ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-183-2009 Observations of high rates of NO<sub>2</sub> – HONO conversion in the nocturnal atmospheric boundary layer in Kathmandu, Nepal Yu Y. 1 4 Galle B. 1 Hodson E. 2 5 Panday A. 2 6 Prinn R. 2 Wang S. 3 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 06 01 2009 9 1 183 223 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/9/183/2009/acpd-9-183-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/183/2009/acpd-9-183-2009.pdf

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), NO<sub>2</sub> (17.9&plusmn;10.2 ppbv), and PM<sub>10</sub> (0.18&plusmn</sub>0.11 mg m<sup>&minus;3</sup>) in urban air in Kathmandu. Surprisingly high ratios of chemically formed secondary [HONO] to [NO<sub>2</sub>] (up to 30%) were found, which indicates unexpectedly efficient chemical conversion of NO<sub>2</sub> to HONO in Kathmandu. The ratios of [HONO]/[NO<sub>2</sub>] 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 NO<sub>2</sub>-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 NO<sub>2</sub> to HONO.

 References 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. Brocken, Water Air Soil Poll., 130, 331–336, 2001. Acker,~K., Moller,~D., Auel,~R., Wieprecht,~W., and Kalass,~D.: Concentrations of nitrous acid, nitric acid, nitrite and nitrate in the gas and aerosol phase at a~site in the emission zone during ESCOMPTE 2001 experiment, Atmos. Res., 74, 507–524, 2005. Adamson,~A W. and Gast,~A P.: Physical Chemistry of Surfaces, 6th ed., John Wiley & Sons, Inc., New York, 1997. Adhikary,~B., Carmichael,~G R., Tang,~Y., Leung,~L R., Qian,~Y., Schauer,~J J., Stone,~E A., Ramanathan,~V., and Ramana,~M V.: Characterization of the seasonal cycle of south Asian aerosols: A~regional-scale modeling analysis,~J. Geophys. Res., 112, D22S22, doi:10.1029/2006JD008143, 2007. Akimoto,~H., Takagi,~H., and Sakamaki,~F.: Photoenhancement of the nitrous acid formation in the surface reaction of nitrogen dioxide and water vapor: extra radical source in smog chamber experiments, Int J. Chem. Kinet., 19, 539–551, 1987. Al-Abadleh,~H A. and Grassian,~V H.: Heterogeneous reaction of \chemNO_2 on hexane soot: A~Knudsen cell and FT-IR study,~J. Phys. Chem. A., 104, 11 926–11 933, 2000. Alcala-Jornod,~C., van den Bergh,~H., and Rossi,~M J.: Reactivity of \chemNO_2 and \chemH_2O on soot generated in the laboratory: a~diffusion tube study at ambient temperature, Phys. Chem. Chem. Phys., 2, 5584–5593, 2000. Ammann,~M., Kalberer,~M., Jost,~D T., Tobler,~L., Rossler,~E., Piguet,~D., Gaggeler,~H W., and Baltensperger,~U.: Heterogeneous production of nitrous acid on soot in polluted air masses, Nature, 395, 157–160, 1998. Andres-Hernandez,~M D., Notholt,~J., Hjorth,~J., and Schrems,~O.: A~DOAS study on the origin of nitrous acid at urban and non-urban sites, Atmos. Environ., 30, 175–180, 1996. Antognozzi,~M., Humphris,~A D L., and Miles,~M J.: Observation of molecular layering in a~confined water film and study of the layers viscoelastic properties, Appl. Phys. Lett., 78, 300–302, 2001. Arens,~F., Gutzwiller,~L., Baltensperger,~U., Gaggeler,~H W., and Ammann,~M.: Heterogeneous reaction of \chemNO_2 on diesel soot