Experimental intercomparison of the absorption cross-sections of nitrous acid (HONO) in the ultraviolet and mid-infrared spectral regions

Nitrous acid, HONO, is an important trace gas in tropospheric photochemistry, because it is a source of OH radicals. In order to obtain HONO concentrations from spectroscopic measurements, the knowledge of accurate absorption cross-sections is essential. Furthermore, the ultraviolet absorption cross-sections of HONO determine its atmospheric photolysis rates, which are still being debated. In particular, in a recent field study focusing on the photolysis frequency of HONO, the absolute values of the ultraviolet absorption cross-sections have been proposed as a potential source for systematic errors. For these reasons, we have compared the absorption cross-sections for HONO in the infrared (IR) and ultraviolet (UV) by performing simultaneous measurements in both spectral regions. Assuming that the IR cross-sections (that show good agreement between different studies) are correct, our study shows a good agreement between the consistent infrared studies and some selected accurate UV spectra published previously (about 6%) while a rather large disagreement (about 22%) is observed when using other UV data sets.