Aerosol composition and source apportionment in the Mexico City Metropolitan Area with PIXE/PESA/STIM and multivariate analysis. - Johnson, K. S.; De Foy, B.; Zuberi, B.; Molina, L. T.; Molina, M. J.; Xie, Y.; Laskin, A.; Shutthanandan, V. (Atmospheric Chemistry & Physics, 2006)