particles, Environ. Sci. Technol., 35, 2191–2199, 2001. Arens,~F., Gutzwiller,~L., Gaggeler,~H W., and Ammann,~M.: The reaction of \chemNO_2 with solid anthrarobin (1,2,10-trihydroxy-anthracene), Phys. Chem. Chem. Phys., 4, 3684–3690, 2002. Ariola,~V., D'Alessandro,~A., Lucarelli,~F., Marcazzan,~G., Mazzei,~F., Nava,~S., Garcia-Orellana,~I., Prati,~P., Valli,~G., Vecchi,~R., and Zucchiatti,~A.: Elemental characterization of PM$_10$, PM$_2.5$ and PM$_1$ in the town of Genoa (Italy), Chemosphere, 62, 226–232, 2006. Aubin,~D G. and Abbatt,~J P D.: Interaction of \chemNO_2 with hydrocarbon soot: Focus on HONO yield, surface modification, and mechanism,~J. Phys. Chem. A, 111, 6263–6273, 2007. Aumont,~B., Madronich,~S., Ammann,~M., Kalberer,~M., Baltensperger,~U., Hauglustaine,~D., and Brocheton,~F.: On the \chemNO_2 plus soot reaction in the atmosphere,~J. Geophys. Res., 104, 1729–1736, 1999. Beine, H. J., Amoroso, A., Dominé, F., King, M. D., Nardino, M., Ianniello, A., and France, J. L.: Surprisingly small HONO emissions from snow surfaces at Browning Pass, Antarctica, Atmos. Chem. Phys., 6, 2569–2580, 2006. Bejan,~I., Abd El Aal,~Y., Barnes,~I., Benter,~T., Bohn,~B., Wiesen,~P., and Kleffmann,~J.: The photolysis of ortho-nitrophenols: a~new gas phase source of HONO, Phys. Chem. Chem. Phys., 8, 2028–2035, 2006. Bongartz,~A., Kames,~J., Schurathy,~U., George,~C., Mirabel,~P., and Pnoche,~J L.: Experimental determination of HONO mass accommodation coefficients using two different techniques,~J. Atmos. Chem., 18, 149–169, 1994. Box,~M A. and Lo,~S Y.: Approximate determination of aerosol size distributions,~J. Appl. Meteorol., 15, 1068–1076, 1976. Carrico,~C M., Bergin,~M H., Shrestha,~A B., Dibb,~J E., Gomes,~L., and Harris,~J M.: The importance of carbon and mineral dust to seasonal aerosol properties in the Nepal Himalaya, Atmos. Environ., 37, 2811–2824, 2003. Cheng,~M T., Horng,~C L., and Lin,~Y C.: Characteristics of atmospheric aerosol and acidic gases from urban and forest sites in central Taiwan, Bull. Environ. Contamin. Toxicol., 79, 674–677, 2007. Davidovits,~P., Jayne,~J T., Duan,~S X., Worsnop,~D R., Zahniser,~M S., and Kolb,~C E.: Uptake of gas molecules by liquids: a~model,~J. Phys. Chem., 95, 6337–6340, 1991. Domine,~F. and Shepson,~P B.: Air-snow interactions and atmospheric chemistry, Science, 297, 1506–1510, 2002. Ehrlich,~C., Noll,~G., Kalkoff,~W D., Baumbach,~G., and Dreiseidler,~A.: PM$_10$, PM$_2.5$ and PM$_1.0$ – Emissions from industrial plants – Results from measurement programmes in Germany, Atmos. Environ., 41, 6236–6254, 2007. Febo,~A., Perrino,~C., and Allegrini,~I.: Measurement of nitrous acid in Milan, Italy, by DOAS and diffusion denuders, Atmos. Environ., 30, 3599–3609, 1996. Finlayson-Pitts,~B J. and Pitts Jr.,~J N.: Chemistry of the upper and lower atmosphere – theory, experiments, and applications, Academic Press, San Diego, 969 pp., 2000. Finlayson-Pitts,~B J., Wingen,~L M., Sumner,~A L., Syomin,~D., and Ramazan,~K A.: The Heterogeneous hydrolysis of \chemNO_2 in laboratory systems and in outdoor and indoor atmospheres: an integrated mechanism, Phys. Chem. Chem. Phys., 5, 223–242, 2003. Flentje,~H., Dubois,~R., Heintzenberg,~J., and