COMPOSITION OF ATMOSPHERIC AIR AND POLLUTANTS - ENVIRONMENT - 2025

The composition of the atmospheric air or atmosphere is defined by the proportion of the different gases contained in it, which has been in constant variation throughout the history of the Earth. The atmosphere of the forming planet contained mainly H ₂ and other gases such as CO ₂ and H ₂ O. About 4.4 billion years ago, the composition of atmospheric air was enriched mainly with CO ₂.

With the appearance of life on Earth, an accumulation of methane (CH ₄) occurred in the atmosphere, since the first organisms were methanogens. Later, photosynthetic organisms appeared, which enriched the atmospheric air with O ₂.

General view of the Earth's atmosphere. Source: Reto Stöckli (land surface, shallow water, clouds) Robert Simmon

The composition of atmospheric air today can be divided into two large layers, differentiated in their chemical composition; the homosphere and the heterosphere.

The homosphere is located from 80 to 100 km above sea level and is made up mainly of nitrogen (78%), oxygen (21%), argon (less than 1%), carbon dioxide, ozone, helium, hydrogen and methane, among other elements present in very small proportions.

The heterosphere is made up of low molecular weight gases and is located above 100 km in altitude. The first layer has molecular N ₂, the second atomic O, the third helium and the last one is made up of atomic hydrogen (H).

History

Studies of atmospheric air began thousands of years ago. The moment primitive civilizations discovered fire, they began to have a notion of the existence of air.

Ancient Greece

During this period, they began to analyze what air is and what it does. For example, Anaxímades of Miletus (588 BC – 524 BC) considered that air was essential for life, since living beings fed on this element.

For his part, Empedocles of Acragas (495 BC – 435 BC) considered that there were four fundamental elements for life: water, earth, fire and air.

Aristotle (384 BC-322 BC) also considered air to be one of the essential elements for living beings.

Discovery of the composition of atmospheric air

In 1773 the Swedish chemist Carl Scheele discovered that air was made up of nitrogen and oxygen (igneous air). Later, in 1774, the British Joseph Priestley determined that air was made up of a mixture of elements and that one of these was essential for life.

In 1776, the Frenchman Antoine Lavoisier called oxygen to the element that he isolated from the thermal decomposition of mercury oxide.

In 1804, the naturalist Alexander von Humboldt and the French chemist Gay-Lussac analyzed the air coming from different parts of the planet. The researchers determined that atmospheric air has a constant composition.

It was not until the late 19th and early 20th centuries, when the other gases that are part of the atmospheric air were discovered. Among these we have argon in 1894, then helium in 1895, and other gases (neon, argon, and xenon) in 1898.

characteristics

Earth's atmosphere, in the background the Moon. Source: NASA, via Wikimedia Commons

The atmospheric air is also known as the atmosphere and it is a mixture of gases that covers the planet Earth.

Origin

Little is known about the origin of Earth's atmosphere. It is considered that after its separation from the sun, the planet was surrounded by an envelope of very hot gases.

These gases were possibly reducing and coming from the Sun, composed mainly of H ₂. Other gases were probably CO ₂ and H ₂ O emitted by the intense volcanic activity.

It is suggested that part of the gases present cooled, condensed and gave rise to the oceans. The other gases remained forming the atmosphere and others were stored in rocks.

Structure

The atmosphere is made up of different concentric strata separated by transition zones. The upper limit of this layer is not clearly defined and some authors place it above 10,000 km above sea level.

The attraction of the force of gravity and the way in which gases are compressed influences their distribution on the earth's surface. Thus, the largest proportion of its total mass (approximately 99%) is located in the first 40 km above sea level.

Layers of the atmosphere. Source: This SVG image was created by Medium69.Cette image SVG a été créée par Medium69.Please credit this: William Crochot

Different levels or layers of atmospheric air have different chemical composition and temperature variations. According to its vertical arrangement, from the closest to the furthest from the Earth's surface, the following layers are known: the troposphere, stratosphere, mesosphere, thermosphere and exosphere.

In relation to the chemical composition of atmospheric air, two layers are defined: the homosphere and the heterosphere.

Homosphere

It is located in the first 80-100 km above sea level, and its composition of gases in the air is homogeneous. In this the troposphere, stratosphere and mesosphere are located.

Heterosphere

It is present above 100 km and is characterized by the composition of the gases present in the air is variable. Matches the thermosphere. The composition of gases varies at different heights.

Composition of primitive atmospheric air

Planetesimal disk. Source: Public Domain, commons.wikimedia.org

After the formation of the Earth, approximately 4,500 million years ago, gases began to accumulate that formed the atmospheric air. The gases came mainly from the Earth's mantle, as well as from the impact with planetesimals (aggregates of matter that originated the planets).

CO build-up

The great volcanic activity on the planet began to release various gases into the atmosphere, such as N ₂, CO ₂ and H ₂ O. Carbon dioxide began to accumulate, since carbonation (the process of fixing atmospheric CO ₂ in the form carbonate) was scarce.

The factors that affected the fixation of CO ₂ at this time were very low intensity rains and a very small continental area.

Origin of life, accumulation of methane (CH

The first living beings that appeared on the planet used CO ₂ and H ₂ to carry out respiration. These early organisms were anaerobic and methanogenic (they produced large amounts of methane).

