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Aerosol composition data, simultaneously collected from collocated samplers at an urban, a rural and a background site in the Tennessee Valley, have been compared for all seasons in 2001. Consistent with previous data, organic aerosols and sulfates are the two largest contributors to fine mass throughout the year at all three sites. Levels of major constituents were not significantly different at the rural and background sites during any season, but levels of organic and elemental carbon were higher at the urban site during fall and winter periods. Seasonal trends at all sites showed maxima in summer for sulfate and significant nitrate levels only in winter, but no significant seasonal trend was observed for organic aerosol mass at any site. Year-to-year variability in aerosol composition at the background site is small compared to day-to-day variability within seasons. The appropriate factor for converting organic carbon to organic mass (conventionally *1.4), used in calculating aerosol chemical composition, may vary with season—larger in warm seasons, smaller in cool seasons. [Copyright &y& Elsevier]