We have seen in Chapter 8 that, even on a global scale, anthropogenic emissions can cause significant perturbations to the budgets of certain trace chemical species in the atmosphere. In urban and industrialized areas, anthropogenic emissions can become so large that the concentrations of various undesirable chemical species (called pollutants) cause significant deterioration in air quality and visibility, and can pose threats to human health. Severe air pollution episodes occur when the rates of emissions or formation of pollutants greatly exceed the rates at which the pollutants are dispersed or destroyed by winds or by vertical transport (e.g., in the presence of a capping temperature inversion) or removed from the atmosphere (e.g., by precipitation or by chemical Reactions).

Sources of anthropogenic pollutants

Combustion (in power plants, smelters, automobiles, and of wood, etc.) is the largest source of air pollutants. On a global scale fossil-fuel combustion is the major source of CO, CO2,NOx, and SO2 (see Figs. 8.1–8.3). Many other pollutants are released into the air by combustion. For example, about 15% of the total emissions of hydrocarbons are from anthropogenic sources; this is because the most common fuels are hydrocarbon compounds (oil, natural gas, coal, and wood). Ideal (or complete) oxidation (or combustion) of hydrocarbon fuel yields only CO2 and H2O. However, for a given quantity of fuel, a precise amount of oxygen is required for complete combustion.

This chapter is concerned with the attenuation by atmospheric gases and aerosols of the incoming shortwave radiation from the Sun (solar radiantion), and the emission and absorption of outgoing longwave radiation (also called infrared, terrestrial, or thermal radiation) from the Earth and its atmosphere. These interactions play important roles in determining the energy balances, and therefore the temperatures, of the Earth's surface and the atmosphere. Also, the absorption of solar radiation by some atmospheric constituents can lead to photochemical reactions, which play crucial roles in atmospheric chemistry.

Figure 4.1 shows current best estimates of the annual global energy balance of the Earth-atmosphere system expressed in terms of 100 units of incoming solar radiation at the top of the Earth's atmosphere (I OA). About 22 of these units arc reflected back into space by clouds, aerosols, and gases, about 20 units are absorbed by the atmosphere, 9 units are reflected from the Earth's surface, and the remaining 49 units are absorbed at the Earth's surface. Measured in the same units, the Earth receives an additional 95 units due to longwave radiation from the atmosphere. Therefore, the total energy received by the Earth is 144 units. Thermal equilibrium at the surface of the Earth is achieved by these 144 units being transferred back to the atmosphere: 114 of them are radiated to the atmosphere as longwave radiation, 23 units as evapotranspiration (which is ultimately released to the atmosphere as latent heat in precipitation), and 7 units are transferred to the atmosphere by heat fluxes associated with turbulence, convection, and so on.

Early studies of atmospheric chemistry emphasized trace gases and homogeneous gas-phase reactions. However, in the latter part of the twentieth century, increasing attention was given to atmospheric aerosols (see Chapter 6), chemical reactions on aerosol surfaces (see Chapter 10), and the role of clouds in atmospheric chemistry. In this chapter we consider how clouds remove particles and gases from the air, some of the chemical reactions that can occur within cloud droplets, and how these processes modify the chemical composition of cloud water and precipitation, as well as some other important properties of clouds. Finally, we will discuss the effects of cloud processing on modifying atmospheric aerosols.

Overview

We will organize our discussion of cloud and precipitation chemistry around the processes illustrated schematically in Figure 7.1.

Clouds form when air becomes slightly supersaturated with respect to liquid water (or in some cases with respect to ice). The most common means by which this is achieved in the atmosphere is through the ascent of air parcels, which results in the expansion and cooling of the air below its dew point. When the air becomes slightly supersaturated with respect to water (by a few tenths of 1%), water vapor begins to condense onto some of the articles in the air to form a cloud of small water droplets. This process, by which a portion of the atmospheric aerosol (called cloud condensation nuclei, or CCN) nucleate the formation of cloud droplets, and are thereby incorporated into cloud water as insoluble and soluble components, is referred to as nucleation scavenging.

