Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 1 2009 10.5194/acpd-9-183-2009 http://www.atmos-chem-phys-discuss.net/9/183/2009/ http://www.atmos-chem-phys-discuss.net/9/183/2009/acpd-9-183-2009.html http://www.atmos-chem-phys-discuss.net/9/183/2009/acpd-9-183-2009.pdf 183 223 2009-01-06 Observations of high rates of NO₂ – HONO conversion in the nocturnal atmospheric boundary layer in Kathmandu, Nepal Y. Yu yongy@uci.edu B. Galle E. Hodson A. Panday R. Prinn S. Wang Optical Remote Sensing, Radio and Space Science, Chalmers University, 41296 Gothenburg, Sweden Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Jet 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 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&plusmn;0.8 ppbv), NO₂ (17.9&plusmn;10.2 ppbv), and PM₁₀ (0.18&plusmn</sub>0.11 mg m^&minus;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. Acker,~K., Moller,~D., Wieprecht,~W., Auel,~R., Kalass,~D., and Tscherwenka,~W.: Nitrous and nitric acid measurements inside and outside of clouds at Mt. 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