Presenter: Dr. Peter A. Alpert
Post-Doc, Paul Scherrer Institute
Ph.D., Stony Brook University, 2013
The main advantage of any microscopy technique is the knowledge of “what is where”, i.e. composition and location. In terms of chemical selectivity and spatial resolution, a technique known as scanning transmission X-ray microscopy coupled to near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) is unmatched and will remain so for years into the future. STXM/NEXAFS uses focused single energy X-rays to a spot size on the order of nanometers onto condensed matter such as films or aerosol particles. Absorption of X-rays by ground state electrons reveals atomistic composition, molecular bonding and oxidation information. Atmospheric aerosol particles typically <1 micron in diameter are made up of organic matter rich in low molecular weight atoms like carbon, nitrogen and oxygen and others compounds such as sea salts and metals. Particles that are collected from ambient air advected over a coastal city, for example, may be impacted by both sea salt from the ocean and soot particles from combustion sources. The presence, mixing and location of soot, inorganic and organic matter within single particles can be directly observed using STXM/NEXAFS, data that has recently been deemed necessary for prediction of light scattering and absorption of a particle population containing soot and thus, radiative forcing calculations. Spectroscopic information is also useful for quantifying chemical bonding in reference material and identifying these in ambient particles, e.g., distinguishing proteinaceous matter from carbohydrates in bioaerosol particles. Recent developments have led to coupling STXM/NEXAFS to small environmental chambers on the order of mm3 in which aerosol particles reside. This has the advantage of extending chemical and spatial information as a function of time during exposure to oxygen, trace gases and light mimicking atmospheric aerosol aging. This talk will present mixing state quantification of atmospheric aerosols and chemical reactions in laboratory generated particles. We argue that all particles are coated by organic material and that diffusion and reaction of trace oxidants must be considered to understand aging and other physical properties, such as cloud particle nucleation.