Formaldehyde (HCHO) is a significant air pollutant in urban environments, impacting both air quality and public health. Mobile monitoring of HCHO allows for accurate source attribution and effective emission control strategies; however, this necessitates instruments with rapid response, high sensitivity, and robustness under mobile operation conditions. This work presents a robust mid-infrared wavelength modulation spectrometer for HCHO measurement. A compact modified Chernin cell with a path length of up to 120 m at 50 cm optical base length was developed to improve detection sensitivity. Real-time rapid background subtraction was implemented to eliminate spectral background structures. Laser current jitter was employed to suppress optical interference fringes and vibration-induced noise. The developed spectrometer achieved a detection limit of 0.89 ppbv with an averaging time of 1 s, corresponding to a detection sensitivity of 3.1 x 10(-9) cm(-1). The spectrometer demonstrates high robustness and offers a reliable solution for precision mobile HCHO monitoring.

Peroxy radicals, RO2, are key species in the atmosphere. They are formed from a reaction of OH radi-cals with hydrocarbons: RH + OH + O2 -> RO2 + H2OIn polluted environments, RO2 radicals react predominantly with NO, leading to formation of NO2, and eventually through photolysis of NO2 to formation of O3.At low NOx concentrations such as in the marine boundary layer or the background troposphere, the life-time of RO2 radicals increases and other reaction pathways such as self- and cross reaction with other RO2 or with HO2 radicals become competitive.To study the reactivity of peroxy radicals, UV absorption spectroscopy has been employed in the past: this technique gives good sensitivity for peroxy radicals, but poor selectivity as these radicals have broad absorption features in the UV. We have established a technique allowing to follow peroxy radicals with a better selectivity compared to UV, but with still good sensitivity by coupling laser photolysis to cw-Cavity Ring Down Spectroscopy in the near IR. Two identical cw-CRDS paths are installed in a recently constructed temperature-controlled photolysis reactor in a small angle with respect to the Excimer photolysis beam, leading to an overlap of around 35 cm between the photolyzed volume and the detection volume. A third detection path for UV absorption measurements is installed in a slightly larger angle, leading to an overlap of around 20 cm between photolysis and absorption volume.Here, we will present the first results obtained in the new reactor: the reaction between RO2 radicals and NO2. This reaction leads in an equilibrium reaction to the formation of RO2NO2 species. If the lifetime of these RO2NO2 are long enough, they will be transported and become a NOx source in remote environments. Therefore, determination of rate- and equilibrium constants of such reactions is important. In this work, two RO2 radicals have been generated simultaneously by 248nm laser photolysis of acetone, leading to roughly 1/3 CH3C(O)O2 radicals and 2/3 CH3O2 radicals. Time-resolved decays have then been observed for both radicals in the presence of different NO2 concentrations. The detection of both RO2 radicals is done simultaneously by high sensitivity cw-CRDS. RO2 concentrations can be decreased to a level where self-reaction becomes negligible at still excellent S/N ratio, making the measurement of RO2 + NO2 reaction straightforward. NO2 is quantified by UV-multipass absorption spectroscopy at 532nm in the photolysis reactor, and concentrations are compared with the calculated ones from the use of calibrated flowmeters.

The hydroperoxyl radicals (HO2) play a crucial role in atmospheric chemistry. Direct in-situ measurement of HO2 concentration using laser spectroscopy has always been a challenge, requiring very high detection sensitivity. In this presentation, we report the development of a frequency-stabilized cavity ring-down spectrometer (FS-CRDS) for the direct measurement of HO2 concentration. The optical cavity of the spectrometer was made of perfluoroalkoxy (PFA) tube with an inner diameter of 9 mm. The distance between the two high reflectivity mirrors (double coated, with reflectivity R = 95% at λ = 632 nm and R = 99.998% at λ = 1506 nm) was about 51.4 cm, with one of the cavity mirrors mounted on a piezo-electric transducer (PZT). A stable red He-Ne laser with a frequency stability of ±2 MHz was used as the reference laser for the cavity length stabilization servo. A 1506 nm fiber laser was used as the probe laser. The probe laser beam was split into two beams: one beam was used to lock the probe laser to the stable cavity using Pound-Drever-Hall (PDH) locking method; the other beam passed sequentially through an acousto-optic modulator (AOM) and a fiber electro-optic modulator (EOM) for cavity ring down spectroscopy (CRDS) measurement. By tuning the frequency of the microwave source drive of the EOM, and using a frequency-agile, rapid scanning spectroscopy method, the laser sidebands were sequentially switched to different optical cavity models, thereby achieving rapid full-spectrum scanning. With a 1 s integration time, the spectrometer achieved a detection sensitivity of about 2.6×10-11 cm-1, which was about 12 times improved compared with normal CRDS system without electronic locking. The corresponding detection limit for HO2 radicals was about 1.2×108 molecule/cm3 (the absorption line for HO2 detection was located at 6638.207 cm-1, with a line strength of 7.09 × 10-21 cm-1/(molecule cm-2)). This work demonstrates that FS-CRDS is a feasible technique for high sensitivity direct measurement of HO2 radicals. Further improvements will be made in the future to enhance detection sensitivity.

