They begin as water droplets coalesce and merge together into clumps, eventually forming what might be called a cumulus precursor like this.

Figure 36

Cumulus clouds quickly grow under the influence of thermals, rising columns of warm air that allow vultures, hawks, and eagles to soar high overhead without flapping their wings. Thermals begin when sunlight heats soil, rocks, and plants, which then warm the adjacent air. Warm air expands and is less dense than cool air. The warm air rises in a process called convection, carrying with it abundant water vapor found near the ground. As the air rises, it eventually encounters cool air above. Cool air is denser and can hold much less water vapor than warm air. As the rising air cools to the dew point, its water vapor condenses into tiny droplets of liquid water, and a cumulus cloud is born.

Figure 37

On warm days, cumulus precursors and their neighbors quickly grow and eventually cover the sky with fluffy cumulus clouds. This fisheye shows a sky full of cumulus clouds on a moist, warm day photograph (see Fig. 38).

Figure 38

Small-to-medium cumulus are warm-weather clouds that usually signal fair weather. Sometimes they barely move across the sky, while other times they move rather rapidly, as shown in this video clip.

The air temperature rises nearly uniformly with elevation, so the bottoms of cumulus clouds are flat. When the air is moist and the temperature is high, cumulus clouds can grow rapidly in size and height. This brightly illuminated cumulus photographed from an airplane is rising above its shaded neighbors on the way to becoming something bigger (see Fig. 39).

Figure 39

A rapidly rising, towering cumulus is known as a cumulus congestus. These clouds are characterized by powerful, turbulent up and drown drafts, and aircraft pilots avoid them.

Figure 40

Cumulus congestus clouds can quickly become big enough to darken the ground below them. Here’s a full-sky view of one doing just that. The photo was made by pointing a camera equipped with a fisheye lens straight up at the base of the cloud (see Fig. 41).

Figure 41

Rain may fall from cumulus congestus clouds, as shown with this that arrived in Central Texas from the Gulf of Mexico one (see Fig. 42).

Figure 42

Cumulus and cumulus congestus can quickly form massive cumulonimbus clouds that generate lightning and drop rain. This newly formed Texas cumulonimbus has the potential to become a much bigger storm (see Fig. 43).

Figure 43

Some cumulonimbus clouds can quickly mushroom into much bigger formations that produce considerable lightning and heavy rain. This one was photographed from east of the Rocky Mountains near Denver, Colorado (see Fig. 44).

Figure 44

The Colorado cumulonimbus quickly expanded into a major storm.

Figure 45

This late-afternoon Texas cumulonimbus was so large that it was necessary to merge three photographs to show all of it (see Fig. 46).

Figure 46

The expanding lower edge of a cumulonimbus is often preceded by a row of low clouds known as scud. Heavy rain often follows the arrival of scud.

Figure 47

Cumulonimbus clouds can generate considerable cloud-to-ground lightning.

Figure 48

Cumulonimbus clouds rise upward as they expand outward. Eventually, the water droplets that form the cloud freeze. The tops of the largest storms can reach the tropopause, and some even pierce into the lower stratosphere. Fast-moving wind blows the tops of these storms into an anvil shape, as shown in this giant Texas cumulonimbus that developed very close to where these words were typed (see Fig. 49).

Figure 49

The top of a major cumulonimbus can exceed 21 km (70,000 feet). A cumulonimbus anvil top can sometimes extend 100 km (62 miles) or more from the storm. Here’s a view of the underside of an anvil more than 65 km (40 miles) from the storm from which it originated (see Fig. 50).

Figure 50

Airplane pilots try to avoid major storms, like the one the passenger plane shown here is flying under (see Fig. 51).

Figure 51

Cumulonimbus clouds are highly turbulent, with warm air moving up and cool air moving down. This photo clearly shows why pilots do their best to stay away from these dangerous storms (see Fig. 52).

Figure 52

Clusters of bubble-like clouds sometimes hang below the base of cumulonimbus clouds. These are called mammatus clouds. Their presence under a cumulonimbus cloud indicates the presence of a potentially violent storm. They can also form under other clouds.

Figure 53

Cumulus Elephants

Cumulus clouds can contain massive quantities of liquid water. Peggy LeMone, a scientist at the National Center for Atmospheric Research in Boulder, Colorado, did the math and found that a typical cumulus cloud contains 450,000 kilograms (500 tons) or more of water. She converted tons to elephants and concluded that a typical cumulus clouds weighs as much as 100 elephants. She then calculated that a large thunderstorm equals about 200,000 elephants, and a hurricane is around 40 million elephants. These facts place an entirely new perspective on children who claim to have seen a cumulus cloud with the shape of an elephant!

Rainbow

When direct sunlight illuminates rain drops as they fall through clean air, it produces a colorful arc in the sky.

Figure 54

Double rainbows sometimes form.

Figure 55

Fogbow

A special class of rainbow is the white, or nearly white, fogbow. This one was photographed from the road to the Mauna Loa Observatory (see Fig. 56).

Figure 56

This section on cumulus clouds began with a photo of wispy precursors. Cumulus clouds often dissipate at sunset, like those shown here that are disintegrating over Hawaii’s Mauna Loa Observatory (see Fig. 57).

Figure 57

Stratus Clouds

These low clouds form blankets over the landscape that can extend for considerable distances. Stratocumulus is a stratus cloud that formed from the merger of many cumulus clouds. Stratus clouds can keep a cool day cool. They can also drop drizzly rain.

This stratus cloud has a very flat bottom (see Fig. 58).

Figure 58

This stratus cloud, which arrived with a cold front, has an undulating base (see Fig. 59).

Figure 59

The best way to show the extent of a stratus cloud is a satellite image or a full-sky fisheye photograph like this (see Fig. 60).

Figure 60

Row Clouds

Moving air can form invisible waves across the