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 anthropogenic activities variation on HONO formation. Therefore, we investigated the impact of anthropogenic activities variation on HONO formation based on a comprehensive observation 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 reduced anthropogenic activities has a stronger influence on secondary HONO formation. NO₃^- photolysis contributed more to HONO during polluted periods, due to higher NO₃^- fractions in PM_2.5 under more polluted conditions. Despite including all known formation pathways in the model, unidentified HONO sources still remain. This is 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 38.4 %, directly demonstrating that reducing anthropogenic activities significantly suppresses HONO formation and provides a scientific basis for the development of air pollution control strategies.