Chapter 1
The Atmosphere

1.1 History and Evolution of Earth's Atmosphere

It is generally believed that the solar system condensed out of an interstellar cloud of gas and dust, referred to as the primordial solar nebula, about 4.6 billion years ago. The atmospheres of Earth and the other terrestrial planets, Venus and Mars, are thought to have formed as a result of the release of trapped volatile compounds from the planet itself. The early atmosphere of Earth is believed to have been a mixture of carbon dioxide (CO2), nitrogen (N2), and water vapor (H2O), with trace amounts of hydrogen (H2), a mixture similar to that emitted by present-day volcanoes.

The composition of the present atmosphere bears little resemblance to the composition of the early atmosphere. Most of the water vapor that outgassed from Earth's interior condensed out of the atmosphere to form the oceans. The predominance of the CO2 that outgassed formed sedimentary carbonate rocks after dissolution in the ocean. It is estimated that for each molecule of CO2 presently in the atmosphere, there are about 105 CO2 molecules incorporated as carbonates in sedimentary rocks. Since N2 is chemically inert, non-water-soluble, and noncondensable, most of the outgassed N2 accumulated in the atmosphere over geologic time to become the atmosphere's most abundant constituent.

The early atmosphere of Earth was a mildly reducing chemical mixture, whereas the present atmosphere is strongly oxidizing. Geochemical evidence points to the fact that atmospheric oxygen underwent a dramatic increase in concentration about 2300 million years ago (Kasting 2001). While the timing of the initial O2 rise is now well established, what triggered the increase is still in question. There is agreement that O2 was initially produced by cyanobacteria, the only prokaryotic organisms (bacteria and archea) capable of oxygenic photosynthesis. These bacteria emerged 2700 million years ago. The gap of 400 million years between the emergence of cyanobacteria and the rise of atmospheric O2 is still an issue of debate. The atmosphere from 3000 to 2300 million years ago was rich in reduced gases such as H2 and CH4. Hydrogen can escape to space from such an atmosphere. Since the majority of Earth's hydrogen was in the form of water, H2 escape would lead to a net accumulation of O2. One possibility is that the O2 left behind by the escaping H2 was largely consumed by oxidation of continental crust. This oxidation might have sequestered enough O2 to suppress atmospheric levels before 2300 million years ago, the point at which the flux of reduced gases fell below the net photosynthetic production rate of oxygen. The present level of O2 is maintained by a balance between production from photosynthesis and removal through respiration and decay of organic carbon (Walker 1977).

Earth's atmosphere is composed primarily of the gases N2 (78%), O2 (21%), and Ar (1%), whose abundances are controlled over geologic timescales by the biosphere, uptake and release from crustal material, and degassing of the interior. Water vapor is the next most abundant constituent; it is found mainly in the lower atmosphere and its concentration is highly variable, reaching concentrations as high as 3%. Evaporation and precipitation control its abundance. The remaining gaseous constituents, the trace gases, represent less than 1% of the atmosphere. These trace gases play a crucial role in Earth's radiative balance and in the chemical properties of the atmosphere.

Aristotle was the first to propose in his book Meteorologica in 347 BC that the atmosphere was actually a mixture of gases and that water vapor should be present to balance the water precipitation to Earth's surface. The study of atmospheric chemistry can be traced back to the eighteenth century when chemists such as Joseph Priestley, Antoine-Laurent Lavoisier, and Henry Cavendish attempted to determine the chemical components of the atmosphere. Largely through their efforts, as well as those of a number of nineteenth-century chemists and physicists, the identity and major components of the atmosphere, N2, O2, water vapor, CO2, and the rare gases, were established. In the late nineteenth-early twentieth century focus shifted from the major atmospheric constituents to trace constituents, that is, those having mole fractions below 10-6, 1 part per million (ppm) by volume. We now know that the atmosphere contains a myriad of trace species. Spectacular innovations in instrumentation over the last several decades have enabled identification of atmospheric trace species down to levels of about 10-12 mole fraction, 1 part per trillion (ppt) by volume.

