Universal Industrial Gases, Inc: Composition of Air - Components & Properties of Air - Answers to "What is air?" - "What is air made up of?" -" What are air products and what are they used for?"

"Air" is the name we give to the mixture of gases which make up the Earth's atmosphere. The composition, physical, and chemical properties of air are very similar everywhere.

It is useful to think of the air which we encounter every day as being a locally-produced mixture of three types of "ingredients":

Standard dry air, which is mainly composed of three gases: nitrogen (about 78%), oxygen (about 21%), and argon (about 1%). Together, these three gases make up 99.96% of dry air. All three can be economically recovered as industrial gas products. Standard dry air also contains a small amount of carbon dioxide, and very small amounts of neon, helium, krypton, hydrogen and xenon (see table below).
Water vapor (humidity). The amount of water vapor in air at ground level can vary quite a bit - from almost zero to about 5 percent. Many factors influence the amount of humidity in the air at a given location and time.
Other constituents (which are usually present in trace amounts) which reflect local conditions

The Composition of Standard Dry Air is shown in Table 1.

The Physical Properties of Standard Dry Air shown in Table 2 reflect the mix of standard air components.

Gas	% by Volume	% by Weight	Parts per Million (by Volume)	Chemical Symbol	Molecular Weight
Nitrogen	78.08	75.47	780805	N₂	28.01
Oxygen	20.95	23.20	209450	O₂	32.00
Argon	0.93	1.28	9340	Ar	39.95
Carbon Dioxide	0.038	0.0590	380	CO₂	44.01
Neon	0.0018	0.0012	18.21	Ne	20.18
Helium	0.0005	0.00007	5.24	He	4.00
Krypton	0.0001	0.0003	1.14	Kr	83.80
Hydrogen	0.00005	Negligible	0.50	H₂	2.02
Xenon	8.7 x 10^-6	0.00004	0.087	Xe	131.30

Local additions to the composition of air can be very site-specific. They depend on the immediate surroundings, including wind direction, time of time of day and season of the year. Non-standard components may be present due to natural processes (biological or geological) or human activities (industrial, transportation, agricultural).)

Some common but "non-standard" components include methane and sulfur dioxide (which can be present in the parts per million range) nitrous oxide, nitrogen dioxide, ammonia and carbon monoxide.

Carbon dioxide (CO₂) is considered to be a "standard" component of air, although the amount of carbon dioxide in air will vary somewhat by location, and by season of the year. Therefore, any "standard" value for background levels of atmospheric carbon dioxide is an approximation. It is more meaningful to assign a range to the concentration of atmospheric carbon dioxide than to use a single value. Currently, 385 ppmv +/- 5 ppmv is a reasonable value. Worldwide, the average concentration of atmospheric carbon dioxide is rising at a rate of about 2 ppmv per year; making it necessary to link expected atmospheric carbon dioxide levels to particular time frames. (See extended discussion of CO2 levels and related topics below)

As air temperature rises, its ability to hold water vapor increases significantly.

The upper limit on water vapor content is predictable for at a given temperature, but the actual amount of water vapor which is present in air at a given time and place will be determined by many factors.

In temperate areas, the water vapor content will typically be between 1 to 2% in spring and fall, less than one-half percent in winter, and as much as 5% on a hot, humid summer day. In humid tropical areas, the water vapor content can be even higher, while in deserts, the water vapor content of air with the same (dry bulb) temperature may be very low. In near-polar regions the water vapor content of air will be less than 0.1%.

Water vapor is less dense than air. (The molecular weight of water is 18.02 vs.28.98 for dry air.) Consequently, when water vapor is mixed with air, the density of the mixture will be less than that of dry air. The difference in density between dry air and saturated air increases with temperature, because the percentage of water in saturated air increases. This difference in density leads to a long-recognized link between fluctuations in barometric pressure and weather conditions. "High" barometric pressure is associated with clear skies, while "low" barometric pressure is associated with cloudy skies and rain.

