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Title of Talk:
Elemental Composition of Freshly Nucleated Particles
Abstract of Talk:
The objective of this research project is to develop a method for real-time sampling and analysis of individual airborne nanoparticles between 5 and 100 nm in diameter. The size range covered by this method is much smaller than existing single particle methods for chemical analysis. Because particles in this size range have had relatively little time to grow or transform in the atmosphere, their chemical compositions should more closely reflect particle formation mechanisms than larger particles that have undergone significant transformation. Chemical composition measurements are performed with the use of a high-energy laser pulse to create a laser induced plasma (LIP) that quantitatively converts the particle into positively charged atomic ions.
Relative to conventional laser ablation which is used in most single particle experiments, the LIP is produced with a 100x higher laser fluence. The higher laser fluence is thought to completely disintegrate the particle into atoms and then quantitatively convert the atoms to positively charged atomic ions. This concept has been confirmed through the study of single particle mass spectra of particle compositions typically found in urban air. In most cases, the measured relative peak areas of different atomic ions match, within experimental error, the expected peak areas based on the elemental composition of the particle. Some important exceptions are noted, for instance transition and heavy metals tended to quench the LIP, inhibiting quantitative analysis. An important advantage of LIP over conventional laser ablation is that the detection efficiency (defined as the number of particles detected divided by the number of particles irradiated with the laser) remains independent of particle size and composition. In particular, ammonium sulfate particles are detected by LIP with the same efficiency as other particle composition types. In contrast, ammonium sulfate is very difficult to detect with conventional laser ablation and has represented an important limitation of the technique in previous field studies.