AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Ionization Efficiency of Evolved Gas Molecules from Aerosol Particles in a Thermal Desorption Aerosol Mass Spectrometer
YU IDE, Kento Uchida, Nobuyuki Takegawa, Tokyo Metropolitan University
Abstract Number: 177 Working Group: Instrumentation and Methods
Abstract Thermal desorption aerosol mass spectrometers (TDAMSs) with electron ionization are widely used to quantitatively measure aerosol chemical compositions. The physical and chemical mechanisms affecting the ionization efficiency of evolved gas molecules are not fully understood. We have performed laboratory experiments to investigate key factors affecting the ionization efficiency using a custom-made TDAMS. Ammonium chloride (NH4Cl) and ammonium iodide (NH4I) are used as test compounds. The ion signals originating from ammonia (NH3) and hydrogen halide (HX) were measured by altering the geometric relationship between the ionizer and vaporization point. The ratio of ion signals of NH3 to HX tended to increase with increasing the divergence angle of evolved gas plumes. Experimental results suggested that spatial broadening of gas molecules could be an important factor affecting the ionization efficiency. To interpret the experimental results, we have developed a numerical model for simulating the dynamics of gas molecules evolved from aerosol particles. The simulation model is composed of two main sections. The first section simulates the elastic collisions of the evolved gas molecules in a small region near the vaporization source (collision domain), where the mean free paths of the molecules are much shorter than those in the surrounding high vacuum environment. The second section simulates the free-molecular dynamics from the boundary of the first section to the ionizer. The ionization efficiencies of ammonia and hydrogen iodide molecules that evolved from NH4I particles were evaluated. Our results suggest that the molecular collisions during the early stage of plume expansion and possible changes in the molecular velocities induced by these collisions could be an important mechanism affecting the observed variability in the ionization efficiency. However, the physical and chemical processes of the vaporization and ionization of aerosol particles in TDAMSs may be too complex to be quantitatively reproduced using simplified numerical models.