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

<p>A well-characterized source of nitrous acid vapour (HONO) is essential for accurate ambient air measurements by instruments requiring external calibration. In this work, a compact HONO source is described in which gas streams containing dilute concentrations of HONO are generated by flowing hydrochloric acid (HCl) vapour emanating from a permeation tube over continuously agitated dry sodium nitrite (<span class="inline-formula">NaNO₂</span>) heated to 50&thinsp;<span class="inline-formula">^∘</span>C. Mixing ratios of HONO and potential by-products including NO, <span class="inline-formula">NO₂</span>, 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, which needs to be kept small (<span class="inline-formula">&lt;4</span>&thinsp;ppmv) to avoid ClNO formation. The source produces gas streams containing HONO in air in <span class="inline-formula">&gt;95</span>&thinsp;% purity relative to other nitrogen oxides. The source output is rapidly tuneable and stabilizes within 90&thinsp;min. Combined with its small size and portability, this source is highly suitable for calibration of HONO instruments in the field.</p>