About 60% of the air pollution in São Paulo and Rio de Janeiro is composed of fine particulate matter emitted by motor vehicles. Together, all other sources of air pollutants – industries, aerosols from the ocean and suspended dust – account for approximately 40% of the air pollution in these two major cities. The findings were reported by the FONTES project, funded by Petrobras and coordinated by researchers José Marcus Godoy, from the Pontifical Catholic University of Rio de Janeiro (PUC-Rio), and Paulo Artaxo, from the Physics Institute at the University of São Paulo (IF-USP). “The good news is that, if we enforce policies that encourage less automobile use, we can significantly reduce air pollution in urban centers,” says Artaxo. The study also discovered that about 20% of the fine particulate matter in the two cities consists of a material called black carbon (soot), produced by incomplete burning of fossil fuels and biomass. Diesel-burning vehicles, like buses, are the biggest emitters of black carbon. To obtain data for the project, measurements were taken between 2011 and early 2014 at eight stations – four located in São Paulo (IF-USP, USP School of Public Health, Ibirapuera Park and Congonhas) and four in the Rio de Janeiro Metropolitan Region (Duque de Caxias, Tijuca, Recreio dos Bandeirantes and Taquara). The air in both cities was shown to be highly homogeneous. The samples obtained from the eight stations had similar compositions, although each location showed some specific characteristics. Another interesting finding: the level of pollution measured in the study was similar to the one verified in 2004, when there were fewer motor vehicles in both cities. This indicates that newer vehicles pollute less.