Objective Formaldehyde (HCHO) is an important air pollutant in both indoor and outdoor environments. Its emission sources include the direct releases from industrial emissions, construction materials, and polymeric resin-based furnishings. HCHO can also be generated through the photochemical oxidation of volatile organic compounds (VOCs) in atmospheric environments. Accurate measurement of formaldehyde concentration is crucial for health effect studies, atmospheric chemistry research, and pollution prevention. Tunable diode laser absorption spectroscopy (TDLAS) is one of the important methods for detecting formaldehyde, which offers advantages such as high sensitivity, high time resolution, and in-situ measurement. However, temperature variation in the operating environment can affect the key parameters of a TDLAS system, thereby impacting system stability and measurement sensitivity. To address this issue, two primary temperature control methods are usually employed. One involves directly controlling the temperature of the multi-pass gas cell (MPC) to maintain optical path length stability. The other entails temperature control of the entire optical path of the TDLAS system, including the multi-pass cell, laser, photodetector, mirrors, and lenses. Temperature control has been demonstrated to be an effective means to improve the performance of TDLAS systems. However, precise temperature control specifically tailored for formaldehyde measurement systems has not been reported before. Methods A TDLAS system with optical path temperature control is developed for formaldehyde measurement. The instrument uses a mid-infrared interband cascade laser emitting at 3.5 mu m as the probe source. The absorption line of formaldehyde is selected at 2831.64 cm-1 with a line strength of S=5.651>< 10(-20) cm(-1)/(moleculecm(-2)). A compact dense spot pattern spherical mirror cell is developed with an optical base length of 17.7 cm, which allows the incident laser beam to reflect 312 times in the multi-pass cell, thereby increasing the effective absorption path length to 55.2 m, with a volume of only 350 mL. The Gaussian beam formula is used to fit the beam radius at different distances to obtain the beam waist position. A focusing lens (f=500 mm) is employed to adjust the beam waist position. The laser beam waist is matched to the center of the multi-pass cell, which is located 479 mm away from the laser. To enhance measurement precision, wavelength-modulated spectroscopy (WMS) and rapid background subtraction techniques are used to minimize the noise in the spectral signal. A semiconductor cooling temperature control box is developed to maintain the operating temperature of the optical system at 32 degrees C. Results and Discussions The temperature inside the temperature control box could be stabilized at 32 degrees C with a high precision of 6 m degrees C (Fig. 6) when the temperature control is turned on. Rapid background subtraction is performed using a three-way solenoid valve, which enables swift switching between the background gas and sample gas. The total time consumed to complete a background subtraction measurement is 16 s. The sample gas signal is difficult to distinguish accurately due to interference from background structures. However, by subtracting the background gas spectrum, the 2f signal can be extracted, which significantly improves the detection sensitivity of the system (Fig. 7). During calibration, the absolute concentrations of diluted HCHO gas in the cell are calculated using direct absorption spectroscopy. The peak-to-peak value of the formaldehyde 2f signal exhibits a linear relationship with volume fraction, which yields a correlation coefficient of 0.9994 (Fig. 8). The performance of the TDLAS instrument is evaluated by measuring a time series of formaldehyde concentrations at a fixed volume fraction. With temperature control enabled, the system achieves a detection precision of 0.25>< 10-9 (1 sigma, 60 s), which is 4.7 times higher than that when temperature control is disabled (Fig. 10). Conclusions The system utilizes a compact, dense spot pattern spherical mirror optical multi-pass cell with a path length of 55.2 m and a volume of only 350 mL to enhance the effective absorption path. The beam profile of the used interband cascade laser emitting at 3.5 mu m is measured, and the laser beam waist is matched to the cell. A semiconductor-based thermoelectric temperature control box is designed to precisely regulate the entire optical path, including the interband cascade laser, multi-pass cell, and detector, which achieves a temperature control precision of 6 m degrees C at the set point of 32 degrees C . By combining wavelength modulation spectroscopy and rapid background subtraction techniques, the system achieves a measurement precision of 0.25x10(-9) (1 sigma, 60 s) under temperature-controlled conditions, which is 4.7 times higher than that under non-temperature-controlled conditions.

