SULFATE AEROSOL LEVELS DERIVED FROM SULFUR CONTENT DETERMINED BY PIXE IN ATMOSPHERIC PARTICLES COLLECTED IN THE MEXICO CITY METROPOLITAN AREA

Abstract

As part of a wide research project aimed to understand and eventually control atmospheric pollution in the Mexico City Metropolitan Area (MCMA), the Secretaría del Medio Ambiente del Gobierno del Distrito Federal (SMA-GDF) operates, since 2003, an automated sampling network of fine airborne particulate matter (FAPM). In this study, the samples collected simultaneously in three sampling sites during years 2003–2004, and those collected (simultaneously) in seven sites during the whole of 2005 and the first three months of 2006, have been analyzed using the PIXE technique. The results have provided ample databases of elemental concentrations (ng/m3) of 13 elements, consistently detected in the samples. As it is well known, Mexico City and its conurbation has been ranked as one of the most densely populated areas in the world, enduring high levels of several pollutants such as PM2.5 fine particles (≤ 2.5 μm) and sulfates among others, which diminish air quality, visibility and endanger the health of the population. According to the emission inventories of the SMA-GDF, around eight thousand tons of SO2 are emitted to the atmosphere in the MCMA every year. Despite continuous efforts of the local authorities aimed to reduce SO2 emissions, S elemental concentrations determined in the PM2.5 samples collected by the fine particle sampling network still show significant levels of this element. Since sulfur compounds, particularly sulfates, are among the most important pollutants affecting the atmosphere, it is compulsory to evaluate its presence and concentrations. As it is has been pointed out by several authors, the ratio of sulfate content to S elemental content in FAPM, such as that determined by PIXE, is fairly constant (approximately three). Using this result, sulfate concentrations were calculated, and these were found in very good agreement with previous experimental results.