PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City

Due to geographical factors, most of the Metropolitan Area of Mexico City features, on average, similar heights above the sea level, climate, wind speed and direction, with very uniform pollution degrees in most of the frequently studied sites. A site with different characteristics, Cuajimalpa de Morelos, was studied. It is located to the West of the urban area at 2760m above sea level, in contrast to other sites (2240m). Here, the wind is mostly directed towards the center of the city. Then, the site should not be affected by pollutants from the Northern/Northeastern industrial zones, so lower aerosol concentrations are expected. In this work, the elemental composition of coarse (PM10-2.5) and fine (PM2.5) fractions of atmospheric aerosol samples collected in Cuajimalpa is studied. The sampling period covered the cold-dry season in 2004–2005 (December 1st, 2004 to March 31, 2005), exposing polycarbonate filters with a Stacked Filter Unit of the Gent design along 24h, every two days. The samples were analyzed with Particle Induced X-ray Emission (PIXE) and X-ray Fluorescence (XRF), to obtain elemental concentrations. The EPA code UNMIX was used to determine the number of possible influencing polluting sources, which were then identified through back-trajectory simulations with the HYSPLIT modeling software. Four sources (mostly related to soil) were found in the coarse fraction, while the fine fraction presented three main sources (fuel oil, industry and biomass burning).