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

Abstract

<p><strong>Abstract.</strong> A cavity-enhanced absorption spectrometer (CEAS) for quantification of atmospheric trace gases that absorb in the cyan region of the electromagnetic spectrum (470 to 540<span class="thinspace"></span>nm), including NO₂ and I₂, is described. The instrument uses a light-emitting diode coupled to a 1<span class="thinspace"></span>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 N₂/He and Ar/He ratios of scattering coefficients and was ~99.98<span class="thinspace"></span>% at its maximum, yielding an effective optical path length of 6.3<span class="thinspace"></span>km. Cross-sections of N₂, O₂, air, Ar, CO₂, and CH₄ scattering and of O₄ absorption were measured and agree with literature values within the measurement uncertainty. Trace gas mixing ratios were retrieved using the spectral fitting software DOASIS from 480 to 535<span class="thinspace"></span>nm. Under laboratory conditions, the 60<span class="thinspace"></span>s, 1σ measurement precisions were &amp;pm;105 and &amp;pm;38 pptv for NO₂ and I₂, respectively. The CEAS sampled ambient air in Ucluelet, BC, in July 2015. CEAS retrievals agreed with independent measurements of NO₂ by blue diode laser cavity ring-down spectroscopy (r² = 0.975), but ambient I₂ concentrations were below the detection limit.</p>