ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-6969-2010 Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective Volkamer R. 1 2 3 Sheehy P. 1 4 Molina L. T. 1 4 Molina M. J. 1 2 Department of Earth, Atmospheric and Planetary Sciences, Massachussetts Institute of Technology, Cambridge, MA, USA Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, USA Department of Chemistry and Biochemistry and CIRES, University of Colorado at Boulder, Boulder, CO, USA Molina Center for Energy and the Environment (MCE<sup>2</sup>), La Jolla, CA, USA 30 07 2010 10 14 6969 6991 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.net/10/6969/2010/acp-10-6969-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/6969/2010/acp-10-6969-2010.pdf

A detailed analysis of OH, HO<sub>2</sub> and RO<sub>2</sub> radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify time-resolved RO<sub>x</sub> (sum of OH, HO<sub>2</sub>, RO<sub>2</sub>) radical production rates from day- and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O<sub>3</sub>), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH- and NO<sub>3</sub> radicals, O<sub>3</sub>; and (4) NO, NO<sub>2</sub>, meteorological and other parameters. The RO<sub>x</sub> production rate was calculated directly from these observations; the MCM was used to estimate further RO<sub>x</sub> production from unconstrained sources, and express overall RO<sub>x</sub> production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO<sub>2</sub> and HO<sub>2</sub> radicals into OH radicals). <br><br> Daytime radical production is found to be about 10–25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: Oxygenated VOC other than HCHO about 33%; HCHO and O<sub>3</sub> photolysis each about 20%; O<sub>3</sub>/alkene reactions and HONO photolysis each about 12%, other sources <3%. Nitryl chloride photolysis could potentially contribute ~15% additional radicals, while NO<sub>2</sub>* + water makes – if any – a very small contribution (~2%). The peak radical production of ~7.5 10<sup>7</sup> molec cm<sup>−3</sup> s<sup>−1</sup> is found already at 10:00 a.m., i.e., more than 2.5 h before solar noon. O<sub>3</sub>/alkene reactions are indirectly responsible for ~33% of these radicals. Our measurements and analysis comprise a database that enables testing of the representation of radical sources and radical chain reactions in photochemical models. <br><br> Since the photochemical processing of pollutants in the MCMA is radical limited, our analysis identifies the drivers for ozone and SOA formation. We conclude that reductions in VOC emissions provide an efficient opportunity to reduce peak concentrations of these secondary pollutants, because (1) about 70% of radical production is linked to VOC precursors; (2) lowering the VOC/NO<sub>x</sub> ratio has the further benefit of reducing the radical re-cycling efficiency from radical chain reactions (chemical amplification of radical sources); (3) a positive feedback is identified: lowering the rate of radical production from organic precursors also reduces that from inorganic precursors, like ozone, as pollution export from the MCMA caps the amount of ozone that accumulates at a lower rate inside the MCMA. Continued VOC reductions will in the future result in decreasing peak concentrations of ozone and SOA in the MCMA.

