近日，來自山東師范大學物理與電子科學學院的聯合研究團隊發表了一篇題為Effects of Temperature and Humidity on the Absorption Spectrum and Concentration of N₂O Using an Open-Path Sensor System的研究論文。

Introduction

Since China’ s proposal of the “carbon peak” and “carbon neutrality” goals, the government and society have attached great importance to the problems of air pollution and global warming. Nitrous oxide (N₂O) is among the six greenhouse gases under the Kyoto Protocol. N₂O content is relatively low compared to carbon dioxide (CO₂), but its global warming potential is about 310 times that of CO₂. In addition, it is destructive to ozone (O₃). There are many reasons for the changes in N₂O concentrations in the atmosphere, which are partly due to anthropogenic activities, such as the widespread use of fertilizers in agricultural activities. The concentrations of other gases in the atmosphere, as well as the wind speed and direction, are all correlated with changes in N₂O concentrations. At the macro level, temperature and humidity are also factors affecting the absorption coefficient of N₂O gas. However, relatively few studies have been conducted on the specific effects of temperature and humidity on N₂O gas, and analysis has also been lacking on the influence of temperature and humidity on the absorption spectrum and the concentration of N₂O. Moreover, some uncertainty and variability remain in the observations of the relationship between N₂O gas concentrations and temperature and humidity. The reasons for these discrepancies may be regional differences, differences in observation methods, and imperfections in data, which are all important bases for measuring the N₂O concentration in atmospheric, medical, combustion, and agricultural processes. Thus, further research and exploration, combined with additional field observations and modeling experiments, can uncover the mechanism of temperature and humidity on the N₂O concentration. Consequently, providing a scientific basis for this concentration is essential for reducing N₂O emissions, controlling climate change, and promoting sustainable development and environmental protection.

簡介

自中國提出“碳峰值”和“碳中和”目標以來，政府和社會對空氣污染和全球變暖問題給予了極大關注。N₂O是《京都議定書》下的六種溫室氣體之一。與二氧化碳（CO₂）相比，N₂O含量相對較低，但其全球變暖潛力約為CO₂的310倍。此外，它對臭氧（O₃）具有破壞性。大氣中N₂O濃度的變化有許多原因，部分原因是人類活動造成的，例如在農業活動中廣泛使用化肥。大氣中其他氣體的濃度以及風速和風向都與N₂O濃度的變化相關。在宏觀水平上，溫度和濕度也是影響N₂O氣體吸收系數的因素。然而，對溫度和濕度對N₂O氣體具體影響的研究相對較少，對溫度和濕度對N₂O吸收譜和濃度的影響分析也不足。此外，在N₂O氣體濃度與溫度和濕度之間的關系觀察中仍存在一些不確定性和變異性。導致這些差異的原因可能是地區差異、觀測方法差異以及數據的不完善，這些都是測量大氣、醫療、燃燒和農業過程中N₂O濃度的重要基礎。因此，進一步的研究和探索，結合更多的現場觀測和建模實驗，可以揭示溫度和濕度對N₂O濃度的機制。因此，為減少N₂O排放、控制氣候變化，促進可持續發展和環境保護提供科學依據至關重要。

Experimental Details

Sensor Setup

Based on WMS technology and an open optical path, an open optical-path detection system for detecting N₂O gas in the atmosphere was built. The schematic diagram is shown in Figure 1. The sensor system is composed of a light-source module, photoelectric Remote Sens. 2023, 15, 5390 4 of 11 detection module, and data processing module. The light-source module mainly consists of signal generation, a laser drive, QCL, and an indication light source. To effectively realize the tunable characteristics of laser emission wavelength, we designed the signal generator plate to generate a high-frequency sine wave signal with a frequency of 10 kHz to realize the modulation function and to generate a low-frequency sawtooth wave signal with a frequency of 10 Hz to realize the scanning function. The two signals are superimposed on the laser driver, controls the temperature and central emission wavelength of QCL and converts it into an injection current acting on the detection light source QCL so that the emission wavelength of QCL is in the tunable range of 2203.7–2204.1 cm^?1.

實驗細節

傳感器設置

基于波長調制光譜學（WMS）技術和開路光學路徑，建立了一種用于檢測大氣中N₂O氣體的開路光學路徑檢測系統。示意圖如圖1所示。該傳感器系統由光源模塊、光電檢測模塊和數據處理模塊組成。光源模塊主要包括信號生成、激光驅動、量子級聯激光器（QCL）和指示光源。為了有效實現激光發射波長的可調特性，我們設計了信號生成器板，生成頻率為10 kHz的高頻正弦波信號以實現調制功能，并生成頻率為10 Hz的低頻鋸齒波信號以實現掃描功能。這兩個信號疊加在激光驅動器上，控制QCL的溫度和中心發射波長，并將其轉化為作用于檢測光源QCL的注入電流，使QCL的發射波長處于2203.7–2204.1 cm^-1的可調范圍內。

Figure 1. Schematic diagram of N₂O open optical sensor system.

項目使用的激光驅動器是寧波海爾欣光電科技有限公司的QC750-Touch^TM量子級聯激光屏顯驅動器。

l集成電流及溫控驅動，功能完備；

l溫度控制驅動采用非PWM式的連續電流輸出控制，大大延長TEC器件的使用壽命；

l多種輸出安全保護機制，保護QCL使用安全：可調電流鉗制、輸出緩啟動、過壓欠壓保護、超溫保護、繼電器短路輸出保護；

l大電流軟鉗制功能，避免誤操作大電流損壞激光管；

lUI界面顯示便于用戶操作使用及數據觀測；

l全自主研發，集成度高，性價比高。

QC750-Touch^TM, Ningbo HealthyPhoton Technology, Co., Ltd.

