The application of single particle inductively coupled plasma mass spectrometry (spICP-MS) for online metallic nanoparticles (NPs) characterization in ambient aerosol remains a challenge. In this study, a hyphenated setup consisting of a rotating disk diluter (RDD) with spICP-MS (RDD-spICP-MS) was used for online sampling and characterization NPs in ambient pressure aerosols. The elemental composition, particle size distribution, and particle number concentration (PNC) of metallic NPs in aerosols could be obtained simultaneously. The RDD allowed sampling of the aerosol at a constant flow rate, adjusting the optimal dilution ratio for characterizing NPs at different PNCs, and exchanging the gas matrix to achieve the requirement of spICP-MS for direct aerosol characterization. In this proof-of-concept study, the feasibility of this setup was tested with AuNPs of different sizes from the argon-based and air-based aerosol. The limit of detection for number concentration is below 30 #/L, which is lower than the regulated environmental level. Moreover, multi-modal samples, as well as the interference of ionic species, were accurately investigated. Finally, AuNPs were spiked with environmental PM10 and Road dust to evaluate the matrix effect as well as the capabilities of the setup to characterize environmental aerosol samples. These results indicate that RDD-spICP-MS has a great potential for online characterization of metallic NPs in the ambient aerosol.