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

Abstract

Abstract. 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 nm), including NO2 and I2, is described. The instrument uses a light-emitting diode coupled to a 1 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 % at its maximum, yielding an effective optical path length of 6.3 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 nm. Under laboratory conditions, the 60 s, 1σ measurement precisions were ±105 and ±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.

Reply on RC1

“…The CEAS system has been described in detail in previous literature (Duan et al, 2018;Fiedler et al, 2003;Gherman et al, 2008;Jordan and Osthoff, 2020;Jordan et al, 2019;Langridge et al, 2006;Liang et al, 2019;Liu et al, 2019;Min et al, 2016;Tang et al, 2020;Ventrillard-Courtillot et al, 2010;Wang et al, 2017a;Yi et al, 2016), so there is a brief introduction to the principle of the instrument. NO2 molecules have a specific absorption structure in the wavelength range of 400-500 nm (Fig.…”

Reply on RC2

“…The CEAS system has been described in detail in previous literature (Duan et al, 2018;Fiedler et al, 2003;Gherman et al, 2008;Jordan and Osthoff, 2020;Jordan et al, 2019;Langridge et al, 2006;Liang et al, 2019;Liu et al, 2019;Min et al, 2016;Tang et al, 2020;Ventrillard-Courtillot et al, 2010;Wang et al, 2017a;Yi et al, 2016), so there is a brief introduction to the principle of the instrument. NO2 molecules have a specific absorption structure in the wavelength range of 400-500 nm (Fig.…”