 References Acker, K., Moller, D., Wieprecht, W., Meixner, F. X., Bohn, B., Gilge, S., Plass-Dulmer, C., and Berresheim, H.: Strong daytime production of OH from HNO<sub>2</sub> at a rural mountain site, Geophys. Res. Lett., 33, L02809, doi:10.1029/2005GL024643, 2006. Alicke, B., Geyer, A., Hofzumahaus, A., Holland, F., Konrad, S., Patz, H. W., Schafer, J., Stutz, J., Volz-Thomas, A., and Platt, U.: OH formation by HONO photolysis during the BERLIOZ experiment, J. Geophys. Res.-Atmos., 108, 8247–8263, 2003. Alicke, B., Platt, U., and Stutz, J.: Impact of nitrous acid photolysis on the total hydroxyl radical budget during the Limitation of Oxidant Production/Pianura Padana Produzione di Ozono study in Milan, J. Geophys. Res.-Atmos., 107, 8196, doi:10.1029/2000JD000075, 2002. Apel, E. C., Emmons, L. K., Karl, T., Flocke, F., Hills, A. J., Madronich, S., Lee-Taylor, J., Fried, A., Weibring, P., Walega, J., Richter, D., Tie, X., Mauldin, L., Campos, T., Weinheimer, A., Knapp, D., Sive, B., Kleinman, L., Springston, S., Zaveri, R., Ortega, J., Voss, P., Blake, D., Baker, A., Warneke, C., Welsh-Bon, D., de Gouw, J., Zheng, J., Zhang, R., Rudolph, J., Junkermann, W., and Riemer, D. D.: Chemical evolution of volatile organic compounds in the outflow of the Mexico City Metropolitan area, Atmos. Chem. Phys., 10, 2353–2375, 2010. Arens, F., Gutzwiller, L., Baltensperger, U., Gaggeler, H. W., and Ammann, M.: Heterogeneous reaction of NO<sub>2</sub> on diesel soot particles, Environ. Sci. Technol., 35, 2191–2199, 2001. Arnold, S. R., Chipperfield, M. P., Blitz, M. A., Heard, D. E., and Pilling, M. J.: Photodissociation of acetone: Atmospheric implications of temperature-dependent quantum yields, Geophys. Res. Lett., 31, L07110, doi:10.1029/2003GL019099, 2004. Arriaga-Colina, J. L., West, J. J., Sosa, G., Escalona, S. S., Ordunez, R. M., and Cervantes, A. D. M.: Measurements of VOCs in Mexico City (1992–2001) and evaluation of VOCs and CO in the emissions inventory, Atmos. Environ., 38, 2523–2533, 2004. Atkinson, R., Aschmann, S. M., Winer, A. M., and Pitts, J. N.: Gas-Phase Reaction of NO<sub>2</sub> with Alkenes and Dialkenes, Int. J. Chem. Kinet., 16, 697–706, 1984. Bass, A. M. and Paur, R. J.: Uv Absorption Cross-Sections for Ozone – the Temperature-Dependence, J. Photochem., 17, p 141, 1981. Bell, M. L., McDermott, A., Zeger, S. L., Samet, J. M., and Dominici, F.: Ozone and short-term mortality in 95 US urban communities, 1987–2000, Jama-J. Am. Med. Assoc., 292, 2372–2378, 2004. Behnke, W., George, C., Scheer, V., and Zetzsch, C.: Production and decay of ClNO2, from the reaction of gaseous N<sub>2</sub>O$_5$ with NaCl solution: Bulk and aerosol experiments, J. Geophys. Res., 102, 3795–3804, 1997. Bernath, P., Carleer, M., Fally, S., Jenouvrier, A., Vandaele, A. C., Hermans, C., Merienne, M. F., and Colin, R.: The Wulf bands of oxygen, Chem. Phys. Lett., 297, 293–299, 1998. Bloss, C., Wagner, V., Bonzanini, A., Jenkin, M. E., Wirtz, K., Martin-Reviejo, M., and Pilling, M. J.: Evaluation of detailed aromatic mechanisms (MCMv3 and MCMv3.1) against environmental chamber data, Atmos. Chem. Phys., 5, 623–639, 2005a. Bloss, C., Wagner, V., Jenkin, M. E., Volkamer, R., Bloss, W. J., Lee, J. D., Heard, D. E., Wirtz, K., Martin-Reviejo, M., Rea, G., Wenger, J. C., and Pilling, M. J.: Development of a detailed chemical mechanism (MCMv3.1) for the atmospheric oxidation of aromatic hydrocarbons, Atmos. Chem. Phys., 5, 641–664, 2005b. Brown, S. S., Osthoff, H. D., Stark, H., Dube, W. P., Ryerson, T. B., Warneke, C., de