Nitrous acid (HONO) plays a significant role in the atmosphere, especially in the polluted troposphere. Its photolysis after sunrise is an important source of hydroxyl free radicals (OH). Measurements of nitrous acid and other pollutants were carried out in the Kathmandu urban atmosphere during January–February 2003, contributing to the sparse knowledge of nitrous acid in South Asia. The results showed average nocturnal levels of HONO (1.7±0.8 ppbv), NO₂ (17.9±10.2 ppbv), and PM₁₀ (0.18±0.11 mg m⁻³) in urban air in Kathmandu. Surprisingly high ratios of chemically formed secondary [HONO] to [NO₂] (up to 30%) were found, which indicates unexpectedly efficient chemical conversion of NO₂ to HONO in Kathmandu. The ratios of [HONO]/[NO₂] at night were found to be much higher than previously reported values from measurements in urban air in Europe, North America and Asia. The influences of aerosol surface, ground reactive surface, and relative humidity on NO₂-HONO chemical conversion were discussed. The high humidity, strong and low inversion layer at night, and high aerosol pollution burden in Kathmandu may explain the particularly efficient conversion of NO₂ to HONO.