The Atmospheric Sulfur Cycle

Another major process at the Earth s surface not involving rapid exchange is the chemical weathering of rocks and dissolution of exposed minerals. In some instances the key weathering reactant is H30 in rainwater (often associated with the atmospheric sulfur cycle), while in other cases H30" comes from high concentrations of CO2, e.g., in vegetated soils. 

Phytoplankton DMS producers <20 Phaeocystis and small autotrophic flagellates Produce dimethylsulfoniopro-pionate (DMSP) and convert it into DMS using an extracellular enzyme (DMSPIyase). Thus, they affect the atmospheric sulfur cycle. Have high P requirement Particularly abundant in coastal areas, where they are often observed in colonies. Calcifiers are also important for the DMS cycle... 

Presented here are, to our knowledge, the first investigations of the reactions of OH, Cl, and NO3 with DMSO. A comparison is made with the corresponding reactions of the radicals with DMS ana the possible implications for the atmospheric sulfur cycle are briefly discussed. 

Obviously our understanding of the involvement of DMSO and DMSO2 in the atmospheric sulfur cycle is at present veiy limited. The results from laboratory and field measurements as briefly discussed here imply that DMSO and DMS02 are important products of the pnotooxidation of DMS which is the major source of reduced sulfur to the marine atmosphere. Further kinetic studies are needed, however, to elucidate the role of DMSO and DMSO2 in the atmospheric sulfur cycle. 

The sixth, seventh, and eighth sections of this volume deal with the atmospheric cycling of biogenic sulfur compounds. This aspect of the sulfur cycle has received a great deal of attention in recent years because of its obvious relationship to the add rain problem and the discovery that natural marine sources constitute a major portion of the total global atmospheric sulfur burden. The chapters in these sections focus on three aspects of this cycle field measurements and techniques used to establish the distributions and fluxes, experimental studies of reaction mechanisms and rates, and numerical simulations of the atmospheric sulfur cycle. Two chapters address the chemical processes involving cloud... 

R8, and R12 should dominate in the troposphere as a whole. The sulfur photochemistry is summarized in Figure 1. The possible importance of Me2S to the atmospheric sulfur cycle should be considered, and more experimental data are needed. 

In the last 150 years the anthropogenic emission of sulfur has increased dramatically, primarily due to combustion processes [1]. In the 1950s anthropogenic emission surpassed natural emission and the atmospheric sulfur cycle is one of the most perturbed biogeochemical cycles [1,2]. The oceans are the largest natural source of atmospheric sulfur emissions, where sulfur is emitted in a reduced form, predominantly as dimethyl sulfide (DMS) and to a much lesser extent carbonyl sulfide (OCS) and carbon disulfide (CS2) [3]. Ocean emitted DMS and CS2 are initially oxidised to OCS, which diffuses through the troposphere into the stratosphere where further oxidation to sulfur dioxide (SO2), sulfur trioxide (SO3) and finally sulfuric acid (H2SO4) occurs [1-4]. 

Peichter J., Lohmann U., and Schult I. (1997) The atmospheric sulfur cycle in ECHAM-4 and its impact on the shortwave radiation. Clim. Dyn. 13, 235—246. 

Bremner J. M. and Steele C. G. (1978) Role of microorganisms in the atmospheric sulfur cycle. In Advances in Microbial Ecology, (ed. M. Alexander) Plenum, New York, pp. 155-201. 

The most important pathway of sulfur through the atmosphere involves injection as a low oxidation state gas and the removal as oxidation state VI sulfate in rainwater (see Fig. 13-2, paths 1,4,5,6, 7, 8,9,10, 12, and 13). Since this pathway involves a change in chemical oxidation state and physical phase, the lifetime of sulfur in the atmosphere is governed by both the kinetics of the oxidation reactions and the frequency of clouds and rain. We will argue below that the overall process is fast - on the order of days -meaning that the atmospheric sulfur cycle is a regional phenomenon and that the distribution of nearly all sulfur species in the atmosphere is necessarily "patchy" over the globe. 

Emissions of sulfur to the atmosphere by humans are almost entirely in the form of SO2. The main sources are coal-burning and sulfide ore smelting. The total anthropogenic flux is estimated to be about 80TgS/year (Ivanov, 1983) and is thus essentially equal in magnitude to the natural flux of low oxidation state sulfur to the atmosphere. Clearly, the atmospheric sulfur cycle is intensely perturbed by human activity. To estimate the spatial extent of this perturbation, we will need some idea of the residence time of sulfur in the atmosphere. 

We have seen that, except for OCS, sulfur species in the atmosphere have residence times that are short (days) such that their geographical distribution is patchy. This perspective on the atmospheric sulfur cycle has important implications. While human... 

SzeND, KoMK. 1980. Photochemistry of COS, CS2, CH3SCH2, and H2S Implications for the atmospheric sulfur cycle. Atmos Environ 14 1223-1239. 

The study of the atmospheric sulfur cycle is a rapidly expanding field because human activity provides an important sulfur dioxide source. In the atmosphere S02 is converted to sulfate containing aerosol particles which can modify the radiation balance of the Earth-atmosphere system, the optical properties and the precipitation forming ability24 of the air. 

Jaeschke, W Georgii, H. W., Claude, H. and Malewski, H., 1978 Contributions of H2S to the atmospheric sulfur-cycle. Pure and Appl. Geophys. 116, 465-475. 

Bremner, J. M., and C. G. Steele (1978). Role of micro organisms in the atmospheric sulfur cycle. Adv. Microb. Ecol. 2, 155-201. 

The major processes involved in the atmospheric sulfur cycle are depicted in Figure 3. The atmospheric sulfur cycle begins with the introduction of a... 

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