Experimental research on instrumentation for measuring automotive exhaust particle concentrations

Due to the absence of domestic automotive exhaust measurement methods, this paper, based on the principle of condensation nucleus growth counting, has developed a nanoparticle counter suitable for automotive exhaust detection. The paper incorporates the peak value counting method and conducts comparative experiments between different counting methods of the nanoparticle counter and the results obtained by electrostatic counting, as well as parallel experiments with a TSI CPC. The following results were obtained: (1) Compared to the traditional rising edge counting method, the peak value counting method significantly improves counting accuracy. (2) When the concentration is below 12, 500 P/cc, the peak value counting method can count more particle signals. (3) In the testing of complex particle clusters in automotive exhaust, the nanoparticle counter can be used for measuring automotive exhaust emissions.


Introduction
High-temperature combustion sources are the primary contributors to atmospheric PM2.5 pollution, with the vast majority of these particles being in the nanoscale range for example nanoparticle concentrations in vehicle exhaust exceed 90% [1].These nanoparticles can penetrate the bloodstream and readily adsorb polluting gases in the atmosphere, forming secondary aerosols, and can also carry viruses and bacteria, posing significant health hazards to humans.For the latest ultra-low emissions from vehicles, aircraft, power plants, and other sources [2][3][4], assessing the harm of exhaust particulates solely based on mass concentration is insufficient.Even though ultrafine particles represent a small proportion by mass, their potential for harm is substantial.In comparison to new vehicles, in-use vehicles emit higher levels of particulate number, making an assessment of exhaust emissions based solely on particulate mass concentration inadequate.It is essential to incorporate the measurement of particulate number concentration.Currently, several countries such as the Netherlands and Finland have initiated the use of particulate number (PN) measurement technology for in-use vehicles' exhaust inspections.In China, there is an urgent need to establish relevant standards and develop measurement equipment and instruments [5].
Pollutants in the exhaust emissions from vehicles include harmful gases (such as carbon dioxide, nitrogen oxides, carbon monoxide, and volatile organic compounds) and particulate matter (such as suspended particles and fine particles) [6,7].These pollutants not only directly affect air quality but can also contribute to environmental issues such as climate change, acid rain, and photochemical smog [3].Furthermore, automotive exhaust emissions pose potential health threats to human beings.When harmful gases and particulate matter in vehicle exhaust enter the human respiratory system, they may lead to respiratory diseases, cardiovascular diseases, allergies, and even cancer.Fine particles, particularly those with a diameter of less than 2.5 micrometers, have high penetration capabilities, can deeply enter the lungs, and be absorbed into the bloodstream, causing more significant health effects [4,[8][9][10].Presently, detection technologies for gaseous pollutants in vehicle exhaust are relatively mature and include laser gas analyzers for measuring carbon dioxide (CO2) and nitrogen oxide (NOx) concentrations [11,12], as well as opacity meters [13].However, with the increasing awareness of the harm of particulate emissions from vehicles [14,15], governments and environmental agencies have imposed more stringent regulations and standards for vehicle emissions, along with higher testing requirements, making particulate matter detection increasingly important.There has been a significant shift in recent years from traditional particle mass (PM) measurements to particle number (PN) measurements for particulate detection [16][17][18], with a particular focus on the quantity and size distribution of fine particles [18,19].Therefore, the independent development of equipment suitable for measuring particulate number concentration in in-use vehicles in China is of great significance.This paper presents a nanoparticle counter designed for automotive exhaust detection based on the condensation nucleus growth counting principle, as shown in Figure 1.The essential components of the CPC (Condensation Particle Counter) include a saturation stage, a condensation stage, and an optical counting unit.The saturation stage operates at a temperature of 50°C and is used to heat isobutyl alcohol to create a stable vapor source.Particles undergo condensation and growth in the condensation stage, which operates at a temperature of 6°C.When the particles to be measured enter the condensation stage, the supersaturated isobutyl alcohol vapor condenses on the particle surfaces due to the temperature difference.This increases particle diameter, transforming them from the nanoscale to submicron or micron levels.Subsequently, the particles enter the optical counting unit, where they are illuminated by a laser and the scattered signal is collected and converted into electrical pulses by photodiodes, ultimately enabling particle counting.

