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Chapter 1
Basic Physics and Chemistry of the Earth’s Atmosphere
1.1Introduction
This chapter gives an introduction to the basic physical and chemical processes in the Earth’s atmosphere. Although there are a number of excellent atmospheric chemistry and physics textbooks [e.g., Brasseur et al., 1999; Jacob, 1999; Andrews, 2000; Salby, 2012], most of them focus only on photochemical processes induced by electromagnetic waves (photons) from sunlight, which can be seen by our eyes. However, there are also many invisible particles such as electrons and other charged particles either originating from outer space (e.g., cosmic rays) or produced from the ionization of atoms and molecules in the atmosphere [Johnson, 1990; Jackman, 1991]. In this Chapter, an introduction to the Earth’s atmosphere is given in Sec. 1.2, followed by a description of various radiation sources (mainly solar radiation and cosmic-ray radiation) in the atmosphere in Sec. 1.3. Subsequently, photon interactions with atmospheric molecules are described briefly in Sec. 1.4, while Sec. 1.5 is devoted to a description of atmospheric ionization. Then, a brief review of ion chemistry in the atmosphere is given in Sec. 1.6. Finally, a summary is given in Sec. 1.7.
1.2The Earth’s atmosphere
The Earth’s atmosphere is vital to sustaining life on its surface. This atmosphere consists of a mixture of ideal gases; the major constituents are molecular nitrogen (N2) and oxygen (O2) by volume. But the minor gases, in particular, water vapor (H2O), carbon dioxide (CO2) and ozone (O3), are also crucial to the ecological system of the Earth.
The Sun is the primary energy source for the Earth. The Sun is a radiating star. Different planets in the Solar system receive various solar radiation intensities, inversely proportional to the squares of their distances from the Sun. The actual intensity of sunlight that reaches the surface depends also on the thickness and composition of the atmosphere of the Earth. While solar radiation makes life on Earth possible, the incoming higher-energy ultraviolet (UV) radiation must be absorbed by the Earth’s atmosphere (especially by the ozone molecules), protecting us from the harmful aspects of solar radiation. The Earth’s nearly ideal positioning in the Solar system and its unique atmospheric constituents render us the benefits of proximity to the Sun without being baked or dried like Venus or Mars.
Fig. 1.1. Typical vertical concentration distributions of chemical constituents in the atmosphere. Based on data from Smith and Adams [1980] and Brasseur et al. [1999].
1.2.1Atmospheric compositions
The Earth’s atmosphere is retained by gravity. As shown in Fig. 1.1, the atmosphere (dry air) is composed of about 78.1% N2, 20.9% O2, 0.93% argon (Ar) gas, 0.04% CO2 (percent by volume), and trace amounts of other gases. A variable percent, 0.001-7%, of water vapor is also present in the atmosphere.
1.2.2Atmospheric layers
The Earth’s atmosphere at altitudes below 100 km can be divided into several layers from lower to higher altitudes: troposphere, stratosphere, mesosphere and thermosphere. With increasing altitudes, the atmospheric temperature decreases in the troposphere, increases in the stratosphere, decreases again in the mesosphere, and increases again in the thermosphere. A typical structure of atmospheric temperature versus altitude below 100 km is shown in Fig. 1.2.
Fig. 1.2. Typical vertical structure of atmospheric temperature in the atmosphere. Based on data from Brasseur et al. [1999].
The troposphere is the region extending from Earth’s surface to between 8-10 km near the polar regions and 16-18 km in tropical regions with some seasonal variations. In the troposphere, the atmospheric temperature decreases with rising altitude. This layer contains about 80% of the entire atmosphere’s mass, and it is the place most daily weather occurs that is observed from the ground. Commercial airlines typically fly at altitudes of about 10 km (30 kft). The tropopause is the highest troposphere where the temperature reaches a minimum and the air is almost completely dry.
Extending from the tropopause to about 50 km is the stratosphere in which the temperature increases with altitude. Ozone and oxygen in the stratosphere absorb much of the UV radiation from the Sun. This is an important protecting layer to living creatures on Earth. The strotopause is the top of the stratosphere where maximum temperature is reached.
Next, the mesosphere is the region extending from 50 km to 80-85 km, in which the temperature falls again with altitude. There are visible meteors between 65 and 120 km above the Earth, disintegrating at altitudes of 50-95 km; millions of meteors enter Earth’s atmosphere daily. The top of the mesosphere is referred to as the mesopause with an altitude of about 80 km where the temperature reaches a second minimum,
Above the mesopause, the thermosphere is the region extending from 80-85 km to more than 500 km, where the temperature increases rapidly with altitude. The International Space Station orbiting the Earth is located at the altitude of 330-400 km in the thermosphere.
Lastly, the ionosphere overlaps the thermosphere as it extends from about 80 km to 480 km. This layer contains electrons, ions and neutral molecules and is the inner edge of the magnetosphere. The magnetosphere is the region around Earth influenced by the geomagnetic field. This region is used to reflect radio signals over long distances and it is where auroral displays occur.
The troposphere is generally called the lower atmosphere; the stratosphere and mesosphere are called the middle atmosphere; and the thermosphere is called the upper atmosphere.
1.2.3Atmospheric temperature profile
The variation of atmospheric temperature with altitude, shown in Fig. 1.2, can be roughly explained by some physical mechanisms. In the upper atmosphere, absorption of solar radiation at short wavelengths (0-100 nm) is an efficient process, resulting in the effective photoionization and photodissociation of O2 and N2 molecules (Fig. 1.3). Thus, the atmospheric temperature increases drastically. Below 100 km, all the neutral gas particles, the electrons and the ions in the plasma are in equilibrium with a...
