The structure and composition of the atmosphere

Table of Contents

1.1. Formation of the Earth atmosphere

1.2. A short history of the atmospheric chemistry

1.3. Atmospheric composition

1.4. Vertical structure of the atmosphere

1.4.1. Vertical change of composition

1.4.2. Vertical temperature changes

1.5. The planetary boundary layer

The total mass of the atmosphere is approximately 5.3·10¹⁸ kg, against the 1.4·10²¹ kg mass of the hydrosphere (oceans, sees, lakes, rivers, groundwater, snow and ice) and 5.98·10²⁴ kg mass of the Earth. The upper border of the atmosphere is not a well-defined altitude. The atmospheric material steadily decreases with height, until it gradually reaches interplanetary space. The atmosphere is an envelope of gases and particles surrounding the Earth. This amount of materials remains around our planet during the rotation around its axis and orbit around the Sun.
The relatively dense area of the Earth's atmosphere is an extremely thin layer compared to the whole atmosphere. Half of the atmosphere’s mass can be found below 5.5 km and around 99% of the air is located in the lower 30 km’s layer. The whole atmosphere actually the same as the range of the magnetosphere (Figure 1.1), which is the result of the interaction between Earth’s natural magnetic field and solar wind^[1].

Figure 1.1: The magnetosphere around the Earth is formed by the interaction between Earth’s natural magnetic field and solar wind

1.1. Formation of the Earth atmosphere

The Earth formed about 4.5 billion years ago, as hot molten rock. It’s first atmosphere probably contained hydrogen and helium and some simple compounds of hydrogen, like ammonia (NH₃) or methane (CH₄). This first atmosphere of the Earth could similar to the atmosphere of Jupiter and Saturn today. Because our planet didn’t have a magnetic field to protect it yet, the intense solar wind from the Sun blew this early atmosphere away. In essence, the Earth was lost its early atmosphere. Meanwhile, as the Earth cooled enough, a solid crust with several active volcanoes was developed about 4.4 billion years ago. By the strong volcanic activity, gases from the hot interior of our planet reached the surface. These gases – basically water vapour, carbon dioxide and ammonia – created the secondary atmosphere of the Earth. The atmospheres of Mars and Venus today – which contain mainly carbon-dioxide – are similar to this early atmosphere of the Earth.
Over hundred millions of years the atmosphere cooled down gradually, therefore most of water vapour condensed and formed the clouds. Precipitation from these clouds created the oceans. Simultaneously, most of atmospheric carbon-dioxide was absorbed by the oceans. At the same time, light from the Sun broke down the ammonia molecules releasing the chemically inactive nitrogen into the atmosphere (Figure 1.2).

Figure 1.2: Probable composition of the atmosphere during the history of the Earth

Based on some theory, the oxygen was first produced in the atmosphere by photochemical dissociation of water vapour by intense ultraviolet radiation:

2H2O + uv radiation → 2H2 + O2.

(R1.1)

However this amount was negligible and free oxygen was probably produced as a by-product of photosynthesis^[2] by tiny organisms known as cyanobacteria (or blue-green algae) from around 2.500 millions years ago. Photosynthesis uses carbon dioxide, water, and light energy releasing organic compounds (carbohydrates) and oxygen:

CO2 + H2O + sunlight → organic compounds + O2

(R1.2)

Initially, the small amount of atmospheric oxygen consumed for oxidation of rocks at the surface (weathering process). Complete oxidation of the surface rocks, oxygen levels in the atmosphere began to grow more intensively. As the atmospheric oxygen reached 1–2% of present oxygen level, ozone (O₃) could form to shield Earth’s surface from intense ultraviolet radiation:

2O2 + uv radiation → O3 + O

(R1.3)

and

O + O2 = O3.

(R1.4)

At this time, primitive plants formed which facilitated the photosynthesis process. Land plants developed a few million (based on new studies around 700 million) years ago and would have removed carbon dioxide (from around 1–5% down to 0.04%) at the same time steadily increased the amount of oxygen in the atmosphere. Oxygen levels fluctuated in the last few million years during various time periods regulated by climate, volcanism and plate tectonics (Holland, 2006). Finally, the atmospheric oxygen level stabilized at around 21% (Figure 1.3).

Figure 1.3: The probable variation of oxygen level in the atmosphere during the history of the Earth

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^[1] Solar wind: a stream of charged particles (electrons and protons) ejected from the upper atmosphere of the Sun.

^[2] Photosynthesis: during this process, plants and other organisms convert light energy from the Sun to chemical energy. Photosynthesis requires sunlight, carbon-dioxide and water. In plants, photosynthesis
occurs mainly within the leaves.

1.2. A short history of the atmospheric chemistry

Greek philosopher, Anaximenes (585–528 BC) declared that air was the primary substance and the source of all other things. Later, Empedocles (ca. 490–430 BC) described the air as one of the four elements (see e.g. May, 2010). This conception was accepted until 18^th century. At this time, a question was arisen: whether air is a compound, or a mixture of individual gases. First scientific studies about atmospheric composition were published in the 1700s. Chemical compounds in the atmosphere were discovered one after the other (Table 1.1) to confirm that air is a mixture of gases (see e.g. Anfossi and Sandroni, 1993).
Table 1.1: Important discoveries of atmospheric elements

Date	Compound	Explorer(s)
1750s	carbon dioxide	Joseph Black
1766	hydrogen	Henry Cavendish
1772	nitrogen	Daniel Rutherford
1774 1772 (published in 1777)	oxygen	Joseph Priestley and Carl Wilhelm Scheele
1840	ozone	Christian Friedrich Schönbein
1894	argon	Lord Rayleigh and William Ramsay

Table 1.2: Some important milestones of atmospheric chemistry in the 20^th century

Date	Explorer(s)	Discovery
1924	Gordon Dobson	developed a spectrophotometer and started the regular measurements of total-column ozone
1930	Sydney Chapman	described theory that explains existence of ozone „layer”
1960	Arie Jan Haagen-Smit	described the emergence of the photochemical smog
1973	James Lovelock	first detected CFC’s (Chlorofluorocarbons) in the atmosphere
1995	Paul Crutzen, Mario Molina and Frank Sherwood Rowland	the Nobel Prize in Chemistry was awarded jointly "for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone".

After the clarification of fundamental composition of the atmosphere till the late 19^th century, the attention was focused on the atmospheric trace gases with very small concentrations. In the 20^th century new research directions were appeared, namely the analysis of temporal variation of trace gas concentrations and investigations of chemical reactions in the atmosphere (Table 1.2).
Today, major challenges of atmospheric chemistry are to describe the relationships and feedbacks between chemistry and climate, as well as the exchange processes between the surface and the atmosphere.