The influence of aerosols on radiative forcing results in the largest uncertainty in the energy terms used to estimate and predict climate change. Light-absorbing particles such as soot (black carbon) play an important role, exerting a warming influence that may be second only to that of CO2. However, the complexity of soot – shape, surface area, porosity, density, and composition – limit detailed knowledge of its chemical, microphysical, and optical properties. On the other hand, this complexity, once quantified and interpreted, is also an unusually rich source of information about conditions experienced in the atmosphere. Systematic measurements on individual particles will provide unique, comprehensive information about soot aging, reactivity, and radiative properties.

The branching, fractal character of soot particles fundamentally influences their reactivity, atmospheric lifetimes, and interaction with radiation. Electron tomography with transmission electron microscopy (TEM) will be used to determine the 3D shapes, surface areas, volumes, and densities of individual soot particles from various sources. Preliminary results show that quantitative measurements from electron tomograms differ from geometrically derived values by one to two orders of magnitude, require revision of the estimated atmospheric lifetime of soot by up to 60%, and indicate that its direct radiative forcing may be underestimated by 30%.

The proposed research is divided into: a) development and refinement of electron tomography to obtain quantitative 3D measurements of individual aerosol particles; b) field studies, with emphasis on samples from Mexico City as an example of emissions from tropical megacities (via the MIRAGE-Mex experiment); c) laboratory studies of synthetic soot prepared under controlled conditions, including use of an environmental cell so that electron microscopy can be done at a specified humidity and temperature; and d) theoretical calculations of radiative effects in light of the field and laboratory observations. Although focused on soot, the results of this project will be applicable to a range of aerosol issues.

The results will provide quantitative information of surface area, volume, density, and fractal dimension of soot particles from different sources and with and without surface coatings. Information about mixing states and contrasts within and outside emission plumes will be developed, as will speciation of associated aerosol particles, calculated soot lifetimes, atmospheric burdens, and optical properties. The direct radiative forcing will be estimated using the GISS GCM II-prime model.