The hydroxyl(OH) free radical is one of the most important oxidants and is at the origin of the majority of chemical transformations in the troposphere. It plays a key role in the formation of ozone and secondary organic aerosols. Accurate and quantitative measurements of OH radical is of great significance to atmospheric chemistry research and air quality control. However, due to the low concentrations in the atmosphere(similar to 10(6) molecule/cm(3)), high reactivity, and very short lifetime(<1 s), current techniques that can be successfully employed for tropospheric OH measurement are extremely limited. Therefore, there is a strong driving force for the development of new techniques. Off-axis Integrated Cavity Output Spectroscopy(OA-ICOS) based on Lambert-Beer law can offer an ultra-long optical pathlength on the order of kilometers utilizing a high-finesse optical resonator with a limited optical base length. In addition, the off-axis paths through optical cavity can actively excite higher-order transverse modes and effectively reduce the influence of cavity mode fluctuation. These advantages make it a powerful tool for sensitive measurement of OH radical. We reported the development of a mid-infrared OA-ICOS experimental setup. A room temperature continuous-wave distributed feedback diode laser emitting at 2.8 mu m was used as the probe laser. The Q(1.5e) transition line of OH radical located at 3 568.52 cm(-1) was selected for detection. The integrated cavity consisted of two 25.4-mm diameter high-reflectivity dielectrically coated plano-concave mirrors (1 m radius of curvature) separated by a distance of 35.8 cm. The measured effective reflectivity of the cavity mirrors was 0.999 3, corresponding to the effective optical path of 512 m. In this mid-infrared OA-ICOS system, the amplified spontaneous emission of the distributed feedback laser, which is usually on the order of nanowatts and is a kind of unwanted broadband radiation outside the highly reflective band of the cavity mirrors, directly passed through the cavity without resonating. This amplified spontaneous emission was received by the detector together with the weak valid absorption signal, causing interference in measurement. It was found that the absorption was about 70 times underestimated due to the amplified spontaneous emission, which needs to be effectively avoided. To reduce the 1/ f noise and improve the sensitivity of this system, wavelength modulation spectroscopy was applied. The laser current was swept and modulated by a triangle wave and a sinusoidal wave around the absorption peak to obtain the second harmonic(2f) signals. The sample of OH radical were generated by the reaction of H2O and O(1D) produced by O-3 photolysis. The concentration of OH in the cavity was determined by a reference absorption line of H2O in the same spectral region at 3 568.55 cm(-1) whose concentration could be calculated by direct absorption spectroscopy. The strong linear relationship (correlation coefficient of 0.999 4)between 2f signals and concentrations was exhibited. Based on a typical spectrum measured under an OH concentration of 4.84x109 molecule/cm(3) and 100 s data acquisition time, the noise level was evaluated by standard deviation from the non-absorption wing, giving a signal-to-noise ratio of about 38. From these results, the detection limit of OH radical was determined to be 1.2x 10(8) molecules/cm(3)(1s). The performance of this spectrometer can be further improved by reducing the residual cavity resonances, using higher reflectivity cavity mirrors to increase the absorption path length and signal intensity, re-injecting to the cavity via a third mirror and improving the effective gain of the detector. In particular, the Wavelength Modulation Off-axis Integrated Cavity Output Spectroscopy (WM-OAICOS) technique in the mid-infrared provides a new direct spectral method for OH radical detection. The successful combination of OA-ICOS and wavelength modulation spectroscopy means that OA-ICOS also can be combined further with other modulation technologies, such as frequency modulation spectroscopy, to achieve shot-noise limited detection.

