Size-Resolved Mass Balance of Aerosol Particles over the São Paulo Metropolitan Area of Brazil

Abstract

During an eight-day period in August of 1999, a nine-stage, micro-orifice, uniform-deposit impactor was used to determine the mass size distribution of atmospheric aerosol particles in the heavily polluted Metropolitan Area of São Paulo (MASP) in Brazil. Particle-induced X-ray emission analysis (PIXE) was used to identify up to 20 trace elements in particles collected in the five lower stages. All nine stages, as well as the after filter, were also analyzed for mass, black carbon, and various inorganic ions. A real-time carbon monitor provided organic and elemental carbon concentrations of PM 10 . For the five lower stages, the crustal material concentration was estimated by the sum of the main oxides and carbonates. In the after filter, PIXE analysis was not performed. The averaged mass balance analysis showed that black carbon was concentrated in the after filter, revealing black carbon particles to be very small in diameter. Ammonium sulfate showed simple unimodal size distribution during the day (peak diameter, 0.38 m) and bimodal size distribution at night (0.38 and 0.59 μ m). The crustal material contribution was greater in larger cutoff diameter stages. No explanation was found for a significant fraction of the fine particulate matter. The difference between the mass concentration and the sum of the component concentrations could be attributed to organic carbon being in fine mode. This could explain 38% of the difference and almost all of the missing diurnal and nocturnal mass. Based on these results and assumptions, we can conclude that the fine mode receives a greater contribution from organic and black carbon than from ammonium sulfate. The three together appear to constitute the bulk of fine particulate matter in the MASP.