Gouw, J. A., Wollny, A. G., Parrish, D. D., Fehsenfeld, F. C., and Ravishankara, A. R.: Aircraft observations of daytime NO3 and N<sub>2</sub>O$_5$ and their implications for tropospheric chemistry, J. Photoch. Photobio. A, 176, 270–278, 2005. Brown, S. S., Ryerson, T. B., Wollny, A. G., Brock, C. A., Peltier, R., Sullivan, A. P., Weber, R. J., Dube, W. P., Trainer, M., Meagher, J. F., Fehsenfeld, F. C., and Ravishankara, A. R.: Variability in nocturnal nitrogen oxide processing and its role in regional air quality, Science, 311, 67–70, 2006. Cantrell, C. A., Davidson, J. A., Mcdaniel, A. H., Shetter, R. E., and Calvert, J. G.: Temperature-Dependent Formaldehyde Cross-Sections in the Near-Ultraviolet Spectral Region, J. Phys. Chem.-US, 94, 3902–3908, 1990. Carter, W. P. L.: Environmental Chamber Studies of Ozone Formation Potentials of Volatile Organic Compounds, in: Environmental Simulation Chambers: Application to Atmospheric Chemical Processes, edited by:Rudzinski, K. and Barnes, I., Proceedings of the NATO Advanced Research Workshop, Zakopane, Poland 1–4 October, 2004, NATO Sciences Series, IV. Earth and Environmental Sciences. Vol. 62. Kluwer Publishing, Doortrecht, The Netherlands, 2005. Davis, D. L., Bell, M. L., and Fletcher, T.: A look back at the London smog of 1952 and the half century since, Environ. Health Persp., 110, A734–A735, 2002. de Foy, B., Varela, J. R., Molina, L. T., and Molina, M. J.: Rapid ventilation of the Mexico City basin and regional fate of the urban plume, Atmos. Chem. Phys., 6, 2321–2335, 2006. Donaldson, D. J., Frost, G. J., Rosenlof, K. H., Tuck, A. F., and Vaida, V.: Atmospheric radical production by excitation of vibrational overtones via absorption of visible light, Geophys. Res. Lett., 24, 2651–2654, 1997. Dunlea, E. J., Herndon, S. C., Nelson, D. D., Volkamer, R. M., Lamb, B. K., Allwine, E. J., Grutter, M., Ramos Villegas, C. R., Marquez, C., Blanco, S., Cardenas, B., Kolb, C. E., Molina, L. T., and Molina, M. J.: Technical note: Evaluation of standard ultraviolet absorption ozone monitors in a polluted urban environment, Atmos. Chem. Phys., 6, 3163–3180, 2006. Dusanter, S., Vimal, D., Stevens, P. S., Volkamer, R., Molina, L. T., Baker, A., Meinardi, S., Blake, D., Sheehy, P., Merten, A., Zhang, R., Zheng, J., Fortner, E. C., Junkermann, W., Dubey, M., Rahn, T., Eichinger, B., Lewandowski, P., Prueger, J., and Holder, H.: Measurements of OH and HO2 concentrations during the MCMA-2006 field campaign – Part 2: Model comparison and radical budget, Atmos. Chem. Phys., 9, 6655–6675, 2009. Dzepina, K., Volkamer, R. M., Madronich, S., Tulet, P., Ulbrich, I. M., Zhang, Q., Cappa, C. D., Ziemann, P. J., and Jimenez, J. L.: Evaluation of recently-proposed secondary organic aerosol models for a case study in Mexico City, Atmos. Chem. Phys., 9, 5681–5709, 2009. Emmerson, K. M., Carslaw, N., and Pilling, M. J.: Urban atmospheric chemistry during the PUMA campaign 2: Radical budgets for OH, HO<sub>2</sub> and RO<sub>2</sub>, J. Atmos. Chem., 52, 165–183, 2005. Emmerson, K. M., Carslaw, N., Carslaw, D. C., Lee, J. D., McFiggans, G., Bloss, W. J., Gravestock, T., Heard, D. E., Hopkins, J., Ingham, T., Pilling, M. J., Smith, S. C., Jacob, M., and Monks, P. S.: Free radical modelling studies during the UK TORCH Campaign in Summer 2003, Atmos. Chem. Phys., 7, 167–181, 2007. Fally, S., Vandaele, A. C., Carleer, M., Hermans, C., Jenouvrier, A., Merienne, M. F., Coquart, B., and Colin, R.: Fourier