机载腔增强吸收光谱系统应用于大气NO_2空间高时间分辨率测量

介绍了一套用于机载平台测量的非相干宽带腔增强吸收光谱(IBBCEAS)系统,并应用于实际大气NO_2空间分布的高时间分辨率观测.为满足机载测量中对时间分辨率的需求,系统采用离轴抛物面镜代替消色差透镜提高光学耦合效率;并运用Allan方差,对系统性能进行了分析.通过腔增强吸收光谱系统与长光程吸收光谱系统对实际大气NO_2的对比测试,两者线性相关系数R~2达到0.86.将IBBCEAS系统应用于机载平台,在时间分辨率为2 s的情况下,探测限达到95 ppt(1σ).通过机载观测,获得了华北地区石家庄等地上空对流层大气NO_2的廓线信息.

Airborne cavity enhanced absorption spectroscopy for high time resolution measurements of atmospheric NO2

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 (20±0.1)℃. The pure PFA material optical cavity and sampling tube are used to reduce wall loss. And we choose the highly reflecting mirrors (reflectivity R>0.9999@440-450 nm) to improve the effective optical path. A 2 μm 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 (1σ) 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 (1σ) 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.