What is the most common gas in the air we breathe? | Reference.com What is the most common gas in the air we breathe? A: Quick Answer The most common gas people breathe from clean air is nitrogen. Air is a mixture of gases, and the air on Earth contains about 78 percent nitrogen and about 21 percent oxygen. Argon, carbon dioxide neon, helium, methane, krypton, hydrogen and xenon are in much smaller amounts. What gases are in the air we breathe? | Reference.com What gases are in the air we breathe? A: Quick Answer In dry air that does not contain pollution, the predominant gas is nitrogen, which makes up 78 percent of the atmosphere. The next two most voluminous gases are oxygen (21 percent) and argon (1 percent). Other gases appearing in trace amounts include carbon dioxide. Full Answer The air naturally contains water vapor as well. The proportion of water vapor in the area varies widely between different areas, depending on the climate. Human activity has had a significant impact on the gases in the atmosphere, particularly through the burning of fuels and the production of pollutant gases. Burning fossil fuels sends a great deal of carbon dioxide into the atmosphere.

Other gases that enter the atmosphere as pollution include sulfur dioxide, nitrogen oxides and carbon monoxides. When fuels do not burn completely, they send solid particles, as with carbon, directly into the air, causing a dirty film to form on buildings and on the inside of lungs. Carbon monoxide is one of the most toxic by-products among these pollutants. Inhaling carbon monoxide can lead to death. However, the other pollutants also are caustic to the environment; sulfur dioxide leads to acid rain, which can maim or even kill crops, trees and other plants. What Gases Make Up the Air We Breathe? | Sciencing What Gases Make Up the Air We Breathe? By Jack Powell Maridav/iStock/Getty Images The Earth’s atmosphere is a layer of gas held in place by gravity, which prevents it from escaping into space. It protects life by absorbing UV radiation, by holding in heat to warm the Earth’s surface and by reducing temperature extremes between day and night. The gases that comprise the atmosphere are commonly referred to as air, which is what all living things on Earth breathe.

Nitrogen: Abundant and Inert It’s a common misconception that oxygen is the most abundant gas in the air breathed on Earth; that honor goes to nitrogen, which makes up 78 percent of the air. Nitrogen occurs as N2 — two nitrogen atoms bonded together. The bond is very strong, making the gas chemically inert. Although inhaled nitrogen passes into the bloodstream, it is not used by the cells in the body. However, since nitrogen is essential for life — it is found in RNA, DNA and proteins — it must be converted to compounds with less stable bonds to be used by animals. One way this happens is through nitrogen fixation in plants. Oxygen: Life-Giving Gas Making up almost 21 percent of the air all living things breathe, oxygen is absorbed by the lungs, or lung-like structures in lower animals, and transported to all cells in the body by the blood. Oxygen is the most unstable, and therefore the most chemically active, gas found in air. Although all animals need oxygen, it can be deadly in higher-than-normal concentrations: Breathing pure oxygen for extended periods leads to oxygen toxicity. In addition to its role in biology, oxygen is essential for combustion, the chemical process responsible for fire.

Argon: Noble Gas The third-most abundant gas in the air on Earth is argon, although it makes up less than 1 percent of air. Argon is classified as a noble gas in chemistry, meaning it is very stable and seldom reacts with other compounds. The argon in the air comes mainly from the decay of potassium-40, a radioactive isotope in the Earth’s crust. The bulk of argon used in science is acquired by fractional distillation of air in its liquid form. Trace Gases There are several additional gases present in the atmosphere in minute amounts. These gases are referred to as trace gases and include water vapor, carbon dioxide, methane, helium, hydrogen and ozone. These gases each have their own purpose and forms of production. Methane, for example, is a powerful greenhouse gas, trapping heat in the earth’s atmosphere. Ozone is found in two distinct layers of the atmosphere: high in the stratosphere, where it blocks harmful ultraviolet light from the sun, and the lower atmosphere, where it is one of the components of smog. References What Is the Chemical Composition of Air? What Is the Chemical Composition of Air? What Is the Chemical Composition of Air?