transform spectroscopy of the O-2 Herzberg bands. III. Absorption cross sections of the collision-induced bands and of the Herzberg continuum, J. Mol. Spectrosc., 204, 10–20, 2000. Fayt, C. and van Roozendael, M.: WinDoas 2.1 – Software User Manual, Brussels: BIRA-IASB, 2001. Finlayson-Pitts, B. J., Ezell, M. J., and Pitts Jr., J. N.: Formation of Chemically Active Chlorine Compounds by Reactions of Atmospheric NaCl Particles with Gaseous N<sub>2</sub>O$_5$ and ClONO<sub>2</sub>, Nature, 337, 241–244, doi:10.1038/337241a0, 1989. Fuhrer, J. and Booker, F.: Ecological issues related to ozone: agricultural issues, Environ. Int., 29, 141–154, 2003. Frost, G. J., Trainer, M., Mauldin, R. L., Eisele, F. L., Prevot, A. S. H., Flocke, S. J., Madronich, S., Kok, G., Schillawski, R. D., Baumgardner, D., and Bradshaw, J.: Photochemical modeling of OH levels during the first aerosol characterization experiment (ACE 1), J. Geophys. Res.-Atmos., 104, 16041–16052, 1999. Garcia, A. R., Volkamer, R., Molina, L. T., Molina, M. J., Samuelson, J., Mellqvist, J., Galle, B., Herndon, S. C., and Kolb, C. E.: Separation of emitted and photochemical formaldehyde in Mexico City using a statistical analysis and a new pair of gas-phase tracers, Atmos. Chem. Phys., 6, 4545–4557, 2006. Geyer, A., Alicke, B., Ackermann, R., Martinez, M., Harder, H., Brune, W., de Carlo, P., Williams, E., Jobson, T., Hall, S., Shetter, R., and Stutz, J.: Direct observations of daytime NO3: Implications for urban boundary layer chemistry, J. Geophys. Res.-Atmos., 108, 4368, doi:10.1029/2002JD002967, 2003. Gomer, T., Brauers, T., Heintz, F., Stutz, J., and Platt, U.: MFC User Manual, Version 1.98, Institut fuer Umweltphysik, University of Heidelberg, 1993. Gregg, J. W., Jones, C. G., and Dawson, T. E.: Urbanization effects on tree growth in the vicinity of New York City, Nature, 424, 183–187, 2003. Griffin, R. J.: Modeling the oxidative capacity of the atmosphere of the South Coast Air Basin of California. 2. HOx radical production, Environ. Sci. Technol. 38, 753–757, 2004. Haagen-Smit, A. J., Darley, E. F., Zaitlin, M., Hull, H., and Nobel, W.: Investigation on injury to plants from air pollution in the Los Angeles area, Plant Physiol., 27, p 18, 1951. Hasson, A. S., Tyndall, G. S., and Orlando, J. J.: A product yield study of the reaction of HO2 radicals with ethyl peroxy (C2H5O2), acetyl peroxy (CH3C(O)O-2), and acetonyl peroxy (CH3C(O)CH2O2) radicals, J. Phys. Chem. A, 108, 5979–5989, 2004. Health Effects Institute: Reanalysis of the Harvard Six Cities Study and the American Cancer Society Study of Particulate Air Pollution and Mortality: A Special Report of the Institute's Particle Epidemiology Reanalysis Project. Library of Congress Catalog Number for the HEI Report Series: WA 754 R432, Health Effects Institute, Cambridge, MA, 2000. IPCC: A Report of Working Group I of the Intergovernmental Panel on Climate Change: Summary for Policymakers, online available at: http://www.ipcc.ch/, 2007. Jauregui, E.: Mexico City's urban heat island revisited, Erdkunde, 47, 185–195, 1993. Jenkin, M. E., Andersen, M. P. S., Hurley, M. D., Wallington, T. J., Taketani, F., and Matsumi, Y.: A kinetics and mechanistic study of the OH and NO2 initiated oxidation of cyclohexa-1,3-diene in the gas phase, Phys. Chem. Chem. Phys., 7, 1194–1204, 2005. Jenkin, M. E., Saunders, S. M., Wagner, V., and Pilling, M. J.: Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part