Abstract
Abstract. This work describes an incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for quantification of HONO and NO2 mixing ratios in ambient air. The instrument is operated in the near-ultraviolet spectral region between 361 and 388 nm. The mirror reflectivity and optical cavity transmission function were determined from the optical extinction observed when sampling air and helium. To verify the accuracy of this approach, Rayleigh scattering cross sections of nitrogen and argon were measured and found to be in quantitative agreement with literature values. The mirror reflectivity exceeded 99.98 %, at its maximum near 373 nm, resulting in an absorption path length of 6 km from a 1 m long optical cavity. The instrument precision was assessed through Allan variance analyses and showed minimum deviations of ±58 and ±210 pptv (1σ) for HONO and NO2, respectively, at an optimum acquisition time of 5 min. Measurements of HONO and NO2 mixing ratios in laboratory-generated mixtures by IBBCEAS were compared to thermal dissociation cavity ring-down spectroscopy (TD-CRDS) data and agreed within combined experimental uncertainties. Sample ambient air data collected in Calgary are presented.
Highlights
Nitrous acid (HONO) has long been recognized as an important tropospheric oxide of nitrogen (Nash, 1974)
Long path absorption photometry (LOPAP), while sensitive with limits of detection (LODs) of < 1 part per trillion (10−12, pptv), is prone to interference from atmospheric NO2 and O3 and conversion of peroxyacetyl nitrate (PAN) (Villena et al, 2011) and peroxynitric acid (HO2NO2) (Legrand et al, 2014)
We report a new incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for quantification of HONO and NO2 in ambient air, nicknamed “HONO detection by optical resonance” (HODOR)
Summary
Nitrous acid (HONO) has long been recognized as an important tropospheric oxide of nitrogen (Nash, 1974). Despite the importance of HONO, accurate and time-resolved (i.e., < 5 min) in situ measurements of ambient HONO mixing ratios remain a challenge, exemplified by discrepancies reported among individual instruments in recent inter-comparison studies (Rodenas et al, 2013; Pinto et al, 2014; Crilley et al, 2019). These discrepancies arise in part as atmospheric HONO measurements by wet chemical techniques or mass spectrometry require external calibration and are prone to interferences.
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