Exhaust emissions from motor vehicles have a significant impact on air quality and human health. Among them, nitrogen oxides, the main pollutants in motor vehicles, are particularly critical and highly sensitive to concentration detection. Commonly used NO detection techniques include chemiluminescence (CL),Fourier transform infrared spectroscopy (FTIR), and differential optical absorption spectroscopy (DOAS). However, none of these can easily fulfil the requirements of miniaturization, portability, high sensitivity, and high selectivity of NO detection systems. Faraday rotation spectroscopy (FRS) detects the concentration of molecules by measuring the changes in the rotational signal, whereby the concentration of the paramagnetic absorbing molecules is obtained from the demodulated spin signals. This paper describes the design of a magnetic rotation absorption cell applied to a motor vehicle exhaust NO detection system based on Faraday rotation spectroscopy, and the performance was examined in terms of the characteristics of the magnetic field it can generate. Simulation experiments were conducted to explore the use of this absorption cell in a motor vehicle exhaust NO detection system.

A magnetic rotation absorption cell model was constructed based on finite element (FEM) simulations and analysis. The magnetic field characteristics generated by the coil were simulated and analyzed, to fabricate the magnetic rotation absorption cell based on the FEM analysis results and parameters of the coil wire: the diameter and number of turns. The development of the magnetic rotation absorption cell was divided into three steps: cavity design, coil design, and window sheet selection. Repeated experiments and step-by-step approximation methods were used to determine whether the three parts of the absorption cell reached the optimal state, as determined by the quality of the collected spectral lines. A Tesla meter was used to check the magnetic field characteristics generated by the absorption cell coil. The consistency between the performances of the designed and simulated absorption cell was analyzed by relative error, and long-term measurements were conducted to check the stability of the magnetic field generated by the absorption cell. The simulation experimental setup of the NO detection system for motor vehicle exhaust gas was built using this absorption cell. The mathematical model of the peak-to-peak value of the rotating signal and NO volume fraction was established by spectral signal calibration, and the minimum detection limit of the system was evaluated by Allan variance for different magnetic induction strengths.

The standard deviations and relative errors of the measured magnetic induction strength indicate that the magnetic field performance of the designed absorption cell is consistent with that of the simulated absorption cell. Measurement results for the magnetic field stability showed that the average value of the generated magnetic induction strength was 25.6 mT; the polar deviation was 65 μT; and the generated magnetic field was very stable. A simulation of the motor vehicle exhaust NO detection system showed that the peak value of the optical rotation signal displayed an excellent linear relationship with NO volume fraction C, with a correlation coefficient of 99.5%, which indicates that the motor vehicle exhaust NO detection system constructed with this absorption cell can detect changes in NO volume fraction. The continuous measurement results and Allan variance calculations of a fixed NO volume fraction with different magnetic induction strengths indicated that the minimum detection limit of the system can be increased by 20% when the magnetic induction strength is increased from 0 to 36.4 mT.

In this study, a model of a magnetic rotation absorption cell was constructed for a motor vehicle exhaust NO detection system, and a single-optical-range magnetic rotation absorption cell with an optical range of 250 mm was designed based on a simulation analysis of the principle. The effective optical range in the magnetic field was 180 mm, and the size was ?60 mm×250 mm. Performance of the cell was examined based on the magnetic induction strength and motor vehicle exhaust NO detection system simulation test results revealing that: 1) The magnetic field performance of the designed absorption cell is consistent with that of the simulated absorption cell, with a relative error of less than 2%. 2) The system built using this absorption cell can accurately detect NO volume fraction changes for magnetic induction strengths of 0?36.4 mT, and the minimum detection limit is increased by 20%. This lays the foundation for the design of a high sensitivity/high selectivity motor vehicle exhaust NO detection system, which can be extended to other paramagnetic molecular detection systems or application scenarios.