Revisiting the Ultra-Violet Absorption Cross Section of Gaseous Nitrous Acid (HONO): New Insights for Atmospheric HONO Budget

Nitrous acid (HONO) is an important source of hydroxyl radicals (OH) in the atmosphere. Precise determination of the absolute ultra-violet (UV) absorption cross-section of gaseous HONO lays the basis for the accurate measurement of its concentration by optical methods and the estimation of HONO loss rate through photolysis. We performed a series of laboratory and field intercomparison experiments for HONO measurement between striping coil-liquid waveguide capillary cell (SC-LWCC) photometer and incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). Specified HONO concentrations prepared by an ultra-pure standard HONO source were utilized for laboratory intercomparisons. Results shows a consistent ~22% negative bias in measurements of the IBBCEAS compared with SC-LWCC photometer. It is confirmed that the discrepancies occurring between these techniques are associated with the overestimation in the absolute UV absorption cross-sections through careful analysis of possible uncertainties. We quantified the absorption cross-section of gaseous HONO (360-390nm) utilizing a custom-built IBBCEAS instrument, and the results was found to be 22%-34% lower than the previously published absorption cross-sections widely used in HONO concentration retrieval and atmospheric chemical transport models (CTMs). This suggests that the HONO concentrations retrieved by optical methods based on absolute absorption cross-sections may have been underestimated by over 20%. Plus, the daytime loss rate and unidentified sources of HONO may also have evidently been overestimated in pre-existing studies. In summary, our findings underscore the significance of revisiting the absolute absorption cross-section of HONO and the re-evaluation of the previously reported HONO budgets.