Working Group I: The Scientific Basis


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5.4.1.2 The IPCC model comparison workshop: sulphate, organic carbon, black carbon, dust, and sea salt

Comparisons of models with observations for sulphate aerosols and other sulphur compounds are particularly relevant for assessing model capabilities because the emissions of sulphur bearing compounds are better known than the emissions of other aerosol compounds (Section 5.2). Thus, comparison can focus on the capabilities of the models to treat transport and oxidation processes. Recent field studies, however, have pointed out the importance of organic aerosol compounds (Hegg et al., 1997), dust aerosols (Li-Jones and Prospero, 1998; Prospero, 1999), and sea salt aerosols (Murphy et al., 1998a). Also, soot is important because it decreases the reflection and increases the absorption of solar radiation (Haywood and Shine, 1995). Furthermore, the magnitude of the indirect effect is sensitive to the abundance of natural aerosols (Penner et al. 1996; ODowd et al., 1999; Chuang et al., 2000). Therefore, an examination of model capability to represent this entire suite of aerosol components was undertaken as part of this report.

Table 5.7: Global emissions specified for the IPCC model intercomparison workshop.
  SC1 SC2 SC3 SC4 SC5  
    A2 A2 B1 A2 IS92a
Year 2000 2030 2100 2100 2100+ natural 2100
Sulphur (as Tg S)a            
  Anthropogenic SO2 69.0 111.9 60.3 28.6 60.3 147.0
  Ocean DMS 25.3 25.3 25.3 25.3 27.0  
  Volcanic SO2 9.6 9.6 9.6 9.6 9.6  
Organic Carbon (as OM)b            
  Anthropogenic 81.4 108.6 189.5 75.6 189.5 126.5
  Natural 14.4 14.4 14.4 14.4 20.7  
Black Carbonb            
  Anthropogenic 12.4 16.2 28.8 12.0 28.8 19.3
Dust (<2 µm diameter)c 400 400 400 400 418.3  
Dust (>2 µm diameter)c 1,750 1,750 1,750 1,750 1,898  
Sea Salt (as Na) (<2 µm diameter)d 88.5 88.5 88.5 88.5 155.0  
Sea Salt (as Na) (>2 µm diameter)d 1,066 1,066 1,066 1,066 1,866  
a Anthropogenic SO2 emissions were the preliminary emissions from Nakicenovic et al. (2000). DMS emissions were the average of the emissions based on Wanninkhof (1992) and those based on Liss and Merlivat (1986) using methods described in Kettle et al. (1999). Volcanic SO2 emissions were those available from the IGAC Global Emissions Inventory Activity (http://www.geiacenter.org) described by Andres and Kasgnoc (1998).
b Organic carbon is given as Tg of organic matter (OM) while black carbon is Tg C. For anthropogenic organic matter, the black carbon inventory of Liousse et al. (1996) was scaled up by a factor of 4. This scaling approximately accounts for the production of secondary organic aerosols consistent with the analysis of Cooke et al. (1999). For 2030 and 2100, the ratio of the source strengths for CO in 2030 and 2100 to that in 2000 was used to scale the source of organic carbon and black carbon at each grid location. For natural organic aerosols, the terpene emissions from Guenther et al. (1995) were assumed to rapidly undergo oxidation yielding a source of aerosol organic matter of 11% by mass per unit C of the emitted terpenes.
c The dust emission inventory was prepared by P. Ginoux.
d The sea salt emissions were those developed by Gong et al. (1997a,b) based on the Canadian Climate Model winds.

Emissions for this model comparison were specified by the most recently available emissions inventories for each component (see Tables 5.2, 5.3, Section 5.2 and Table 5.7). Eleven aerosol models participated in the model intercomparison of sulphate, and of these, nine treated black carbon, eight treated organic carbon, seven treated dust, and six treated sea salt. Eight scenarios were defined (see Table 5.7). The first, SC1, was selected to provide good estimates of present day aerosol emissions. SC2 was defined to simulate aerosol concentrations in 2030 according to preliminary estimates from the IPCC SRES A2 scenario (Nakic´enovic´, et al., 2000). SC3 was defined to simulate the draft A2 scenario in 2100 and SC4 to simulate the draft B1 scenario in 2100. SC1-SC4 used present day chemistry and natural emissions. In addition, we estimated possible future changes in emissions of the natural components DMS, terpenes, dust and sea salt in 2100 in SC5 for the A2 scenario and in SC8 for the B1 scenario. Scenario SC6 also estimated changes in emissions of other gas phase components associated with the production of sulphate in the A2 scenario in 2100 (see Chapter 4) and SC7 estimated changes in climate (temperature, winds and precipitation patterns) as well. Table 5.7 also shows the estimates of anthropogenic emissions in 2100 associated with the IS92a scenario. Some of the participants also provided estimates of direct and indirect forcing. These estimates, together with the range of predicted concentrations among the models, help to define the uncertainty due to different model approaches in aerosol forcing for future scenarios. The models, participants, scenarios they provided, and the aerosol components treated are summarised in Table 5.8.

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