PM2.5 Haracterisation By Scanning ElectronMicroscopy (SEM) And Its Correlation WithDiverse Particle Emission Sources

In the Valley of Mexico Metropolitan Zone (VMMZ), there are over 20 million people, more than 4.5 million vehicles and about 35,000 industries and services. Daily average fuel consumption has been estimated as 50 million liters which generates thousands of tons of different air pollutants. Among these, an annual average of 5,499 ton of PM2.5 which cause health and visibility problems. PM2.5 sampling was performed at the Air Quality Sampling Station of UAMAzcapotzalco, Northwest VMMZ, with a low volume sampler, from Monday to Tuesday and Thursday to Friday, during 24-hour periods for 14 weeks in 2004 (from January to June and September to October). Samples were prepared cutting each filter into three sections which were later observed at the Scanning Electron Microscope (SEM). Several PM2.5 were identified. Among these, the chosen ones were those with a good definition of morphology, texture, porosity and size. Once the morphology was described, data were correlated with emission sources considering wind speed direction data as well as particles spatial distribution on the VMMZ maps. In most of the micrographs, observed particles had a spheroidal or ovoidal shape, with smooth or rugged and porous surface depending on their origin. Based on morphology information, it was determined that most of the particles came from combustion processes, both from mobile sources in the downtown area of Mexico City, and from point and area sources Northeast of the VMMZ. Metals in major proportion were Zn and Pb. Metals in minor proportion were