Karbach,~H J.: Retrieval of aerosol properties from boundary layer extinction measurements with a~DOAS system, Geophys. Res. Lett., 24, 2019–2022, 1997. Geiger,~H., Kleffmann,~J., and Wiesen,~P.: Smog chamber studies on the influence of diesel exhaust on photosmog formation, Atmos. Environ., 36, 1737–1747, 2002. George,~C., Strekowski,~R S., Kleffmann,~J., Stemmler,~K., and Ammann,~M.: Photoenhanced uptake of gaseous \chemNO_2 on solid-organic compounds: a~photochemical source of HONO?, Faraday Discuss., 130, 195–210, 2005. Gerecke,~A., Thielmann,~A., Gutzwiller,~L., and Rossi,~M J.: The chemical kinetics of HONO formation resulting from heterogeneous interaction of \chemNO_2 with flame soot, Geophys. Res. Lett., 25, 2453–2456, 1998. Giri,~D., Murthy,~V K., Adhikary,~P R., and Khanal,~S N.: Ambient air quality of Kathmandu Valley as reflected by atmospheric particulate matter concentrations (PM$_10$), Int J. Environ. Sci. Tech., 3, 403–410, 2006. Gomiscek,~B., Hauck,~H., Stopper,~S., and Preining,~O.: Spatial and temporal variations of PM$_1$, PM$_2.5$, PM$_10$ and particle number concentration during the AUPHEP-project, Atmos. Environ., 38, 3917–3934, 2004. Grassian,~V H.: Heterogeneous uptake and reaction of nitrogen oxides and volatile organic compounds on the surface of atmospheric particles including oxides, carbonates, soot and mineral dust: implications for the chemical balance of the troposphere, Int. Rev. Phys. Chem., 20, 467–548, 2001. Grassian,~V H.: Chemical reactions of nitrogen oxides on the surface of oxide, carbonate, soot and mineral dust particles: implications for the chemical balance of the troposphere,~J. Phys. Chem. A., 106, 860–877, 2002. Gregg,~S J. and Sing,~K S W.: Adsorption, surface area and porosity, 2nd ed., Academic Press, London, 1982. Gustafsson, R. J., Orlov, A., Badger, C. L., Griffiths, P. T., Cox, R. A., and Lambert, R. M.: A comprehensive evaluation of water uptake on atmospherically relevant mineral surfaces: DRIFT spectroscopy, thermogravimetric analysis and aerosol growth measurements, Atmos. Chem. Phys., 5, 3415–3421, 2005. Gustafsson,~R J., Orlov,~A., Griffiths,~P T., Cox,~R A., and Lambert,~R M.: Reduction of \chemNO_2 to nitrous acid on illuminated titanium dioxide aerosol surfaces: implications for photocatalysis and atmospheric chemistry, Chem. Commun., 3936–3938, 2006. Hao,~N., Zhou,~B., Chen,~D., and Chen,~L M.: Observations of nitrous acid and its relative humidity dependence in Shanghai,~J. Environ. Sci., 18, 910–915, 2006. Harrison,~R M. and Kitto,~A.-M N.: Envidence for a~surface source of atmospheric nitrous acid, Atmos. Environ., 28, 1089–1094, 1994. Harrison,~R M., Peak,~J D., and Collins,~G M.: Tropospheric cycle of nitrous acid,~J. Geophys. Res., 101, 14429–14439, 1996. Harrison,~R M. and Collins,~G M.: Measurements of reaction coefficients of \chemNO_2 and HONO on aerosol particles,~J. Atmos. Chem., 30, 397–406, 1998. He,~Y., Zhou,~X L., Hou,~J., Gao,~H L., and Bertman,~S B.: Importance of dew in controlling the air-surface exchange of HONO in rural forested environments, Geophys. Res. Lett., 33, L02813, doi:10.1029/2005GL024348, 2006. Heland,~J., Kleffmann,~J., Kurtenbach,~R., and Wiesen,~P.: A~new