Atmospheric impacts of vehicular emissions in São Paulo, Brazil

São Paulo is a large megacity (18 million people) with 7 million vehicles, with a peculiar vehicle fleet with significant ethanol emissions. A mixture of 24% of ethanol in the gasoline and a large fraction of vehicles running pure ethanol in flex fuel vehicles makes the city particularly interesting from the point of view of aerosol formation. A long term experiment was designed to analyze the physico-chemical properties of aerosols in São Paulo, as well as to apportion aerosol sources, with emphasis on vehicular emissions. Aerosol size distribution in the size range of 10 to 600 nm were measured with a Scanning Mobility Particle Sizer (SMPS). Optical properties are being measured with a TSI Nephelometer and a Thermo MAAP (Multi Angle Absorption Photometer). The composition of fine and coarse mode aerosols are being measured with XRF analysis and a Sunset instrument measure organic and elemental carbon in quartz filters. An Aerosol Chemical Speciation Monitor (ACSM) is used to characterize organic aerosols, in parallel with a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) to determine VOC concentrations. The measured total particle concentration typically varies between 10,000 and 50,000 per cm-3 being the lowest late in the night and highest around noon after peak vehicle emissions. Clear diurnal patterns in aerosol optical properties were observed. Scattering and absorption coefficients typically range between 20 and 100 Mm-1 at 450 nm, and between 10 to 40 Mm-1 at 637 nm, respectively, both of them peaking at 7:00 local time, the morning rush hour. The corresponding single scattering albedo varies between 0.50 and 0.85, indicating a significant contribution of primary absorbing particles to the aerosol population. During the first month a total of seven new particle formation events were observed with growth rates ranging from 9 to 25 nm h-1. Interestingly enough there were also events where condensed vapors were evaporating from the condensed phase thus shrinking the size of the particles in all sizes. Organic aerosol dominates completely the aerosol mass, with 70-80% of PM1. Diesel powered vehicles are also important, and are associated with black carbon emissions. The study allowed the aerosol characterization of a unique fueled fleet emission.