Abstract: Significance　With the accelerating global momentum toward addressing climate change and achieving carbon neutrality, controlling mobile source exhaust emissions has emerged as a critical component of air quality improvement. The European Union's recently released Euro 7 emission standards, as well as China's next-generation standards currently under development, impose more stringent requirements regarding solid particle number (PN). These regulations not only lower the cut-off value of particle size measurement from 23 nm to 10 nm, but also introduce unprecedented limits on non-exhaust emissions, such as those arising from braking and tire wear. Against this backdrop, the importance of particle number monitors for mobile source emission (referred to as the PN measurement instrument) as essential equipment for evaluating the emission compliance of engines and complete vehicles is becoming increasingly prominent. However, compared to mature international commercial products, China still lags behind in the research and development of core components, fundamental theoretical research, and calibration technologies for PN measurement instruments. Therefore, a systematic review of the technical status and development trends of PN measurement instruments and their critical components, including volatile particle remover (VPR) and particle detector (PD), is of significant guiding value for China. And such kind of analysis is essential for overcoming technical bottlenecks in 10 nm particle measurement and promoting the independent research, development, and innovation of high-end domestic testing equipment in China. Progress　The PN measurement instrument is primarily composed of two key subsystems: VPR and PD. Regarding VPR technology, to accommodate the detection of particles smaller than 10 nm and effectively prevent the re-condensation of volatile matter, the mainstream approach has progressively evolved from the traditional evaporation tube (ET) method to the catalytic stripper (CS) method. By combining catalytic oxidation with sulfur trapping, the CS method achieves a more thorough removal of hydrocarbons and sulfur-containing volatile compounds. Specifically, the optimally designed CS units now demonstrate high solid particle transmission efficiencies comparable to those of ETs for particles larger than 30 nm, while offering superior removal performance for volatile components. Regarding PD, detection technologies are primarily categorized into two classes: condensation particle counters (CPCs) and diffusion charging (DC) sensors. CPCs operate by enlarging particles via supersaturated vapor condensation to facilitate optical counting. Although they have the advantage of high measurement accuracy, their complex structure necessitates specific optimization of the growth tube and optical scattering detectors to accommodate the characteristics of mobile source emissions, such as high concentrations and rapid transient changes. Comparatively, DC technology has garnered widespread attention due to its compact structure and lower cost. Its development has evolved from steady-state diffusion charging (SSDC) to modulated diffusion charging (MDC), and subsequently to modulated precipitation diffusion charging (MPDC). MPDC technology, in particular, employs Electrostatic Precipitator (ESP) for modulation. By leveraging the correlation between particle electrical mobility and particle size, it effectively counteracts the inherent positive proportionality between particle charge and size. This mechanism achieves an ideal response characteristic within a specific size range, where the output current signal is directly proportional to the particle number concentration and substantially independent of particle size. In addition, the counting efficiency curve of MPDC sensors can be flexibly tuned by adjusting aerosol flow rate and precipitation voltage, thereby meeting various regulatory lower detection limits, such as 10 nm or 23 nm. Conclusions and Prospects　In response to the imminent implementation of more stringent emission standards, the critical objective in the development of PN measurement instruments is to enhance the detection capability and accuracy of ultrafine particles below 10 nm. Regarding VPR, future designs must prioritize minimizing losses of particles of 10 nm and above caused by diffusion and thermophoresis, while ensuring high volatile removal efficiency. Furthermore, corresponding quality control methodologies must be established. For CPC, further optimization of growth tube and detection system is required to improve the activation efficiency of particles with varying material properties, which is essential to accommodate the dynamic variations in the composition and concentration characteristics of mobile source emissions. Concerning detectors based on diffusion charging principle, although MPDC technology has partially mitigated size dependency through parameter tuning, the influence of particle size distribution on measurement results has not been completely eliminated. Therefore, future research should focus on refining theoretical models for methods such as MPDC and systematically optimizing key parameters through experiments. These efforts aim to fundamentally improve measurement reliability within complex, real-world emission environments.In summary, through sustained breakthroughs in these critical technologies, it is anticipated that domestic PN measurement instruments in China will narrow the performance gap with the state-of-the-art international products, and ultimately provide robust metrological support for the implementation of the next generation of national emission regulations.

Key words: mobile emission, particulate matter, number concentration, condensation particle counter, diffusion charging