Multiphase Reaction of Ozone with HONO/NO₂^- on Surfaces: Effects on Indoor HONO and Ozone

Nitrous acid (HONO) and ozone (O₃) are two important indoor pollutants that affect the indoor oxidation capacity. Previous field studies have observed an inverse correlation between these two pollutants indoors, but the specific mechanism remains unclear. Given the semivolatile behavior of HONO, a possible mechanism is its multiphase reaction with ozone. In this study, we measured ozone uptake on surface HONO/NO₂^- under environmentally relevant conditions in a flow tube. The ozone deposition velocities (v_d = 0.002 ± 0.001-0.3 ± 0.005 cm s^-1) and uptake coefficients (γ = (0.2 ± 0.1) × 10^-6-(2.0 ± 0.2) × 10^-4) depend on reactant concentrations, relative humidity, and reaction time but are less affected by illumination. The lifetimes of gaseous HONO and ozone are approximately 10 min due to this multiphase reaction under indoor conditions, which is a significant sink for HONO and O₃ as compared to those of other indoor reactions and air exchange. This study for the first time revealed the previously overlooked vital role of the reaction of surface HONO/NO₂^- with O₃ in affecting both indoor HONO and O₃ and has significance for understanding the multiphase chemistry of HONO and O₃, with implications for outdoor surfaces and model studies to better constrain HONO sinks.