Elements in the Atmosphere By Anne Marie Helmenstine, Ph.D. Question: What Is the Chemical Composition of Air? Nearly all of the Earth's atmosphere is made up of only five gases : nitrogen, oxygen, water vapor, argon, and carbon dioxide . Several other compounds also are present. Although this CRC table does not list water vapor , air can contain as much as 5% water vapor, more commonly ranging from 1-3%. The 1-5% range places water vapor as the third most common gas (which alters the other percentages accordingly). Answer: This is composition of air in percent by volume, at sea level at 15°C and 101325 Pa. Nitrogen -- N2 -- 78.084% Air we breathe: Air Composition Composition of clean & polluted air Before you can understand air pollution, you need to have an understanding of clean air. Composition of Clean Air In the fall of 2002 I got to go to Switzerland. On the morning of November 6th, I had to leave the village of Zermatt, but wanted to take one more look at the Matterhorn. The air was absolutely clear.

Cleaner than I have ever seen air anywhere. The Matterhorn jumped out at you. By the way, Zermatt does not allow automobiles. People walk or ride electric vehicles. The train that goes there and the trolley that takes you to the ski lifts also run off of electricity. Here are 10 gases that make up clean air: In order of highest to lowest concentration they are Nitrogen, Oxygen, Argon, Carbon dioxide, Neon, Helium, Methane (CH4), Krypton, Hydrogen, and Xenon. Five of them travel alone, so we call them atoms. For example, a helium balloon contains atoms of helium (He), but an oxygen tank contains molecules of oxygen (O2). When there's two or more atoms are bonded together, they are called molecules. NITROGEN (N2) #1: 8 out of 10 atoms (or molecules) of air are made of nitrogen. Nitrogen gas can't be seen but when it is cooled to -320 °F (-195°C) it turns to a liquid, which you can see. Being this cold makes it handy for many things.

Shown here, it is being used to quickly freeze cream and sugar to make ice cream. NITROGEN #2: Liquid nitrogen is also used to cool certain metals to the point where they become superconductors. Superconductors conduct electricity (electrons) without any resistance. In this picture a rectangular shaped magnet had been dropped onto a metal disk that had been cooled enough so that it was a superconductor. As the magnet fell toward the metal disk, the magnetism in the magnet caused currents of electricity to flow inside the metal disk. The currents created in the metal disk by the falling magnet made that part of the metal disk a magnet itself. The falling magnet then feels the repulsion of the magnet it just created. The falling magnet will then stop just above the metal disk. Electricity continues to flow in the metal disk constantly creating a mirror image magnet that repels the real magnet. This will continue forever as long as the metal is cool enough to stay a superconductor. NITROGEN #3: Oxygen reacts with many things including paint. Therefore, expensive works of art and other rare relics are stored in pure nitrogen.

Nitrogen gas is fairly inert and doesn't react (combine) with most other substances, so it is used to protect items and keep oxygen away.. Bad Nitrogen NITROGEN goes bad: Under high temperatures, like in a jet engine or car engine, nitrogen will combine with oxygen to form a class of toxic compounds called nitrogen oxides. The simplest having one nitrogen and one oxygen (NO). Others have two nitrogens and one oxygen (N2O), one nitrogen and two oxygens (NO2), and the fourth has two of each (N2O2). Their names (in order) are nitrogen oxide, dinitrogen oxide, nitrogen dioxide, and dinitrogen dioxide. Nitrogen dioxide has a brownish appearance and is often what you see in polluted cities. Oxygen #1: 2 out of 10 atoms/molecules of air is oxygen. Oxygen is produced by plants and is consumed by animals. Animals need oxygen to "burn" the food they eat in order to get the calories (energy) they need. The way animals use oxygen to burn food is different than a fire, but it produces the same products of carbon dioxide and water.

Of all of the gases that make up air, oxygen is the most reactive. Oxygen is Everywhere in This Photo OXYGEN #2: Oxygen is everywhere. As a gas it makes up 20% of the air. Some oxygen gas is dissolved in water, and some oxygen gas is in the pores in the ground. However, there's a lot more oxygen in this picture that we don't normally consider. 90% of the weight of the water in the river is oxygen atoms. Remember water is H2O. The one oxygen in H2O weighs 8 times has much as the two hydrogen atoms combined. The trees and grass are made of carbon, oxygen, hydrogen, and nitrogen, with oxygen accounting for about half of a plant's weight. The rocks, the soil, and the mountains contain metals and silicon that is bound to oxygen, which makes up about 1/2 of their weight. Think about it; one half of a mountain's weight is from oxygen. The only thing in the picture that doesn't contain oxygen are the metal rails. OXYGEN #3: In the movie Total Recall, there was an ancient alien factory that could release the oxygen from the Martian soil. This is actually possible.

