可在线校准的大气CO_2浓度光声光谱监测系统研究

Fs光声光谱系统的谐振频率和池常数通常在实验室由标准气体标定得到,但在实际应用中,由于标准气体本身的不确定度以及与被测气体成分的不同、环境温湿度的变化,使得现场测量中谐振频率和池常数与实验室标定结果有偏差,从而导致测量结果不准确。为了解决以上问题,提出了基于大气中氧气的在线校准技术,并将该技术用于检测大气中二氧化碳浓度的光声光谱系统。大气中氧气浓度恒定为20.964%,通过探测氧气在763.73nm附近的扫频信号及峰值信号,实现共振频率和池常数的在线校准。该系统中光声池为直径6mm,长度100mm的一阶纵向共振模式结构。理论上分析了环境温湿度、气体成分对光声池性能的影响,同时给出了用标准气体、室内空气和室外空气标定的谐振频率和池常数,在标定结果的基础上,测量得到室内和室外的二氧化碳浓度值。实验结果显示,与校准过的气体分析仪的测量值相比,用被测大气中的氧气标定的谐振频率和池常数计算的二氧化碳浓度更准确,相对误差小于1%,远小于实验室标准气体标定计算的浓度相对误差。创新处在于,直接利用大气中的氧气对光声池的池常数和共振频率进行在线校准,有效的减小了标准气体标定带来的误差,以及环境变化带来系统漂移,提高光声系统在线监测的准确性和可靠性。

Research on On-Line Calibration Based Photoacoustic Spectrometry System for Monitoring the Concentration of CO2 in Atmosphere

Resonate frequency and cell constant of photoacoustic spectrum system are usually calibrated by using standard gas in laboratory ,whereas the resonate frequency and cell constant will be changed in‐situ ,leading to measurement accuracy errors , caused by uncertainties of standard gas ,differences between standard and measured gas components and changes in environmen‐tal condition ,such as temperature and humidity .As to overcome the above problems ,we have proposed an on‐line atmospheric oxygen‐based calibration technology for photoacoustic spectrum system and used in measurement of concentration of carbon diox‐ide in atmosphere .As the concentration of atmospheric oxygen is kept as constant as 20.96% ,the on‐line calibration for the photoacoustic spectrum system can be realized by detecting the swept‐frequency and peak signal at 763.73 nm .The cell of the PAS has a cavity with length of 100 mm and an inner diameter of 6 mm ,and worked in a first longitudinal resonant mode .The influence of environmental temperature and humidity ,gas components on the photoacoustic cell’s performance has been theoreti‐cally analyzed ,and meanwhile the resonant frequencies and cell constants were calibrated and acquired respectively using stand‐ard gas ,indoor air and outdoor air .Compared with calibrated gas analyzer ,concentration of carbon dioxide is more accurate by using the resonant frequency and cell constant calculated by oxygen in tested air ,of which the relative error is less than 1% , much smaller than that calculated by the standard gas in laboratory .The innovation of this paper is that using atmospheric oxy‐gen as photoacoustic spectrum system’s calibration gas effectively reduces the error caused by using standard gas and environ‐mental condition changes ,and thus improves the on‐line measuring accuracy and reliability of the photoacoustic spectrum sys‐tem .