Nitrous acid (HONO) is a reactive gas that plays an important role in atmospheric chemistry. However, accurately quantifying its direct emissions and secondary formation in the atmosphere, as well as attributing it to specific nitrogen sources, remains a significant challenge. In this study, we developed a novel method using stable nitrogen and oxygen isotopes (δ¹⁵N; δ¹⁸O) for apportioning ambient HONO in an urban area in North China. The results show that secondary formation were the dominant HONO formation processes during both day and night, with the NO₂ heterogeneous reaction contributing 59.0 ± 14.6% in daytime and 64.4 ± 10.8% at nighttime. A Bayesian simulation demonstrated that the average contributions of coal combustion, biomass burning, vehicle exhaust, and soil emissions to HONO were 22.2 ± 13.1%, 26.0 ± 5.7%, 28.6 ± 6.7%, and 23.2 ± 8.1%, respectively. We propose that the isotopic method presents a promising approach for identifying nitrogen sources and the secondary formation of HONO, which could contribute to mitigating HONO and its adverse effects on air quality.

HONO; Nitrogen isotope; Oxygen isotope; Sources; Formation