Objective Hydroxyl radical (center dot OH), the most significant oxidant in the atmosphere, initiates oxidation reactions of most natural and anthropogenic trace gas species, determines the atmospheric lifetimes of these pollutants, and regulates the atmosphere's self-cleaning capacity. Time-resolved measurements of center dot OH provide an essential tool for researching chemical reaction kinetics and field measurements of atmospheric center dot OH total reactivity, which is crucial for understanding ozone formation and secondary organic aerosols. The pump -probe technique represents a vital method for time-resolved center dot OH measurements. This technique employs a 266 nm UV photolysis laser to generate center dot OH and initiate its chemical reaction with reactants while synchronously detecting center dot OH in another optical path. Using an optical multi-pass cell (MPC) to increase the overlap path length between the detection optical path and the UV photolysis beam effectively enhances pump-probe detection sensitivity. Several research groups have implemented Herriott-type multi-pass cells for pump- probe applications. Although these multi-pass cells provide powerful tools for pump-probe technology, their effective utilization efficiencies remain relatively low compared to designed path lengths, limiting further improvements in detection sensitivity. This study developes a high-efficiency Herriott pump-probe cell and constructed a pump-probe system based on the cell for time-resolved center dot OH measurements. Methods The spot distribution pattern of the Herriott cell is investigated. A pump-probe MPC with an optical path utilization efficiency of 75.4% is developed. Based on the cell, a Faraday rotation spectroscopy system for time-resolved measurement of center dot OH is constructed (Fig. 5). center dot OH radicals are generated through the photolysis of O3 and H2O at 266 nm. The system uses a 2.8 mu m continuous-wave distributed feedback (cw-DFB) laser as the probe light source. The Q(1.5e) line of center dot OH at 3568.523 cm-1 is selected as the detection absorption line, with a line intensity of S=9.023>< 10-20 cm-1/(moleculecm-2). By measuring the beam waist position of the laser (Fig. 4) and matching it with the multi-pass cell, the problem of beam divergence is solved. The stability and detection precision are evaluated by Allan deviation analysis. The kinetic rate constant for the reaction between center dot OH and CH4 is measured. The dynamic monitoring performance of the system is tested in a photochemical smog chamber. Additionally, the system is applied to real atmospheric field observation. Results and Discussions The distributions of the reflection spots on the mirror surface and at the cell center position are simulated under reflection angles of 50.4 degrees, 79.2 degrees, 122.4 degrees, and 158.4 degrees, respectively (Fig. 2). When the reflection angle is set to 158.4 degrees, the system achieved an effective absorption path length of 28.5 m, with an overlapping efficiency of 75.4%. The red light test demonstrates that positioning the laser beam waist outside the multi-pass cell results in significant beam dispersion after several reflections, preventing the formation of a clear and complete spot pattern (Fig. 3). When the laser beam waist matches the cell center, a distribution of 25 reflection spots, including the light-through hole, is obtained on the mirror surface with relatively uniform spot sizes. The Allan deviation analysis (Fig. 7) of zero air measurement indicates a measurement precision of 0.22 s-1 with an acquisition time of 60 s, improving to 0. 14 s-1 and 0.11 s-1 at averaging times of 180 s and 300 s, respectively. The statistical histogram exhibits a normal distribution, indicating system stability without obvious drift. The measured reaction rate constant for center dot OH+CH4 is 6.49(-1.1, +1.3)>< 10-15 cm3 molecule-1 s-1 (Fig. 8). The time series of kO'H monitored in the smoke chamber correlate well with calculated values from measured CO particle concentration (Fig. 9), demonstrating good agreement in numerical values and change trends with a slope of 0.95 and a linear correlation coefficient of R2=0.97. The daily variation of atmospheric kO'H is measured in the Shouxian area in May 2024 (Fig. 10). The daily average value of kO'H is 18.4 s-1, with peaks of 19.6 s-1 at 06:00 and 21.1 s-1 at 19:00, respectively. A trough of 15.8 s-1 occurres at 14:00. Conclusions A pump-probe MPC with an optical base length of 77.2 cm achieves an overlap efficiency of 75.4 degrees o. The ray propagation in the cell is confirmed using red light. Through precise alignment of the incident laser beam's waist position with the cell center, the beam maintains consistent propagation during multiple reflections, producing 25 uniformly distributed spots on the mirror surface. The beam waist position of the 2.8 mu m cw-DFB laser is determined and aligned with the cell center. A Faraday rotation spectroscopy system is established for time-resolved center dot OH measurements. Allan deviation analysis reveals a measurement precision of the center dot OH decay rate at 100 mbar of 0.22 s-1 (1 sigma, 60 s). The measured reaction rate constant for center dot OH+CH4 demonstrates strong agreement with the recommended values from the International Association of Pure and Applied Chemistry (IUPAC). The system's deviation from dynamic measurements of kO'H in the smog chamber remains below 5 degrees o. The daily variation of atmospheric kO'H is monitored in the Shouxian area in May 2025.

