Observational Evidence of the Recycling Mechanism for pNO ₃ ^– -HONO-O ₃ during Anthropogenic NO _<i>x</i> Emission 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 (NOx) 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 NOx emission scenarios, respectively. Our results demonstrate that COVID-19 restrictions, combined with the holiday effect in 2022, suppressed HONO production from NOx-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 NOx 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 NOx emission reduction alone is insufficient to mitigate HONO and O3 pollution.