Ionization efficiency of evolved gas molecules from aerosol particles in a thermal desorption aerosol mass spectrometer: Numerical simulations

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 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 ammonium iodide 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. Copyright © 2019 American Association for Aerosol Research

配备电子电离源的热解析气溶胶质谱仪（Thermal Desorption Aerosol Mass Spectrometers，TDAMSs）被广泛应用于气溶胶化学成分的定量测定。目前学界尚未完全阐明影响脱附气体分子电离效率的物理与化学机制。本研究开发了一款用于模拟气溶胶颗粒脱附产生的气体分子动力学过程的数值模型。该模拟模型包含两大核心模块：第一模块模拟脱附气体分子在汽化源附近小区域（碰撞区域）内的弹性碰撞过程——该区域内分子的平均自由程远小于周围高真空环境中的平均自由程；第二模块则模拟从第一模块边界至电离源的自由分子动力学过程。本研究评估了碘化铵颗粒脱附产生的氨与碘化氢分子的电离效率。研究结果表明，羽流扩张早期阶段的分子碰撞，以及此类碰撞引发的分子速度变化，可能是导致电离效率出现观测差异的重要机制。但TDAMSs中气溶胶颗粒的汽化与电离物理化学过程极为复杂，难以通过简化数值模型实现定量复现。© 2019 美国气溶胶研究协会（American Association for Aerosol Research）