instrument to measure gaseous nitrous acid (HONO) in the atmosphere, Environ. Sci. Technol., 35, 3207–3212, 2001. Honrath,~R E., Lu,~Y., Peterson,~M C., Dibb,~J E., Arsenault,~M A., Cullen,~N J., and Steffen,~K.: Vertical fluxes of \chemNO_x, HONO, and \chemHNO_3 above the snowpack at Summit, Greenland, Atmos. Environ., 36, 2629–2640, 2002. Hu,~M., Zhou,~F M., Shao,~K S., Zhang,~Y H., Tang,~X Y., and Slanina,~J.: Diurnal variations of aerosol chemical compositions and related gaseous pollutants in Beijing and Guangzhou,~J. Environ. Sci. Heal. A., 37, 479–488, 2002. Huang,~G., Zhou,~X L., Deng,~G H., Qiao,~H C., and Civerolo,~K.: Measurements of atmospheric nitrous acid and nitric acid, Atmos. Environ., 36, 2225–2235, 2002. IPCC: Climate Change 2001: The Scientific Basis: Contributation of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge,~UK, 2001. Israelachvili,~J N. and Pashley,~R M.: Molecular layering of water at surfaces and origin of repulsive hydration forces, Nature, 306, 249–250, 1983. Kanda,~Y. and Taira,~M.: Chemiluminescent method for continuous monitoring of nitrous-acid in ambient air, Anal. Chem., 62, 2084–2087, 1990. Kang,~C M., Kang,~B W., and Lee,~H S.: Source identification and trends in concentrations of gaseous and fine particulate principal species in Seoul, South Korea,~J. Air Waste Manag., 56, 911–921, 2006. Karagulian,~F. and Rossi,~M J.: Heterogeneous chemistry of the \chemNO_3 free radical and \chemN_2O_5 on decane flame soot at ambient temperature: reaction products and kinetics,~J. Phys. Chem. A., 111, 1914–1926, 2007. Kirchner,~U., Scheer,~V., and Vogt,~R.: FTIR spectroscopic investigation of the mechanism and kinetics of the heterogeneous reactions of \chemNO_2 and H\chemNO_3 with soot,~J. Phys. Chem. A., 104, 8908–8915, 2000. Kirchner,~W., Welter,~F., Bongartz,~A., Kames,~J., Schweighoefer,~S., and Schurath,~U.: Trace gas exchange at the air/water interface: measurements of mass accommodation coefficients,~J. Atmos. Chem., 10, 427–449, 1990. Kirchstetter,~T W., Harley,~R A., and Littlejohn,~D.: Measurement of nitrous acid in motor vehicle exhaust, Environ. Sci. Technol., 30, 2843–2849, 1996. Kleffmann,~J., Becker,~K H., and Wiesen,~P.: Heterogeneous \chemNO_2 conversion processes on acid surfaces: possible atmospheric implications, Atmos. Environ., 32, 2721–2729, 1998a. Kleffmann,~J., Becker,~K H., and Wiesen,~P.: Investigation of the heterogeneous \chemNO_2 conversion on perchloric acid surfaces,~J. Chem. Soc. Faraday T., 94, 3289–3292, 1998b. Kleffmann,~J., Heland,~J., Kurtenbach,~R., Lorzer,~J., and Wiesen,~P.: A~new instrument (LOPAP) for the detection of nitrous acid (HONO), Environ. Sci. Poll. Res, 48–54, 2002. Kleffmann, J. and Wiesen, P.: Heterogeneous conversion of NO<sub>2</sub> and NO on HNO<sub>3</sub> treated soot surfaces: atmospheric implications, Atmos. Chem. Phys., 5, 77–83, 2005. Kleffmann,~J.: Daytime sources of nitrous acid (HONO) in the atmospheric boundary layer, Chem. Phys. Chem., 8, 1137–1144, 2007. Kondo,~A., Shrestha,~M., Kaga,~A., Inoue,~Y., and Sapkota,~B.: Comparison of field observation with water tank experiment on air pollution concentration in Katmandu valley, Adv. Air Poll., 11, 