Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
Department of Chemistry, University of Helsinki, FIN-00014 Helsinki, Finland.
Faraday Discuss. 2013;165:75-89. doi: 10.1039/c3fd00031a.
Formation of secondary atmospheric aerosol particles starts with gas phase molecules forming small molecular clusters. High-resolution mass spectrometry enables the detection and chemical characterization of electrically charged clusters from the molecular scale upward, whereas the experimental detection of electrically neutral clusters, especially as a chemical composition measurement, down to 1 nm in diameter and beyond still remains challenging. In this work we simulated a set of both electrically neutral and charged small molecular clusters, consisting of sulfuric acid and ammonia molecules, with a dynamic collision and evaporation model. Collision frequencies between the clusters were calculated according to classical kinetics, and evaporation rates were derived from first principles quantum chemical calculations with no fitting parameters. We found a good agreement between the modeled steady-state concentrations of negative cluster ions and experimental results measured with the state-of-the-art Atmospheric Pressure interface Time-Of-Flight mass spectrometer (APi-TOF) in the CLOUD chamber experiments at CERN. The model can be used to interpret experimental results and give information on neutral clusters that cannot be directly measured.
二次大气气溶胶粒子的形成始于气相分子形成小分子团簇。高分辨率质谱能够从分子尺度向上探测和化学表征带电团簇，而对电中性团簇的实验探测，特别是作为化学成分测量，直到 1nm 以下甚至更小的粒径，仍然具有挑战性。在这项工作中，我们用一个动态碰撞和蒸发模型模拟了一组既带有电荷又不带电荷的小分子团簇，由硫酸和氨分子组成。根据经典动力学计算了团簇之间的碰撞频率，并从没有拟合参数的第一性原理量子化学计算推导出蒸发速率。我们发现，用在 CERN 的 CLOUD 室实验中使用的最先进的大气压接口飞行时间质谱仪（APi-TOF）测量的负团簇离子的稳态浓度的模型与实验结果吻合良好。该模型可用于解释实验结果，并提供无法直接测量的中性团簇的信息。