Extreme heat will cause the oxygen to separate from the metals and non-metals that it was bound to. Most of Martian soil is iron oxide, which would turn to iron metal and oxygen gas at high temperatures. In the pictures above, Arnold presses the touch sensitive on "button" of the oxygen generator. Next he gets blown out of the factory into the thin Martian air, which causes the air in his body to expand. Fortunately, the oxygen rushes out of the top of the mountain that housed the factory and they show Mars getting an atmosphere similar to Earth's. Actually, this atmosphere would be pure oxygen, which is not good to breath for long periods and would present extreme fire hazards. It's lacking the 80% nitrogen. ARGON: Argon is 1 out of 100 atoms/molecules of air. Argon is an inert gas that is used to pressurize light bulbs. Being inert it doesn't react with the tungsten metal, which makes the filament. The argon gas also helps keeps the tungsten metal atoms on the surface of the filament from jumping off the filament.

Argon gas is also used in welding to push away the air so that the oxygen in the air won't react (oxidize) the metal being welded. CARBON DIOXIDE: Carbon dioxide only makes 3 out of 10,000 atoms/molecules of air; however, it supplies the carbon that plants use to make leaves, trunks, roots, etc. It's hard to believe that the weight of giant redwood trees came from the small amount of carbon dioxide gas in the air. NEON: 2 out 100,000 atoms/molecules of air. Neon is a clear inert gas; however, high voltage can strip electrons off neon atoms. As the electrons fall back into place, they give off ultraviolet light (black light), which you can't see. However, the glass tubes are coated with different phosphor powders, which glow when ultraviolet light hits them. HELIUM: 5 out of 1,000,000 atoms or molecules of air are helium. Helium is the second lightest gas (hydrogen is lightest). It is used to fill toy balloons and weather balloons.

It is so inert that there is no danger of igniting nor will it combine with other elements. For this reason it is used in welding by having helium gas surround an electric arc that is melting a metal. The flow of helium gas keeps the oxygen away from the metal so that the oxygen won't oxidize (rust) the metals being welded. METHANE: 2 out of 1,000,000 atoms/molecules of air are methane molecules. Bacteria is a big source of methane gas and this type of bacteria is found in termites (see huge termite mound on the left), cattle (cow flatulence), rice paddies, swamps (called swamp or marsh gas), and in the sea bed. In the sea bed, methane can be found trapped in ice (see person holding ice that's on fire). Methane also comes from petroleum fields and is the natural gas you cook with. An embarrassing source of methane is from the eating of beans. KRYPTON: 1 out of 1,000,000 atoms/molecules of air is krypton gas. You may remember krypton as the name of the planet that Superman came from.

The pieces of the planet that fell as meteorites were called kryptonite. The element krypton is used in lamps in a similar way that argon is used (see above). The word krypton comes from "Krypto" meaning hidden. The gas krypton was in a way hidden because it took quite a while for scientists to discover it. HYDROGEN: 1 out of 2,000,000 atoms/molecules of air is hydrogen. Hydrogen is the lightest element, which is why is was the preferred gas to fill blimps and dirigibles. Unfortunately, hydrogen is very flammable, which led to the catastrophic tragedy of Germany's Hindenberg airship. XENON: 87 atoms out of 1,000,000,000 (one billion) atoms/molecules of air are xenon. Like argon and krypton, xenon is used as the gas in lamps. This one is for a car's headlamp. These are probably the ones that look more blue. WATER VAPOR: The other gas we didn't list before is water vapor. The concentration of the other gases are pretty consistent, but water vapor varies greatly.

