The hydroperoxyl radical (HO2) is an important oxidant, playing a crucial role in atmospheric chemistry processes such as free radical cycling and ozone formation. However, the direct measurement of HO2 using laser absorption techniques presents significant challenges due to its low atmospheric concentration, high reactivity, and short lifetime. In this paper, a high-sensitivity narrow-linewidth cavity ring-down spectroscopy (CRDS) system for HO2 detection was developed. By employing dual locking of the feedback phase and laser current to the resonant cavity, the probe laser linewidth was successfully narrowed from 2.3 MHz to 12.3 kHz, and the frequency noise was reduced by 4 to 5 orders of magnitude. Through cavity length adjustments, a continuous laser frequency tuning range of 4.5 GHz was achieved. This narrow linewidth laser significantly heightened the trigger efficiency of the CRDS system. Allan deviation analysis at a fixed laser frequency demonstrated that an improved sensitivity of 3.24 × 10-11 cm-1 could be obtained with an averaging time of 0.48 s, which corresponds to an HO2 detection limit of 1.29 × 108 molecule per cm3. The developed system features a simple design and provides a valuable technical reference for future research on direct HO2 measurements.

Quantitative measurements of atmospheric total OH reactivity (kOH′) provide crucial insights into atmospheric photochemistry. However, widespread application of total OH reactivity measurements is challenging due to insufficient equipment and the complexity of existing instrumentation. In this work, we report the development of a portable laser-flash photolysis Faraday rotation spectroscopy (LP-FRS) instrument for real-time and in situ measurement of kOH′. To achieve efficient overlapping between the pump and probe laser and realize a long effective absorption path length, thus enabling high-sensitivity measurement, a specific Herriott-type pump–probe optical multi-pass cell was designed. The instrument's optical box dimensions were 130 cm × 40 cm × 35 cm. The obtained effective absorption path was ∼ 28.5 m in a base length of 77.2 cm. The kOH′ detection precisions of the LP-FRS instrument were 2.3 and 1.0 s−1 with averaging times of 60 and 300 s, respectively. The kOH′ measurement uncertainty was evaluated to be within 2 s−1. Field measurement was performed, and the difference between the measured kOH′ and the model simulated from the measured reactive species was analysed. The developed portable LP-FRS instrument extends the measurement methods of atmospheric total OH reactivity and has certain advantages in terms of cost, operation, and transportation, which will play an increasingly important role in future atmospheric chemistry research.

Atmospheric chemistry research and atmospheric measurement techniques have mutually promoted each other and developed rapidly in China in recent years. Cavity-based absorption spectroscopy, which uses a high-finesse cavity to achieve very long absorption path-length, thereby achieving ultra-high detection sensitivity, plays an extremely important role in atmospheric chemistry research. Based on the Beer–Lambert law, this technology has the unique advantages of being non-destructive, chemical-free, and highly selective. It does not require any sample preparation and can quantitatively analyze atmospheric trace gases in real time and in situ. In this paper, we review the following: (1) key technological advances in different cavity-based absorption spectroscopy techniques, including cavity ring-down spectroscopy, cavity-enhanced absorption spectroscopy, cavity attenuated phase shift spectroscopy, and their extensions; and (2) applications of these techniques in the detection of atmospheric reactive species, such as total peroxy radical, formaldehyde, and reactive nitrogen (e.g., NOx, HONO, peroxy nitrates, and alkyl nitrates). The review systematically introduces cavity-based absorption spectroscopy techniques and their applications in atmospheric chemistry, which will help promote further communication and cooperation in the fields of laser spectroscopy and atmospheric chemistry.