This table illustrates the impact of temperature on maximum potential water vapor content and air density at a pressure of one atmosphere.

Air Density and Maximum Water Vapor Content at Various Temperatures

Air Temperature (dry bulb)		Maximum Water Vapor (PPM_V)	Maximum Water Vapor (PPM_W)	Dry Air Density (Pounds/ Cu Ft)	Saturated Air (Pounds/ Cu Ft)	Density Difference Dry vs. Saturated
˚F	˚C	Maximum Water Vapor (PPM_V)	Maximum Water Vapor (PPM_W)	Dry Air Density (Pounds/ Cu Ft)	Saturated Air (Pounds/ Cu Ft)	Density Difference Dry vs. Saturated
20	-7	3,400	2,100	0.0826	0.0823	0.4%
30	-1	5,500	3,400	0.0809	0.0805	0.5%
40	4	8,400	5,200	0.0793	0.0787	0.8%
50	10	12,300	7,600	0.0778	0.0768	1.3%
60	16	17,800	11,100	0.0763	0.0749	1.9%
70	21	25,400	15,800	0.0748	0.0730	2.5%
80	27	35,900	22,300	0.0734	0.0709	3.5%
90	32	50,800	31,600	0.0721	0.0686	5.1%

Water vapor is the component of air which can vary the most from place to place, from day to day, even hour to hour. The amount of water vapor in air will literally change with the weather and other variables such as proximity to large bodies of water, and the temperature of the water in those nearby lakes, seas and oceans.

Both "relative" and "absolute" humidity measurements have been developed to quantify the amount of water vapor present in atmospheric air.

Relative humidity is closely linked to human comfort. Both very dry air and very humid air are less comfortable than air which has a relative humidity of about 50%.
Absolute humidity is most useful for engineering calculations such as sizing air compression and air drying / de-humidifying equipment.

The body of knowledge and techniques which allow physical and thermodynamic properties of air to be calculated for any set of temperature and pressure conditions are known as psychrometry.

Useful definitions and relationships related to water content and other properties of air are summarized below - and presented in more detail on the psychrometry page.

Dry-Bulb Temperature:	Dry bulb temperature is what is usually meant by "air temperature". It is measured with a normal thermometer.
Dew Point:	Dew point is the temperature at which water vapor begins to condense out of the air. Dew points can be defined and specified for ambient air or for compressed air.
Wet-Bulb Temperature:	Wet bulb temperature is never higher than dry bulb temperature. They are equal when air is at its dew point or saturation temperature. The difference between the dry bulb and wet bulb temperatures is an indicator of the humidity level. Wet-bulb temperature is the lowest temperature that water will reach by evaporative cooling, and that temperature is almost always lower than dry bulb. Wet bulb temperature is a critical parameter for sizing, and measuring the performance of evaporative-cooled cooling water systems.
Relative Humidity:	Relative humidity, when at dew point conditions, is 100%. Otherwise, relative humidity is the percentage of the amount of water vapor actually present in the air, to the maximum amount that the air could hold under those temperature and pressure conditions. This measurement is highly correlated with human comfort - with about 50% being most comfortable .
Absolute Humidity:	With the aid of a psychrometric chart, or its computerized equivalent, absolute values for water content such as weight fraction of ambient air, or weight-per-unit-volume of ambient air can be determined for any combination of dry bulb and wet bulb temperatures, or combination of dry bulb temperature and relative humidity. This measurement is required to design various types of moisture removal or humidification systems.

Psychrometric Charts and Online Psychrometric Calculations: Psychrometric charts graphically represent the thermodynamic properties of air. They depict inter-relationships between multiple properties, such as temperature, moisture content, density and energy (enthalpy). Charts can be drawn for various elevations. For each elevation two known properties allow determination of all other properties plotted on the chart.
Download psychrometric charts in pdf format, drawn for sea level (one atmosphere) conditions. Choose charts in: Inch-Pound (I-P) units or International System (SI) Units Or use an online psychrometric calculator to generate properties for air. Specify two properties (e.g. dry bulb temperature and % relative humidity) and calculate other thermodynamic and physical values. The calculator will correct for elevation (or ambient pressure).