493–502, 2002. Kurtenbach,~R., Becker,~K H., Gomes,~J A G., Kleffmann,~J., Lorzer,~J C., Spittler,~M., Wiesen,~P., Ackermann,~R., Geyer,~A., and Platt,~U.: Investigations of emissions and heterogeneous formation of HONO in a~road traffic tunnel, Atmos. Environ., 35, 3385–3394, 2001. Labban,~R., Veranth,~J M., Chow,~J C., Engelbrecht,~J L P., and Watson,~J G.: Size and geographical variation in PM$_1$, PM$_2.5$ and PM$_10$: source profiles from soils in the western United States, Water Air Soil Poll., 157, 13–31, 2004. Lahoutifard, N., Ammann, M., Gutzwiller, L., Ervens, B., and George, C.: The impact of multiphase reactions of NO<sub>2</sub> with aromatics: a modelling approach, Atmos. Chem. Phys., 2, 215–226, 2002. Lammel,~G. and Perner,~D.: The Atmospheric aerosol as a~source of nitrous-acid in the polluted atmosphere,~J. Aerosol Sci., 19, 1199–1202, 1988. Lammel,~G. and Cape,~J N.: Nitrous acid and nitrite in the atmosphere, Chem. Soc. Rev., 25, 361–369, 1996. Lammel,~G.: Formation of Nitrous Acid: Parameterization and Comparison with Observations, Max-Planck-Institut für Meteorologie, Hamburg, Germany, 286, 1–36, 1999. Lee,~J H. and Tang,~I N.: Accommodation coefficient of gaseous \chemNO_2 on water surfaces, Atmos. Environ., 22, 1147–1151, 1988. Li,~C S. and Lin,~C H.: PM$_1$/PM$_2.5$/PM$_10$ characteristics in the urban atmosphere of Taipei, Aerosol Sci. Tech., 36, 469–473, 2002. Liao,~W., Case,~A T., Mastromarino,~J., Tan,~D., and Dibb,~J E.: Observations of HONO by laser-induced fluorescence at the South Pole during ANTCI 2003, Geophys. Res. Lett., 33, L09810, doi:09810.01029/02005GL025470, 2006a. Liao,~W., Hecobian,~A., Mastromarino,~J., and Tan,~D.: Development of a~photo-fragmentation/laser-induced fluorescence measurement of atmospheric nitrous acid, Atmos. Environ., 40, 17–26, 2006b. Liu,~Y S., Chen,~R., Shen,~X X., and Mao,~X L.: Wintertime indoor air levels of PM$_10$, PM$_2.5$ and PM$_1$ at public places and their contributions to TSP, Environ. Int., 30, 189–197, 2004. Livingston,~J M. and Russell,~P B.: Retrieval of aerosol size distribution moments from multiwavelength particulate extinction measurements,~J. Geophys. Res., 94, 8425–8433, 1989. Longfellow,~C A., Ravishankara,~A R., and Hanson,~D R.: Reactive uptake on hydrocarbon soot: focus on \chemNO_2,~J. Geophys. Res., 104, 13 833–13 840, 1999. Matsumoto,~M. and Okita,~T.: Long term measurements of atmospheric gaseous and aerosol species using an annular denuder system in Nara, Japan, Atmos. Environ., 32, 1419–1425, 1998. Mertes,~S. and Wahner,~A.: Uptake of nitrogen-dioxide and nitrous-acid on aqueous surfaces,~J. Phys. Chem., 99, 14 000–14 006, 1995. Miranda,~P B., Xu,~L., Shen,~Y R., and Salmeron,~M.: Icelike water monolayer adsorbed on mica at room temperature, Phys. Rev. Lett., 81, 5876–5879, 1998. Msibi,~I M., Li,~Y., Shi,~J P., and Harrison,~R M.: Determination of heterogeneous reaction probability using deposition profile measurement in an annular reactor: application to the \chemN_2O_5/\chemH_2O reaction,~J. Atmos. Chem., 18, 291–300, 1994. Nienow,~A M. and Roberts,~J T.: Heterogeneous chemistry of carbon aerosols, Annu. Rev. Phys. Chem., 57, 105–128, 2006. Notholt,~J. and