Selection of N₂O Transitions

To achieve effective detection of N₂O gas molecules, we need to select the absorption line intensity and the emission central wavelength of the laser. First, combined with the HITRAN-2016 database, the wave number range of 2000–2250 cm^?1 was selected to analyze the region of the absorption spectral line intensity of N₂O, and then carbon monoxide (CO), carbon dioxide (CO₂), and water (H₂O) molecules were simulated and analyzed, as shown in Figure 2. Within this wave number range, the absorption spectra of CO₂ were mainly distributed within the 2000–2081 cm^?1 range, and the absorption spectra of CO gas were distributed within the 2025–2200 cm^?1 wave number range. The absorption spectra of N₂O gas were distributed before the 2020 cm?1 wave number range. The absorption spectra of N₂O gas molecules were mainly distributed in the 2200–2250 cm^?1 wave number range, and they were far from the absorption spectra of water vapor and other gases, reducing interference. At around 2203.7 cm^?1 , the absorption spectra of N₂O gas were the strongest. Therefore, we set the position of the N₂O absorption line to 2203.7333 cm^?1, which was used as the wave number of the QCL emission center. The corresponding spectral line intensity was 7.903 × 10^?19 (cm^?1 .mol^?1 ). The central current and temperature of QCL were set at 330 mA and 36.0 ?C, respectively.

N₂O躍遷的選擇

為了有效檢測N₂O氣體分子，我們需要選擇吸收線強度和激光的發射中心波長。首先，結合HITRAN-2016數據庫，選擇了2000–2250 cm^?1的波數范圍，以分析N₂O吸收光譜線強度的區域，然后對一氧化碳（CO）、二氧化碳（CO₂）和水（H₂O）分子進行了模擬和分析，如圖2所示。在這個波數范圍內，CO₂的吸收光譜主要分布在2000–2081 cm^?1范圍內，CO氣體的吸收光譜分布在2025–2200 cm^?1波數范圍內。H₂O氣體的吸收光譜分布在2020 cm^?1波數范圍之前。N₂O氣體分子的吸收光譜主要分布在2200–2250 cm^?1波數范圍內，遠離水蒸氣和其他氣體的吸收光譜，減少了干擾。在2203.7 cm^?1左右，N₂O氣體的吸收光譜達到峰值。因此，我們將N₂O吸收線的位置設置為2203.7333 cm^?1，用作QCL發射中心的波數。相應的光譜線強度為7.903 × 10^?19（cm^?1·mol^?1）。QCL的中心電流和溫度分別設置為330 mA和36.0 ℃。

Figure 2. The intensity distribution of absorption lines of N₂O, CO, CO₂, and H₂O in the range of 2000–2250 cm^?1.

Conclusions

In this study, we investigated the effects of temperature and humidity on the concentration of N₂O and its absorption spectra using an open-path sensor system. By combining theoretical analysis and field monitoring, we first conducted monitoring of N₂O in a campus environment, analyzing the effects of temperature on its concentration and absorption spectra. We discovered that the concentration of N₂O would increase correspondingly with the increase in temperature. The influence of humidity on N₂O concentration was monitored under the condition that the ambient temperature of the laboratory remained unchanged. The concentration of N₂O was negatively correlated with humidity. The 2f and 1f signals under different temperature and humidity levels were extracted for analysis. We found that the higher the temperature, the smaller the peak value of the 2f and the 1f signals, which accords with the trend of the Gaussian function changing with temperature. Under different humidity conditions, the lower the humidity, the larger the 2f signal peak; the higher the humidity, the smaller the 2f signal. This study is of great significance for analyzing the relationship between N₂O and environmental parameters such as temperature and humidity. We hope that our research findings can assist environmental agencies in formulating more effective environmental policies for different environments. In the future, we can use QCL to analyze the relationship between N₂O and other environmental and gas parameters.

結論

在本研究中，我們利用開路傳感器系統研究了溫度和濕度對N₂O濃度及其吸收光譜的影響。通過理論分析和現場監測相結合，我們首先在校園環境中進行了N₂O監測，分析了溫度對其濃度和吸收光譜的影響。我們發現隨著溫度升高，N₂O濃度相應增加。在實驗室環境中，保持環境溫度不變的條件下監測了濕度對N₂O濃度的影響。N₂O濃度與濕度呈負相關。在不同溫度和濕度水平下提取并分析了2f和1f信號。我們發現溫度越高，2f和1f信號的峰值越小，這與高斯函數隨溫度變化的趨勢相符。在不同濕度條件下，濕度越低，2f信號峰值越大；濕度越高，2f信號越小。這項研究對分析N₂O與溫度、濕度等環境參數之間的關系具有重要意義。我們希望我們的研究結果能夠協助環境機構為不同環境制定更有效的環境政策。未來，我們可以利用QCL來分析N₂O與其他環境和氣體參數之間的關系。

參考：

Effects of Temperature and Humidity on the Absorption Spectrum and Concentration of N₂O Using an Open-Path Sensor System, Remote Sens. 2023, 15, 5390.