Sources and formation of HONO from an isotopic perspective

Gaseous nitrous acid (HONO) is a key precursor of hydroxyl radicals (·OH) in the atmosphere, significantly influencing atmospheric oxidation capacity and regional air pollution. The generation mechanisms of HONO are complex, involving direct emissions, photolysis, and secondary formation pathways, yet the contribution of each source remains unclear. Particularly, the concentration of HONO exhibits considerable variation across different atmospheric layers and environmental conditions, making its specific generation pathways and contribution rates a challenging research topic. In recent years, stable isotope techniques (such as isotope fractionation and isotope labeling) have gradually become powerful tools for studying the sources and formation mechanisms of HONO. Through isotope tracing methods, researchers are able to identify different sources of HONO and reveal secondary formation pathways. This review summarizes the current progress in HONO research based on isotope analysis, with a focus on isotope fractionation techniques and their applications in atmospheric environments. By reviewing research findings across various regions and environmental conditions, this paper provides new insights into the mechanisms of HONO formation and offers scientific support for the development of future air pollution control strategies.