Abstract: The results showed that the average HONO concentration during the study period was 2.55±1.34 ppb, with two high-HONO events accompanied by elevated concentrations of PM_2.5 and O₃. Using a photochemical box model (F0AM) coupled with an updated heterogeneous chemical mechanism based on the Master Chemical Mechanism version 3.3.1 (MCM3.3.1), the main sources and formation pathways of HONO were explored. During daytime, the major sources of HONO were nitrate photolysis, the homogeneous reaction of NO with OH, and the photochemically enhanced heterogeneous reaction of NO₂ on aerosol surfaces, with average formation rates (contribution ratios) of 2.70 (55.8%), 0.53 (10.8%), and 0.05 (10.6%) ppb h^-1, respectively. At night, the contribution of heterogeneous reactions gradually increased, including the heterogeneous reactions of NO₂ on ground and aerosol surfaces, as well as the enhanced uptake of NO₂ on aerosol surfaces in the presence of NH₃, with average formation rates (contribution ratios) of 0.07 (41.3%), 0.03 (20.3%), and 0.03 (20.0%) ppb h^-1, respectively. The primary removal pathway for HONO during daytime was photolysis, whereas dry deposition dominated at night. Further simulation analyses revealed that incorporating the new HONO mechanism significantly enhanced the formation and loss rates of O₃. The sensitivity of O₃ formation shifted from a volatile organic compound (VOC)-dominated regime to a synergistic control regime involving both VOCs and NOx. Elevated HONO concentrations and their associated source–sink processes not only accelerated O₃ production but also modified its formation sensitivity by dynamically regulating the ratios of O₃ precursors (VOCs/NOx). Therefore, preventing and controlling springtime ozone pollution in Beijing requires a correct understanding of the feedback mechanisms of HONO chemistry on O₃ formation.