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Foi realizada a amostragem de aerossol em dez locais distribuídos em diferentes regiões da cidade do Rio de Janeiro, de setembro/2003 a setembro/2004, em uma base semanal com amostragens de 24 horas. As frações do aerossol PM 2.5 e PM 2.5-10 foram amostradas usando um sistema com filtros em série. Foram determinadas: a massa do aerossol, carbono elementar, a concentração dos íons solúveis, assim como a concentração total de alumínio, cálcio, ferro, magnésio, potássio, sódio e titânio. A concentração mássica média anual do PM 10 variou de 18 a 38 μg m -3 , sem violação dos padrões brasileiros de qualidade do ar. Para as frações fina e grossa, entre 70 e 90% da massa medida foi identificada como sendo devido à poucas fontes, sendo que a poeira do solo explica 37-63% da massa da fração grossa (PM 2.5-10 ). Por outro lado, o carbono elementar e o aerossol secundário representam proporções elevadas (45-58%) da massa da fração fina (PM 2.5 ).Aerosol sampling was performed at ten sites distributed in different location of Rio de Janeiro city, from September/2003 to Sepetember/2004, on a weekly basis with 24 h sampling time interval. PM 2.5 and PM 2.5-10 aerosol particles were sampled using Stacked Filter Units (SFU). Aerosol mass, black carbon, water soluble ions concentration as well as total aluminum, calcium, iron, magnesium, potassium, sodium and titanium concentration were determined. Mean annual PM 10 mass concentration ranged from 18 to 38 μg m -3 , without violation of the Brazilian air quality standards. For fine and coarse modes, between 70 and 90% of the measured mass was apportioned to relatively few sources, soil dust accounts for 37-63% of the coarse mode mass. On the other hand, black carbon and secondary aerosol, related to anthropogenic sources as fuel burning, represents very high proportions (45-58%) of the PM 2.5 mass.Keywords: Brazil, air pollution, aerosol, source apportionment IntroductionThe city of Rio de Janeiro is one of the twenty biggest urban agglomerations in the world, with 11 million inhabitants in the metropolitan area and the first, among the Brazilian cities, in respect to the population density, with 1700 hab/km 2 . 1,2 Among the metropolitans regions existing in the country, Rio de Janeiro has the highest degree of urbanization, 96.8%, and responds for 80% of the internal income of the State and 13% of the Country. 1,3 Factors such as local topography, irregular occupation of the space, presence of open sea and a fraction of the urban area at the Guanabara Bay influence the urban atmosphere of the city of Rio de Janeiro. These factors result in an efficient, however complex, regimen of atmospheric circulation, combining winds with the canalizations originated from its topography, and, consequently, contributing to an irregular distribution and dispersion of pollutants. The climatological evaluation of the atmospheric circulation in Rio de Janeiro shows that the highest wind direction frequency is from SouthSoutheast to Northeast sectors, along the year, with average wind velocity o...