Weixiong Zhao

Combined absorption–emission spectroscopy is demonstrated using a single VIPA spectrometer coupled to a supercontinuum source. The system captures a ~36 nm bandwidth (742–778 nm) at picometer resolution. In a low-pressure glow discharge, N ₂ emission calibration, O I 777 nm triplet absorption (2.2% consistency), and linewidth analysis distinguishing instrumental/Doppler broadening from Ar I self-absorption (broadening factors up to ~1.5) are performed. Joint retrieval of temperature (953 ± 49 K) and instrumental width, along with time-division absorption–emission measurements, is demonstrated. Long-term stability makes this approach suitable for quasi-steady plasmas.

A novel broadband, high-resolution spectroradiometer based on a virtually imaged phased array (VIPA) has been developed to meet the stringent requirements for high-precision vertical profiles of key atmospheric constituents. The instrument operates within the spectral range of 7535–7680 cm ⁻¹ , achieving a spectral resolution of 0.023 cm ⁻¹ (690 MHz) with an integration time as short as 400 ms. Using this system, high-resolution atmospheric transmittance spectra in the 7535–7680 cm ⁻¹ band were successfully measured, and the vertical profile of water vapor was retrieved using the optimal estimation method (OEM). The results demonstrate that the novel VIPA-based spectroradiometer enables rapid acquisition of high-resolution atmospheric transmittance spectra over a broad spectral range, providing a robust and effective new approach for remote sensing of key atmospheric constituents. To the best of our knowledge, this represents the first successful application of a VIPA-based spectroradiometer for retrieving vertical profiles of atmospheric constituents.

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.

Nitrogen oxides (NOx) including nitric oxide (NO) and nitrogen dioxide (NO2), are significant reactive nitrogen compounds in the tropospheric atmosphere. The circulation of NOx and free radicals are coupled and interacted with each other, which has an important influence on atmospheric oxidation capacity and the formation of air pollution. Emissions from agricultural sources are the main source of atmospheric NOx. However, due to the limitations of high-precision measurement method of NOx and its flux measurement technique, the flux intensity of NOx emissions from agricultural sources and their effects on atmospheric oxidation are still unclear.In this presentation, we will report the latest development of amplitude-modulated cavity-enhanced absorption spectroscopy (AM-CEAS) and its application to NOx flux measurement using eddy covariance (EC) method.The new AM-CEAS based instrument was operated at 406 nm and TTL-modulated at 5 kHz, using phase-sensitive detection for ultra-sensitive absorption measurement. The detection precisions of the instrument were 78 pptv and 30 pptv, respectively, in 0.1 s and 1 s data acquisition time, for NO2 measurement. Combining with Cavity Ring-Down Spectroscopy (CRDS), the AM-CEAS method can eliminate the calibration process of mirror reflectivity, thus realizing NO2 absolute concentration measurements. Building upon single-channel system, a dual-channel NO2 detector has been developed. It utilizes ozone titration in one channel to convert NO to NO₂, allowing for NOx measurement by calculating the difference between the channels.Using the NOx analyzer, an EC NOx flux observation system was constructed at a wheat field of Shouxian county, and was used for measuring the atmospheric NOx and its flux intensity in summer when ozone pollution is severe. The performance of the flux observation system was evaluated and the resulting NOx fluxes were also analyzed. Based on the advantages of high sensitivity, and minimal maintenance of AM-CEAS, the flux observation system demonstrated good performance, and provided the potential for investigating NOx flux using the EC method across diverse temporal scales.

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.

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/ $\surd $ 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.

Ozone (O₃) concentration is a function of the catalytic cycles of RO_x radical (hydroxyl (OH), hydroperoxyl (HO₂) and organic peroxy (RO₂) radical) and NO_x (NO and NO₂). Total OH reactivity (k′_OH) and total peroxyl radicals (RO₂* = RO₂ + HO₂) play an important role in this cycle and have a major impact on O₃ mitigation. The currently used atmospheric models generally underestimate the measured values of k′_OH and RO₂* concentrations, therefore the ozone production processes remains unclear. This paper presents observation of k′_OH, RO₂*, and ancillary trace species from February and March 2022 in Beijing, covering the 24th Olympic Winter Games period. We aim to accurately predict the ozone production processes by exploring the impact of k′_OH on the RO₂* chemistry. During the Games period, observed k′_OH were significantly higher than calculations, and average RO₂* concentrations at noontime of 6.9 pptv were observed, which is about 1.3–10 times higher than previous winter observations. A zero‐dimensional model underestimated the observed RO₂*. Constraining k′_OH to the measured value by introducing the oxidation chemistry of unknown or unmeasured volatile organic compounds (VOCs) species (X) can effectively close the gap between modeled and measured RO₂*. By considering measured k′_OH and RO₂*, the ozone production potential increased by a factor of 1.3–2.1, the O₃ production regime sensitivity showed a significant trend shift from VOCs to NO_x limited, and enhanced the absolute ozone production sensitivity on VOCs by an order of magnitude. This means that simultaneously measurement of k′_OH and RO₂* provides a more accurate approach to O₃ control.

A narrow-linewidth cavity ring-down spectroscopy system for HO₂ detection was developed. The laser linewidth was reduced by optical feedback through locking the laser current and feedback phase, achieving a tenfold improvement in sensitivity.