1Optical Remote Sensing, Radio and Space Science, Chalmers University of Technology, 41296 Gothenburg, Sweden
2Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
3Jet Propulsion Laboratory, Pasadena, CA 91109, USA
*now at: Monitoring & Laboratory Division, California Air Resources Board, 9528 Telstar Avenue, El Monte, CA 91731, USA
**now at: Atmospheric and Oceanic Program, Princeton University, Princeton, NJ 08544, USA
***now at: Ecological Process Modelling Unit, Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, 8903, Switzerland
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 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.