Objective Formaldehyde is the most abundant aldehyde in the troposphere and a primary indoor pollutant, classified as a human carcinogen. High- sensitivity on-line measurement of formaldehyde is critical for monitoring atmospheric environments and indoor pollution. High- resolution and accurate formaldehyde spectrum are essential for developing high- sensitivity detection instruments and improving spectral inversion accuracy. Therefore, it is of great significance to obtain a high- quality formaldehyde spectrum for formaldehyde research. Fourier transform infrared (FTIR) spectroscopy is a commonly used infrared spectral technique. However, the traditional FTIR spectrometer with an incoherent thermal light source suffers from low sensitivity and requires long averaging time to improve sensitivity. The optical frequency comb is essentially a pulsed laser with the advantages of wide spectrum, high brightness, and good collimation. It can replace the thermal light source in traditional FTIR spectrometer and improve the detection sensitivity by combining a multi- pass cell or an optical resonant cavity. Therefore, we built a Fourier transform spectrometer based on an optical frequency comb to measure the broadband spectrum of formaldehyde and have a quantitative analysis, including measurements in the presence of water interference. Methods Leveraging the advantages of the optical frequency comb and the FTIR in detecting wide- range molecular absorption spectra, an FC-FTIR spectrometer is built to measure the formaldehyde spectrum near 3.5 mu m. The comb source used is FC1500-250-WG and mid-IR optical frequency comb, generating an infrared laser with a center wavelength of 3200 nm by difference frequency conversion. The laser is collimated into the Herriott-type optical multi- pass cell. The optical base length of the cell is only 1.2 m, and an effective absorption path of 60 m can be obtained after multiple reflections. The comb beam exiting the cell is then focused into the FTIR spectrometer for formaldehyde spectrum detection. Results and Discussions The constructed FC-FTIR spectrometer successfully measures the broadband infrared spectrum of formaldehyde in the 2730-2970 cm-1 band (Fig. 3). The sensitivity of the instrument reaches 3x10-8 cm-1 Hz-1/2 (Fig. 4), and the detection limit of formaldehyde is 414x10-9. The wavenumber accuracy is better than 150 MHz, allowing for precise quantitative analysis of formaldehyde concentration with an uncertainty of approximately 9 degrees o -11 degrees o (Fig. 5). At the same time, the absorption spectra of formaldehyde in the presence of a large amount of water are measured (Fig. 6), confirming that the device's spectral range and resolution are adequate for detecting various species. Conclusions In this paper, an FC-FTIR spectrometer is built. The broadband spectra of formaldehyde in the 3.5 mu m spectral range are obtained at low pressure and room temperature. The sensitivity of the system is 3x10-8 cm-1Hz-1/2, corresponding to the formaldehyde detection sensitivity of 414x10-9. At the same time, the formaldehyde absorption spectrum under water signal interference is measured with prominent band characteristics, and the concentration of formaldehyde can be accurately obtained. The mid- infrared FC-FTIR device can not only detect formaldehyde but also detect greenhouse gases and pollution gases in the atmosphere, which is expected to take an advantage in the field of multi- species dynamic concentration monitoring. This device combines the high- resolution, wide- spectrum measurement and quantitative analysis benefits of traditional FTIR spectrometer with enhanced system sensitivity, reduced response time, and improved frequency accuracy and precision. In addition, the sharp spectral bands of the molecular species obtained by the measurement make it possible to use all the peaks with complete shapes for multi- line fitting in data processing. With its capability for fast acquisition and high- sensitivity broadband spectral data in molecular fingerprint regions, the mid- infrared FC-FTIR spectrometer is expected to gradually replace traditional FTIR spectrometer in molecular spectroscopy.

The virtually imaged phased array (VIPA) spectrometer uses the orthogonal dispersion method and has the advantages of compact structure, high spectral resolution, and wide wavelength coverage. It has been widely...

Optical freciuency comb-based Fourier transform spectroscopy has the features of broad spectral bandwidth, high sensitivity, and multiplexed trace gas detection, which has valuable application potential in the fields of precision spectroscopy and trace gas detection. Here, we report the development of a mid-infrared Fourier transform spectrometer based on an optical frequency comb combined with a Herriott-type multipass cell. Using this instrument, the broadband absorption spectra of several important molecules, including methane, acetylene, water molecules and nitrous oxide, are measured by near real-time data acquisition in the 2800-3500 cm(-1) spectral region. The achieved minimum detectable absorption of the instrument is 4.4xl0(-8) cm(-1)Hz(-1//2) per spectral element. Broadband spectra of H2O are fitted using the Voigt profile multispectral fitting technique and the consistency of the concentration inversion is 1%. Our system also enables precise spectroscopic measurements, and it allows the determination of the spectral line positions and upper state constants of N2O in the (0002)-(1000) band, with results in good agreement with those reported by Toth [Appl. Opt. 30, 5289 (1991)].

