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

Abstract

Abstract. 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 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 (DOAS intelligent system) from 480 to 535 nm. Under laboratory conditions, the 60 s, 1σ measurement precisions were ±124 and ±44 pptv for NO2 and I2, respectively. The IBBCEAS instrument sampled ambient air in Ucluelet, BC, Canada, in July 2015. IBBCEAS 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.

A compact incoherent broadband cavity-enhanced absorption spectrometer for trace detection of nitrogen oxides, iodine oxide and glyoxal at levels below parts per billion for field applications

“…The losses of the cavity mirrors are assumed to be constant between the acquisition of the reference and the sample spectrum. Mie scattering is minimized with a 5 µm particle filter in the gas inlet, while Rayleigh scattering losses were calculated to be 2.55 × 10 −7 cm −1 at 445 nm at 25 • C and 1 atm (Kovalev and Eichinger, 2004) and thus negligible with respect to the cavity losses normalized by the cavity length 1−R d = 2.28 × 10 −6 cm −1 . Therefore, the light transmitted through the cavity is mainly affected by the absorption of the gas species, which leads to well-defined absorption spectral features, α i (λ), that are analysed in real time by a linear multicomponent fit routine.…”

Quantification of nitrous acid (HONO) and nitrogen dioxide (NO2) in ambient air by broadband cavity-enhanced absorption spectroscopy (IBBCEAS) between 361–388 nm

“…The instrument is comprised of a light source, collimating 75 optics, a resonant cavity, an optical filter, a fibre collimator, a specialized fibre bundle, and a grating spectrometer. Many instrument components, including the sample cell design, are identical to the instrument described by Jordan et al (2019) with differences noted below.…”

“…The grating spectrometer (spectrograph and camera) has been described by Jordan et al (2019). The spectrograph is configured with a 1200 groove cm -1 grating, blazed at 500 nm and positioned at 350 nm central wavelength with a spectral coverage from 291.9 to 408.2 nm.…”

“…3.2). For ambient air measurements, zero air was generated using a custom-built generator (Jordan et al, 2019). The IBBCEAS sampled zero air every 10 140 min for a duration of 2 min.…”

Quantification of nitrous acid (HONO) and nitrogen dioxide (NO2) in ambient air by broadband cavity-enhanced absorption spectroscopy (IBBCEAS) between 361 and 388 nm

“…The prime example is open-path differential optical absorption spectroscopy (DOAS), with LODs typically in the range of 10-100 pptv with integration times of several minutes (Tsai et al, 2018). A considerable advantage of open-path instruments is the lack of any inlet and associated chemistry, such as loss of N. Jordan and H. D. Osthoff: Quantification of HONO and NO 2 by IBBCEAS HONO due to partitioning onto inlet walls (Duan et al, 2018) or HONO formation, for example from reaction of NO 2 with soot particles (Longfellow et al, 1999;Kalberer et al, 1999;Indarto, 2012) that may have deposited on the inlet lines and particle filter. Open-path DOAS, however, only provides concentrations averaged over an absorption path that is multiple kilometres long.…”