The Beishan region near Shiquanhe Town in Ngari, western Xizang (Tibet), represents a typical alpine desert ecosystem on the Qinghai-Xizang Plateau. However, depth-related patterns of soil microbial communities and their physicochemical controls remain insufficiently understood. Here, microbial community composition and functional attributes were examined across three soil horizons-topsoil (0-20 cm), subsoil (20-40 cm), and deep subsoil (40-60 cm)-sampled in May 2024 prior to artificial greening. High-throughput 16S rRNA and ITS sequencing combined with physicochemical analyses revealed clear vertical stratification: bacteria were dominated by Proteobacteria and Actinobacteriota, and fungi by Ascomycota. Bacterial diversity was higher in the topsoil, whereas fungal diversity exhibited a gradual increase with soil depth; however, these trends did not reach statistical significance (p > 0.05). Functional predictions indicated predominantly aerobic heterotrophic bacteria and a shift from pathogenic to saprotrophic fungi with depth. Multivariate analyses (RDA, CCA, BRT) consistently identified soil pH and moisture as fundamental habitat constraints, and organic carbon, available phosphorus, and available potassium as physicochemical drivers with nonlinear threshold responses. These results highlight soil pH, moisture, and nutrient status (N, P, K) as primary determinants of microbial community assembly and provide guidance for microbially informed ecological restoration in alpine desert ecosystems.

Biosensing technology serves as a cornerstone in biomedical diagnostics, environmental monitoring, personalized medicine, and wearable devices, playing an indispensable role in precise detection and real-time monitoring. Compared with traditional sensing platforms, functional nanomaterials-by virtue of their ultra-large specific surface area, exceptional optoelectronic properties, and superior catalytic activity-significantly enhance the sensitivity, selectivity, and response speed of biosensors. This has enabled ultrasensitive, rapid, and even in situ detection of disease biomarkers, pollutants, and pathogens. This review summarizes recent advances in five key categories of functional nanomaterials-metallic, semiconductor, carbon-based, two-dimensional, and stimulus-responsive materials-for advanced biosensing applications. It elucidates the structure-property relationships governing sensing performance, such as the surface plasmon resonance of gold nanoparticles and the high carrier mobility of graphene, and analyzes the core mechanisms behind optical sensing, electrochemical sensing, and emerging multimodal sensing strategies. With a focus on medical diagnostics, wearable health monitoring, and environmental and food safety surveillance, the review highlights the application value of functional nanomaterials across diverse scenarios. Current research is progressively moving beyond single-performance optimization toward intelligent design, multifunctional integration, and real-world deployment, though challenges related to industrial application remain. Finally, the review outlines existing issues in the development of functional nanomaterial-based biosensors and offers perspectives on the integration of nanomaterials with cutting-edge technologies and the construction of novel sensing systems. This work aims to provide insights for the rational design of functional nanomaterials and the cross-disciplinary translation of biosensing technologies.

Experimental and observational evidence indicates that spring warming advances the green-up onset of woody plants, while winter warming delays it. However, evidence for herbaceous plants is limited, leaving a gap in our understanding of how their green-up onset respond to winter warming, which complicates predictions of phenological shifts under long-term climate change. Particularly in the alpine grasslands of the Tibetan Plateau (TP), there are strong debates over whether green-up onset is influenced by winter temperatures. We conducted a three-year in situ manipulative winter warming experiment in an alpine grassland in the central TP (4550 m above sea level) and found that the experimental winter warming delayed green-up onset for three out of four species (three dominant and one common in the TP alpine grasslands) and for the community, by reducing chill accumulation and increasing growing degree-days. This effect was more pronounced in species with earlier green-up onset, indicating a stronger reliance on chilling cues to avoid frost risk. However, the convergent cross mapping method did not detect a causal effect of winter temperature on green-up onset from long-term ground and satellite observations across the TP. Our findings indicate the impacts of winter warming on the green-up onset of alpine herbaceous plants on the TP with implications for improving phenology models.