	The I-P and SI psychrometric charts and the online psychrometric calculator are provided under license from the Linric Company.

Elevation (Altitude) Affects Air Pressure and Temperature:

For convenience, much information about air is specified for "sea level" or "one atmosphere" conditions. But much human activity takes place at higher elevations - in come cases only a few feet or meters above sea level - in other cases hundreds or thousands of feet or meters higher.

Almost all of our activities take place in the lowest level of the Earth's atmosphere - the Troposphere. The Troposphere extends to a height of about 6 kilometers (4 miles) at the poles, and to 11 kilometers (36,000 feet or 7 miles) near the equator. The Troposphere has similar composition at all levels, but its average pressure, temperature and density decrease with altitude.

In the lowest levels of the atmosphere, air temperature decreases with elevation - on average, about 3.56˚F for every 1000 feet, or 6.5˚C for each 1000 meters of elevation. Atmospheric pressure drops as well - about 0.5 psia for each 1000 feet of elevation, or about 1.1 kPa for each 100 meters. Density decreases rapidly with altitude, as density is proportional to the product of pressure and temperature.

Average Atmospheric Pressure at Typical City and Industrial Plant Elevations

Feet	Meters	psia	Atm	Bar (a)	kPa	kg/cm2	In Hg	Mm Hg
0	0	14.7	1.00	1.013	101	1.03	29.9	760
500	150	14.4	0.98	0.994	99.4	1.01	29.4	747
1000	300	14.2	0.96	0.976	97.6	1.00	28.9	734
1500	450	13.9	0.94	0.956	95.6	0.98	28.3	719
2000	600	13.7	0.93	0.939	93.9	0.96	27.8	706
2500	750	13.4	0.91	0.923	92.3	0.94	27.3	694
3000	900	13.2	0.89	0.906	90.6	0.92	26.8	681
3500	1070	12.9	0.88	0.888	88.8	0.91	26.3	668
4000	1220	12.7	0.86	0.871	87.1	0.89	25.8	655
4500	1370	12.4	0.85	0.858	85.8	0.87	25.4	645
5000	1520	12.2	0.83	0.842	84.2	0.86	24.9	633
5500	1680	12.0	0.81	0.825	82.5	0.84	24.4	620

Nitrogen, Oxygen and Argon, the three primary constituents of air, can be recovered from air in air separation plants. Each of these gases is valued for particular properties. These gases are used extensively in industry, the medical field, and in some products used in homes.

Oxygen is valued for reactivity. It supports biological processes and combustion. It finds numerous uses in steelmaking and other metals refining and fabrication processes, as well as in chemicals, pharmaceuticals, petroleum processing, glass and ceramic manufacture, pulp and paper manufacture, healthcare, environmental protection through treatment of municipal and industrial effluents, and in other miscellaneous uses.

Oxygen is often used, with or instead of air, to increase the amount of oxygen available for combustion or biological activity. It is used to assist breathing in hospitals and other places. In industry, oxygen is used to increase reaction rates, which leads to greater throughput in existing equipment and smaller sizes for new equipment. In some cases, such as industrial and municipal effluent treatment, oxygen is first converted to ozone, its even more reactive form, prior to use; which enhances reaction rates and ensures full reactions take place with undesired compounds.

Gaseous nitrogen is valued for inertness. It is used to shield potentially reactive materials from contact with oxygen.

Liquid nitrogen is valued for coldness plus inertness. Vaporizing liquid nitrogen and warming the gas to ambient temperature absorbs a large quantity of heat. Liquid nitrogen combines inertness with intense cold, making it an ideal coolant for certain applications. One of these is food freezing, where very rapid freezing results in minimal cell damage from ice crystals, leading to improved appearance, taste and texture. Liquid nitrogen is also used to facilitate machining or fracturing of soft or heat sensitive materials. These include plastics, certain metals, pharmaceuticals, and even used tires.

