Organic aerosols (OAs) represent a critical component of the atmosphere, impacting global climate, regional air pollution, and human health. Due to the complex mixtures included in the single particle, measurement of organic aerosol is a significant challenge. Mass spectrometry which provides high sensitivity and wide molecular mass range is well-suited for the analysis of organic aerosols. However, traditional aerosol mass spectrometry (AMS) techniques using electron ionization could generate extensive fragment, which is adverse to the molecular identification of organic particle. Over the past decade, AMS instruments employing soft ionization sources, such as chemical ionization, photoionization, and electrospray ionization, have been developed; their mass spectral simplicity provides unprecedented molecular level detail of the chemistry of OAs.