| HO2 is an important intermediate in the atmospheric oxidation cycle,the study on its chemical mechanism is helpful to analyze the causes of atmospheric oxidative and combined pollution,so as to be applied to the control and governance of environmental pollution.Due to the characteristics of high reactivity,short lifetime and low concentration of HO2 in the atmosphere,it is a grand challenge to accurately measure its concentration.Most of the commonly used measuring devices of HO2 adopt indirect measurement methods,and they will be chemically disturbed when applied to complex atmospheric environment,which will affect the accuracy of measurement.This is also a bottleneck that restricts the profound study of radical chemistry,so it is necessary to develop a measuring system without secondary disturbances.Considering that the absorption spectroscopy technique will not induce additional chemical reactions and has high detection sensitivity,it is suitable for real-time and online measurement of HO2.To realize the absorption spectrum of HO2 detection in real atmosphere,in this paper,a miniaturized cavity ring down spectroscopy system was independently developed combined with the continuous wave cavity ring-down spectroscopy.By measuring the absorption peak at 6638.205 cm-1 to quantify the concentration of HO2,minimizing the cavity to increase the displacement rate,the loss in the process of HO2 injection was reduced,and the measurement system can be improved by combining the magnetic rotation technology and cavity locking technology.The details are as follows:1.A continuous wave cavity ring-down system was established by using a distributed feedback laser as the emission light source.We had improved the structure of the optical cavity.The integrated optical cavity module increases the stability of the system;Teflon tube was used as the resonant cavity to increase the gas displacement rate and reduce the injection loss;the data acquisition system was optimized to obtain the cavity ring down spectrum of HO2.The detection sensitivity of 8.1 ×10-10 cm-1 was achieved with acquisition time of 30 s,corresponding to a detection limit of 3.9×109 molecule·cm-3 at a pressure of 100 mbar.2.The Faraday rotation spectrum was combined with cavity ring down spectrum using the paramagnetic effect of HO2.The self-designed DC solenoid magnet has a length of 250 mm and a uniform field strength length of more than 100 mm.The two orthogonal polarized ring-down signal was measured by the dual-path differential detection method,thus obtaining the Faraday rotating cavity ring-down spectrum signal of HO2.At the acquisition time of 100 s and the magnetic field of 385.2 Gs,the minimum detectable magnetic rotation angle was 9.2 nrad,corresponding to the detection limit of 1.1×109 molecule·cm-3 at the pressure of 35 mbar.3.Combined cavity ring-down spectroscopy with frequency locking technology,the laser frequency was locked to the absorption of acetylene by frequency transmission through a high precision optical resonator.The wavelength of the locked laser was 6638.205 cm-1,which is located at the absorption peak of HO2.The results show that the frequency locking has improved the stability of the system,and the system detection sensitivity can reach 3.1×10-10 cm-1 with the acquisition time of 140 s,corresponding to HO2 detection limit of 1.5×109 molecule·cm-3。4.The cavity mirror with reflectivity of 99.9987%was used to improve the effective absorption optical path.The contribution of various noises to the ring-down signal was analyzed.The ring-down signal with high signal-to-noise ratio was obtained by using the acquisition card with lower noise.It was verified that the Erbium-doped fiber laser with narrower line width can obtain higher detection sensitivity,reaching 3.9×10-12 cm-1 with the acquisition time of 0.2 s,and the theoretical detection limit corresponding to HO2 was 1.9×107 molecular cm-3,which is an order of magnitude higher than that of distributed feedback laser under the same acquisition time. |
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