Objective Carbon dioxide (CO2) is a principal byproduct of hydrocarbon fuel combustion. Real-time detection of CO2 can evaluate combustion temperature and efficiency, playing a crucial role in combustion diagnosis. Compared with the probe method and other contact techniques, laser absorption spectroscopy offers rapid, precise and non-intrusive measurement of CO2 in combustion environments. This method has attracted increasing attention and research, becoming a mainstream technology for combustion diagnosis. Among various approaches, combining a broadband laser source with broadband absorption spectrum measurement allows capturing more sample absorption characteristics, especially when sample absorption is weak or subject to interference from other absorbents, providing the advantage of multi-wavelength absorption spectrum detection. The virtual image phase array (VIPA) spectrometer, characterized by its wide spectral range and high resolution, represents a novel type of orthogonal dispersion spectrometer. However, when directly applying the VIPA spectrometer to gas parameter inversion, the measured spectral frequency axis exhibits deviations from theoretical values due to the nonlinear dispersion of the VIPA element and discrete sampling by the array detector, leading to reduced accuracy in gas inversion. This paper presents a spectral inversion accuracy optimization algorithm based on particle swarm optimization (PSO) aimed at enhancing the precision of CO2 detection using the VIPA spectrometer for wide-spectrum CO2 detection. Methods The CO2 measurement system, centered around the VIPA spectrometer, primarily consists of two components: the CO2 concentration detection part and the gas preparation part. Light emitted by a supercontinuum light source, after filtration through a 1.42-1.45 mu m filter, combined with a fiber collimator, enters a Chernin-type optical absorption multi-pass cell with an optical path length of 4 m. An optical fiber coupler directs the light exiting the multi-pass cell into a single-mode fiber, which is then connected to the VIPA spectrometer's fiber interface. Initially, the Voigt absorption line model for the CO2 molecule is established by the HITRAN database. The peak position of the absorption model and the experimental peak's pixel position are fitted using a cubic polynomial to achieve preliminary calibration of the frequency axis. Subsequently, the PSO algorithm corrects the peak position of the simulated spectrum line to ensure optimal agreement between the simulated and measured spectra. Finally, the gas volume fraction is determined through the least square method. During peak position correction of PSO algorithm, the spectrum is divided into several sub-intervals using the trough of the spectrum line as the cut-off point. Adjacent sub-intervals with peak spacing less than 1 cm(-1) are grouped into a single fitting interval, and each interval's peak is corrected individually. Results and Discussions The cubic polynomial fitting spectrum extraction algorithm yields a frequency axis with a position deviation ranging from 0?0.1 cm(-1) compared to the theoretical positions [Fig. 4(c)]. Residual analysis indicates that frequency axis calibration deviations are the primary source of these discrepancies. Given the disparity between the measured spectrum's frequency axis and the theoretical spectrum, the PSO algorithm is used to adjust peak positions (Fig. 5). As iterations increase, peak position distribution stabilizes, with the algorithm generally converging by the 30th iteration. The reliability of the PSO peak correction algorithm for gas volume fraction retrieval is examined by measuring CO2 concentrations of 30%, 40%, 50% and 60% within the range of 6900 to 6990 cm(-1). Without PSO correction, the average deviation of inversion is 33.27% (Fig. 8), and the maximum relative error reaches 35.43%. The average deviation of inversion after PSO correction is 1.81%, and the maximum relative error is 2.58%. The accuracy of the inversion is significantly improved after PSO correction of the peak value. Conclusions To address the issue of substantial parameter inversion errors due to insufficient spectrometer frequency axis calibration accuracy, an optimization algorithm of absorption spectrum inversion accuracy based on PSO is introduced in our study. By employing the PSO algorithm to adjust the simulated peak positions of the measured spectrum line of pure gas, an optimal match between simulated and measured spectral lines is achieved. Using corrected peak positions, simulated absorption lines serve as a basis for solving the volume fraction as an independent variable through least squares fitting to experimental lines. Pre- and post-peak correction fitting outcomes for pure CO2 measurement and simulation spectra demonstrate that the PSO-based peak correction algorithm effectively enhances peak location accuracy and reduces fitting residuals. According to CO2 measurement data spinning 30%-60% volume fractions, the average deviation in corrected volume fraction inversion stands at 1.81%, with an average root mean square error of 1.01x10(-5), indicating the method's efficacy in improving the inversion accuracy of volume fraction and verifying the algorithm's applicability to VIPA spectral parameter inversion. This algorithm also offers reference value for gas parameter inversion optimization in other spectrometers.