Quantum cascade laser (QCL) plays an important role in mid-infrared detection because of the high output power and wide coverage range. However, due to the fluctuation of laser wavelength caused by the sensitivity of the laser to changes in the external environment, the peak-to-peak frequency drift is as high as 180 MHz within the observed time of 400 s, which affects the performance of the QCL to some extent and reduces the accuracy of molecular spectral detection. Frequency locking has been widely applied to the mid-infrared areas. In this paper, a QCL frequency locking system based on gas absorption was developed. Taking 5. 3 pm QCL as an example, the laser frequency is locked to the absorption peak of nitric oxide (NO) molecule at 1 875. 812 8 cm i by modulating laser wavelength. The principle of error signal generation was introduced, and the advantages of using the third harmonics as an error signal for frequency locking were analyzed. The NO absorption signal with a high signalto-noise ratio (SNR) was obtained using a NO absorption cell with a length of 30 cm. The conversion coefficient between the third harmonic voltage and the laser frequency was calibrated. The locking process was introduced in detail and explored the significance of proportional, integral, differential parameters of the feedback loop during the locking process, and the locking parameters had been given in detail. Disturbing the locking system, with the recovery time better than 40 ms demonstrate that the locking system can respond quickly and remain stable against external disturbances. In addition, the stability of the frequency locking system was also verified by the fluctuation of the error signal with the voltage-frequency conversion coefficient. A frequency drift better than 673 kHz (16, 10 ms integration time) was achieved. The Allan variance analysis results show that when the integrated time is extended to 100 s, the frequency drift is lower than 4. 5 kHz ( corresponding to stability of 8 X 10 "), effectively improving the laser frequency's long-term stability. This method of directly modulating laser frequency without an external modulator simplified the system and improved the stability of the optical detection system.

A portable sensor with an optical case dimension of 46 x 28 x 16 cm3 based on laser absorption spectroscopy for highly sensitive measurement of formaldehyde was reported. A compact improved spherical mirror multi-pass cell consisting of two 5-cm diameter spherical mirrors separated by 17.7 cm was developed. The cell offered a sample volume of 350 mL and an optical absorption path length of 50.6 m, giving a high path length-to-volume ratio of 14.6 cm-2. The sampling response time was less than 1 s, allowing the application of rapid spectral background subtraction technique to effectively suppress optical fringes. The achieved detection limit was 650 pptv (1 & sigma;, 1 s), corresponding to a minimum detection absorption coefficient of 2.3 x 10-9 cm -1. The capability of the HCHO sensor was demonstrated by two days of continuous indoor air monitoring. The developed portable sensor is suitable for mobile and fast indoor air quality measurements.

A new experimental set-up for highly sensitive detection of transient radicals was established based on cavity ring-down spectroscopy (CRDS) combined with flash photolysis. The mode coupling between the laser and the cavity was achieved by scanning the cavity length of the self-designed optical resonant cavity, and the ring -down time was obtained by fitting the decay curve based on the linear regression summation (LRS) algorithm. The minimum detectable absorption of the system of 5.05x10-11 cm(-1) was achieved with effective optical path of 20.3 km and acquiring time of 6.25 s. HO2 radicals were generated by using a 266 nm Nd & COLRATIO;YAG laser photolysis of O-3/C2H2 mixtures. The characteristic absorption of HO2 radicals at 6638.203 cm-1 was measured, and the absorption spectrum with a resolution of 0.002 cm(-1) was obtained. Under the condition of 2 kPa, the absorption cross section of HO(2 )radicals at 6638.203 cm-1 was 3.3x10-19 cm2/molecule by measuring the decay of the HO2 radicals self-reaction, and the corresponding absorption line strength was 6.02x10-21cm(-1)/(molecule/cm(2)). Keywords Cavity ring-down spectroscopy; Laser-flash photolysis; Hydroperoxyl radical; Absorption cross section

