High-purity nitrous acid (HONO) generation and quantification using broadband cavity-enhanced absorption spectroscopy (BBCEAS)

Abstract. Nitrous acid (HONO) is a key atmospheric precursor to hydroxyl radicals (OH), which drive oxidation processes in the troposphere. Accurate quantification of HONO is essential for modelling atmospheric chemistry but is often challenging due to low ambient concentrations, significant wall losses from instruments with inlets, and interference from co-produced nitrogen dioxide (NO2). Laboratory generation methods frequently struggle to produce HONO in both high purity and usable quantities. This study introduces a simple, scalable method for generating high-purity HONO relative to NO2 via the reaction of hydrochloric acid (HCl) and sodium nitrite (NaNO2) in a multi-bubbler system. The method incorporates a thermal “catch and release” mechanism to selectively trap HONO and separate it from NO2 prior to analysis. While the melting point of NO2 is well-established at −9 °C, the melting point of HONO was monitored and determined to be −4 °C, enabling the thermal separation mechanism to function effectively. Broadband cavity-enhanced absorption spectroscopy (BBCEAS) was used for detection and quantification in the 329–344 nm region. Systematic optimization of bubbler temperatures, carrier gas flow rates, capture durations, and moisture levels yielded HONO purities greater than 96 %. This method enables reliable and flexible production of high-purity HONO relative to NO2, making it well-suited for use in laboratory-based chamber studies and instrument calibration applications.