A lightweight broadband cavity-enhanced spectrometer for NO₂ measurement on uncrewed aerial vehicles

<p>We describe the design and performance of a lightweight broadband cavity-enhanced spectrometer for measurement of NO<span class="inline-formula">₂</span> on uncrewed aerial vehicles and light aircraft. The instrument uses a light-emitting diode (LED) centered at 457 nm, high-finesse mirrors (reflectivity <span class="inline-formula">=0.999963</span> at 450 nm), and a grating spectrometer to determine optical extinction coefficients between 430 and 476 nm, which are fit with custom spectral fitting software and published absorption cross sections. The instrument weighs 3.05 kg and has a power consumption of less than 35 W at 25 <span class="inline-formula">^∘</span>C. A ground calibration unit provides helium and zero air flows to periodically determine the reflectivity of the cavity mirrors using known Rayleigh scattering cross sections. The precision (1<span class="inline-formula"><i>σ</i></span>) for laboratory measurements is 43 ppt NO<span class="inline-formula">₂</span> in 1 s and 7 ppt NO<span class="inline-formula">₂</span> in 30 s. Measurement of air with known NO<span class="inline-formula">₂</span> mixing ratios in the range of 0–70 ppb agreed with the known values within 0.3 % (slope <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mn mathvariant="normal">0.997</mn><mo>±</mo><mn mathvariant="normal">0.007</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="78pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="070c85d9bb34c1e2574f428ab72c4964"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-6643-2022-ie00001.svg" width="78pt" height="10pt" src="amt-15-6643-2022-ie00001.png"/></svg:svg></span></span>; <span class="inline-formula"><i>r</i>²=0.99983</span>). We demonstrate instrument performance using vertical profiles of the NO<span class="inline-formula">₂</span> mixing ratio acquired on board an uncrewed aerial vehicle between 0 and 110 m above ground level in Boulder, Colorado.</p>