PM2.5 filter sampling was conducted on a daily basis at the HKUST Air Quality Research Supersite (AQRS) for one year from March 2011 to February 2012. Approximately one fifth of the filter samples were subjected to full chemical analysis including major ions, elements, organic carbon (OC), elemental carbon (EC), and non-polar organic compounds (NPOCs). The major ions (sulfate, nitrate, and ammonium) were compared with those measured online by a MARGA system and the two sets of data were found in agreement within 25% or better. The major PM2.5 components (crustal materials, organic matter, soot, ammonium sulfate, ammonium nitrate, and non-crustal trace elements) accounted for 90% of the measured mass with sulfate being the most abundant (32.0%), followed by organic matter (23.5%) and ammonium (11.8%). The monthly variation patterns for different components suggested variable regional/super-regional sources, reflecting variation of transport contribution caused by shifts in synoptic weather conditions. Receptor modeling analysis by Positive Matrix Factorization revealed that secondary sulfate formation process (annual average of 31%), biomass burning (23%), and secondary nitrate formation process (13%) were the three dominant contributing sources to the observed PM2.5 at HKUST AQRS throughout the sampling year. The PM2.5 mass concentrations of all the individual sampling days were within the recently-proposed AQOs standards by the Hong Kong government (35 mu g/m(3) for annual average and 75 mu g/m(3) for 24-hr average) while approx. 52% of the sampling days were recorded with PM2.5 concentrations exceeding the WHO health 24-hr standards of 25 mu g/m(3). Major composition and source analysis showed that the increased mass concentrations on high PM days were mainly caused by air pollutant transport from the outside-Hong Kong regions. Results from this study indicate the importance of regional/super-regional strategies such as reduction in SO2, NO2 (precursors for secondary inorganic aerosols) and restricting biomass burning for lowering PM2.5 in Hong Kong.