Intercomparison of the DOAS and LOPAP techniques for the detection of nitrous acid (HONO)

Recent studies have demonstrated that nitrous acid (HONO) is a source of hydroxyl radicals (OH) in the boundary layer not only early in the morning but also throughout the day. Despite its importance, all known instruments to detect HONO in the atmosphere suffer either from the great experimental effort necessary or from the possibility of significant interferences. In addition, only a few instruments are sensitive enough to detect low HONO concentrations during daytime. Accordingly, validated and sensitive measurements of nitrous acid are of paramount importance to describe the oxidation capacity of the atmosphere. Up to now, in intercomparisons of these chemical detectors with the well accepted DOAS technique significantly higher concentrations have been detected during the day with the chemical detectors, the discrepancy being attributed to interferences. In the present study, a recently developed HONO instrument (LOPAP) was validated against the DOAS technique, both in the field and in a large smog chamber under various conditions. Since sampling artefacts were minimised and interferences were measured and corrected for by the LOPAP instrument excellent agreement was obtained between these techniques during daytime as well as night-time. It is demonstrated that chemical instruments, which do not measure and do not correct interferences may significantly overestimate daytime concentrations of HONO caused by unknown interferences, which are particularly important during daytime, when HONO concentrations are low. For the DOAS technique, the possibility of HONO impurities in the NO2 reference spectra used for the spectral analysis needs to be treated actively in the evaluation process, to avoid a potential negative interference at low HONO/NO2 ratios. A simple procedure is presented that eliminates this possible error source in DOAS measurements of HONO in the atmosphere.