The hydroperoxyl radical (HO2) plays a key role in atmospheric chemistry. It reacts with NO to generate hydroxyl radical (OH) and nitrogen dioxide (NO2), resulting in the HOx (= OH + HO2) cycle that governs the atmospheric oxidation capacity and the formation of air pollution, and a net production of ozone that determines the troposphere ozone budget. Quantitative measurement of its absolute concentration is very important. However, due to its short lifetime and low concentration (typically about 108 to 109 molecule/cm3 under atmospheric condition), most of the currently used methods are indirect methods that require chemical conversion; direct measurement remains very challenging.The cavity ring-down spectroscopy (CRDS) technique uses high reflectivity mirrors to increase the effective absorption pathlength to tens of kilometers, enabling very high detection sensitivity; the absolute concentration of the target absorbers can be quantitatively determined by the Beer-Lambert law, providing a powerful tool for direct measurement of free radicals. In this work, we report the development of a portable cavity ring-down spectrometer for direct and absolute measurement of HO2 radical concentration using a distributed feedback (DFB) diode laser operating at 1506 nm. At a pressure of 100 mbar, a detection limit of ~ 7.3×107 molecule/cm3 (1σ, 10s) was achieved with a ring-down time (τ0) of 136 μs. The corresponding detection sensitivity was 1.5×10-11 cm-1, which was close to the state-of-the-art performance.In cavity ring down spectroscopy, the coupling efficiency of the laser beam into the cavity depends on the laser frequency tuning speed and the ratio of the laser linewidth to the cavity mode width. For the DFB laser system, the laser linewidth (~ 2 MHz) was about thousands of times larger than that of the cavity mode (~ 1.2 kHz), which results in the conversion of laser phase noise into amplitude fluctuation, making the cavity injection noisy and limiting the improvement of detection sensitivity. Here, by replacing the DFB diode laser with a narrow linewidth erbium-doped fiber (EDF) laser, the amplitude fluctuation caused by the laser phase noise was reduced and the cavity mode injection efficiency was improved. The sensitivity was improved to 3.9×10-12 cm-1 with a short data-acquisition time of 0.2 s. The one order of magnitude improvement makes further ambient applications look promising.

We report the development of a portable cavity ring-down spectrometer (CRDS) for direct and absolute measurement of HO2 radical concentration using a distributed feedback (DFB) diode laser operating at 1506 nm. The spectrometer has a compact design with all optics in a 1000 × 400 × 140 mm3 box. At a pressure of 100 mbar and a ring-down time (τ0) of 136 µs, the detection limit of the CRDS spectrometer was ∼ 7.3 × 107 molecule/cm3 (1σ, 10s). The corresponding detection sensitivity was 1.5 × 10-11 cm-1, which was close to the state-of-the-art performance. By replacing the DFB diode laser with a narrow linewidth erbium-doped fiber (EDF) laser, the amplitude fluctuation caused by the laser phase noise was reduced and the cavity mode injection efficiency was improved. The sensitivity was improved to 3.9 × 10-12 cm-1 with a short data-acquisition time of 0.2 s. Compared with the DFB laser, the improvement was nearly an order of magnitude. The use of the narrow linewidth laser is attractive. The instrument can achieve very high sensitivity without the need for a complex locking technique, ensuring simple and ease of use in future field applications.

We report the development of an optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) instrument for OH detection at 2.8 μm using a DFB diode laser. Two different approaches, symmetry analysis and wavelength modulation, were performed to achieve laser frequency locking to the cavity mode. Compared with the symmetry analysis method, the wavelength modulation method continuously locked the laser frequency to the cavity mode and eliminated decoupling the laser from the cavity mode. A detection sensitivity of 1.7×10-9 cm-1 was achieved in a 25 s sampling time and was about 3 times better than that of the symmetry analysis method. The corresponding OH detection limit was ∼ 2×108 molecule/cm3. Further improvement can be achieved by using higher reflectivity mirrors and other high-sensitivity approaches, such as frequency modulation spectroscopy and Faraday rotation spectroscopy.

光学多通池具有结构简单、光路对准容易、光谱通用性高、鲁棒性好及成本低等优势,是增加吸收光程最有效的手段之一,可以显著提高吸收光谱的探测灵敏度.简要总结了不同类型光学多通池的基本原理,重点介绍了中国科学院安徽光学精密机械研究所张为俊研究员团队使用光线追迹研制的200 m光程像散镜池、59.7 m光程Herriott池、大于200 m光程密集光斑型球面镜池以及带有便携稳定调节机构的Chernin池,详述了这些多通池的研制方法及其在激光吸收光谱探测中的应用.

