Total source (Tg S/yr) 95.7
Anthropogenic emission 63.7-92.0 66.5
Biomass burning 2.2-2.9 2.3
Volcanoes 3.4-8.5 5.5
Photochemical production 10.0-24.7 16.9
Total sink (TgS/yr) 91.2
Gas-phase oxidation 6.1-16.8 9.2
In-cloud oxidation 23.3-55.5 42
Dry deposition 16.0-55.0 35.5
Wet deposition 0-19.9 9.0
Total atmospheric burden (Tg S) 0.2-0.6 0.4
Lifetime (days) 0.6-2.6 1.5 Sulfate
Total source (TgS/yr) 50.6
Anthropogenic emission 0-3.5 1.4
Gas-phase production 6.1-16.8 9.2
In-cloud processing 23.3-57.8 40.0
Dry deposition 3.7-17.0 6.7
Wet deposition 34.6-61.0 44.5
Total atmospheric burden (Tg S) 0.3-0.96 0.63
Note-. Models are from Langner and Rodhe (1991), Pham et al. (1995), Feichter et al. (1995), Chin et al. (1996), Chuang et al. (1997), Koch et al. (1999), and Barth et al. (1999).
"Asa result of using median values derived from several different models to define the total source and sink for S02 and sulfate, these values do not necessarily balance.
sulfate budget in the middle and upper troposphere. This is due to volcanoes providing direct injection of sulfur gases to upper altitudes where species such as S02 typically have lifetimes an order of magnitude longer than that in the boundary layer. The major impact of global volcanic emissions at high altitude is a conclusion also reached by Graf et al. (1998). These investigators found that the global mean radiative forcing by volcanic sulfate aerosols was actually comparable to anthropogenic aerosols.
Table 3 summarizes the global S02-sulfate budget results based on the modeling results from several groups (e.g. Langner and Rodhe, 1991; Pham et al, 1995; Feichter et al, 1996; Chin et al, 1996, 2000; Chuang et al, 1997; Koch et al, 1999; Barth et al, 2000). As one might expect, there are a number of differences among these models, especially in their handling of meteorological fields and para-meterizations. Even so, all still agree on certain key points. For example, all models assign 70 to 75% of sulfate precursor emissions to anthropogenic activities; 30 to 45% of the primary S02 is also estimated to be removed by dry deposition; and finally, it is agreed that concerning the oxidation of S02 to sulfate, 65 to 85% of the total is dominated by in-cloud processes. Among the important areas where significant uncertainties still exist is that of fully understanding the levels of S02 and sulfate in remote marine regions. As discussed in Section 3, of particular concern is assessing the relative contributions at free tropospheric altitudes of sulfate derived from DMS oxidation versus that from volcanoes and long-range transport of surface-generated continental sources.
The presences of sulfur in the stratosphere in the form of a sulfate aerosol layer, or Junge layer, was first reported in the early 1960s (Junge et al., 1961). Since its discovery, there have been substantial advances in understanding the effects of stratospheric sulfur on climate and atmospheric chemistry. The primary importance of stratospheric sulfur is that it affects Earth's radiative balance. Aerosols can directly scatter incoming solar radiation back to space. This results in a cooling of Earth's surface. By absorbing outgoing infrared radiation, however, they can also cause a warming of the stratosphere. These effects have been observed after major volcanic eruptions (e.g., Labutzke and McCormick, 1992). Stratospheric aerosols can also have an indirect effect on the radiative balance by acting as CCN. For example, they are involved in forming polar stratospheric clouds (PSCs) and possibly in the development of large-scale cirrus clouds. In addition, stratospheric aerosols may play a significant role in stratospheric chemistry by providing surfaces upon which heterogeneous reactions take place. Such reactions appear to be centrally important as a means of modulating stratospheric ozone levels (see, e.g., Hofmann and Solomon, 1989).
The composition of stratospheric aerosols appears to be mainly sulfate (Rosen, 1971). The most likely source of this sulfate is oxidation of S02 to form H2S04(g) as discussed above in Section 3. This oxidation step would then be followed by nuclea-tion and condensation. On the basis of numerous observations of stratospheric aerosols over the past 30 years, volcanic eruptions that inject large amounts of S02 directly into the stratosphere are now believed to be one of the dominant sources of stratospheric sulfate aerosols. However, because of the presence of a persistent background of aerosol even during periods when no major volcanic eruptions occurred, there has been considerable speculation concerning other possible sources of this aerosol.