Vertical profiles of aerosol light scattering (bscat), absorption (babs), as well as the single scattering albedo (SSA, ω), play an important role in the effects of aerosols on climate, air quality, and local photochemistry. However, direct measurement of the vertical profiles of these optical parameters remains challenging. Although some aircraft observations have been carried out, there is still large uncertainty. In this presentation, we will report the development of an unmanned-aerial-vehicle (UAV)-borne cavity-enhanced aerosol single scattering albedometer (CEA-albedometer) operating at λ = 532 nm for simultaneous in-situ measurements of the vertical distributions of bext, bscat, babs, and ω. The achieved detection precisions in laboratory were 0.38, 0.21, and 0.43 Mm-1 for bext, bscat, and babs, respectively, for a 1 s data acquisition time. The UAV used here was a petrol-powered hexacopter with a flight height of up to 1.5 km and a speed of up to 20 m/s. The maximum take-off weight of the UAV was ~ 150 kg, and about 30 kg of equipment can be carried. The full load flight endurance time was about 80 minutes depending on the temperature and wind. It was controlled by a GPS module with a precision of 0.1 m in the horizontal direction and 0.05 m in the vertical direction. The concept of using UAVs for atmospheric research began in the 1990s. Nowadays, it has developed rapidly, and various instruments have demonstrated impressive performance. The UAV flight platform reported here demonstrated good performance and will become a valuable and powerful tool for atmospheric boundary layer research.

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.

Highly integrated data acquisition and processing (DAQP) systems play an important role in promoting the miniaturization of high-performance optical instruments. Currently, such systems are mainly based on digital signal processors or advanced reduced instruction-set compute machines, which have the advantages of low-power consumption and embedded integration, but still face certain limitations in processor performance, interface capacity, and edge computing capabilities. In this work, we report the development of a novel DAQP system based on a system-on-module and field-programmable gate array architecture, which combines the functionalities of signal generation, high-speed data acquisition, and lock-in amplifier (LIA). It had two signal generation channels with a maximum output frequency of 125 million samples/s, four signal acquisition channels with a maximum sampling rate of 65 million samples/s, and a four-channel digital LIA (DLIA) with a demodulation frequency range from dc to 5 MHz. By using the exponential moving average (EMA) filtering algorithm in DLIA, the compilation resource consumption of the filter was reduced. When the modulation frequency was set to 20 kHz and the input voltage range was set to +/- 1 V, the measured input-referred noise of the DLIA was about 39 nV/root Hz. In order to verify the high-speed acquisition and modulation demodulation functions of the DAQP system, it was applied to amplitude modulation cavity-enhanced absorption spectroscopy, achieving portable and high-precision NO2 measurements and providing an effective means for the miniaturization of optical instruments.

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.

Vertical profiles of aerosol light scattering (bscat), absorption (babs), as well as the single scattering albedo (SSA, ω), play an important role in the effects of aerosols on climate, air quality, and local photochemistry. High-precision in-situ measurements of the vertical profiles of these properties are challenging and therefore uncommon. We report here the development of a portable cavity-enhanced albedometer operating at λ = 532 nm for use aboard an unmanned aerial vehicle (UAV). Multi-optical parameters, bscat, babs, extinction coefficient bext, and ω, can be measured simultaneously in the same sample volume. The achieved detection precisions in laboratory were 0.38, 0.21, and 0.43 Mm-1 for bext, bscat, and babs, respectively, for a 1 s data acquisition time. The albedometer was installed on an hexacopter UAV and simultaneous in-situ measurements of the vertical distributions of bext, bscat, babs, and ω were realized for the first time. Here we report a representative vertical profile up to a maximum height of 702 m with a vertical resolution of better than 2 m. The UAV platform and the albedometer demonstrate good performance and will be a valuable and powerful tool for atmospheric boundary layer research.

Coincidental realization of broadband spectral coverage and high resolution in one spectrometer system has always been a challenge. Here, we report the development of a high-resolution visible CCD spectrometer based on the virtually imaged phase array (VIPA) technique. By using a thin glass plate and a reflective grating, a two-dimensional cross-dispersion was realized. A broadband coverage of ∼14.94 THz and a high resolution of ∼1 GHz at 632.996 nm were achieved with a simple structure. The effects of the surface quality of VIPA etalon, the pixel size of the CCD camera, the pinhole size of the input beam, and the focal length of the imaging lens on the resolution of the spectrometer and the transverse spot size on the detector plane were considered. A comparison between the experimental results by changing the imaging lens and the theoretical calculation results proved a better simulation of these two parameters, which is a helpful contribution to the design and construction of a VIPA spectrometer. The developed spectrometer will provide a useful tool for the study of high-resolution spectroscopy and for simultaneous multi-species trace detection.