开放光路调制LED宽带腔增强吸收光谱技术定量探测大气NO3自由基方法研究

Regardless many interests in nocturnal NO3 radical chemistry, concentration data of NO3 in the different atmosphere are scarce as it has been proven difficult to be quantitively detected due to the very low concentrations and the high spatial and temporal variability of NO3 radical. The project will study the quantitative measurements of NO3 radical by using open-path incoherent broadband cavity enhanced absorption spectroscopy to meet demand for highly sensitive detection. A detection system based on this spectroscopy, which is in open cavity mode and uses a LED as light source, is built. The LED's output intensity is modulated to on-line determine absorption path length by using phase shift cavity ring-down spectroscopy. An iterative methodology for calculating water vapour absorption cross sections and DOAS technique are introduced to an algorithmic model which is made to retrieve NO3 concentrations and able to effectively remove the absorption structures of other species, especially water vapour, from the measured absorption spectra. By measuring NO3 sample in the laboratory and analyzing the effects of system parameters on detection sensitivity, the performance of the system is further improved to be capable of quantitatively detecting NO3 radical in the atmosphere. The primary value of studying this project is to provide a new measuring technique for quantitively detecting atmospheric NO3 and scientific data for nocturnal NO3 radical chemistry research.

NO3自由基夜间化学过程备受关注，然而不同大气环境下的NO3浓度信息比较匮乏，究其原由是NO3自由基浓度非常低并且时空变化快，导致一般测量技术很难准确定量探测。针对NO3自由基的高灵敏度探测要求，本项目拟采用开放光路非相干宽带腔增强吸收光谱技术，研究大气NO3自由基的高灵敏定量探测问题。建立以LED为光源的开放光路探测系统，通过对LED光强进行调制，以相移腔衰荡光谱法在线获得开放光路下吸收光程信息。将计算水汽有效吸收截面的迭代算法与DOAS方法相结合，建立能够有效扣除水汽等多种物质干扰的NO3自由基浓度反演算法模型。在此基础上，开展实验室测量并分析系统参数对探测灵敏度的影响，优化系统设计从而实现大气NO3自由基的高灵敏定量探测。项目研究最终不仅为大气NO3自由基浓度定量提供新的技术手段，而且为揭示夜间NO3自由基化学过程提供科学数据。

NO3自由基夜间化学过程备受关注，然而不同大气环境下的NO3浓度信息比较匮乏，究其原由是NO3自由基浓度非常低并且时空变化快，导致一般测量技术很难准确定量探测。针对NO3自由基的高灵敏度探测要求，本项目采用开放光路非相干宽带腔增强吸收光谱技术，研究了大气NO3自由基的高灵敏定量探测问题。项目研究内容和结果主要包括：1）大气NO3自由基高灵敏IBBCEAS吸收光谱的获取研究。建立了一套基于红光LED光源的IBBCEAS测量装置，研究了LED光谱的温度特性，自行设计LED温度控制装置实现了±0.1℃的控制精度。2）吸收光程的在线标定方法研究。研究了基于调制LED的相移腔衰荡技术和基于O2-O2吸收测量技术的两种在线标定方法。对红光LED辐射光强进行方波调制，当调制频率为10kHz时，得到相应的相移为18度，对应腔长50cm时的平均吸收光程为1.5km。在蓝光波段，采用O2-O2吸收测量标定出腔长1.3m时的平均吸收光程为1.1km。3）开展基于DOAS技术的NO3浓度反演算法研究，采用迭代计算水汽吸收截面方法，将水汽从吸收光谱中进行有效扣除。4）开展基于腔衰荡技术的大气NO3自由基外场测量，为IBBCEAS技术测量NO3自由基提供数据参考。在测量时间10s的情况下，NO3的探测限约为3.2pptv，测量不确定性约8%。项目研究不仅为大气NO3自由基浓度定量提供新的技术手段，而且为揭示夜间NO3自由基化学过程提供科学数据。