D D Davis G Chen And M Chin

1 INTRODUCTION

The focus of this chapter is that of providing the reader with an overview of atmospheric sulfur. It will address the issues of where sulfur comes from, how it is processed, and how it gets returned to the planetary surface. It will also endeavor to show how sulfur, during its atmospheric cycle, plays a significant role in helping to maintain a stable global environment.

Sulfur is an element that is essential to life on this planet. Living organisms at nearly all levels of sophistication ingest sulfur from their environment, mainly in the form of sulfate or amino acid sulfur. But living organisms not only ingest sulfur, they also have a decisive impact on the chemical forms and total burden that is found in the atmosphere. During the process known as assimilatory sulfate reduction, microorganisms and plants use sulfate to build sulfur-containing proteins for purposes of storing energy and to support cell growth. During food digestion, animals are able to generate energy from the catabolism of these proteins, breaking them down to their chemical building blocks, the amino acids. The further breakdown of these compounds results in the release of volatile sulfur back to the environment.

Some microorganisms living in anoxic environments, such as tidal flats, obtain energy from using sulfate as an electron acceptor instead of 02. This process is called disimilatory sulfate reduction. Hydrogen sulfide (H2S) released during this process often combines with iron minerals to form pyrite, FeS, resulting in its incorporation into sediment layers. Alternatively, the H2S may react with buried organic matter, thus forming a source of sulfur in fossil oil and coal deposits. In general, the turnover of sulfur in dissimilatory processes is several orders of magnitude faster than in assimilatory processes. The biological sulfur cycle is therefore mainly controlled by anaerobic sulfate reducing bacteria. [For further details on the

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

assimilatory and dissimilatory biological processes, the reader is referred to reviews by Krouse and McCready (1979) and Andreae and Jaeschke (1992).]

Sulfur, having six valence electrons, has the potential for existing in the atmosphere in a wide range of oxidation states, ranging from —2 to +6, with the most common states being —2, +4, and +6. In most remote global locations, atmospheric sulfur is found at concentration levels of only 1 ppbv (part-per billion by volume) or less. However, for continental regions, particularly those experiencing significant industrial development, concentrations can reach upwards of 100 to 200 ppbv. Similar levels can be found in regions under the influence of active volcanoes. This trend in global sulfur levels is a strong reflection of the distribution of the major sources of sulfur as illustrated in Figure 1. From this abbreviated picture of the atmospheric sulfur cycle, the most critical members of the atmospheric sulfur family are identified as dimethyl sulfide (DMS), sulfur dioxide (S02), sulfuric acid (H2S04), and aerosol sulfate (S042-). The latter species is typically found in the form of condensation nuclei (CN) or cloud condensation nuclei (CCN). Two of the major primary sulfur sources are shown as S02, emitted from the burning of large quantities of fossil fuel or, alternatively, from volcanoes, and DMS, which predominantly is released from the world's oceans. This source is obviously dispersed over a much larger global surface area than is primary S02, leading to much lower concentration levels of sulfur over remote regions. In the latter context, shown also in Figure 1, is the most recently identified remote sulfur source, emissions from ships (Corbett and Fischbeck, 1997; Corbett et al., 1999). This new source, however, is significantly smaller than the three previously discussed.

Among the central points revealed in Figure 1 is the fact that the atmosphere can be viewed as a large oxidizing chemical reactor in which sulfur, emitted from Earth's surface, enters the atmosphere in a chemically reduced oxidation state (typically —2 and +4) is oxidized to the +6 state, and then in ionic form (i.e., higher solubility) is returned to the biosphere, thus closing the cycle. The processes responsible for oxidizing sulfur are shown as occurring by both gas phase as well as heterogeneous reactions. Once in the +6 oxidation state, this sets the stage for the final contribution from atmospheric sulfur toward maintaining a stable global environment, namely, its impact on the planetary radiation budget. As shown in Figure 1 atmospheric aerosols, which are predominately composed of sulfur, can have a significant impact on the planet's climate via their influence on direct scattering of incoming solar radiation and by their controlling the radiative characteristics and formation rates of clouds (Charlson et al., 1992).

In Section 2 of this chapter, we will expand on the source inventories for DMS and S02 as well as present inventories for several less important primary sulfur source species, including those for H2S, carbony sulfide (OCS), and carbon disulfide (CS2). Of particular significance will be the hemispheric distributions of these collective sulfur sources and how they manifest themselves in observed concentration levels. Section 3 will also explore in greater detail the oxidation processes responsible for converting the dominant sulfur species (i.e., DMS and S02) into forms that result in their removal. Section 4 will combine the source inventory data presented in Section 3 and the chemical transformation information discussed in Section 2 in exploring global distributions of S02 and S042-. Finally, in Section 5,

2 CHEMICAL FORMS. SOURCES. AND CONCENTRATION LEVELS 127

2 CHEMICAL FORMS. SOURCES. AND CONCENTRATION LEVELS 127

Figure I Simplified tropospheric sulfur cycle: J The three largest documented global sulfur sources: Ocean emissions, volcanoes, and fossil fuel burning. 3 The most critical specics involved in the cycling of sulfur DMS. SO:. il:S04. and S04:_ (as CN. and CCN). (2 The major chemical processes in ihc cycling of tropospheric sulfur encompass gas-phase and heterogeneous reactions. @ Among the important environmental impacts of tropospheric sulfur: formation of new particles and promotion of aerosol growth. Both are critical factors in Earth's radiation budget.

we present an overview of sources, sinks, and transformations of sulfur in the stratosphere with a special emphasis on sulfur sources responsible for maintaining the "background" level of stratospheric aerosol.

The authors note that because of the more fundamental chemical nature of Section 3, the discussion in this section is necessarily presented in greater detail than are other sections. The reader may choose, therefore, to by-pass this section. As the text is configured, this can be done without a major loss in grasping the larger global picture of atmospheric sulfur and how this element is critically coupled to the larger planetary environment.

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