Projections of Climate Forcing by Sulfate, Organic Aerosols, Dust, and Sea Salt: Results from the Ipcc Model Inter- Comparison Workshop

Comparisons of models with observations for sulfate aerosols and other sulfur compounds are particularly relevant for assessing model capabilities because the emissions of sulfur bearing compounds are better known than the emissions of other aerosol compounds. 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 and Prospero, 1998]; and sea salt aerosols [Murphy et al., 1998]. Moreover, the presence of absorbing aerosols, primarily in the form of soot, is important because it lowers the single scattering albedo of the aerosol, leading to a decrease in the reflection and an increase in the absorption of solar radiation. Furthermore, the magnitude of the indirect effect is sensitive to the abundance of natural aerosols represented in the models [Penner et al., 1999]. Therefore, an examination of model capability to represent this entire suite of aerosol components was undertaken as part of this report. Emissions for this model comparison were specified by the most currently available emissions inventories for each component (see Table1). Ten aerosol models participated in the model inter-comparison of sulfate, eight treated black carbon, 7 treated organic carbon, 6 treated dust, and 5 treated sea salt. Eight scenarios were defined. The first, SC1, was selected to provide good estimates of present-day aerosol emissions. SC2 used present day chemistry and natural emissions to simulate aerosol concentrations in 2030 according to the IPCC SRES A2 scenario (SRES, 1999). SC3 used present day chemistry and natural emissions to simulate the A2 scenario in 2100 and SC4 used present day chemistry and natural emissions to simulate the B1 scenario in 2100. In addition, we examined 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 SC7 for the B1 scenario. Scenario SC6, also examined changes in emissions of other gas phase components associated with the production of sulfate in the A2 scenario in 2100 (see Chapter 4). Finally, SC8 is provided to show estimates for anthropogenic emissions in 2100 associated with the IS92a scenario. Some of the participants also provided estimates of direct and indirect forcing. The estimates, together with the range of predicted concentrations among the models, help to define the uncertainty in aerosol forcing for future scenarios. Not all participants were capable of producing results for all 5 aerosol compounds. The models participants, scenarios they provided, and the aerosol components treated in each model are summarized in Table 2.