宽带腔增强吸收光谱技术应用于大气NO3自由基的测量

NO3自由基是夜间大气化学中最重要的氧化剂,控制着多种痕量气体成分的氧化及去除,了解NO3自由基的化学过程对研究灰霾等大气污染过程意义重大.NO3自由基浓度低、活性强,实现大气NO3自由基的高灵敏度准确测量相对困难.本文介绍了大气NO3自由基的宽带腔增强吸收光谱定量方法,采用红光LED作为宽带腔增强吸收光谱系统光源,设计低损耗且适合国内高颗粒物环境的采样气路,并通过LED光源测试确定最佳工作电流和温度;通过采用白天的大气谱作为背景光谱参与NO3自由基的光谱拟合过程,减少水汽对NO3自由基光谱反演的干扰;通过对镜片反射率和有效腔长进行标定,对系统性能进行Allan方差分析,该宽带腔增强吸收光谱系统在光谱采集时间为10 s的情况下,NO3自由基极限探测灵敏度为0.75 pptv,总测量误差约为16%.在合肥开展了实际大气NO3自由基观测,观测期间NO3自由基的浓度范围从低于探测限到23.4 pptv,NO3自由基浓度呈现夜间高、白天低的特征,符合NO3变化规律,表明该宽带腔增强吸收光谱系统能够用于实际大气NO3自由基的高灵敏度测量.

Broadband cavity enhanced absorption spectroscopy for measuring atmospheric NO3 radical

NO3 radical is the most important oxidant in atmospheric chemistry at night,and it controls the oxidation and removal of various trace gas components in the atmosphere.The understanding of the chemical process of NO3 radical is of great significance for studying the atmospheric pollution processes such as haze.The NO3 radical has a low concentration and strong activity,so it is relatively difficult to measure accurately.We report here in this paper an instrument for unambiguously measuring NO3 based on broadband cavity enhanced absorption spectroscopy(BBCEAS).To achieve the robust performance and system stability under diverse conditions,this BBCEAS instrument has been developed,with efficient sampling,and resistance against vibration and temperature change improved,and the BBCEAS instrument also has low-power consumption.The 660-nm-wavelemngth light-emitting diode(LED)is used as a light source of the BBCEAS system.The sampling gas path with low loss and suitable for domestic high-particle environment is designed.Through the LED light source test,the optimal working current and temperature can be obtained to achieve the acquisition of NO3 absorption spectrum with high signal-to-noise ratio.Considering the fact that the water vapor absorption is an important interference factor for the measurement of NO3 radical by BBCEAS,the daytime atmospheric measurement spectrum is used as a background spectrum,and participates in spectral fitting of NO3 to reduce the effect of water vapor.The mirror reflectivity and effective cavity length are calibrated,and the Allan variance analysis is also carried out.The reflectance of the mirror can reach about 0.99993 at 662 nm(NO3 absorption peak),and the corresponding theoretical effective optical path can reach more than 7 km,which can meet the measurement requirements of atmospheric NO3 radicals.The detection limit(1s)of 0.75 pptv for NO3 is achieved with an acquisition time of 10 s and a total measurement error of about 16%.The atmospheric NO3 radical observation is carried out in Hefei.During the observation period,the highest NO3 concentration is 23.4 pptv,demonstrating the promising potential applications in in-situ,sensitive,accurate and fast simultaneous measurements of NO3 in the future by using the developed broadband cavity enhanced absorption spectroscopy.