Methane accumulated in the atmospheric air, because its decomposition was very slow. It decomposes by photolysis and in a nearly oxygen-free atmosphere, this process can take up to 10,000 years.

According to some geological records, about 3.5 billion years ago there was a decrease in CO ₂ in the atmosphere, which has been associated with the fact that the air rich in CH ₄ intensified the rains, favoring carbonation.

Large oxidative event (accumulation of O

It is considered that about 2.4 billion years ago the amount of O ₂ on the planet reached significant levels in the atmospheric air. The accumulation of this element is associated with the appearance of photosynthetic organisms.

Photosynthesis is a process that allows the synthesis of organic molecules from other inorganic ones in the presence of light. During its occurrence, O ₂ is released as a by-product.

The high photosynthetic rate produced by cyanobacteria (first photosynthetic organisms) was changing the composition of the atmospheric air. The large amounts of O ₂ that were released returned to the atmosphere increasingly oxidizing.

These high levels of O ₂ influenced the accumulation of CH ₄, since it accelerated the photolysis process of this compound. As methane in the atmosphere fell dramatically, the planet's temperature dropped and glaciation occurred.

Another important effect of the accumulation of O ₂ on the planet was the formation of the ozone layer. Atmospheric O ₂ dissociates under the effect of light and forms two atomic oxygen particles.

Atomic oxygen recombines with molecular O ₂ and forms O ₃ (ozone). The ozone layer forms a protective barrier against ultraviolet radiation, allowing the development of life on the earth's surface.

Atmospheric nitrogen and its role in the origin of life

Nitrogen is an essential component of living organisms, as it is necessary for the formation of proteins and nucleic acids. However, atmospheric N ₂ cannot be used directly by most organisms.

Nitrogen fixation can be biotic or abiotic. It consists of the combination of N ₂ with O ₂ or H ₂ to form ammonia, nitrates or nitrites.

The N ₂ contents in atmospheric air have remained more or less constant in the Earth's atmosphere. During the CO ₂ accumulation period, N ₂ fixation was basically abiotic, due to the formation of nitrogen oxide, formed by the photochemical dissociation of H ₂ O and CO ₂ molecules that were the source of O ₂.

When atmospheric CO ₂ levels decreased, nitrogen oxide formation rates decreased dramatically. It is considered that during this time the first biotic routes of N ₂ fixation originated.

Current atmospheric air composition

Atmospheric air is made up of a mixture of gases and other quite complex elements. Its composition is mainly affected by altitude.

Homosphere

The chemical composition of dry atmospheric air at sea level has been found to be fairly constant. Nitrogen and oxygen make up about 99% of the mass and volume of the homosphere.

Atmospheric nitrogen (N ₂) is in a proportion of 78%, while oxygen constitutes 21% of the air. The next most abundant element in atmospheric air is argon (Ar), which occupies less than 1% of the total volume.

Components of atmospheric air. Source: https://commons.wikimedia.org/wiki/File:Proporci%C3%B3n_de_gases_de_la_atm%C3%B3sfera.svg?uselang=es#filelinks Modified.

There are other elements that are of great importance, even when they are in small proportions. Carbon dioxide (CO ₂) is present in a proportion of 0.035% and water vapor can vary between 1 and 4%, depending on the region.

Ozone (O ₃) is found in a proportion of 0.003%, but it forms an essential barrier for the protection of living beings. Also in this same proportion we find various noble gases such as neon (Ne), krypton (Kr) and xenon (Xe).

In addition, there is presence of hydrogen (H ₂), nitrous oxides and methane (CH ₄) in very small quantities.

Another element that is part of the composition of atmospheric air is the liquid water contained in clouds. Likewise, we find solid elements such as spores, pollen, ashes, salts, microorganisms and small ice crystals.

Heterosphere

At this level, altitude determines the predominant type of gas in atmospheric air. All gases are light (low molecular weight) and are organized in four different layers.

It is seen that as the height increases, the more abundant gases have a lower atomic mass.

Between 100 and 200 km of altitude, there is a greater abundance of molecular nitrogen (N ₂). The weight of this molecule is 28.013 g / mol.

The second layer of the heterosphere is made up of atomic O and is located between 200 and 1000 km above sea level. Atomic O has a mass of 15,999, being less heavy than N ₂.

Later, we find a helium layer between 1000 and 3500 km high. Helium has an atomic mass of 4.00226.

The last layer of the heterosphere is made up of atomic hydrogen (H). This gas is the lightest in the periodic table, with an atomic mass of 1.007.

References

1. Katz M (2011) Materials and raw materials, Air. Didactic Guide Chapter 2. National Institute of Technological Education, Ministry of Education. Buenos Aires. Argentina. 75 pp
2. Monks PS, C Granier, S Fuzzi et al. (2009) Atmospheric composition change-global and regional air quality. Atmospheric Enviroment 43: 5268-5350.
3. Pla-García J and C Menor-Salván (2017) The chemical composition of the primitive atmosphere of planet Earth. Chem 113: 16-26.
4. Rohli R and Vega A (2015) Climatology. Third Edition. Jones and Bartlett Learning. New York, USA. 451 pp.
5. Saha K (2011) The Earth's Atmosphere, its physics and dynamics. Springer-Verlag. Berlin, Germany 367 pp.