Raes,~F.: Test of insitu measurements of atmospheric aerosols and trace gases by long path transmission spectroscopy,~J. Aerosol Sci., 21, S193–S196, 1990. Notholt,~J., Hjorth,~J., and Raes,~F.: Long path field-measurements of aerosol parameters and trace gas concentrations – formation of nitrous-acid during foggy periods,~J. Aerosol. Sci., 22, S411–S414, 1991. Notholt,~J., Hjorth,~J., and Raes,~F.: Formation of \chemHNO_2 on aerosol surfaces during foggy periods in the presence of NO and \chemNO_2, Atmos. Environ., 26, 211–217, 1992a. Notholt,~J., Hjorth,~J., Raes,~F., and Schrems,~O.: Simultaneous long path field-measurements of \chemHNO_2, \chemCH_2O and aerosol, Ber. Bunsen. Phys. Chem., 96, 290–293, 1992b. Opitz,~A., Scherge,~M., Ahmed,~S I U., and Schaefer,~J A.: A~comparative investigation of thickness measurements of ultra-thin water films by scanning probe techniques,~J. Appl. Phys., 101, 064310, doi:10.1063/1061.2712155, 2007. Panday,~A.: The Diurnal cycle of air pollution in the Kathmandu valley, Nepal, Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA, Page 100 pp., 2006. Park,~S S., Hong,~S B., Jung,~Y G., and Lee,~J H.: Measurements of PM$_10$ aerosol and gas-phase nitrous acid during fall season in a~semi-urban atmosphere, Atmos. Environ., 38, 4265–4265, 2004. Perner,~D. and Platt,~U.: Detection of nitrous-acid in the atmosphere by differential optical-absorption, Geophys. Res. Lett., 6, 917–920, 1979. Pitz,~M., Cyrys,~J., Karg,~E., Wiedensohler,~A., Wichmann,~H.-E., and Heinrich,~J.: Variability of apparent particle density of an urban aerosol, Environ. Sci. Technol., 37, 4336–4342, 2003. Ponche,~J L., George,~C., and Mirabel,~P.: Mass transfer at the air/water interface: mass accommodation coefficients of \chemSO_2, \chemHNO_3, \chemNO_2 and \chemNH_3,~J. Atmos. Chem., 16, 1–21, 1993. Qin,~M., Xie,~P H., Liu,~W Q., Li,~A., Dou,~K., Fang,~W., Liu,~H G., and Zhang,~W J.: Observation of atmospheric nitrous acid with DOAS in Beijing, China,~J. Environ. Sci., 18, 69–75, 2006. Ramanathan,~V., Li,~F., Ramana,~M V., Praveen,~P S., Kim,~D., Corrigan,~C E., Nguyen,~H., Stone,~E A., Schauer,~J J., Carmichael,~G R., Adhikary,~B., and Yoon,~S C.: Atmospheric brown clouds: hemispherical and regional variations in long-range transport, absorption, and radiative forcing,~J. Geophys. Res., 112, D22S21, doi:10.1029/2006JD008124, 2007. Ramazan,~K A., Wingen,~L M., Miller,~Y., Chaban,~G M., Gerber,~R B., Xantheas,~S S., and Finlayson-Pitts,~B J.: New experimental and theoretical approach to the heterogeneous hydrolysis of \chemNO_2: key role of molecular nitric acid and its complexes,~J. Phys. Chem. A, 110, 6886–6897, 2006. Regmi,~R P., Kitada,~T., and Kurata,~G.: Numerical simulation of late wintertime local flows in Kathmandu valley, Nepal: Implication for air pollution transport,~J. Appl. Meteorol., 42, 389–403, 2003. Reisinger,~A R.: Observations of \chemHNO_2 in the polluted winter atmosphere: possible heterogeneous production on aerosols, Atmos. Environ., 34, 3865–3874, 2000. Rohrer, F., Bohn, B., Brauers, T., Brüning, D., Johnen, F.