Water-soluble inorganic ions of size-differentiated atmospheric particles from a suburban site of Mexico City

During the MILAGRO campaign, March 2006, eight-stage cut impactors were used to sample atmospheric particles at Tecamac (T1 supersite), towards the northeast edge of the Mexico City Metropolitan Area, collecting fresh local emissions and aged pollutants produced in Mexico City. Particle samples were analyzed to determine total mass concentrations of Ca2+, Mg2+, NH4 (+), K+, Cl-, SO4 (2-), and NO3 (-). Average concentrations were 22.1 +/- 7.2 mu g m(-3) for PM10 and 18.3 +/- 6.2 mu g m(-3) for PM1.8. A good correlation between PM10 and PM1.8, without influence from wind patterns, indicates that local emissions are more important than the city's pollution transported to the site, despite the fact that Tecamac is just 40 km away from Mexico City. A lack of diurnal patterns in the PM2.5/PM1.8 ratio supports this conclusion. The inorganic composition of particles suggests that vehicles, soil resuspension, and industries are the main pollutant sources. Finally, the particles were found to be neutralized, in agreement with observations in the Mexico City Metropolitan Area.