Observational Evidence of the Recycling Mechanism for pNO 3 – -HONO-O 3 during Anthropogenic NO x Emission Reduction Scenarios

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Nitrous acid (HONO) enhances the atmospheric oxidative capacity by generating hydroxyl radicals (OH), which contribute to secondary pollutants such as ozone (O 3 ) 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-O 3 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 (pNO 3 – ) 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 O 3 and dinitrogen pentoxide (N 2 O 5 ) levels, which subsequently elevated nocturnal pNO 3 – formation, establishing a positive feedback loop within the HONO-O 3 cycle. Our findings provide observational evidence of the recycling mechanism for atmospheric pNO 3 – -HONO-O 3 chemistry and suggest that moderate NO x emission reduction alone is insufficient to mitigate HONO and O 3 pollution.

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