Observational Evidence of the Recycling Mechanism for pNO₃^–-HONO-O₃during Anthropogenic NO_xEmission Reduction Scenarios

Nitrous acid (HONO) enhances the atmospheric oxidative capacity by generating hydroxyl radicals (OH), which contribute to secondary pollutants such as ozone (O₃) and particulate matter (PM). These pollutants further modulate HONO formation pathways, creating a complex feedback loop. However, the mechanism by which nitrogen oxide (NO_x) emission reduction affects the HONO-O₃ cycle is not fully clarified. To address this knowledge gap, we conducted two distinct field campaigns during the Spring Festival periods in 2022 and 2023, representing low and high NO_x emission scenarios, respectively. Our results demonstrate that COVID-19 restrictions, combined with the holiday effect in 2022, suppressed HONO production from NO_x-related reactions but enhanced its generation via particulate nitrate (pNO₃^–) photolysis. In contrast, an opposite trend was observed in 2023. Chemical transport model simulations and sensitivity analyses further revealed that moderate anthropogenic NO_x reduction elevated O₃ and dinitrogen pentoxide (N₂O₅) levels, which subsequently elevated nocturnal pNO₃^– formation, establishing a positive feedback loop within the HONO-O₃ cycle. Our findings provide observational evidence of the recycling mechanism for atmospheric pNO₃^–-HONO-O₃ chemistry and suggest that moderate NO_x emission reduction alone is insufficient to mitigate HONO and O₃ pollution.