B): tropospheric degradation of aromatic volatile organic compounds, Atmos. Chem. Phys., 3, 181–193, 2003. Jenouvrier, A., Merienne, M. F., Coquart, B., Carleer, M., Fally, S., Vandaele, A. C., Hermans, C., and Colin, R.: Fourier transform spectroscopy of the O-2 Herzberg bands – I. Rotational analysis, J. Mol. Spectrosc., 198, 136–162, 1999. Jobson, B. T., Volkamer, R. A., Velasco, E., Allwine, G., Westberg, H., Lamb, B. K., Alexander, M. L., Berkowitz, C. M., and Molina, L. T.: Comparison of aromatic hydrocarbon measurements made by PTR-MS, DOAS and GC-FID during the MCMA 2003 Field Experiment, Atmos. Chem. Phys., 10, 1989–2005, 2010. Kleffmann, J., Gavriloaiei, T., Hofzumahaus, A., Holland, F., Koppmann, R., Rupp, L., Schlosser, E., Siese, M., and Wahner, A.: Daytime formation of nitrous acid: A major source of OH radicals in a forest, Geophys. Res. Lett., 32, L05818, doi:10.1029/2005GL022524, 2005. Kleffmann, J., Lorzer, J. C., Wiesen, P., Kern, C., Trick, S., Volkamer, R., Rodenas, M., and Wirtz, K.: Intercomparison of the DOAS and LOPAP techniques for the detection of nitrous acid (HONO), Atmos. Environ., 40, 3640–3652, 2006. Kraus, A., Rohrer, F., and Hofzumahaus, A.: Intercomparison of NO2 photolysis frequency measurements by actinic flux spectroradiometry and chemical actinometry during JCOM97, Geophys. Res. Lett., 27, 1115–1118, 2000. Kurtenbach, R., Ackermann, R., Becker, K. H., Geyer, A., Gomes, J. A. G., Lorzer, J. C., Platt, U., and Wiesen, P.: Verification of the contribution of vehicular traffic to the total NMVOC emissions in Germany and the importance of the NO<sub>3</sub> chemistry in the city air, J. Atmos. Chem., 42, 395–411, 2002. Kwan, A. J., Crounse, J. D., Clarke, A. D., Shinozuka, Y., Anderson, B. E., Crawford, J. H., Avery, M. A., McNaughton, C. S., Brune, W. H., Singh, H. B., and Wennberg, P. O.: On the flux of oxygenated volatile organic compounds from organic aerosol oxidation, Geophys. Res. Lett., 33, L15815, doi:10.1029/2006GL026144, 2006. Lei, W., de Foy, B., Zavala, M., Volkamer, R., and Molina, L. T.: Characterizing ozone production in the Mexico City Metropolitan Area: a case study using a chemical transport model, Atmos. Chem. Phys., 7, 1347–1366, 2007. Lei, W., Zavala, M., de Foy, B., Volkamer, R., and Molina, L. T.: Characterizing ozone production and response under different meteorological conditions in Mexico City, Atmos. Chem. Phys., 8, 7571–7581, 2008. Lei, W., Zavala, M., de Foy, B., Volkamer, R., Molina, M. J., and Molina, L. T.: Impact of primary formaldehyde on air pollution in the Mexico City Metropolitan Area, Atmos. Chem. Phys., 9, 2607–2618, 2009. Li, G., Lei, W., Zavala, M., Volkamer, R., Dusanter, S., Stevens, P., and Molina, L. T.: Impacts of HONO sources on the photochemistry in Mexico City during the MCMA-2006/MILAGO Campaign, Atmos. Chem. Phys. Discuss., 10, 4143–4188, 2010. Li, S. P., Matthews, J., and Sinha, A.: Atmospheric hydroxyl radical production from electronically excited NO<sub>2</sub> and H<sub>2</sub>O, Science, 319, 1657–1660, 2008. Martinez, M., Harder, H., Kovacs, T. A., Simpas, J. B., Bassis, J., Lesher, R., Brune, W. H., Frost, G. J., Williams, E. J., Stroud, C. A., Jobson, B. T., Roberts, J. M., Hall, S. R., Shetter, R. E., Wert, B., Fried, A., Alicke, B., Stutz, J., Young, V. L., White, A. B., and Zamora, R. J.: OH and HO2 concentrations, sources, and loss rates during the Southern Oxidants Study in Nashville, Tennessee, summer 1999, J. Geophys. Res.