The mass concentrations of PM1 (particles less than 1.0 mu m in aerodynamic diameter), organic carbon (OC), elemental carbon (EC), water-soluble ions, and up to 25 elements were reported for 24 h aerosol samples collected every sixth day at a roadside sampling station in Hong Kong from October 2004 to September 2005. Annual average PM1 mass concentration was 44.5 +/- 19.5 mu g m(-3). EC, OM (organic matter, OC x 1.2), and SO4- were the dominant components, accounting for similar to 36%, similar to 26%, and similar to 24% of PM1, respectively. Other components, i.e., NO3-, NH4+, geological material, trace elements and unidentified material, comprised the remaining similar to 14%. Annual average OC/EC ratio (0.6 +/- 0.3) was low, indicating that primary vehicle exhaust was the major source of carbonaceous aerosols. The seasonal variations of pollutants were due to gas-particle partitioning processes or a change in air mass rather than secondary aerosol produced locally. Vehicle exhaust, secondary aerosols, and waste incinerator/biomass burning were dominant air pollution sources, accounting for similar to 38%, similar to 22% and similar to 16% of PM1, respectively. Pollution episodes during summer (May-August) which were frequently accompanied by tropical storms or typhoons were dominated by vehicle emissions. During winter (November-February) pollution episodes coincided with northeasterly monsoons were characterized by secondary aerosols and incinerator/biomass burning emissions. (C) 2011 Elsevier B.V. All rights reserved.