•An especial linear TOF MS for vacuum arc discharge ion source was developed.•The linear TOF MS has a wire type ion gate and a 1.5 m drift tube.•The accelerating voltage of ion beam reaches 40 kV and the resolution is 120 (FWHM).•The compositions of ions in the first few μs of discharge process, with O+, C+, O2+ and C2+ dominating.•It provides an effective detection means for plasma because of relatively high resolution.A special linear time-of-flight mass spectrometer, with relatively high mass resolution of 120 full width at half maximum (FWHM), for vacuum arc discharge ion source was developed. The instrument comprises a two-electrode ion extraction system, an ion gate, a drift tube, an einzel lens system and a micro channel plate (MCP) detector. Wire type ion gate is used for precise extraction of high energy (up to 40 keV) ion beam, and the drift tube length is 1.5 m. The extracted ion bunch is focused by the einzel lens system before it arrives the MCP detector, in order to improve the detection efficiency. The low mass ions in plasma extracted during the first few microseconds of discharge process are mainly O+ (m/z 16), C+ (m/z 12), O2+ (m/z 8) and C2+ (m/z 6). The instrument we have described can be a valuable diagnostic tool for high energy ion beam composition. It can be used both in fundamental research and in technological applications such as ion implantation.

The extremely high concentrations of PM2.5 (particulate matter with an aerodynamic meter ≤ 2.5 μm) during severe and persistent haze events in China have been closely related to the formation of secondary aerosols (SA). New particle formation (NPF) is the critical initial step of SA formation. New particles are commonly formed from gas-phase precursors (e.g., SO2, volatile organic compounds) via nucleation and initial growth, in which molecular clusters with a mobility diameter smaller than 3 nm (hereafter referred to nanoscale molecular clusters) will be involved throughout the whole process. Recently, significant breakthroughs have been obtained on NPF studies, which are mostly attributed to the technical development in the real-time analysis of size-resolved number concentration and chemical composition of nanoscale molecular clusters. Regarding the detection of size-resolved number concentrations of nanoscale molecular clusters, both methods and instruments have been well built up; practical application in laboratory-scale experiments and field measurements have also been successfully demonstrated. In contrast, real-time analysis of chemical composition of nanoscale molecular clusters has still encountered the great challenges caused by the complex organic compositions of the clusters, and improvement of present analytical strategies is urgently required. The better understanding in NPF will not only benefit the atmospheric modeling and climate predictions but also the source control of SA.