-Atmos., 108, 4617–4633, 2003. Merienne, M. F., Jenouvrier, A., Coquart, B., Carleer, M., Fally, S., Colin, R., Vandaele, A. C., and Hermans, C.: Fourier transform spectroscopy of the O-2 Herzberg bands – II. Band oscillator strengths and transition moments, J. Mol. Spectrosc., 202, 171–193, 2000. Merienne, M. F., Jenouvrier, A., Coquart, B., Carleer, M., Fally, S., Colin, R., Vandaele, A. C., and Hermans, C.: Improved data set for the Herzberg band systems of O-16(2), J. Mol. Spectrosc., 207, 120–120, 2001. Molina, L. T. and Molina, M. J.: Air Quality in the Mexico Megacity: An Integrated Assessment, Kluwer Academic Publishers, Dordrecht, The Netherlands, 105–136, 2002. Molina, L. T., Kolb, C. E., de Foy, B., Lamb, B. K., Brune, W. H., Jimenez, J. L., Ramos-Villegas, R., Sarmiento, J., Paramo-Figueroa, V. H., Cardenas, B., Gutierrez-Avedoy, V., and Molina, M. J.: Air quality in North America's most populous city – overview of the MCMA-2003 campaign, Atmos. Chem. Phys., 7, 2447–2473, 2007. Molina, L. T., Madronich, S., Gaffney, J. S., Apel, E., de Foy, B., Fast, J., Ferrare, R., Herndon, S., Jimenez, J. L., Lamb, B., Osornio-Vargas, A. R., Russell, P., Schauer, J. J., Stevens, P. S., and Zavala, M.: An overview of the MILAGRO 2006 campaign: Mexico City emissions and their transport and transformation, Atmos. Chem. Phys. Discuss., 10, 7819–7983, 2010. Nel, A.: Air Pollution–Related Illness: Effects of Particles, Science, 308, 804–806, 2005. Oke, T. R., Spronken-Smith, R. A., Jauregui, E., and Grimmond, C. S. B.: The energy balance of central Mexico City during the dry season, Atmos. Environ., 33, 3919–3930, 1999. Orphal, J.: A critical review of the absorption cross-sections of O<sub>3</sub> and NO<sub>2</sub> in the ultraviolet and visible, J. Photoch. Photobio. A, 157, 185–209, 2003. PAPA, Public Health and Air Pollution in Asia (PAPA) Special Report 15: Health Effects of Outdoor Air Pollution in Developing Countries of Asia. Library of Congress Catalog Number for the HEI Report Series: WA 754 R432, Health Effects Institute, Cambridge, MA, 2004. Platt, U.: Differential Optical Absorption Spectroscopy, in Monitoring by Spectroscopic Techniques, edited by M. W. Sigrist, Wiley & Sons, New York, 27–84, 1994. Platt, U., Perner, D., Harris, G. W., Winer, A. M., and Pitts, J. N.: Observations of Nitrous-Acid in An Urban Atmosphere by Differential Optical-Absorption, Nature, 285, 312–314, 1980. Pope, C. A., Burnett, R. T., Thun, M. J., Calle, E. E., Krewski, D., Ito, K., and Thurston, G. D.: Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution, Jama-J. Am. Med. Assoc., 287, 1132–1141, 2002. Ramanathan, V., Crutzen, P. J., Kiehl, J. T., and Rosenfeld, D.: Aerosols, Climate, and the Hydrological Cycle, Science, 294, 2119–2124, 2001. 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 NO2: Key role of molecular nitric acid and its complexes, J. Phys. Chem. A, 110, 6886–6897, 2006. Ren, X. R., Harder, H., Martinez, M., Lesher, R. L., Oliger, A., Simpas, J. B., Brune, W. H., Schwab, J. J., Demerjian, K. L., He, Y., Zhou, X. L., and Gao, H. G.: OH and HO2 Chemistry in the urban atmosphere of New York City, Atmos. Environ., 37, 3639–3651, 2003. Roberts, J. M., Osthoff, H. D., Brown, S. S., and Ravishankara, A. R.: N<sub>2</sub>O$_5$ oxidizes chloride to Cl<sub>2</sub> in acidic atmospheric aerosol, Science, 321, p 1059, 2008. Robinson, A. L., Donahue, N. M., Shrivastava, M. K., Weitkamp, E. A., Sage, A. M., Grieshop, A. P., Lane, T. E., Pierce, J. R., and