We report the development of an optical feedback linear cavity-enhanced absorption spectroscopy instrument for HO2 detection using a distributed feedback (DFB) diode laser operating at 1506 nm. A direct and accurate method of reflectivity measurement based on the analysis of cavity mode signals was proposed. A differential circuit was used to judge the zero crossing point of the optical feedback cavity mode in the center of the frequency locking region, and shift the laser operating current to the non-resonant region. In this way, a ring-down signal was obtained with a time of 17.9 μs, corresponding to an effective absorption pathlength of 5.37 km. Combining the standing wave condition, the relationship between cavity length and drive voltage of the PZT mounted on the cavity rear mirror is translated into a correlation between the transmitted light wavenumber and the PZT voltage. The spectral resolution was improved from 290 MHz to 97 MHz by precisely tuning the PZT voltage. The achieved detection sensitivity of the system was 7 × 10− 10 cm− 1 with a data acquisition time of 10.6 s. The absorption spectrum of HO2 at 6638.205 cm− 1 was measured at a cell pressure of 50 mbar with a detection limit of 3.24 × 109 molecule/cm3.

We present a novel mid-infrared frequency-modulated Faraday rotation spectrometer (FM-FRS) for highly sensitive and high bandwidth detection of OH radicals in a photolysis reactor. High frequency modulation (up to 150 MHz) of the probe laser using an electro-optical modulator (EOM) was used to produce a modulation sideband on the laser output. An axial magnetic field was applied to the multi-pass Herriott cell, causing the linearly polarized light to undergo Faraday rotation. OH radicals were generated in the cell by photolyzing a mixture of ozone (O3) and water (H2O) with a UV laser pulse. The detection limit of OH reaches 6.8 × 108 molecule/cm3 (1σ, 0.2 ms) after 3 and falling to 8.0 × 107 molecule/cm3 after 100 event integrations. Relying on HITRAN absorption cross section and line shape data, this corresponds to minimum detectable fractional absorption (Amin) of 1.9 × 10-5 and 2.2 × 10-6, respectively. A higher signal-to-noise ratio and better long-term stability was achieved than with conventional FMS because the approach was immune to interference from diamagnetic species and residual amplitude modulation noise. To our knowledge, this work reports the first detection of OH in a photolysis reactor by FM-FRS in the mid-infrared region, a technique that will provide a new and alternative spectroscopic approach for the kinetic study of OH and other intermediate radicals.

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.