Using a fish-eye lens, I took this picture of a double rainbow in Geneva, Switzerland. It had rained all day, but just before the sun went down, the clouds parted enough to allow the sunshine to strike the droplets of liquid water still in the air. Water in vapor form (gas) is not visible. One Last Look at Composition of Air: Imagine the volume of air in a typical classroom that is 30 feet by 30 feet with a 10 foot high ceiling. Also assume, we separated all the gases. Oxygen would cover the room to about 2 feet deep. Nitrogen would fill almost to the ceiling (another 8 feet minus a couple of inches). Argon gas would fill a one inch layer over the whole room. The remaining gases fill the last one inch. Carbon dioxide has about the same volume of one student. Neon is 1.5 gallons. Helium would fill a one liter bottle. Methane gas would fill someone's 1/2 liter bottle. Krypton would fill a 12 oz soda can. Hydrogen would fill about half of a 12 oz soda can. And xenon gas would have the volume of a pencil's eraser.

Composition of Polluted Air Particulates Part of the below tutorial is from Lab 1 (Air We Breathe) The image on the left is from one of the dust storms that hit Mesa and Phoenix. We all know that breathing that dust will be bad for us. Even on days without a dust storm, there are plenty of particulates in the air which aren't healthy. Surprisingly, it's not just what the particulates are made of, but the size of the particulates that also affects its effect on the lungs. Note: In this context, "particulates" mean solid particles that are suspended in the air. The lungs contain thousands of sacs called alveoli. This is where oxygen passes through the lining of the alveoli and enters the blood. This is also where carbon dioxide leaves the blood. Our nose hairs and mucous in the nasal passages and the bronchial tubes try to block particles that we breath in. However, if the particles are too small, they get by these defenses and get to the alveoli. If the particles are even smaller, they pass through the lining of the alveoli and even get into the blood stream.

(See image below) The human hair is about 70 to 100 millionths of a meter in diameter. A millionth (micro) of a meter is called a micrometer or micron for short. The symbol for a millionth (micro) is µ. So the human hair is 70µm to 100µm in diameter. Particles over 10µm are usually trapped by nose hairs and mucous in the nasal passages. So their danger is less. However, particles between 10µm and 2µm get past those defenses and get deep into the lungs. These are such things as dust particles, mold, mold spores, and pollen. They can irritate the lungs and cause severe reactions. Smaller particles (0.1µm to 2.5µm) are small enough to pass through the lung lining and enter the blood stream and reach all organs of the body. As you might guess, this can cause even greater health issues.

These smaller particles can be soot (from combustion of wood, gasoline, diesel, etc.), aerosols (from sulfuric acid forming in the atmosphere and reacting with other particles), and finely powdered metal (from car brake pads, metal sanding, etc.). The air inside and outside our home contains numerous particulates (particles suspended in the air). Below are some, plus their size in microns. Anything under 10 microns is not visible to the eye and is more hazardous. By the way, dust mites live in your bedding and pillows. They eat the flakes of skin that you shed. Human Hair .................(70 - 100 microns) Pet Dander ..................(0.5 - 100 microns) Pollen ...........................(5 - 100 microns) Dust Mite Debris ........ (0.5 - 50 microns) Household Dust .......... (.05 - 100 microns) Skin Flakes ...................(0.4 - 10 microns) Bacteria....................... (0.35 - 10 microns) As you can see, most of these dip down into the unhealthiest range of under 2.5µm.

As a defense, filters are employed to trap these particles. You may have heard of HEPA filters for vacuum cleaners and room air filters. HEPA stands for "High Efficiency Particulate Arresting". Arresting meaning stopping. These filters were first used to capture radioactive particles in the air at the facility where the first atomic bomb was being built. HEPA filters will only miss 3 out 10,000 particles 0.3 microns in size. For larger size particles, it misses even fewer of them. It uses a mesh of fibers to accomplish this. Fibers can be made from plastic, glass, or cellulose (like cotton fibers). The fibers actually have openings much larger than the particles that it can capture. So it doesn't act like a sieve that can only capture particles larger than the sieve openings. As particles move through the fiber mesh, they get wedged in the narrow spaces where the fibers cross each other. The faster moving air actually helps particles get wedged in between fibers. That's probably why it is called High Efficiency Particulate Arresting. Of course, particles larger than the openings will get caught between the fibers.