-J., Wahner, A., and Kleffmann, J.: Characterisation of the photolytic HONO-source in the atmosphere simulation chamber SAPHIR, Atmos. Chem. Phys., 5, 2189–2201, 2005. Sapkota,~B. and Dhaubhadel,~R.: Atmospheric turbidity over Kathmandu valley, Atmos. Environ., 36, 1249–1257, 2002. Schuttlefield,~J D., Cox,~D., and Grassian,~V H.: An investigation of water uptake on clays minerals using ATR-FTIR spectroscopy coupled with quartz crystal microbalance measurements,~J. Geophys. Res., 112, D21303, doi:10.1029/2007JD008973, 2007. Seinfeld,~J H. and Pandis,~S N.: Atmospheric chemistry and physics: from air pollution to climate change, Wiley & Sons, New York, 250–253, 1998. Seisel,~S., Lian,~Y., Keil,~T., Trukhin,~M E., and Zellner,~R.: Kinetics of the interaction of water vapour with mineral dust and soot surfaces at $T=298 \unitK$, Phys. Chem. Chem. Phys., 6, 1926–1932, 2004. Seisel,~S., Pashkova,~A., Lian,~Y., and Zellner,~R.: Water uptake on mineral dust and soot: a~fundamental view of the hydrophilicity of atmospheric particles?, Faraday Discuss., 130, 437–451, 2005. Sharma,~C K.: Urban air quality of Kathmandu valley \qutKingdom of Nepal, Atmos. Environ., 31, 2877–2883, 1997. Sharma,~T., Rainey,~R C., Neumann,~C M., Shrestha,~I L., Shahi,~K B., Shakya,~A., and Khatri,~S.: Roadside particulate levels at 30 locations in the Kathmandu Valley, Nepal, Int J. Environ. Pollut., 17, 293–305, 2002. Spindler,~G., Muller,~K., Bruggemann,~E., Gnauk,~T., and Herrmann,~H.: Long-term size-segregated characterization of PM$_10$, PM$_2.5$, and PM$_1$ at the IfT research station Melpitz downwind of Leipzig (Germany) using high and low-volume filter samplers, Atmos. Environ., 38, 5333–5347, 2004. Stemmler,~K., Ammann,~M., Donders,~C., Kleffmann,~J., and George,~C.: Photosensitized reduction of nitrogen dioxide on humic acid as a~source of nitrous acid, Nature, 440, 195–198, 2006. Stemmler, K., Ndour, M., Elshorbany, Y., Kleffmann, J., D'Anna, B., George, C., Bohn, B., and Ammann, M.: Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol, Atmos. Chem. Phys., 7, 4237–4248, 2007. Stockwell,~W R. and Calvert,~J G.: The Mechanism of \chemNO_3 and HONO formation in the nighttime chemistry of the urban atmosphere,~J. Geophys. Res., 88, 6673–6682, 1983. Stutz,~J., Alicke,~B., Ackermann,~R., Geyer,~A., Wang,~S H., White,~A B., Williams,~E J., Spicer,~C W., and Fast,~J D.: Relative humidity dependence of HONO chemistry in urban areas,~J. Geophys. Res., 109, D03307, doi:10.1029/2003JD004135, 2004. Sumner,~A L., Menke,~E J., Dubowski,~Y., Newberg,~J T., Penner,~R M., Hemminger,~J C., Wingen,~L M., Brauers,~T., and Finlayson-Pitts,~B J.: The Nature of water on surfaces of laboratory systems and implications for heterogeneous chemistry in the troposphere, Phys. Chem. Chem. Phys., 6, 604–613, 2004. Syomin,~D A. and Finlayson-Pitts,~B J.: HONO decomposition on borosilicate glass surfaces: implications for environmental chamber studies and field experiments, Phys. Chem. Chem. Phys., 5, 5236–5242, 2003. Takenaka,~N., Terada,~H., Oro,~Y., Hiroi,~M., Yoshikawa,~H., Okitsu,~K., and Bandow,~H.: A~new method for the measurement of trace amounts of HONO in the atmosphere using an air-dragged aqua-membrane-type denuder and fluorescence detection, Analyst, 129, 1130–1136, 2004. Trebs, I., Meixner, F. X., Slanina, J., Otjes, R., Jongejan, P., and