A mini broadband cavity enhanced absorption spectrometer for nitrogen dioxide measurement on the unmanned aerial vehicle platform

We developed a small size and light weight analyzer based on the broadband cavity enhanced absorption spectroscopy for measuring nitrogen dioxide (NO2) with high spatial-temporal resolution and high accuracy. The instrument utilizes a LED centered at 450 nm, a short optical cavity, and a spectrometer to determine the extinction coefficients span 435–465 nm, allowing us to retrieve the mixing ratio of NO2 by absorption cross-section. The measurement precision (1σ) for NO2 is 98 ppt in 2 s under laboratory conditions. The uncertainty is estimated at 6% mainly attributed to cross-section and the mirror reflectivity. Good consistency was achieved by comparing with commercial chemiluminescence detector and cavity attenuation phase shift spectrometer (CAPS) (both R2 = 0.99). We further mounted the instrument on an unmanned aerial vehicle (UAV), and maintained the UAV flight at the height of 20 m, the comparative measurement with a CAPS set in a neighbor building with the same sampling height showed a good correlation (R2 = 0.97), which confirmed the instrumental feasibility and stability on the UAV platform. At last, we present a successful NO2 vertical measurement test, the good performance indicates this portable technique has great potential for vertical measurement. In particular, the mini-CEAS technique can further extend to measure the vertical profiles of other trace gas species (such as nitrous acid) with strong optical structural absorptions, which would largely promote the understanding of atmospheric chemistry on a vertical scale.