10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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PIXE Analysis of PM2.5 Atmospheric Aerosols in a Two Year Monitoring in Mexico City
VALTER ARMANDO BARRERA, Raul Venancio Diaz, Javier Miranda, Giulia Calzolai, Silvia Nava, Martina Giannoni, Franco Lucarelli, CONACYT- CIACYT/UASLP, Mexico
Abstract Number: 923 Working Group: Aerosol Chemistry
Abstract After more than 25 years of local air pollution monitoring in Mexico City, Particle Matter (PM) is one pollutant that is not decreasing. This is due to many factors, like the well-known geography, the Ozone all year production with the subsequent reactivity in the local atmosphere, and natural or anthropogenic sources; the Metropolitan Area of Mexico City (MAMC), is still growing without control (8 M inhabitants plus 12 M inhabitants in Mexico City basin). Moreover, the number of vehicles (more than 4.5 M) increases with no regulation and finally, there are industrial zones around the megacity.
Last decades, multiple elemental studies have been done for different PM fractions (Miranda et al., 1992, 1998, 2004, 2005; Chow et al., 2002; Barrera et al., 2012; Diaz et al., 2014). However, there is a lot to know of the chemical composition of the PM pollutant in Mexico city in the recent years, mainly the fine fraction or PM2.5.
This work presents a Particle Induced X-ray Emission (PIXE) analysis and a source apportionment study of PM2.5 in a two-year monitoring campaign, collecting samples in 4 sites to provide a wider knowledge of elemental concentrations. Monitoring sites were located at the southwest and southeast city for year 2009 and for 2010 at southwest and downtown Mexico City. Almost 200 PM2.5 samples were collected on a 24 h (from midnight to midnight) sampling base each two days, by using two Teom samplers (medium-volume 1 m3/h), depositing the particles onto 47 mm diameter Teflon filters, allowing the application of the PIXE technique. Meteorological parameters were obtained from the local network.
PIXE analyses were performed at the 3 MV Tandetron accelerator of the INFN-LABEC laboratory, with a recently installed external beam set-up composed by a Silicon Drift Detector with a large active area (80 mm2) and 450 mm thickness for low Z element, with proton deflector and an identical SDD detector used just for the detection of medium-high energy X-rays, thus reducing the irradiation time and generating better statistical data (Lucarelli et al., 2014). Each sample was irradiated for ~90 s with a 3.0 MeV proton beam (~2 mm2 spot, 10 nA to 150 nA intensity). PIXE spectra were fitted using the GUPIX code (Maxwell et al., 1995) and elemental concentrations were obtained by a calibration curve from a set of thin standards of known areal density (Micromatter Inc.). The lighter elements (Na, Mg, Al and Si) concentrations were corrected for self-absorption effects (Calzolai et al., 2010). Also, PIXE technique provide a better analysis for several trace elements that are very useful for aerosol source apportionment (e.g., V, Ni, Cu, Zn, Pb) (Maenhaut, 2015). Uncertainties were determined by a sum of independent uncertainties or an expanded uncertainty method from each analysis (Espinosa et al., 2010).
More than 20 elements were identified by PIXE, representing more than 12% of the PM2.5 total mass in the four sites. Maximum concentrations episodes were registered in the two corresponding monitoring zones at Feb 12th, 2009 and June 1st, 2010, mainly due to different industrial zones near the MAMC.
Different multivariate methods were used to determine the number of possible influencing polluting sources for each site, which were then identified through back-trajectory simulations with the HYSPLIT modeling software (Draxler & Rolph, 2010).
Acknowledgements. Barrera V. acknowledges the support of CONACYT, Mexico (Grant No. 208131).