The reservoirs of chemical species in the Earth system are the solid Earth, the hydrosphere (oceans and fresh water), the cryosphere (ice and snow), the biosphere, and the atmosphere. Chemical species can be transferred between these reservoirs. We have already seen that such transfers played crucial roles in the evolution of Earth's atmosphere. Since, under steady-state conditions, a chemical species cannot accumulate indefinitely in any one of the reservoirs, there must be continual cycling of species through the various reservoirs. This is termed biogeochemical cycling

In this chapter we will consider briefly the tropospheric portions of the biogeochemical cycles of carbon, nitrogen, and sulfur. We have chosen these cycles not only because they are of considerable importance in atmospheric chemistry but because the productivity of many terrestrial and aquatic organisms depends on the availability of these elements (as well as on oxygen and hydrogen). We will be concerned with relatively rapid interchanges involving the atmosphere and other reservoirs (generally the oceans and the biosphere). For simplicity, we will assume the various cycles are independent, although in the atmosphere they can interact with each other and are coupled in the stoichiometry of living matter and biochemical processes. A few words of caution are needed. Many aspects of global chemical cycles are not well understood, and the magnitudes of the various emission fluxes are not necessarily equivalent to the importance of the species, since atmospheric residence times must also be taken into account.

In this chapter we will consider the sources, transformations, transport, and sinks of chemicals in the natural troposphere. Our emphasis will be on gases; aerosols will be considered in more detail in Chapter 6.

Sources

The principal natural sources of gases in the troposphere are the biosphere, the solid Earth, the oceans, and in situ formation in air from other chemical species. These sources are discussed, in turn, in the following subsections.

Biological

Even though the biosphere contains only a small fraction of the total reservoirs of most chemicals on Earth, it plays a major role in determining the abundances and transport of many gases in the atmosphere. The smells associated with flowers and other forms of vegetation provide direct evidence that biota emit chemicals into the air. Some important biological sources of trace gases that enter the atmosphere are:

• Photosynthesis in plants (Reaction (1.1)), which is responsible for virtually all of the oxygen in the atmosphere.

• Respiration (the reverse of Reaction (1.1)), which releases CO2 into the air. This can be seen, on a seasonal basis, in Figure 1.1 where the decline of CO2 in the summer months is due to its uptake by plants during photosynthesis. The rise of CO2 in winter and early spring (Fig. 1.1) is due to respiration, and the decay of leaf litter and other dead plant material. This “breathing” of the atmosphere can also be detected in diurnal fluctuations in CO2; CO2 concentrations in forests can be ~35ppmv higher at night than in the day.

• […]

The following suggestions for further general readings in atmospheric chemistry are listed in approximately increasing order of difficulty and/or comprehensiveness.

Hobbs, P. V., Basic Physical Chemistry for the Atmospheric Sciences, 2nd edition, Cambridge University Press, New York, 2000.

Brimblecombe, P., Air Composition and Chemistry, Cambridge University Press, Cambridge, 1996.

Mészáros, E., Atmospheric Chemistry: Fundamental Aspects, Elsevier Scientific Publishing Co., Amsterdam, 1981.

Harrison, R. M., S. J. de Mora, S. Rapsomanikis, and W. R. Johnston, Introductory Chemistry for the Environmental Sciences, Cambridge University Press, Cambridge, 1991.

Schlesinger, W. H., Biogeochemistry: An Analysis of Global Change, Academic Press, New York, 1997.

Wayne, R. P., Chemistry of Atmospheres, Oxford University Press, Oxford, 1991.

Warneck, P., Chemistry of the Natural Atmosphere, Academic Press, 1988.

Finlayson-Pitts, B. J., and J. N. Pitts, Jr., Atmospheric Chemistry, John Wiley and Sons, Inc., New York, 1986.

