Few things are as sure as the air: it is all around us, invisible, odorless, and essential to all life on Earth. Its very uniformity made the ancient Greeks classify it as one of the four basic elements: earth, water, fire, and air. Yet the air is actually a combination of gaseous elements that have a remarkable uniformity in terms of their contribution to the whole. While Earth's atmosphere has no definitive ending point, but instead thins out to near nothingness several hundred kilometers above the surface, virtually all of the atmosphere exists below100 km. The constituent elements are primarily nitrogen and oxygen, with a small amount of argon. Below a height of about 86 kilometers, the three main gaseous elements, which together account for about 99.9% of the total atmosphere, exist in essentially constant proportion to the total: nitrogen comprises 78% of air by volume, oxygen comprises 21% of air by volume, and argon comprises another 0.9%.
The remaining 0.1% of the atmosphere consists of the trace constituents. These include water vapor, carbon dioxide, ozone, methane, various oxides of nitrogen, neon, and helium. They are called trace gases because they exist in small amounts.
Near the surface, water vapor can be as high as 2-3% of the gaseous portion of the atmosphere in a warm ground fog. (The 0.1% figure cited above for trace constituents is a global average.) In the stratosphere, the layer of the atmosphere where temperatures increase with height and vertical motions are weak, water vapor is typically a few parts water per million molecules of air by volume (ppmv). Ozone can be vanishingly small near the surface and as high as 10 ppmv in the stratosphere. Typical global averages of the other trace gases in descending order are: carbon dioxide (0.03% or 300 ppmv), neon (0.002% or 20 ppmv), helium (5 ppmv), and methane (1.7ppmv).
2.2.1 Water vapor -- Essential to all life, water in the lowest level of the atmosphere in various forms (vapor, liquid, solid) creates many of the observed weather features, like clouds, rain, snow, ice, and fog. As we shall see, this lowest level of the atmosphere is called the troposphere, and it is separate from the next highest level, called the stratosphere. Water vapor concentrations can vary, as stated above, from nearly 3 percent at the surface to almost nothing high up, depending on weather, altitude, and season. As a consequence of its properties and its abundance in the troposphere, water vapor concentration is one of the most important considerations in understanding meteorology and climate.
Water vapor amounts in the stratosphere are much lower, typically on the order of 4 to 6 parts per million by volume (0.004 to 0.006 %). Yet even this water vapor plays a significant role in the energy budget of the atmosphere. Water vapor is radiatively active, absorbing and reradiating the thermal (or infrared) energy from the surface. It also plays a key role in the formation of particles in the stratosphere, such as aerosols and at very low temperatures, special types of clouds known as polar stratospheric clouds.
2.2.2 Methane -- Though methane has a global average of just 1.7 ppmv of air in the well-mixed layer of the lower atmosphere, it displays, like other trace gases, considerable spatial variation in concentration at the surface. Near its source regions, concentrations are much higher; in sink regions, concentrations are lower than the global average. Two source regions for methane are feedlots, specifically, cattle flatulence, and rice paddies. Both sources reside at the surface. As a result, methane at higher altitudes must be mixed up from below. Such vertical mixing of air in the stratosphere is relatively slow, thus limiting the supply of methane there. On the loss side, reactions of methane with the hydroxyl radical OH reduce its concentration in the stratosphere below the global average of 1.7 ppmv.
2.2.3 Ozone -- Ozone is critical to life on earth because it blocks harmful ultraviolet (UV) radiation from the Sun reaching the surface. Ozone absorbs harmful UV radiation from the Sun in the 200-300 nanometer (i.e. 2-3 x 10-7 meter) wavelength region (see Chapter 4). In so doing, ozone is responsible for life as we know it. Ozone forms 10 to 40 kilometers above the ground in a region where intense UV radiation from the Sun breaks apart (photodissociates) the diatomic oxygen molecule O2. Absorption of solar radiation occurs in the stratosphere, causing it to be warmer than it would otherwise be. Indeed, it is because of this ozone layer that the temperatures rise with height, giving rise to the structure of the stratosphere. It is contrasted with the troposphere (see Section 3).
Ozone is formed by the reaction of an O atom (created by the photodissociation of O2) and an O2 molecule to form O3. In addition to creating ozone, UV radiation also destroys it, as do a number of chemical reactions. Although ozone is constantly being created and destroyed, the amount of ozone at a given location remains relatively constant. This amount of ozone, also known as the steady state concentration of ozone, peaks at roughly 5 x 1012 molecules per cubic centimeter at around 30 km in altitude. Considerable natural variation in ozone occurs, however, due to transport.
Ozone is also produced at the surface in two ways, both of which are related to human activities. One way is in the form of photochemical smog that arises from industrial pollution reacting with sunlight. Another is in so-called "biomass burning". This refers to burning of jungle, savannah, and existing farm land. It is primarily due to human agricultural needs, though lightening can also trigger wildfires that result in biomass burning-related ozone creation. These forms of ozone are unhealthy, while stratospheric ozone, by blocking dangerous UV light, is essential for life on Earth's surface. Our focus here is on this beneficial ozone created by energetic UV light, principally in the stratosphere.
The trace gases gases such as carbon dioxide, water vapor, ozone, and methane, as well as the manmade chlorofluorocarbons (CFC's), are all also categorized as "greenhouse gases." This is because each one of them has the ability to affect Earth's energy balance and change the temperature at the surface and in the atmosphere. They do this through the way in which they absorb incoming shortwave energy (radiation) from the sun and reradiate it upwards and downwards as longwave radiation in the form of infrared or thermal energy, which we can feel as sensible heat.
