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Abstract. A well-characterized source of nitrous acid vapour (HONO) is essential foraccurate ambient air measurements by instruments requiring externalcalibration. In this work, a compact HONO source is described in which gasstreams containing dilute concentrations of HONO are generated by flowinghydrochloric acid (HCl) vapour emanating from a permeation tube overcontinuously agitated dry sodium nitrite (NaNO2) heated to 50 ∘C. Mixing ratios of HONO and potential by-productsincluding NO, NO2, and nitrosyl chloride (ClNO) were quantified by Fourier transform infrared (FTIR) and thermal-dissociation cavity ring-down spectroscopy (TD-CRDS). A key parameter is the concentration of HCl, whichneeds to be kept small (&lt;4 ppmv) to avoid ClNO formation. Thesource produces gas streams containing HONO in air in &gt;95 %purity relative to other nitrogen oxides. The source output is rapidlytuneable and stabilizes within 90 min. Combined with its small size and portability, this source is highly suitable for calibration of HONOinstruments in the field.

A compact, high-purity source of HONO validated by Fourier transform infrared and thermal-dissociation cavity ring-down spectroscopy