In some of the chapters that follow, there are detailed discussions on many individual trace gases such as CO, the oxides of nitrogen, and tropospheric ozone. Additionally, these three trace gases are of particular interest as they interact to a major degree to determine the oxidizing capacity of the troposphere. One of the most important
This relatively simple catalytic cycle shows how the global budgets of CO and ozone in the troposphere are intertwined if there is a sufficient amount of NO and N02 present in the atmosphere. Since it has already been demonstrated that the CO
and CH4 budgets are linked, one of the driving questions in atmospheric chemistry is the determination of what percentage of all these trace gases is natural, what fraction is anthropogenic, how have these budgets been perturbed over the past decades and centuries, and, lastly, how much will these budgets change in the coming decades. Through the complex interactions of these trace gases, the oxidizing capacity of the troposphere can be determined and possibly even predicted.
Earth is the only planet in this solar system where there is an oxidizing atmosphere, and although nearly all carbon emitted to the atmosphere eventually ends up as completely oxidized C02, and all hydrogen-containing trace species end up as H20, some interesting and often complex chemistry takes place along these oxidation pathways. Sulfur and nitrogen also eventually become oxidized, and the final products result in the formation of acids that, being soluble, contribute to the formation of acid rain. In recent years, chemical analysis of rain, fog, clouds, and dew have shown that the aqueous chemistry is as equally challenging and even more complicated than gas-phase chemistry. A complete understanding of aqueous-phase chemistry is still evolving, but many of the basic principles have become fairly well established and are described in more detail in some of the chapters in this section.
The most important species in clouds and precipitation is the hydrogen ion, whose concentrations can be indicated by specifying solution acidity, or pH. The presence of atmospheric C02 assures that nearly all atmospheric water droplets will be acidic; natural and anthropogenic nitrogen and sulfur increase the acidity (i.e., lower the pH value) to at least pH 5.0. Many urban areas experience pH levels nearer 4.0. Cloud and fog droplets are nearly always more acidic than rain, apparently because smaller cloud drop sizes inhibit dilution of the acidic constituents. In some fogs, the pH of the droplets has been measured as low as 1.7.
Organic compounds in the atmosphere also contribute to cloud acidity. Formaldehyde (CH20), often found in high concentrations where urban pollution is present, is a key tropospheric species with sufficiently high solubility that it can affect the acidity of rain. In remote regions, forests are known to emit large quantities of isoprene (C5Hg), which can react with OH or 03 to form more complex aldehydes (RCHO), and also resulting in the measurement of acid rain in the range of pH 5.0. Other carbon-, nitrogen-, and sulfur-containing acids also exist and are discussed in the chapters on acid rain, reactive nitrogen species, and sulfur species. The micro-physics by which these trace gases are converted to both gaseous and aqueous forms of these acids is also treated explicitly in several chapters in this section.
Returning to the central theme of this overview, a considerable amount of research has been conducted over the past several decades to determine the budgets of carbon monoxide, methane, nitrogen oxides, and tropospheric ozone and to determine how these budgets are affected by human activity. Because these species do interact with each other, a series of different conclusions has been reached regarding human influence on tropospheric chemistry cycles. In the early 1970s, a series of research studies extrapolated Levy's initial hypothesis to postulate that many important chemical processes in the unpolluted atmosphere were dominated by (natural) methane chemistry. Since those initial studies, however, the atmospheric chemistry community has come to recognize that the natural background atmosphere and the concentrations of naturally occurring trace species such as methane, tropo-
8 STRATOSPHERE-TROPOSPHERE CHEMICAL AND CLIMATE INTERACTION 21
spheric ozone, and carbon monoxide have increased dramatically, at rates comparable to, and, in most cases, even more than, carbon dioxide, the hallmark of proof of the concept of global change.
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