Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 4 2006 10.5194/acpd-6-5999-2006 http://www.atmos-chem-phys-discuss.net/6/5999/2006/ http://www.atmos-chem-phys-discuss.net/6/5999/2006/acpd-6-5999-2006.html http://www.atmos-chem-phys-discuss.net/6/5999/2006/acpd-6-5999-2006.pdf 5999 6040 2006-07-10 Implementation of a Markov Chain Monte Carlo Method to inorganic aerosol modeling of observations from the MCMA-2003 Campaign. Part II: Model application to the CENICA, Pedregal and Santa Ana sites F. M. San Martini E. J. Dunlea R. Volkamer T. B. Onasch J. T. Jayne M. R. Canagaratna D. R. Worsnop C. E. Kolb J. H. Shorter S. C. Herndon M. S. Zahniser D. Salcedo K. Dzepina J. L. Jimenez J. M. Ortega K. S. Johnson G. J. McRae L. T. Molina M. J. Molina Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado at Boulder, Boulder, CO, USA Aerodyne Research Inc., Billerica, MA, USA Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Cuernevaca, Morelos, México Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA Department of Chemistry and Biochemistry, University of Colorado-Boulder, Boulder, CO, USA Molina Center on Energy and the Environment, La Jolla, CA, USA now at: the Board on Chemical Sciences and Technology, National Academies, Washington D.C., USA now at: Sandia National Laboratory, Livermore, CA, USA now at: University of California San Diego, La Jolla, CA, USA A Markov Chain Monte Carlo model for integrating the observations of inorganic species with a thermodynamic equilibrium model was presented in Part I of this series. Using observations taken at three ground sites, i.e. a residential, industrial and rural site, during the MCMA-2003 campaign in Mexico City, the model is used to analyze the inorganic aerosol and ammonia data and predict gas phase concentrations of nitric and hydrochloric acid. In general the model is able to accurately predict the observed inorganic aerosol concentrations at all three sites. The agreement between the predicted and observed gas phase ammonia concentration is excellent. The NO_z concentration calculated from the NO_y, NO and NO₂ observations is of limited use in constraining the gas phase nitric acid concentration given the large uncertainties in this measure of nitric acid and additional reactive nitrogen species. Focusing on the acidic period of 9–11 April identified by Salcedo et al. (2006), the model accurately predicts the aerosol phase observations during this period with the exception of the nitrate predictions after 10:00 a.m. (CDT) on 9 April, where the model underpredicts the observations by, on average, 20%. For periods when the aerosol chloride observations are consistently above the detection limit, the model is able to both accurately predict the aerosol chloride predictions and provide well-constrained HCl (g) concentrations. When the aerosols are aqueous, the most likely concentrations of HCl (g) are in the sub-ppbv range. The most likely predicted concentration of HCl (g) was found to reach concentrations of order 10 ppbv if the aerosols are dry. Finally, the atmospheric relevance of HCl (g) is discussed in terms of its indicator properties for the possible influence of chlorine-mediated photochemistry in Mexico City.