A broadband cavity-enhanced spectrometer for atmospheric trace gas measurements and Rayleigh scattering cross sections in the cyan region (470–540 nm)

<p>An incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for quantification of atmospheric trace gases that absorb in the cyan region of the electromagnetic spectrum (470 to 540&thinsp;nm), including <span class="inline-formula">NO₂</span> and <span class="inline-formula">I₂</span>, is described. The instrument uses a light-emitting diode coupled to a 1&thinsp;m optical cavity consisting of a pair of mirrors in stable resonator configuration. Transmitted light is monitored using a grating spectrometer and charge-coupled device array detector. The average mirror reflectivity was determined from the <span class="inline-formula">N₂∕He</span> and <span class="inline-formula">Ar∕He</span> ratios of scattering coefficients and was <span class="inline-formula">∼99.98</span>&thinsp;% at its maximum, yielding an effective optical path length of 6.3&thinsp;km. Cross sections of <span class="inline-formula">N₂</span>, <span class="inline-formula">O₂</span>, air, Ar, <span class="inline-formula">CO₂</span>, and <span class="inline-formula">CH₄</span> scattering and of O<span class="inline-formula">₄</span> absorption were measured and agree with literature values within the measurement uncertainty. Trace gas mixing ratios were retrieved using the spectral fitting software DOASIS (DOAS intelligent system) from 480 to 535&thinsp;nm. Under laboratory conditions, the 60&thinsp;s, <span class="inline-formula">1<i>σ</i></span> measurement precisions were <span class="inline-formula">±124</span> and <span class="inline-formula">±44</span>&thinsp;pptv for <span class="inline-formula">NO₂</span> and <span class="inline-formula">I₂</span>, respectively. The IBBCEAS instrument sampled ambient air in Ucluelet, BC, Canada, in July 2015. IBBCEAS retrievals agreed with independent measurements of <span class="inline-formula">NO₂</span> by blue diode laser cavity ring-down spectroscopy (<span class="inline-formula"><i>r</i>²=0.975</span>), but ambient <span class="inline-formula">I₂</span> concentrations were below the detection limit.</p>

Abstract licence: CC BY

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