How they do this, and what is meant by longwave and shortwave radiation is explored in depth in Chapter 4.
Trace gases like methane, carbon dioxide, and CFCs are all increasing in our atmosphere. Figure 2.01 shows a plot of methane measurements made in Cape Meares, Oregon between 1979 and 1992 which shows roughly a 3% increase in just 5 years. (Figure taken from Khalil, et al, 1993.) The measurements are in parts per billion by volume, or ppbv.
Other measurements of methane from all over the globe show a similar trend. Measurements of the chemical composition of bubbles in ice cores dug up in the Arctic and Antarctic show that methane concentrations remained constant at about 0.7 ppmv for thousands of years and the increase has only occurred relatively recently on geological time scales. The current global average is 1.7 ppmv.
Likewise are chlorofluorocarbons concentrations increasing. Unlike the other trace gases, CFC's do not occur naturally. They have been used in spray propellants, as well as various refrigerator and air conditioning coolants and cleaning solvents. CFC's have been shown to play a role in stratospheric ozone destruction. It is over Antartica that their ozone-destroying ability via a series of catalytic reactions is best known. This is explored in depth in Chapters 5 and 11.
In addition to gaseous species in the atmosphere, there are suspended solid or liquid particulates as well. Known as aerosols, they are typically on the order of magnitude of micrometers to millimeters in size. Dust, sea salt, and volcanic emissions are natural sources for aersols. Sulfur dioxide from volcanic eruptions and from power plant emissions react to form tiny sulfuric acid droplet aerosols. These particles form cloud condensation nuclei for cloud water droplets and frozen cloud ice crystals. Sulfuric acid droplets high in the stratosphere for the so-called Junge Layer. Aerosols are also involved in the formation of polar stratospheric clouds (PSC's) composed of water and hydrates of nitric acid. Aerosols may have significant atmospheric consequences due to their influence on Earth's radiation balance. PSC's are intimately connected to the well-known Antarctic polar ozone hole via heterogeneous chemical reactions that are explored in Chapter 11. Aerosols also influence the global radiation budget by scattering visible wavelength solar radiation and absorbing infrared (thermal) radiation emitted by Earth and the atmosphere.
Another atmospheric component is ions. Ions are compounds that are electrically imbalanced due to the gain or loss of electrons. They are present at all altitudes in the atmosphere, though they are produced by different phenomena. In the troposphere, ions are produced by lightning, cosmic rays, and the decay of radioactive elements in Earth's crust. In the stratosphere and mesosphere, charged particles (protons) from the Sun can penetrate and cause some ionization of chemical compounds.
So where did Earth's atmosphere come from anyway?
The solid Earth first condensed from the gas and dust of the primordial solar nebula, along with the Sun and its other retinue of planets and their satellites, about 4.5 billion years ago. The embryonic or proto-Earth swept up the material of the gaseous solar nebula in a process of accretion over many millions of years. Collisions with meteors and comets were intense and frequent. Occasionally, there were collisions with very large bodies, called planetesimals, which blew apart the proto-Earth, only to have it recoalesce, the heavier materials (like nickel and iron) sinking below the ligher, carbon containing elements. Many of the gases now present on Earth were carried here via collisions with these other celestial bodies. Eventually, the bombardment subsided as the growing Earth swept up most of the smaller bodies and remaining gases in its orbit around the infant Sun. Heavier gases became trapped in the congealing molten rocks. Lighter gases captured by Earth, such as methane, ammonia, and hydrogen, formed Earth's first atmosphere. Completely anoxic or lacking in oxygen, it is known as the primary atmosphere. The intense ultraviolet radiation present in the early solar system broke apart the methane and ammonia molecules, freeing hydrogen atoms. Being light in weight, the hydrogen was able to escape Earth's atmosphere, leaving Earth depleted in hydrogen.
Meanwhile, gases originally trapped below the surface began to enter the atmosphere through volcanic eruptions and other openings in the crust. The atmosphere began to fill with carbon dioxide, nitrogen, and water vapor. Oceans developed as water vapor condensed and fell as rain in ceaseless storms. Large amounts of carbon dioxide was removed from the atmosphere and sequestered in carbonate rocks and in sea water. Photolysis of carbon dioxide and water vapor yielded some atmospheric oxygen. Initially, however, every oxygen atom that was produced reacted quickly with rocks on the surface and gases already present in the atmosphere. Life developed early on in the oceans. Initial life forms on the planet were various types of bacteria for thrived in the anoxic environment. Oxygen therefore did not begin to accumulate in Earth's atmosphere until simple photosynthetic plants (types of algae) and cyanobacteria (blue-green algae) appeared, which produced oxygen as a byproduct.
The presence of free oxygen in the atmosphere is itself intimately associated with the development of these algal and cyanobacterial life forms. Their appearance about 2.5 billion years ago allowed carbon dioxide to be removed from the atmosphere and replaced with free oxygen (O2).
As oxygen built up, so too did ozone. In Figure 2.02, we see a simulation of the possible evolution of life together with the evolution of oxygen and ozone in Earth's atmosphere [taken from Wayne, R.P., Chemistry of Atmospheres, New York: Oxford Press, 1991, p. 404].
Models of the chemical balance of ozone and the evolution of oxygen in Earth's atmosphere indicate that the atmosphere reached the 50% level of ozone about 1 billion years ago. At this time, oxygen was still just a fraction of a percent of what it is today. By about 400 million years ago, the atmospheric content of oxygen and ozone probably closely resembled present values. The first land animals also arose about 400 million years ago. Some scientists have hypothesized that the development of higher life forms was necessarily linked to the evolution of the protective ozone layer.