Argon is valued for its total inertness. It is used in critical industrial processes such as producing high quality stainless steels and growing impurity-free silicon crystals for semi-conductor manufacture. It is used as a shield gas in critical welding applications, as a filler gas for light bulbs and as a heavier-than-air filler gas for the space between glass panels in high-efficiency multi-pane windows.

The so-called "rare" gases Neon, Krypton and Xenon, are present in very low concentrations in air, which makes them economically recoverable only in large air separation plants. All of these gases are monatomic. They are valued for chemical inertness and for their light emitting properties when electrically charged.

The boiling point of Neon is significantly lower than nitrogen (lower than all the gases except helium and hydrogen). It can be used as a very low temperature working fluid in refrigeration cycles.

Krypton and Xenon have additional uses that take advantage of their inertness and high molecular weight (83.80 and 131.30, respectively). Krypton and Xenon are about two to three times as heavy as argon (molecular weight 39.95) and approximately three to four times as heavy as nitrogen (not a true inert gas) at 28.0. Krypton and Xenon are used in very high insulating value multi-pane windows as they minimize heat loss due to convection between the panes. These gases are also used in light bulbs, where these heavy filler gases slow evaporation of the hot tungsten filament, leading to longer useful operating life and/or the ability to use higher efficiency, thinner filaments while still retaining acceptable operating life.

Many aspects of air separation design and operation are determined by the properties of air.

Air separation plants are designed to handle expected concentrations of non-standard air components safely, and in a manner which ensures the purity of protects. When the local atmosphere is known to contain (or potentially contain) unusually high concentrations of gases such as sulfur oxides, CO2, hydrogen, ammonia, methane or higher hydrocarbons, additional purification (impurity removal) processes are added to air compression and air separation systems.

Average atmospheric pressure at a plant site (which is primarily a function of elevation) must be known to properly select air compression machinery. Air compressors are typically equipped with inlet guide vanes or other means of capacity control, but still have a limited range of operation. The maximum volume of inlet air that can be processed (as measured in actual cubic feet or cubic meters per minute) is similar at any inlet pressure. To deliver the same amount of air (measured in Pounds, Tons, Kilograms, Standard Cubic Feet, Normal Cubic Meters, etc.) when operating at high elevations, a compressor must process greater physical volumes of inlet air than would be required if the compressor were being operated near sea level (about 3.4% more for each 1000 feet or 1.1% more for each 100 meters). To achieve the same discharge pressure when the inlet pressure is lower than it would be at sea level, the pressure ratio across the compressor must be higher; which requires more power.

Weather-related variations in atmospheric pressure have only minor impacts on the performance of installed air separation systems. However, seasonal and daily variations in air temperature and humidity can have significant impacts on plant operations:

Temperature Impact on Compression Capacity: Cold, dry winter air can be 10% denser than dry summer air. Since air compression equipment has limited ability to adjust the volume of inlet air that will be compressed, lower density summer air decreases the air handling and production capacities of air separation systems while higher density, cooler winter air results in greater air compression capacity (as measured in standard cubic feet, standard cubic meters, or by weight).
Humidity Impact on "Dry Air" Delivery: The effective "dry air" compression capacity of air separation plants is reduced on humid days. Water vapor entering an air separation system must be removed to meet product specifications and to avoid operating problems. Since humidity can represent as much as 5% of the volume of ambient air on a hot, humid summer day, the ability to deliver "dry air" to the air separation equipment can be reduced by as much as 5% on such days.
Density Impact on Discharge Pressure: The pressure that is developed across a centrifugal compressor is a function of the density of the gas being compressed. Less-dense hot and humid air results in less pressure increase.   Conversely, higher discharge pressures are available when compressing cool dry winter air.

Because of these atmospheric effects, the effective production capacity of air separation equipment can be measurably reduced on hot, humid days compared to winter and yearly-average conditions.

To assure satisfactory operation, the effects of ambient air temperature and moisture content must be taken into account when sizing new equipment, relocating used equipment, and comparing plant operating data taken at different times.