利用虚像相位阵列(VIPA)的强色散能力发展而来的VIPA光谱仪是一种新型的高性能光谱测量装置,兼具宽光谱覆盖范围、高时间分辨率及高光谱分辨率等优点.基于60 GHz自由光谱区(FSR)的石英VIPA,研制了一台可用于红光波段光谱测量的VIPA光谱仪,并测量了633 nm多模激光器的出射光谱.在100μs的时间分辨率下,获得了单次3.1 nm测量谱宽、2.0 pm(1.5 GHz)的高光谱分辨率,连续4 h运行显示光谱漂移小于0.50 pm.通过采用衍射光栅扫描装置、更短曝光速度的面阵CCD和更大FSR的VIPA,有望进一步提高光谱仪性能.

We report the development of a static magnetic field Faraday rotation spectrometer for NO detection. A 5.33 µm continuous-wave quantum cascade laser was used as the probing laser. Line absorption at 1875.81 cm−1 (2Π3/2Q(3/2), υ=1←0) was chosen for the detection. By using a Chernin type multipass cell, a detection precision of 1.15 ppbv (1σ, 1s) was achieved with an absorption pathlength of 108 m. This value was reduced to 0.43 ppbv by increasing the data-acquisition time to 15 s.

The total OH reactivity (kOH') is an important parameter for quantitative assessment of the atmospheric oxidation capacity. Although laboratory measurement of kOH' has been achieved 20 years ago, the instruments required are often costly and complex. Long-term atmospheric observations remain challenging and elusive. In this work, a novel instrument combining laser-flash photolysis with a mid-infrared Faraday rotation spectrometer (LFP-FRS) has been developed for the measurement of kOH' and for studying gas phase free radical kinetics. The reactor is composed of a Herriott-type optical multipass cell, and OH radicals were generated by flash photolysis of ozone with a 266 nm pulsed Nd:YAG laser. The decay of the OH signal was directly measured with a time-resolved FRS spectrometer at 2.8 μm. The overlapping path length between the pump beam and probe beam was 25 m. High performance was achieved by subtracting the signals before and after flash photolysis to eliminate interferences caused by H2O absorption and background drift. The optimum precisions (1σ) of OH concentration and kOH' measurement were 4 × 106 molecules cm-3 and 0.09 s-1 over data acquisition times of 56 and 112 s, respectively. The performance of the system was evaluated by the reaction of OH with CO and NO. The measured rate coefficients (kOH+CO and kOH+NO) were in good agreement with values reported in the literature. The developed LFP-FRS provides a new, high precision, and highly selective tool for atmospheric chemistry research of OH radicals and other transient paramagnetic free radicals such as HO2 radicals.

Kinetic studies of the reactions between center dot OH and alkanes were performed with laser-flash photolysis and Faraday rotation spectrometer ( LFP-FRS ) . The center dot OH was generated by flash photolysis of ozone with a 266 nm pulsed laser, the decay of the center dot OH concentration was directly measured with a time-resolved FRS spectrometer at 2.8 mu m under the pseudo-first-order, and the reaction rate constant of methane with center dot OH at 5 kPa was measured to be 6. 59 x 10(-15) cm(3) /( molecule . s ) . Meanwhile , the rate constants of methane, ethane and propane reacted with center dot OH under different pressure conditions were compared. The reaction process was simulated by implementing several existed reaction channels into a chemical kinetic model, which provided a basis for studying the important atmospheric chemical mechanism and reaction process of center dot OH radical.

We report the development of an improved spherical mirror multipass-cell-based interband cascade laser (ICL) spectrometer for ambient formaldehyde (HCHO) detection. The multipass cell consists of two easily manufactured spherical mirrors that are low cost, and have a simple structure, large mirror area utilization, and dense spot pattern. Optical interference caused by the multipath cell was largely reduced, resulting in good sensitivity. Using wavelength modulation spectroscopy (WMS), a detection precision (${1} \sigma $1σ) of 51 pptv in 10 s was achieved with an absorption pathlength of 96 m, which compared favorably with the performance of other state-of-the-art instruments. The precision can be further improved by using a long absorption pathlength configuration and by removing fringe-like optical noise caused by the collimation lens. Ambient application of the developed spectrometer was demonstrated.