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A Markov Chain Monte Carlo model for integrating the observations ofinorganic species with a thermodynamic equilibrium model was presented inPart I of this series. Using observations taken at three ground sites, i.e. aresidential, industrial and rural site, during the MCMA-2003 campaign inMexico City, the model is used to analyze the inorganic particle and ammoniadata and to predict gas phase concentrations of nitric and hydrochloricacid. In general, the model is able to accurately predict the observedinorganic particle concentrations at all three sites. The agreement betweenthe predicted and observed gas phase ammonia concentration is excellent. TheNOz concentration calculated from the NOy, NO and NO2observations is of limited use in constraining the gas phase nitric acidconcentration given the large uncertainties in this measure of nitric acidand additional reactive nitrogen species. Focusing on the acidic period of9–11 April identified by Salcedo et al. (2006),the model accurately predicts the particle phase observations during thisperiod with the exception of the nitrate predictions after 10:00 a.m. (Central Daylight Time, CDT)on 9 April, where the model underpredicts theobservations by, on average, 20%. This period had a low planetaryboundary layer, very high particle concentrations, and higher than expectednitrogen dioxide concentrations. For periods when the particle chlorideobservations are consistently above the detection limit, the model is ableto both accurately predict the particle chloride mass concentrations andprovide well-constrained HCl (g) concentrations. The availability ofgas-phase ammonia observations helps constrain the predicted HCl (g)concentrations. When the particles are aqueous, the most likelyconcentrations of HCl (g) are in the sub-ppbv range. The most likelypredicted concentration of HCl (g) was found to reach concentrations oforder 10 ppbv if the particles are dry. Finally, the atmospheric relevanceof HCl (g) is discussed in terms of its indicator properties for thepossible influence of chlorine-mediated photochemistry in Mexico City

atmospheric chemistry and physics

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