A review of measurements and model simulations of atmospheric nitrous acid

Ambient nitrous acid (HONO) plays a crucial role in the atmosphere's oxidative capacity, significantly impacting air quality and climate. This study reviews the current understanding of HONO formation mechanisms, including in-situ and vertical gradient measurements, as well as the temporal, spatial, and vertical characteristics of HONO and its modeling approaches. HONO concentrations exhibit significant diurnal variation based on sources and sinks in different environments. Typically, concentrations are higher near the ground and decrease with altitude. Additionally, this study examines the incorporation of contemporary HONO chemical mechanisms into box models, regional and global chemical transport models (CTMs), and chemistry-climate models. Models often underestimate observations due to uncertainties in heterogeneous HONO formation and varying measurement techniques. Finally, this review identifies key challenges for future HONO measurements and modeling efforts. Significant opportunities remain to enhance our fundamental understanding of HONO. Precision and accuracy are important for advancing HONO observation measurement techniques. Simultaneously, the representation of HONO in state-of-the-art models helps us better quantify atmospheric oxidation capacity and air quality impacts.