They call these mechanical filters because the particulates are mechanically held (blocked or wedged) in the filter. This is a granule of activated charcoal. The pits seen are about 100 microns in size; however, these are the large pits. There are others thousands of times smaller. So activated charcoal can act as a mechanical filter by blocking particles larger than its smallest pores. However, it can also trap certain substances that are so small that they can pass right through the pores. Activated charcoal does this because certain substance are attracted to the surface of the carbon. In other words, the carbon in activated charcoal acts like fly paper to certain substances. Vapors from solvents like gasoline or paint thinner stick to the surfaces and are trapped. They call this adsorption (spelled with a "d"). Absorption (spelled with a "b") is when a thing soaks into something. Adsorption is when it sticks to its surface. This is the famous brown air of Phoenix. It's appearance is from a mixture of dust, smoke, other particulates, and nitrogen dioxide (which is brown).

Smoke from wood fires: The smoke from a campfire or from a forest fire is quite dangerous partially because of the abundance of very small particles that are in the smoke. Most particles in wood smoke are smaller than the 2.5 microns (2.5µm). These particles don't normally get coughed up. They are so small that they get stuck into the lining of the alveolus sacs. Perhaps the most dangerous component of wood smoke is the countless airborne particles (particulates). Airborne particles smaller than 2.5 microns are harmful because they are tiny enough to lodge inside lung tissue, while particles larger than 2.5 microns are coughed out. Researchers have discovered that the particles found in wood smoke are almost all smaller than 2.5 microns. Nonfatal heart attacks Some cancers One place to find particulates in the air is to look for what particulates have settled out of the air and onto our cars. In the dry climate in Arizona, particulates are not washed out of the air from rain, so cars get dusty pretty quickly. Also, the car seems to accumulate more road grime and dust when it's driven on the highways and streets.

A few cars filter the air coming through the vents, but that still doesn't help when you roll down the windows. Again, most cars don't have a car cabin air filter. SAMPLE OF OUTDOOR PARTICLES: Below is a sample taken from a dusty car using clear tape. What is mostly seen are lot of small particles of clear silica (sand/quartz). The photo doesn't show how clear these little particles appear. Also, the few cylindrical particles present look like pieces of plant material. When there was white paper behind the tape, many small black particles were seen. They, again, are likely soot particles and even particles from tires. So outside seems to have more mineral dust, more soot, and tire particles. POLLEN: Pollen is one of the common particulates in the air. For people with allergies, pollen is one of the biggest triggers of allergic reactions. Pollen is stored on the anthers, which sit on the ends of stalks called filaments. The yellowish white spheres are pollen. The tiny white specks that you see in the photo are just adhesive from the tape that was used to capture the pollen. DUST from SOIL: Fine particles in dirt are often blown into the air by wind and vehicles.

Dust from soil is one of the common particulates in the air, especially in Phoenix and the surrounding area. The soil sample collected shows a variety of minerals. Many are too large to get airborne, but the smaller ones can become dust pollution. Soil is mostly quartz with hundreds of other minerals. The darker grains are likely iron oxides or silicon dioxide (quartz) with iron or magnesium impurities. Soil will also have some pieces of plant material. None is that noticeable in this photo. Some soils are rich in organic (plant) materials, but Arizona soil doesn't have that much plant material in its soil. Here is the same sample without the white paper background. You can see that it changes the look of the particles. The lighter particles stand out more. The grains of minerals have a rough look. They also look clear, white, black, or may have some color in it. If it is a bright color, then it is probably not a mineral, perhaps a paint chip or piece of some insect. SOOT and SMOKE: The other particles that are often seen in air samples are carbon particles (soot). These come from unburnt gasoline and unburnt diesel.

Fireplaces also put out a lot of soot, but fortunately in Phoenix fireplaces aren't used that much. Forest fires also put out a tremendous amount of ash and soot. So it's just not the fire that is dangerous about a forest fire; it's the smoke. When we think of diesel smoke, we usually think of the large semi trucks. However, there are thousands of pickups that have diesel engines just in any one city. The small trucks also put out diesel smoke. Not as much as the large trucks nor as much as the truck in this photo. Rather sadly, some diesel truck owners actually program their truck's microcontroller to produce these large amounts of smoke when they want to. Nearly every school bus runs on diesel also. If the engines are cold, old, or not properly tuned, then a lot of diesel smoke gets breathed by school children around that bus. The ultimate air pollution happens when a person smokes. The smoke is deliberately inhaled deep into the lung. The very small particles of smoke penetrate into the lung tissue. This is a section of lung tissue from a smoker. You can see how the carbon particles get lodged around the alveoli.

