Comparing Sprayed Water Microdroplets with Several Ionization Methods on the Capability of Ionizing Ambient Air

Microdroplet chemistry has been utilized in different fields due to a plethora of unique properties such as the existence of a high electric field up to 10⁹ V/m at the surface of the microdroplets or between two oppositely charged microdroplets. Such a high field endows microdroplets with the ability of ionizing ambient air. In this study, we compare the ionization capability of microdroplets with several other ionization methods, including photoionization (vacuum ultraviolet photons, X-ray), nuclear decay (α-ray from ²⁴¹Am), and discharge ionization (piezoelectric discharge, dielectric barrier discharge, and arc discharge) using mass spectrometry to detect ionic products. The mass spectra show striking similarity. These results reveal the high ionization capability of water microdroplets and rationalize the observation of various radical and ion related reactions initiated by water microdroplets. The high ionizing nature of water microdroplets also sheds light on the chemistry of the origin of life.