Atmospheric aerosol particles affect the Earth's radiative balance both directly through the upward scatter of solar radiation and indirectly as cloud condensation nuclei (CCN). The natural aerosol derived from biogenic sulfur emissions has been substantially perturbed by anthropogenic aerosols, particularly sulfates from SO2 emissions and organic condensates and soot from biomass and fossil fuel combustion. The global mean radiative forcing due to the direct effect of anthropogenic sulfate aerosol particles is calculated to be of comparable magnitude (approximately -0.3 to -1.1 watt m-2) but opposite in sign to the forcing due to anthropogenic CO2 and the other greenhouse gases (Charlson et al., 1991, 1992; Kiehl and Briegleb, 1993; Penner et al., 1994). More uncertain is the radiative forcing due to the indirect cloud-mediated effects of aerosol particles (Boucher and Lohmann, 1995). Although aerosol particles have a potential climatic importance over and down wind of industrial regions that is equal to that of anthropogenic greenhouse gases, they are still poorly characterized in global climate models. This is a result of a lack of both globally distributed data and a clear understanding of the processes linking gaseous precursor emissions, atmospheric aerosol properties, and the spectra of aerosol optical depth and cloud reflectivity. At this time, tropospheric aerosols pose one of the largest uncertainties in model calculations of the climate forcing due to anthropogenic changes in the composition of the atmosphere. Clearly, considerable attention must be focused on quantifying the processes controlling the natural and anthropogenic aerosol and on defining and minimizing the uncertainties in the calculated climate forcings.
Reducing the uncertainties in estimates of aerosol forcing of climate will require a combination of laboratory experiments, long-term continuous and short-term intensive field studies, satellite observations and modelling analyses (Penner et al., 1994). Although much of this work can be accomplished by single laboratory groups, intensive process studies that seek to obtain closure (internal consistency) between various measured and modeled aerosol properties often require a large number of research platforms and investigators. The IGAC Project provides the international cooperation and leadership needed for these large process studies. The Aerosol Characterization Experiments (ACE) are envisioned as a series of international field studies aimed at quantifying the role of the chemical and physical processes that control the evolution and properties of the atmospheric aerosol that are relevant to radiative forcing and climate. The ultimate goal of this series of process studies is to provide the necessary data to incorporate aerosols into global climate models and to reduce the overall uncertainty in the calculation of climate forcing by aerosols. The strategy to achieve this goal mandates improved understanding of the multiphase atmospheric chemical system including gas and aerosol exchange at the ocean surface. ACE-1, the first experiment, is aimed at the minimally polluted marine troposphere (Figure 1). ACE-2 will focus on the polluted marine atmosphere.
Figure 1. ACE-1 Study area. The ACE-1 operations center and aircraft base will be located in Hobart. Ground-based measurements will be made at Cape Grim, Macquarie Island and Baring Head (!). Aircraft and ship operations will take place in the region between 38-58°S and 130-170°E. Upper air soundings will be made from Cape Grim, the R/V Discoverer, the R/V Southern Surveyor and the regional meteorological stations (þ).