Pandis, S. N.: Rethinking organic aerosols: Semivolatile emissions and photochemical aging, Science, 315, 1259–1262, 2007. Ruiz-Suarez, L. G., Castro, T., Mar, B., Ruiz-Santoyo, M. E., and Cruz, X.: Do We Need An Ad Hoc Chemical Mechanism for Mexico-City Photochemical Smog, Atmos. Environ. A-Gen., 27, 405–425, 1993. Salcedo, D., Onasch, T. B., Dzepina, K., Canagaratna, M. R., Zhang, Q., Huffman, J. A., DeCarlo, P. F., Jayne, J. T., Mortimer, P., Worsnop, D. R., Kolb, C. E., Johnson, K. S., Zuberi, B., Marr, L. C., Volkamer, R., Molina, L. T., Molina, M. J., Cardenas, B., Bernabé, R. M., Márquez, C., Gaffney, J. S., Marley, N. A., Laskin, A., Shutthanandan, V., Xie, Y., Brune, W., Lesher, R., Shirley, T., and Jimenez, J. L.: Characterization of ambient aerosols in Mexico City during the MCMA-2003 campaign with Aerosol Mass Spectrometry: results from the CENICA Supersite, Atmos. Chem. Phys., 6, 925–946, 2006. Sander, S. P., Friedl, R. R., Golden, D. M., Kurylo, M. J., Moortgat, G. K., Keller-Rudeck, H., Wine, P. H., Ravishankara, A. R., Kolb, C. E., Molina, M. J., Finlaysson-Pitts, B. J., Huie, R. E., and Orkin, R. L.: Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies Evaluation Number 15, JPL Publication, 06-2, 1–522, 2006. Saunders, S. M., Jenkin, M. E., Derwent, R. G., and Pilling, M. J.: Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part A): tropospheric degradation of non-aromatic volatile organic compounds, Atmos. Chem. Phys., 3, 161–180, 2003. Sheehy, P. M., Volkamer, R., Molina, L. T., and Molina, M. J.: Oxidative capacity of the Mexico City atmosphere – Part 2: A RO$_\rm x$ radical cycling perspective, Atmos. Chem. Phys., 10, 6993–7008, doi:10.5194/acp-10-6993-2010, 2010. Shirley, T. R., Brune, W. H., Ren, X., Mao, J., Lesher, R., Cardenas, B., Volkamer, R., Molina, L. T., Molina, M. J., Lamb, B., Velasco, E., Jobson, T., and Alexander, M.: Atmospheric oxidation in the Mexico City Metropolitan Area (MCMA) during April 2003, Atmos. Chem. Phys., 6, 2753–2765, 2006. Singh, H. B., Kanakidou, M., Crutzen, P. J., and Jacob, D. J.: High-Concentrations and Photochemical Fate of Oxygenated Hydrocarbons in the Global Troposphere, Nature, 378, 50–54, 1995. 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. Stephens, S., Madronich, S., Wu, F., Olson, J. B., Ramos, R., Retama, A., and Muñoz, R.: Weekly patterns of México City's surface concentrations of CO, NO<sub>x</sub>, PM$_10$ and O<sub>3</sub> during 1986–2007, Atmos. Chem. Phys., 8, 5313–5325, 2008. 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.-Atmos., 109, D03307, doi:10.1029/2003JD004135, 2004. Stutz, J., Kim, E. S., Platt, U., Bruno, P., Perrino, C., and Febo, A.: UV-visible absorption cross sections of nitrous acid, J. Geophys. Res.-Atmos., 105, 14585–14592, 2000. Vandaele, A. C., Simon, T., Giolmont, J., Carleer, C., and Colin, R.: SO<sub>2</sub> absorption cross section measurement in the UV using fourier transform spectrometer, J. Geophys. Res., 99, 25599–25605, 1994. Velasco, E., Lamb, B., Westberg, H., Allwine, E., Sosa, G., Arriaga-Colina, J. L., Jobson, B. T., Alexander, M. L., Prazeller, P., Knighton, W. B., Rogers, T. M., Grutter, M., Herndon, S. C., Kolb, C. E., Zavala, M., de Foy, B., Volkamer, R., Molina, L. T., and Molina, M. J.: Distribution, magnitudes, reactivities, ratios and diurnal patterns of volatile organic compounds in the Valley of Mexico