Signal correction work
The traditional counting method relies on counting based on the rising edge of the signal, as illustrated in Figure 2. Using a set threshold, it counts an event when the signal transitions from below the threshold to above it.Consequently, this rising edge counting method may miss counts when dealing with multiple pulse signals within a certain range.This situation arises because after the particle concentration increases, particles overlap when passing through the optical path of the optical counter, causing the received optical signal to not fall back to the bottom noise.At the same time, overlapping particles will also interact with each other to generate more scattered light, resulting in more peak signals, which to some extent leads to technical difficulties in the case of particle overlap.
To address this issue, the peak value counting method is introduced, as depicted in Figure 3.The peak value counting method is based on counting the peaks of the pulse signals.By identifying local maxima in the existing signal acquisition frequency, it calculates the wave peaks and counts them, ensuring accurate frequency identification for the current signal.Therefore, the peak value counting method does not suffer from counting losses and effectively enhances the accuracy of the counting results.

Experimental testing and validation
As shown in Figure 4, to validate the improved nanoparticle counter, comparative experiments were conducted at different particle concentrations using a self-developed nanoparticle counter and a TSI electrostatic counter.The experimental setup included an aerosol generator for generating stable particle sources, a Differential Mobility Analyzer (DMA) used for particle size selection, the Electrostatic counter (ELT) used as the reference instrument for comparison, and the self-developed nanoparticle counter.In the experiments, different concentrations of 50 nm sodium chloride nanoparticles were generated by adjusting the sheath-to-aerosol flow ratio of the DMA and the mass concentration of the sodium chloride solution.The counting results of the self-developed nanoparticle counter and the electrostatic counter were simultaneously tested and compared.As depicted in Figure 5, the experimental results clearly demonstrate that, at concentrations exceeding 10000 P/cc, the self-developed nanoparticle counter using the rising edge counting method yields significantly lower counts compared to the electrostatic counter (ELT).This discrepancy arises because, at higher particle concentrations, a greater number of particles trigger high plateau signals, producing multiple consecutive peaks that cannot be accurately counted using a single rising edge detection.The introduction of the peak value counting method effectively corrected the counting issue.As shown in the graph, the counting results obtained after employing the peak value counting method closely align with those of the electrostatic counter.
After implementing the peak value counting method, the counting results of the self-developed nanoparticle counter and the electrostatic counter exhibited a more consistent trend over time and demonstrated a positive correlation.It is worth noting that when the particle number concentration falls within the range of 12500 to 15750 P/cc, the measurement results of the self-developed nanoparticle counter and the electrostatic counter are nearly identical.At lower concentrations, the nanoparticle counter produces measurements higher than the electrostatic counter, while at concentrations above 15700 P/cc, the nanoparticle counter yields measurements lower than those of the electrostatic counter.To verify the detection performance of this system on complex particle clusters in automotive exhaust, the exhaust was diluted to a concentration below 20, 000 P/cc using a dilution system.The self-developed nanoparticle counter and TSI Model 3775 CPC were simultaneously tested.During a 2-minute test, the vehicle's engine speed was varied from a low speed of 700 r/s to a high speed of 4000 r/s, then reduced back to low speed, with 30-second intervals maintained at 4000 r/s and 2000 r/s.The test results, as shown in Figure 6, indicate that the self-developed BH CPC exhibits good consistency with the TSI 3775 CPC.
It still has good consistency between high-speed and low-speed changes, and the response speed of the self-developed nanoparticle counter to concentration changes is similar to that of TSI CPC.Therefore, it is believed that the nanoparticle counter, with the help of a diluter, can measure the number and concentration of automotive exhaust particles.

Conclusion
Based on the principle of condensation nucleus growth counting, this paper has developed a nanoparticle counter for automotive exhaust detection, achieving the following results: (1) Compared to the traditional rising edge counting method, the peak value counting method significantly improves counting accuracy.
(2) When the concentration is below 12, 500 P/cc, the peak value counting method can count more particle signals.
(3) In testing complex particle clusters in automotive exhaust, the self-developed BH CPC (Condensation Particle Counter) results closely match the counts obtained with the TSI CPC, indicating that the BH CPC can be used for measuring automotive exhaust emissions.

Figure 1 .
Figure 1.Schematic representation of the nanoparticle counter instrument.

Figure 4 .
Figure 4. Schematic diagram of nanoparticle counter and electrometer testing experiment.

Figure 5 .
Figure 5.Comparison of different counting methods.

Figure 6 .
Figure 6.Nanoparticle Counter Automotive Exhaust Gas Detection Results.