Characterization of size-differentiated inorganic composition of aerosols in Mexico City

Size-differentiated atmospheric aerosol particles were collected during December 2000–October 2001 in Mexico City (19°N, 99°W) using a micro-orifice uniform deposit impactor. Sulfate and ammonium, which were correlated, were major features of the size distributions. The more predominant mode was at 0.32±0.1 μm, aerodynamic diameter. This peak of concentration is likely the result of condensation of secondary aerosol components from the gas phase. During part of the rainy season (April and June), a larger mode was found at 0.56±0.2 μm, aerodynamic diameter. This peak of concentration, identified as a droplet mode, is probably the result of aqueous-phase reactions (i.e., oxidation of sulfur dioxide in liquid droplets). During August, the peak of concentration was observed at both size ranges of the accumulation mode. Overall, ion balances were achieved with a small deficit of cations, except for the April and June samples, where a significant amount of excess sulfate was present as a result of moderate–high activity of the neighboring volcano Popocatepetl, as well as ambient conditions that favored production of sulfate (moderate–high relative humidity values). Based on the analysis of the ammonia/sulfate molar ratios, the ammonia concentrations were sufficient to fully neutralize sulfate concentrations, except for the April and June samples. During these months, ammonium bisulfate, letovicite and H2SO4(aq) (or a solution of the corresponding ions) were the dominant form of sulfate present in both fine and coarse modes. The acidic nature of these particles (with NH3/H2SO4 molar ratio less than 2) is potentially important in assessing health effects of inhaled particles.