during the MCMA 2002 & 2003 field campaigns, Atmos. Chem. Phys., 7, 329–353, 2007. Volkamer, R., Etzkorn, T., Geyer, A., and Platt, U.: Correction of the oxygen interference with UV spectroscopic (DOAS) measurements of monocyclic aromatic hydrocarbons in the atmosphere, Atmos. Environ., 32, 3731–3747, 1998. Volkamer, R., Platt, U., and Wirtz, K.: Primary and Secondary Glyoxal Formation from Aromatics: Experimental Evidence for the Bicycloalkyl-Radical Pathway from Benzene, Toluene, and p-Xylene, J. Phys. Chem. A, 105, 7865–7874, 2001. Volkamer, R., Molina, L. T., Molina, M. J., Shirley, T., and Brune, W. H.: DOAS measurement of glyoxal as an indicator for fast VOC chemistry in urban air, Geophys. Res. Lett., 32, L08806, doi:10.1029/2005GL022616, 2005a. Volkamer, R., Spietz, P., Burrows, J. P., and Platt, U.: High-resolution absorption cross-section of Glyoxal in the UV/vis and IR spectral ranges, J. Photoch. Photobio. A, 172, 35–46, 2005b. Volkamer, R., Jimenez, J. L., San Martini, F., Dzepina, K., Zhang, Q., Salcedo, D., Molina, L. T., Worsnop, D. R., and Molina, M. J.: Secondary organic aerosol formation from anthropogenic air pollution: Rapid and higher than expected, Geophys. Res. Lett., 33, L17811, doi:10.1029/2006GL026899, 2006. von Friedeburg, C., Wagner, T., Geyer, A., Kaiser, N., Vogel, B., Vogel, H., and Platt, U.: Derivation of tropospheric NO<sub>3</sub> profiles using off-axis differential optical absorption spectroscopy measurements during sunrise and comparison with simulations, J. Geophys. Res.-Atmos., 107, 4168, doi:10.1029/2001JD000481, 2002. Wall, K. J., Schiller, C. L., and Harris, G. W.: Measurements of the HONO photodissociation constant, J. Atmos. Chem., 55, 31–54, 2006. West, J. J., Zavala, M. A., Molina, L. T., Molina, M. J., San Martini, F., McRae, G. J., Sosa-Iglesias, G., and Arriaga-Colina, J. L.: Modeling ozone photochemistry and evaluation of hydrocarbon emissions in the Mexico City metropolitan area, J. Geophys. Res.-Atmos., 109, D19312, doi:10.1029/2004JD004614, 2004. Witonsky, S. K., Canagaratna, M. R., Coy, S. L., Steinfeld, J. I., Field, R. W., and Kachanov, A. A.: The 3 nu(1) overtone band of trans-nitrous acid: Rotational and perturbation analysis and absolute intensity, J. Chem. Phys., 115, 3134–3143, 2001. Zavala, M., Herndon, S. C., Slott, R. S., Dunlea, E. J., Marr, L. C., Shorter, J. H., Zahniser, M., Knighton, W. B., Rogers, T. M., Kolb, C. E., Molina, L. T., and Molina, M. J.: Characterization of on-road vehicle emissions in the Mexico City Metropolitan Area using a mobile laboratory in chase and fleet average measurement modes during the MCMA-2003 field campaign, Atmos. Chem. Phys., 6, 5129–5142, 2006. Zhang, Q., Jimenez, J. L., Canagaratna, M. R., Allan, J. D., Coe, H., Ulbrich, I., Alfarra, M. R., Takami, A., Middlebrook, A. M., Sun, Y. L., Dzepina, K., Dunlea, E., Docherty, K., deCarlo, P. F., Salcedo, D., Onasch, T., Jayne, J. T., Miyoshi, T., Shimono, A., Hatakeyama, S., Takegawa, N., Kondo, Y., Schneider, J., Drewnick, F., Borrmann, S., Weimer, S., Demerjian, K., Williams, P., Bower, K., Bahreini, R., Cottrell, L., Griffin, R. J., Rautiainen, J., Sun, J. Y., Zhang, Y. M., and Worsnop, D. R.: Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes, Geophys. Res. Lett., 34, L13801, doi:10.1029/2007GL029979, 2007. Zhou, X. L., Civerolo, K., Dai, H. P., 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.-Atmos., 107, 4590, doi:10.1029/2001JD001539, 2002.