Carbonaceous particles are major components of atmospheric aerosols, with importance for climate, visibility, and health. They are produced in large amounts globally by biomass burning and are significant contributors to light absorption and thus radiative transfer and resultant heating of the atmosphere. In spite of this importance, optical properties of carbonaceous aerosol particles generated through biomass burning are represented with a large uncertainty in global climate models, and thus major issues remain about the role they play in climate change. The proposed ARM aircraft-based field campaign provides an integrated opportunity to resolve some of the uncertainties and ambiguities related to the physical and chemical properties of particles generated through biomass burning. Atmospheric aerosol studies can, in general, be divided into remote (satellite, lidar, etc.) and sample investigations. The latter can be subdivided into on-line and off-line measurements. Each type of analysis provides important, complementary information, and all are required for a comprehensive understanding of the nature and role of carbonaceous aerosol particles for influencing climate and climate change. We propose to do off-line measurements using transmission electron microscopy (TEM). Although more time consuming and personnel-intensive than the on-line methods, TEM measurements provide unique, exquisitely detailed information about the individual particles that are measured in large quantities, effectively more or less in bulk, by on-line methods (Psfai and Buseck, 2010). The latter provide good statistical reliability but require all sorts of inferences about the nature of the particles being measured. We will determine morphological, chemical, hygroscopic, and perhaps optical properties of aerosols generated by biomass burning. The morphological information will be both twodimensional, as is typical of most microscopy images and that have many of the characteristic of shadows in that they lack depth data, and three-dimensional (3D). The electron tomographic measurements will provide 3D data, including the presence and nature of pores, interstices, and whether the individual particles are coated by or embedded within other materials (Adachi et al., 2010). These microphysical properties will be determined for particles as a function of time and distance from the respective sources in order to obtain detailed information regarding the time evolution of changes during aging. In the process of the above measurements, we hope to provide insight into the results of measurements made with the SP2 soot photometer, which has produced important but, in some cases, unpredicted and surprising results. The issue is that the SP2, like an aerosol mass spectrometer (AMS), destroys the particles that are being measured. A consequence is that unexpected results must be explained by making simplifying inferences about the particles producing these results, just as with an AMS one needs to reconstruct the measured particles by combining the fragments produced during the analyses. Making parallel TEM measurements on the same sample sets should go a long way to testing and, hopefully, confirming the assumptions and inferences required by the on-line measurements, although of course the results can not be predicted. An interesting outcome of the project is that it should bring more clarity to the relationship between black carbon (BC), which is used for most if not all global climate models, and soot, a well-known product in the combustion community. The problem arises because BC is measured indirectly through optical (absorption) techniques. It uses materials such as graphite and fullerenes as standards because there is no material that can be provided that is itself recognized as a BC standard. That is because BC is not a known material, although there is a widespread assumption that it is equivalent to soot (Jacobson, 2001; Bond and Bergstrom, 2006; IPCC, 2007). In contrast, material and microphysical properties of soot (e.g., morphology, composition, crystallographic structure, average dimensions and mixing states) have been well described in the aerosol literature (Psfai et al., 1999; Wentzel et al., 2003; Adachi et al., 2007; Adachi and Buseck, 2008). Thus, in contrast to BC, soot has definable material properties. Reconciling the relationship will be a possible outcome of the proposed project. Our experience with such sampling and analyses comes from participation in numerous campaigns that included SAFARI 2000 in southern Africa, MILAGRO in and around Mexico City and extending to biomass burns in Yucatan, and CalNex in southern California. We would use the same samplers as in those campaigns.
|Effective start/end date||4/1/13 → 6/30/15|
- US Department of Energy (DOE): $205,250.00