Elemental composition and morphology of individual particles separated by size and hygroscopicity with the TDMA

Particles from the Minneapolis atmosphere were segregated according to hygroscopicity using a tandem differential mobility analyzer (TDMA) and collected with a specially designed aerodynamic focusing impactor for elemental analysis. Areal deposit densities obtained using the focusing impactor are up to a factor of 100 greater than those obtained using a conventional single-jet impactor, thereby reducing required sampling times by the same factor. A Philips CM30 scanning transmission electron microscope (STEM) equipped with an EDAX super ultra-thin-window detector was used to analyze for carbon and heavier elements. For the limited sample of 0.3-0.4 pm summertime aerosols examined in this study, less hygroscopic particles included chain agglomerates (similar to 55%), irregular shapes (similar to 33%), spheres and flakes (< 10% each) and contained mostly carbon, while more hygroscopic particles were liquid droplets that contained sulfur and sometimes carbon or ionic species such as sodium or potassium.