Andreae, M. O.: Real-time measurements of ammonia, acidic trace gases and water-soluble inorganic aerosol species at a rural site in the Amazon Basin, Atmos. Chem. Phys., 4, 967–987, 2004. Trick,~S.: Formation of nitrous acid on urban surfaces: a~physical chemical perspective, Fakultät für Naturwissenschaften und Mathematik, University of Heidelberg, Heidelberg, p 290, 2004. Vallius,~M J., Ruuskanen,~J., Mirme,~A., and Pekkanen,~J.: Concentrations and estimated soot content of PM$_1$, PM$_2.5$, and PM$_10$ in a~subarctic urban atmosphere, Environ. Sci. Technol., 34, 1919–1925, 2000. Vecera,~Z. and Dasgupta,~P K.: Measurement of ambient nitrous-acid and a~reliable calibration source for gaseous nitrous-acid, Environ. Sci. Technol., 25, 255–260, 1991. Vogel,~B., Vogel,~H., Kleffmann,~J., and Kurtenbach,~R.: Measured and simulated vertical profiles of nitrous acid – Part II. model simulations and indications for a~photolytic source, Atmos. Environ., 37, 2957–2966, 2003. Wang,~S H., Ackermann,~R., Spicer,~C W., Fast,~J D., Schmeling,~M., and Stutz,~J.: Atmospheric observations of enhanced \chemNO_2-HONO conversion on mineral dust particles, Geophys. Res. Lett., 30, 1595, doi:10.1029/2003GL017014, 2003. World Bank: Urban air quality management strategy in Asia (URBAIR), Kathmandu valley., Technical paper 378, 100 pp., 1997. Wormhoudt,~J., Herndon,~S C., Yelvington,~P E., Miake-Lye,~R C., and Wey,~C.: Nitrogen oxide (NO/\chemNO_2/HONO) emissions measurements in aircraft exhausts,~J. Propul. Power, 23, 906–911, 2007. Yu,~Y., Geyer,~A., Xie,~P H., Galle,~B., Chen,~L M., and Platt,~U.: Observations of carbon disulfide by differential optical absorption spectroscopy in Shanghai, Geophys. Res. Lett., 31, L11107, doi:11110.11029/GL019543, 2004. Yu,~Y., Panday,~A., Hodson,~E., Galle,~B., and Prinn~R.: Monocyclic aromatic hydrocarbons in Kathmandu during the winter season, Water Air Soil Poll., 191, 71–81, 2008a. Yu,~Y., Ezell,~M J., Zelenyuk,~A., Imre,~D., Alexander,~M L., Ortega,~J., D'Anna,~B., Harmon,~C W., Johnson,~S N., and Finlayson-Pitts,~B J.: Photooxidation of $\alpha $-pinene at high relative humidity in the presence of increasing concentrations of \chemNO_x, Atmos. Environ., 42, 5044–5060, 2008b. Zhang, J., Wang, T., Chameides, W. L., Cardelino, C., Kwok, J., Blake, D. R., Ding, A., and So, K. L.: Ozone production and hydrocarbon reactivity in Hong Kong, Southern China, Atmos. Chem. Phys., 7, 557–573, 2007. Zhou,~X., Civerolo,~K., Dai,~H., Huang,~G., Schwab,~J., and Demerjian,~K.: Summertime nitrous acid chemistry in the atmospheric boundary layer at a~rural site in New York State,~J. Geophys. Res., 107, 4590, doi:10.1029/2001JD001539, 2002. Zhou,~X L., Beine,~H J., Honrath,~R E., Fuentes,~J D., Simpson,~W., Shepson,~P B., and Bottenheim,~J W.: Snowpack photochemical production of HONO: a~major source of OH in the Arctic boundary layer in springtime, Geophys. Res. Lett., 28, 4087–4090, 2001. Zhou,~X L., Huang,~G., Civerolo,~K., Roychowdhury,~U., and Demerjian,~K L.: Summertime observations of HONO, HCHO, and \chemO_3 at the summit of Whiteface Mountain, New York,~J. Geophys. Res., 112, D08311, doi:10.1029/2006JD007256, 2007.