Gasoline engines also put out soot especially if the gasoline to air mixture is rich (too much gasoline). The inside of a tailpipe of a car is a good place to find carbon particles (soot). Here is the sample from the tail pipe. Soot formed from unburnt gasoline or diesel are very small particles. They look like tiny black or gray specks under the microscope. In extreme magnification, they look like spheres. The reddish color in this image is probably from rust particles (iron oxides) coming from metal in the exhaust system. As mentioned before, the small size of these smoke particulate make them more dangerous because they can lodge in the alveoli of the lungs. Some ash particles from cigarettes or burning wood can be larger than the typical soot particles. They are also black or dark gray. Some have a bluish tint. These ashes in the photo were caught in a wad of cotton using an air pump. The strands are the cotton fibers. Again, the larger particles are normally caught by the nose or sinus. Unfortunately, smoking means the smoke goes directly into the mouth where there is little filtering. So it gets into the lungs without much stopping it.

FIBERS: The air we breathe also has a lot of fibers and hairs in it. These short strands of fiber are from such things as clothes, rags, carpet, and paper towels. The hairs may be from pets or people, and even insects. Cotton fibers look like flat ribbons that got twisted. These are cotton fibers at 250x magnification. Synthetic fibers like nylon, polyester, and others are more round than cotton fibers. This magnification is about 100x. Long fibers don't get in the air easily. It's the short ones that we often breathe. This photo was shown above. The fibers that are more bent and twisted are likely cotton fibers. These have color because they are dyed cotton. A few fibers are less bent. Those are likely synthetic fibers. On the left is a synthetic fiber at about 250x magnification. Notice is it is not flat and twisted like cotton fiber. You can see some turquoise dye on (or in) the fiber. On the right is a strand of human or animal hair. Human and animal hair have a channel that runs through the middle of it. In high magnifications, scales on the outside of the hair can be seen.

DANDER AND DANDRUFF: Both dander and dandruff are pieces of dead skin. Dander normally refers to the dead skin that comes off of animals like pet dogs and cats. Dandruff is dead skin from the scalp. Dandruff is usually larger and flatter than dander. If you are interested in more information about microscopy, you can visit the website developed by the National High Magnetic Field Laboratory at the Florida State University. Earth's Atmosphere | UCAR Center for Science Education Earth's Atmosphere The layered structure of Earth's atmosphere is visible in this sunset view from the International Space Station. Credit: Image Science & Analysis Laboratory, NASA Johnson Space Center Earth's atmosphere is a mixture of gases that surrounds our home planet. Besides providing us with something to breathe, the atmosphere helps make life on Earth possible in several ways. It shields us from most of the harmful ultraviolet (UV) radiation coming from the Sun, warms the surface of our planet by about 33° C (59° F) via the greenhouse effect, and largely prevents extreme differences between daytime and nighttime temperatures. The atmosphere is a mixture of many different gases.

Nitrogen and oxygen are by far the most common; dry air is composed of about 78% nitrogen (N2) and about 21% oxygen (O2). Argon, carbon dioxide (CO2) , and many other gases are also present in much lower amounts; each makes up less than 1% of the atmosphere's mixture of gases. The atmosphere also includes water vapor. The amount of water vapor present varies a lot, but on average is around 1%. There are also many small particles - solids and liquids - "floating" in the atmosphere. These particles, which scientists call "aerosols", include dust, spores and pollen, salt from sea spray, volcanic ash, smoke, and more. The atmosphere grows thinner (less dense and lower in pressure) as one moves upward from Earth's surface. It gradually gives way to the vacuum of outer space. There is no precise "top" of the atmosphere. Air becomes so thin at altitudes between 100 and 120 km (62-75 miles) up that for many purposes that range of heights can be considered the boundary between the atmosphere and space.