The importance of carbonyl sulfide (OCS) as a stratospheric aerosol source was first proposed by Crutzen (1979). As noted in Section 2, carbonyl sulfide is the most abundant sulfur compound in the atmosphere. It is emitted at Earth's surface by natural and anthropogenic sources, and it is also formed by the oxidation of carbon disulfide (CS2) (Chin and Davis, 1993). Recall, however, that because of its chemical inertness in the troposphere, it is found to have a near uniform mixing ratio (i.e., 500pptv) throughout this region. Because of this, significant quantities of OCS are transported to the stratosphere where it undergoes photodecomposition and/or oxidation via reactions with 0(3P) atoms and OH radicals. The resulting product S02, like that from volcanic injections, is then converted to sulfate aerosol. Early modeling studies supported Crutzen's hypothesis and showed that the flux of OCS into the stratosphere was sufficient to maintain the background sulfate aerosol layer.
Twenty years later, with a far more extensive set of OCS atmospheric observations and with improved laboratory reaction rate data, Chin and Davis (1995) reanalyzed the stratospheric significance of OCS as a source of background sulfate aerosols. They compared the flux of OCS calculated in a one-dimensional model with the flux needed to sustain the background aerosol level. Historically, the background level has been estimated from the ratio of background aerosol mass to aerosol lifetime. Departing from earlier analyses, Chin and Davis (1995) found that OCS could provide only 20 to 50% of the required sulfur. This conclusion was based on two important insights: (1) The so-called background aerosol layer observed during volcanic quiescent periods still contained a significant amount of residual volcanic aerosol, and (2) important sources other than OCS quite likely were also contributing to background sulfate aerosol levels. Although a more recent one-dimensional model study, which included microphysical processes, proposed that a sustainable background sulfate layer could indeed be maintained by OCS oxidation (Zhao et al., 1995), Weisenstein et al. (1997) report results that are much closer to those given earlier by Chin and Davis. Weisenstein et al., using a global two-dimensional model, found that OCS oxidation could only account for half of the background sulfur loading. They also found that convective transport of S02 in the tropical troposphere could provide the other half of the background sulfate aerosol. In a still more recent study Mills et al. (1999), based on new measurements by Wilson et al. (1999), have suggested that in addition to S02, tropospheric sulfate aerosol at the tropopause could make a significant contribution to the sulfate loading of the stratosphere during quiescent periods. As shown in Figures 9a and 9b, the concentrations of S02 and sulfate at the Northern Hemisphere tropopause can reach 50 and lOOpptv, respectively. Quite clearly, there are still important aspects of the so-called background aerosol layer issue that are still unresolved.
The variability in background sulfate aerosol levels has also drawn considerable attention since human activities may have already perturbed the natural background level. For example, there have been reports published indicating that we could be experiencing as much as a 6 to 8% per year increase in background levels. Although the initial speculation was focused on these increases being tied to anthropogenic emissions of OCS, upon further reflection this explanation has been largely rejected. This follows from the fact that there has been no significant long-term trend in OCS concentrations in the troposphere over the last 20 years. Hofmann (1991) has noted, however, that the increase in background aerosol mass is closely related to increases in sulfur emissions from high-altitude aircraft. Another possible anthropogenic source would involve the direct transport of S02 and sulfate from the troposphere, as discussed earlier. The anthropogenic fraction of sulfate in the Northern Hemisphere's upper troposphere can vary from 20% in January to 60 to 80% in July (Chin et al., 2000). Thus, an increase in anthropogenic S02 emissions could have made an impact on the stratospheric aerosol level.
A quite different perspective on background stratospheric aerosol trends has been put forward by Chin and Davis (1995). They have raised the question whether one can even reliably define a baseline value for aerosol in an environment that is continually being disturbed by new volcanic injections of sulfur. They point out that there were only 2 years in the 10-year record cited by Sedlacek et al. (1983) and 2 years in the 18-year observations by Hofmann (1990) that could be identified as "volcanic quiescent" periods. In neither case, however, was it possible to convincingly show that the aerosol or sulfate levels observed during these periods were free of any significant volcanic influence. Given the multiyear residence time of volcanic aerosols and the frequency of minor volcanic injections, Chin and Davis (1995) argued that overall there still remains a serious question whether a true background sulfate aerosol level (i.e., one largely uninfluenced by volcanic emissions) has as yet been observed. Thus, critical to any future analyses designed to show the role of tropospheric sulfur compounds in forming stratospheric sulfur aerosol will be the further elucidation of the volcanic component of the so-called background aerosol layer.
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