Airborne cavity enhanced absorption spectroscopy for high time resolution measurements of atmospheric NO2
Authors: Liang Shuai-Xi  , Qin Min  , Duan Jun  , Fang Wu  , Li Ang  , Xu Jin  , Lu Xue  , Tang Ke  , Xie Pin-Hua  , Liu Jian-Guo  , Liu Wen-Qing  ,   Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, and Hefei Institutes of Physical Sciences, Chinese Academy of Sciences, Hefei 230031, China;,   University of Science and Technology of China, Hefei 230026, China;,   CAS Center for Excellence in Regional Atmospheric Environment, Xiamen 361021, China
Published: December 2020

Nitrogen dioxide (NO2) is an important trace gas in the troposphere and plays a vital role in many aspects of the chemistry of the atmosphere. Accurate measurement of NO2 is the primary step to understand its role in atmospheric chemistry and to establish effective pollution prevention policies. Relatively few measurements of the NO2 profile in troposphere by using point-type instruments with high temporal resolution have been carried out in China. Due to the relatively poor measurement environment on airborne platform, the measurement system requires good anti-vibration ability, stability and environmental adaptability. A home-built incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) on the airborne platform is presented in this paper, and applied to high temporal resolution observations of the actual atmospheric NO2 spatial distribution. According to the strong absorption of NO2 in a wavelength range from 449 nm to 470 nm, we choose a high-power 457 nm light-emitting diode (LED) as a light source. A Peltier is used to control LED temperature and to stabilize the LED temperature at (200.1)℃. The pure PFA material optical cavity and sampling tube are used to reduce wall loss. And we choose the highly reflecting mirrors (reflectivity R0.9999@440-450 nm) to improve the effective optical path. A 2 m filter is used at the inlet of instrument to remove most of the particulate matter in the sample flows, which reduce the effect of particulate matter on the effective path length. In order to meet the requirement for time resolution in airborne measurement, we use an off-axis paraboloic mirror instead of an achromatic lens to improve the optical coupling efficiency. The reflectivity of the highly reflecting mirror is calibrated by the difference in Rayleigh scattering between He and N2. And the optimum averaging time of the IBBCEAS instrument is confirmed to be 1000 s by the Allan variance analysis. Detection limit (1) of 10 ppt for NO2 is achieved with an optimum acquisition time of 1000 s. Concentrations of NO2 are recorded and compared with data from a long path different optical absorption spectroscopy instrument, and the results show good agreement with each other. The linear correlation coefficient R2 is 0.86 in a slope of 0.92 with an offset of -0.402 ppb. The IBBCEAS system is deployed on an airborne platform, and the detection limit is 95 ppt (1) with a time resolution of 2 s. The profile of tropospheric NO2 by airborne observation is obtained over Shijiazhuang in Northern China. IBBCEAS system in the airborne platform shows good stability.