Objective Broadband and high-resolution spectroscopy plays a significant role in many research fields such as atmospheric trace gas detection, industrial monitoring, precision measurement, and basic physics and chemistry. Large spectral bandwidth allows for the simultaneous detection of multiple species, which enables a single instrument to have many functions. However, detection techniques that can provide a pm-level spectral resolution over a wide bandwidth still need to be further studied. The virtually imaged phased array (VIPA) is a plane-parallel etalon, where the input beam is injected at an angle through an entrance window on the front face. The multiple reflections occur within the VIPA etalon. The emerging light interferes to make different frequencies exit at different angles. VIPA spectrometer is an orthogonal dispersion system composed of VIPA and grating and can achieve spectral coverage of tens of nm in a single frame and spectral resolution of pm. In the past years, the VIPA spectrometer has been widely applied in high-precision broadband spectral measurement. However, practical applications of VIPA spectrometer face the following problems. First, some algorithms that employ gas absorption to calibrate the VIPA spectrometer ignore the instrument lineshape function (ILS), and second, these algorithms are difficult to calibrate when weakly absorbed. Additionally, the adjustment structure of the VIPA spectrometer can still be improved. Our paper reports an improved near-infrared spectrometer based on the VIPA and presents the experimental details and performance evaluation. The broadband and high-resolution measurement technology of CO2 in 1. 43-1. 45 mu m is carried out by combining the supercontinuum source and multi-pass cell. The results verify the reliability of the system and the accuracy of the improved data processing algorithm. Methods The experimental system mainly consists of a supercontinuum laser, a Chernin muti-pass cell, and a VIPA spectrometer. The broadband light is collimated by the aspheric collimator. Then the emergent light is reflected eight times inside the gas cell and finally connects to the interface of the VIPA spectrometer by a single-mode fiber to acquire the CO2 absorption spectrum. The experimental source is a supercontinuum laser with a spectral coverage of 0. 47-2. 4 mu m. The Chernin cell is composed of five pieces of plano-concave mirrors with a radius of 0. 5 m. To obtain CO2 absorption of appropriate intensity and avoid absorption saturation, the mirror angle of the Chernin cell is adjusted to realize the reflection number of 8 and the optical path of 4 m. The VIPA spectrometer is made of high-strength hard aluminum alloy with dimensions of 400 mmx280 mmx120 mm. The main improvements of the spectrometer structure are as follows. The adjusting structure of the cylindrical lens and the collimator is combined to change the incident optical axis, and the offaxis aberrations of the VIPA spectrometer are reduced. The adjusting structure of the imaging lens is improved and the CCD leads to more compact spectrometer. Meanwhile, the grating rotation structure is added and the spectral coverage of the VIPA spectrometer is extended. The system employs pure N-2 absorption as the background image (I-0) and pure CO2 absorption as the signal image (I). The algorithm subtracts the dark image from each of the signal and background images and then adopts Eq. (10) to subtract the baseline to get the absorption image. Finally, the algorithm extracts the one-dimensional spectra according to the rules shown in Fig. 2 and realizes the absorption spectral inversion. Results and Discussions The fitting residual of the CO2 absorption spectrum at 6971. 0021 cm(-1) is 3x10(-3) [Fig. 4(c)], which verifies the correctness of the improved algorithm with the spectral resolution of the VIPA spectrometer being 4. 5 pm [Fig. 4(d)]. By generalizing unimodal fitting to multimodal fitting, the broadband theoretical absorption spectrum can be obtained by line-by-line integration [Fig. 5(a)]. The minimum fitting residual of the whole spectrum (1.43-1.45 mu m) is 5. 31x10(-1), proving that the developed VIPA spectrometer can be utilized for broadband and high-resolution spectral measurement of gases. The standard deviation (SD) of the baseline is 2. 68x10(-1) [Fig. 5(a)], and the detection limit of CO2 molecules corresponding to the highest absorption peak of line intensity is 1. 85x10(-1), which can be improved by increasing the optical path. Conclusions A high-resolution near-infrared VIPA spectrometer with a relatively simple structure, a spectral resolution of 4. 5 pm, and a spectral coverage of 25 nm in a single frame is developed. Improving the adjustment structure of the VIPA spectrometer makes the spectrometer more compact, reduces the off-axis aberrations, and extends the actual spectral coverage of the VIPA spectrometer. In terms of the data processing algorithm, the extraction accuracy of weak signals is improved by adding image enhancement algorithms, and the accuracy of gas parameter inversion is improved by considering the ILS. Finally, the broadband and high-resolution measurement technology of CO2 in 1. 43-1. 45 mu m is carried out by combining the supercontinuum source and multi-pass cell. The fitting results of the single absorption peak at 6971. 0021 cm(-1) verify the spectral resolution of the VIPA spectrometer. The accuracy and reliability of the VIPA spectrometer applied to the measurement of broadband and high-resolution gas absorption spectrum are verified by comparing the measured absorption spectrum with the theoretical absorption spectrum. In the future, the VIPA spectrometer combined with optical cavity can realize broadband and high-resolution spectral measurement of trace gases.

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.

Hydroxyl (OH) radicals play a vital role in the degradation of trace gases and pollutants in the troposphere and in controlling the atmospheric oxidation capacity. Due to its short lifetime and low concentration, interference-free high sensitivity in situ OH monitoring by laser spectroscopy represents a challenge. In this presentation, we will report the development of Faraday rotation spectroscopy (FRS) instruments operating at 2.8 µm for quantitative measurement of OH concentrations in an atmospheric simulation chamber and the total atmospheric OH reactivity (k’OH). The Q (1.5) double lines (2Π3/2 (ν=1<-0)) at 3568 cm-1 were selected for the detection. Different detection methods have been studied. The FRS technology relies on the particular magneto-optic effect observed for paramagnetic species (including most radicals and some compounds with unpaired electrons), which can significantly reduce excess laser noise and makes it capable of enhancing the detection sensitivity and mitigation of spectral interferences from diamagnetic species in the atmosphere. With the use of a multipass enhanced FRS, a detection limit of 3.2 × 106 OH/cm3 (2σ, 4s) was achieved with an absorption path length of 108 m. We demonstrated that FRS method provides a unique method for atmospheric chemistry research.