Seinfeld, J. H., and S. N. Pandis, Atmospheric Chemistry and Physics, John Wiley and Sons, Inc., New York, 1998.

Units for chemical abundance

Before discussing the chemical composition of the atmosphere, we need to describe the various units that can be used for expressing the amounts (or abundances) of chemicals in the atmosphere.

The most common unit for expressing the abundance of gases in air is mixing ratio by volume that is, the fraction of the volume of the air occupied by a gas. Depending on the magnitude of the mixing ratio, the most convenient units can be percent by volume, parts per million by volume (ppmv),parts per billion by volume (ppbv), or parts per trillion by volume (pptv), where, 1 ppmv = 1 unit of volume per 106 units (or 10−6), I ppbv = 1 unit of volume per 109 units (or 10−9), and 1 pptv = 1 unit of volume per 1012 units (or 10−12). Note that we use the American definitions of billion (109) and trillion (1012). It is very convenient to express the abundance of a gas as a mixing ratio by volume, because, from the gas equation, we know that the volumes occupied by different gases at the same temperature and pressure are proportional to the numbers of molecules of the gases.

Apart from cloud and precipitation particles, which have relatively large sizes, the Earth's atmosphere consists a mixture of gases, small solid particles, and small liquid droplets. Mixtures of air with small solid particles and small droplets are called aerosols The small particles and droplets themselves (called aerosol particle, but often loosely referred to as simply aerosols) are important not only in air chemistry but in determining visibility, the formation of cloud particles, atmospheric radiation, and atmospheric electricity. In this chapter we will be concerned primarily with chemical aspects of atmospheric aerosols, but we will start by describing two of their important physical attributes, namely, their concentrations and size distributions.

Aerosol concentrations and size distributions

Total number and mass concentrations

One of the oldest and most convenient techniques (which in various modified forms is still in use) for counting the number concentrations of atmospheric aerosols is the Aitken1 nucleus counter. In this instrument, humid air is expanded rapidly so that it cools and becomes supersaturated by several hundred percent with respect to water. At these high supersaturations, water condenses onto virtually all of the particles in the air to form a cloud of small water droplets in the chamber of the counter. The number concentration of droplets in this cloud (which is close to the total number concentration of aerosol particles) can be determined automatically by optical techniques or by allowing the droplets to settle onto a substrate where they can be counted. The concentration of aerosol particles measured with an Aitken nucleus counter is referred to as the Aitken (or condensation) nucleus count (CN count for short).

This short book is a companion volume and a natural extension to my textbook entitled Basic Physical Chemistry for the Atmospheric Sciences (Cambridge University Press, 1995; second edition published in 2000). Together these two books provide material for a first (undergraduate or graduate) course in atmospheric chemistry; they should also be suitable for self-study.

In Basic Physical Chemistry for the Atmospheric Sciences the groundwork was laid for courses in atmospheric chemistry and other areas of environmental chemistry. The present book provides a short introduction to the subject of atmospheric chemistry itself. Twenty years ago this subject was a minor branch of the atmospheric sciences, pursued by relatively few scientists. Today, atmospheric chemistry is one of the most active and important disciplines within meteorology, and one with which every geoscientist and environmental scientist should have some familiarity.

The emphasis of this book is on the basic principles of atmospheric chemistry, with applications to such important environmental problems as air pollution, acid rain, the ozone hole, and global change. In keeping with the pedagogical approach of its companion volume, model solutions are provided to a number of exercises within the text. In an appendix, readers are invited to test their skills on further exercises. Answers to all of the exercises and worked solutions to the more difficult ones, are provided.

Thanks are due to Halstead Harrison for allowing me to use some of his exercises, and to Richard Gammon, Dean Hegg, Daniel Jaffe, Robert Kotchenruther, Conway Leovy, Donald Stedman, and Stephen Warren for reviewing various portions of this book. I thank also the National Science Foundation and the National Aeronautics and Space Administration for their support of my own research on atmospheric chemistry.