The extraordinary pace of the recent increases in atmospheric trace gases can be seen when current levels are compared with those of the distant past. Such comparisons can be made for CO2 and CH4, whose histories can be reconstructed from their concentrations in bubbles of air trapped in ice in such perpetually cold places as Antarctica and Greenland. With gases that are long-lived in the atmosphere and therefore distributed rather uniformly over the globe, such as CO2 and CH4, polar ice core samples reveal global average concentrations of previous eras. Analyses of bubbles in ice cores show that CO2 and CH4 concentrations remained essentially unchanged from the end of the last ice age some 10,000 years ago until roughly 300 years ago, at mole fractions close to 260 and 0.7 ppm by volume, respectively. Activities of humans account for most of the rapid changes in the trace gases over the past 200 years-combustion of fossil fuels (coal and oil) for energy and transportation, industrial and agricultural activities, biomass burning (the burning of vegetation), and deforestation.

These changes have led to the definition of a new era in Earth's history, the Anthropocene (Crutzen and Steffen 2003). Records of atmospheric CO2, CH4, and N2O show a clear increase since the end of the eighteenth century, coinciding more or less with the invention of the steam engine in 1784. The global release of SO2, from coal and oil burning, is at least twice that of all natural emissions. More nitrogen is now fixed synthetically and applied as fertilizers in agriculture than fixed naturally in all terrestrial ecosystems. The Haber-Bosch industrial process to produce ammonia from N2, in many respects, made the human explosion possible.

The emergence of the Antarctic ozone hole in the 1980s provided unequivocal evidence of the ability of trace species to perturb the atmosphere. The essentially complete disappearance of ozone in the Antarctic stratosphere during the austral spring is now recovering, owing to a global ban on production of stratospheric ozone-depleting substances. Whereas stratospheric ozone levels eroded in response to human emissions, those at ground level have, over the past century, been increasing. Paradoxically, ozone in the stratosphere protects living organisms from harmful solar ultraviolet radiation, whereas increased ozone in the lower atmosphere has the potential to induce adverse effects on human health and plants.

Levels of airborne particles in industrialized regions of the Northern Hemisphere have increased markedly since the Industrial Revolution. Atmospheric particles (aerosols) arise both from direct emissions and from gas-to-particle conversion of vapor precursors. Aerosols can affect climate and have been implicated in human morbidity and mortality in urban areas.

Atmospheric chemistry comprises the study of the mechanisms by which molecules introduced into the atmosphere react and, in turn, how these alterations affect atmospheric composition and properties (Ravishankara 2003). The driving force for chemical changes in the atmosphere is sunlight. Sunlight directly interacts with many molecules and is also the source of most of the atmospheric free radicals. Despite their very small abundances, usually less than one part in a billion parts of air, free radicals act to transform most species in the atmosphere. The study of atmospheric chemical processes begins with determining basic chemical steps in the laboratory, then quantifying atmospheric emissions and removal processes, and incorporating all the relevant processes in computational models of transport and transformation, and finally comparing the predictions with atmospheric observations to assess the extent to which our basic understanding agrees with the actual atmosphere. Atmospheric chemistry occurs against the fabric of the physics of air motions and of temperature and phase changes. In this book we attempt to cover all aspects of atmospheric chemistry and physics that bear on air pollution and climate change.

1.2 Climate

Viewed from space, Earth is a multicolored marble: clouds and snow-covered regions of white, blue oceans, and brown continents. The white areas make Earth a bright planet; about 30% of the sun's radiation is reflected immediately back to space. The surface emits infrared radiation back to space. The atmosphere absorbs much of the energy radiated by the surface and reemits its own energy, but at lower temperatures. In addition to gases in the atmosphere, clouds play a major climatic role. Some clouds cool the planet by reflecting solar radiation back to space; others warm the earth by trapping energy near the surface. On balance, clouds exert a significant cooling effect on Earth.

The temperature of the earth adjusts so that the net flow of solar energy reaching Earth is balanced by the net flow of infrared energy leaving the planet. Whereas the radiation budget must balance for the...