However, there are very thin but measurable traces of atmospheric gases hundreds of kilometers/miles above Earth's surface. There are several different regions or layers in the atmosphere. Each has characteristic temperatures, pressures, and phenomena. We live in the troposphere , the lowest layer, where most clouds are found and almost all weather occurs. Some jet aircraft fly in the next higher layer, the stratosphere , which contains the jet streams and the ozone layer. Higher still are the mesosphere , thermosphere and exosphere . © 2011 UCAR Atmosphere | Atmospheric Gases | Aerosols | Greenhouse Gases Atmospheric Composition Air Air is a mixture of gases and aerosols that composes the atmosphere surrounding Earth. The primary gases of air include nitrogen (78%) and oxygen (21%). Trace gases and aerosols make up the remaining 1% of air. The trace gases include the noble gases argon, neon, helium, krypton and xenon; hydrogen; and the greenhouse gases.

The aerosols are solid or liquid particles having diameters in the region of 0.001 to 10 microns (millionth of a metre), and include dust, soot, sea salt crystals, spores, bacteria, viruses and a plethora of other microscopic particles, which may be natural or man-made. Earth maintains an atmosphere through its gravitational pull. Consequently, most air is found in the lowest 10 kilometres of the atmosphere. Experienced mountain climbers are aware of how thin the air becomes, and may carry oxygen tanks to assist breathing at high altitudes. Within the lower atmosphere, however, air remains remarkably uniform in composition, as a result of efficient recycling processes and turbulent mixing in the atmosphere. Atmospheric Gases There are a number of atmospheric gases which make up air. The main gases are nitrogen and oxygen, which make up 78% and 21% of the volume of air respectively. Oxygen is utilised primarily by animals, including humans, but also to a small degree by plants, in the process of respiration (the metabolism of food products to generate energy). The remaining 1% of the atmospheric gases is made up of trace gases.

These include the noble gases, very inert or unreactive gases, of which the most abundant is argon. Other noble gases include neon, helium, krypton and xenon. Hydrogen is also present in trace quantities in the atmosphere, but because it is so light, over time much of it has escaped Earth's gravitational pull to space. The remaining trace gases include the greenhouse gases, carbon dioxide, methane, nitrous oxide, water vapour and ozone, so-called because they are involved in the Earth natural greenhouse effect which keeps the planet warmer than it would be without an atmosphere. Oxygen The gas oxygen (O 2 ), composed of molecules of two oxygen atoms, occupies 21% of the Earth's atmosphere by volume. It is colorless, odorless, and tasteless. Oxygen also comprises 86% of the oceans and 60% of the human body, and is the third most abundant element found in the Sun. Almost all plants and animals require oxygen for respiration to maintain life. Oxygen is very reactive and oxides of most elements are known. A chemical reaction in which an oxide is formed is known as oxidation. The rate at which oxidation occurs varies with the element with which oxygen is reacting.

Rust, or iron oxide, for example forms relatively slowly, over days or weeks. Burning or combustion, however, involves a very rapid oxidation. Carbon in fossil fuels, for example, can be quickly oxidised to carbon monoxide and carbon dioxide, with a considerable amount of heat being given off. We can convert this heat into useful energy for heating, electricity and locomotion. Within the stratosphere, oxygen molecules combine with free oxygen atoms to form ozone (O 3 ). Ozone absorbs ultraviolet (UV) radiation from the Sun, and protects life on Earth from its damaging effect. Although abundant between 19 and 30 km altitude, the air at these levels in the atmosphere is thin. If all the ozone in the stratosphere was compressed to ordinary atmosphere pressure at ground level, it would occupy a layer only 3 mm thick. Nitrogen The gas nitrogen (N 2 ), composed of molecules of two nitrogen atoms, occupies 78% of the Earth's atmosphere. It is colorless, odorless, and tasteless. Nitrogen is as important as it is common. It's essential to the nutrition of plants and animals.

Nitrogen is a constituent in all proteins and in the genetic material (DNA) in all organisms. The low content of nitrogen in most soils exists in stark contrast to the abundance of nitrogen in air. This is because gaseous nitrogen molecules have very strong bonds linking the atoms together, making the gas chemically stable and unusable by most biological organisms. Some species of bacteria absorb nitrogen from the air and convert it into ammonium, which plants can use. This process, called nitrogen fixation, is the principal natural means by which atmospheric nitrogen is added to the soil. Legumes, such as beans, can fix nitrogen from the atmosphere. This is accomplished by nitrogen-fixing bacteria living in nodules on the plant roots. Nitrogen molecules in the atmosphere can also be broken by the energy generated by lightning strikes and volcanic action. Whenever lightning flashes in the atmosphere, some nitrogen combines with oxygen and forms the gas nitric oxide (NO). This nitric oxide is converted to nitric acid, which is highly soluble in water and falls to the ground in rainwater, to be absorbed by soils. Globally, however, nitrogen-fixing bacteria are a far more significant source of fixed nitrogen.

