Spontaneous Oxidation of Nitrous Acid to Nitric Acid in Supermicron Aqueous Droplets Is Acid-Accelerated

It is generally assumed that acidic submicron atmospheric aerosol particles do not constitute a significant sink for nitrous acid (HONO), as this weak acid would remain protonated and volatile, yet the uptake of HONO to larger less acidic particles is unexplored. Experiments on optically tweezed aerosol did not observe HONO gas uptake but instead revealed rapid oxidation of HONO to HNO₃ in droplets of initial pH between 0 and 7.75. This oxidation was spontaneous at room temperature with no oxidant added and occurred over a subminute time scale. The reaction is accelerated under acidic conditions of pH < 2. We hypothesize that protonated HONO is restricted to the interfacial region while NO₂^– is not, and HONO is therefore oxidized to HNO₃ following a second-order rate dependence on the HONO concentration. The oxidation of HONO can thus be self-catalyzed in weakly buffered aerosol. Less acidic droplets (pH > 5.0) displayed an approximately 2 orders of magnitude lower conversion rate of HONO to HNO₃, likely due to deprotonation and then slower oxidation of NO₂^– directly to NO₃^– with a first-order dependence on the NO₂^– concentration. Production of HNO₃ can drive a liquid–liquid phase separation of secondary organic aerosol, but an organic shell phase did not prevent oxidation of HONO to HNO₃. This rapid conversion of HONO to HNO₃ at the droplet interface due to an acidity-based transition in the reaction mechanism could represent a significant new sink for HONO and a source of strong inorganic acids in the atmosphere that are more readily removed through deposition.