The concentration of carbon dioxide in the atmosphere which is shown in the table above (380 ppmv) is representative of recent measurements.
This is an historically high value for CO2 concentration in air. It is higher than any level experienced by modern man - and possibly higher than any atmospheric levels reached for many hundreds of thousands, if not millions of years.
Unlike other components of "standard air", the concentration of carbon dioxide is constantly changing. Annually, the concentration of atmospheric carbon dioxide rises and falls in a seasonal pattern over a range of about 6 ppmv; or about one-and-a-half percent of its average value. Over periods lasting 70,000 years or more, it rises and falls within a characteristic range. During periods of global cooling and widespread glaciation, the concentration is typically near 180 ppmv. During inter-glacial warm periods, it rises to around 300 ppmv.
The carbon dioxide concentration in air was around 200 ppmv about 11,000 years ago, after the end of the last glacial period. It rose to 300 ppmv early in the Twentieth Century. It has continued to rise to its current value at an increasingly rapid rate, which is now near 2 ppmv / year, or an annual growth rate of about 0.5%.
There is every reason to believe the concentration of CO₂ in the atmosphere will continue to increase for several decades, at the very least.
Many human activities, including the use of carbon-containing fuels for heating, industry and transportation, and wide-spread de-forestation, have undoubtedly contributed to the recent rapid rise in atmospheric carbon dioxide concentration.
What can be done, and what should be done, about the rising concentration of CO2 in the atmosphere are matters of intense global research and debate.

Over its long history, the Earth has been no stranger to atmospheric change, temperature change, and the appearance and disappearance of species of plants and animals.

A huge body of historical evidence shows a clear correlation between the amount of carbon dioxide in the Earth's atmosphere and average annual temperature on the planet.

Not all changes in global temperatures have been caused by changes in the CO₂ content of the air. Some major temperature swings (which lasted many millions of years) have been caused by disruption to the circulation patterns in the world's oceans (e.g. the "Gulf Stream", "California Current", and "Humboldt Current"). These flows transport warm waters toward the poles and return cold water to equatorial regions. They have major impacts on local temperatures around the world. On a very long (geological) time scale, there have been several times when large land masses have accumulated, which caused very drastic changes in ocean current patterns, and world climate. There is current concern that rapidly melting polar ice stores can make cold ocean waters less dense, and slow down the heat transfer between the polar and equatorial regions of the Earth.

Perspective on the Long-term Evolution and Stabilization of the Earth's Atmosphere:

It is instructive to point out that the Earth's atmosphere has changed drastically over time.

The air that we are accustomed to breathing is the product of billions of years of evolving life forms interacting with the seas, the land, and the air :

About 4 billion years ago, the Earth had an atmosphere which was mostly CO₂.
The development of anaerobic bacterial photosynthesis and blue-green algae ("cyanobacteria") about 3.5 billion years ago resulted in the release of oxygen to the atmosphere and the sequestering of carbon, lowering the level of CO₂ in the atmosphere. The change in composition had a" reverse greenhouse effect" on the earth's energy balance; producing a prolonged period (about 500 million years) of widespread glaciation between 2.5 and 2.0 billion years ago.
As photosynthesizing plants and other life forms, such as sea creatures appeared, more carbon dioxide was removed and sequestered in organic matter and as carbonate in shells. Eventually, oxygen levels increased to the vicinity of the current level, paving the way for new life forms to emerge and evolve which were dependent upon oxygen in the air.
Oxygen and carbon dioxide concentrations in the atmosphere have been maintained at near current levels for millions of years through many dynamic, and largely-self regulating, inter-relationships between the land, the seas, the air and various life forms.

Oxygen is reactive. Without replenishment of gaseous oxygen by photosynthesis, it would disappear as it combined with other el

Air: Its Composition and Properties The Source of Industrial Gas Products Nitrogen, Oxygen, Argon

Air: Its Composition and Properties
The Source of Industrial Gas Products
Nitrogen, Oxygen, Argon