Trace Gases Most of our atmosphere is made up of nitrogen (78% by volume) and oxygen (21% by volume). The remaining 1% of the atmospheric gases are known as trace gases because they are present in such small concentrations. The most abundant of the trace gases is the noble gas argon (approximately 1% by volume). Noble gases, which also include neon, helium, krypton and xenon, are very inert and do not generally engage in any chemical transformation within the atmosphere. Hydrogen is also present in trace quantities in the atmosphere, but because it is so light, over time much of it has escaped Earth's gravitational pull to space. Despite their relative scarcity, the most important trace gases in the Earth's atmosphere are the greenhouse gases. Most abundant in the troposphere, these gases include carbon dioxide, methane, nitrous oxide, water vapour and ozone, so-called because they are involved in the Earth natural greenhouse effect which keeps the planet warmer than it would be without an atmosphere. Apart from water vapour, the most abundant greenhouse gas (by volume) is carbon dioxide.

Despite being present in only 380 parts per million by volume of air, carbon dioxide and the other greenhouse gases help to keep the Earth 33�C warmer than it would otherwise be without an atmosphere. Through emissions of greenhouse gases however, mankind has enhanced with natural greenhouse effect which may now be leading to a warming of the Earth climate. Whilst ozone behaves like a greenhouse gas in the troposphere, in the stratosphere where its abundance is most significant within the ozone layer, it helps to filter out the incoming ultraviolet radiation from the Sun, protecting life on Earth from its harmful effects. Air within the stratosphere is thin however. If all the ozone in the stratosphere was compressed to ordinary atmosphere pressure at ground level, it would occupy a layer only 3 mm thick. Other trace gases in the atmosphere arise from natural phenomena such as volcanic eruptions, lightning strikes and forest fires. Gases from these sources include nitric oxide (NO) and sulphur dioxide (SO 2 ). In addition to natural sources of nitric oxide and sulphur dioxide there are now many man-made sources, including pollutant emissions from cars, agriculture and electricity generation through the burning of fossil fuels.

During the 20th century other man-made processes have put completely new trace gases into the atmosphere, for example the chlorofluorocarbons (CFCs) which damage the ozone layer. Aerosols Aerosols are solid or liquid particles dispersed in the air, and include dust, soot, sea salt crystals, spores, bacteria, viruses and a plethora of other microscopic particles. Collectively, they are often regarded as air pollution, but many of the aerosols have a natural origin. They are conventionally defined as those particles suspended in air having diameters in the region of 0.001 to 10 microns (millionth of a metre). They are formed by the dispersal of material at the surface (primary aerosols), or by reaction of gases in the atmosphere (secondary aerosols). Primary aerosols include volcanic dust, organic materials from biomass burning, soot from combustion and mineral dust from wind-blown processes. Secondary aerosols include sulphates from the oxidation of sulphur-containing gases during the burning of fossil fuels, nitrates from gaseous nitrogen species, and products from the oxidation of volatile organic compounds (VOCs).

Although making up only 1 part in a billion of the mass of the atmosphere, they have the potential to significantly influence the amount of sunlight that reaches the Earth�s surface, and therefore the Earth's climate. Although the abundance of aerosols varies over short time scales, for example after a volcanic eruption, over the long term the atmosphere is naturally cleansed through mixing processes and rainfall. Cleansing is never complete however, and there exists a natural background level of aerosols in the atmosphere. The average time spent in the atmosphere by aerosols is dependent upon their physical and chemical characteristics, and the time and location of their release. Natural sources of aerosols are probably 4 to 5 times larger than man-made ones on a global scale, but regional variations of man-made aerosol emissions may change this ratio significantly in certain areas, particularly in the industrialised Northern Hemisphere. At certain times of the year, the natural background level of aerosols may increase, for example, during the growing season, when large quantities of pollen are released into the atmosphere.

Question: What is the most common gas in the air we breathe?
Answer: Nitrogen