1Optical Remote Sensing, Radio and Space Science, Chalmers University, 41296 Gothenburg, Sweden
2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
3Jet Propulsion Laboratory, Pasadena, CA 91109, USA
*now at: Department of Chemistry, UC Irvine, Irvine, CA 92697-2025, USA
**now at: Ecological Process Modelling Unit, Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, 8903, Switzerland
***now at: Atmospheric and Oceanic program, Princeton University, Princeton, NJ 08544, USA
Abstract. 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 nights are much higher than previously reported values from measurements in urban air in Europe, North America and Asia. The influence of aerosol plumes, relative humidity, aerosol surface and ground reactive surface, temperature on NO2-HONO chemical conversion were discussed. The high humidity, strong and low inversion layer at night, and serious aerosol pollution burden may explain the particularly efficient conversion of NO2 to HONO.