Measurement report: Anthropogenic activities reduction suppresses HONO formation: direct evidence for secondary pollution control

Abstract. Nitrous acid (HONO) is a key precursor of atmospheric hydroxyl radicals (OH) and significantly influences the formation of secondary pollutants, making it essential for understanding and controlling air pollution. While many studies have focused on its formation mechanisms, few have explored the impact of variations in anthropogenic activities on HONO formation. Therefore, we investigated the impact of variations in anthropogenic activities on HONO formation based on comprehensive observations conducted in urban Beijing during autumn and winter of 2022. During clean periods with a 53 % drop in Traffic Performance Index, HONO, CO, and NO2 levels decreased by 2–3 times compared to polluted periods and significantly lower than previously reported wintertime levels in Beijing. Source apportionment revealed that NO2 heterogeneous reaction on ground was the dominant HONO source across all periods. Vehicle emissions contributed more to HONO during clean periods, suggesting that reducing anthropogenic activities has a stronger influence on secondary HONO formation. Particulate nitrate (pNO3) photolysis contributed more to HONO during polluted periods, due to higher pNO3 fractions in PM2.5 under more polluted conditions. Despite including all known formation pathways in the model, unidentified HONO sources still remained. This was strongly associated with intense solar radiation and high OH concentrations at daytime, as well as elevated NH3 concentrations at nighttime. Emission reduction simulations further revealed that a 50 % NOx reduction during polluted periods could lower HONO by up to 46.3 %, directly demonstrating that reducing anthropogenic activities significantly suppresses HONO formation and provides a scientific basis for the development of air pollution control strategies.