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Twenty-four-hour PM2.5 and PM10 samples were collected simultaneously at a highly-trafficked roadside station in Hong Kong every sixth day from October 2004 to September 2005.Mass concentrations of PM2.5,PM10-2.5 (or PMcoarse,defined as PM10 – PM2.5),organic carbon (OC),elemental carbon (EC),water-soluble ions,and up to 25 elements were determined.Investigation on the chemical compositions and potential sources of aerosols shows distinct differences between PM2.5 and PM10-2.5.Annual average mass concentration was 55.4±25.5 μg m-3 and 25.9±15.5 μg m-3 for PM2.5 and PM10-2.5,respectively.EC,OM (OM=OC×1.4),and ammonium sulfate (NH4)2SO4 comprised over ~82% of PM2.5,accounting for ~29%,~27%,and ~25% of the PM2.5 mass,respectively.Low OC/EC ratios (less than 1) in PM2.5 suggested influence from fresh diesel-engine exhaust.Seven factors were resolved for PM2.5 data by Positive Matrix Factorization (PMF) Model,including vehicle emissions (~29%),secondary aerosol (~27%),waste incinerator/biomass burning (~23%),residual oil combustion (~10%),marine aerosol (~6%),industrial exhaust (~4%),and resuspended road dust (~1%).PM10-2.5 showed significant difference in chemical composition as compared to PM2.5,with geological material and trace elements being the most abundant components (~28%),followed by unidentified material (~26%),OM(~12%),sea salt (~10%),and ammonium nitrate (NH4NO3,~10%).EC and (NH4)2SO4,the major components (>54%) in PM2.5,each accounted for only ~7% of PM10-2.5.Average OC/EC ratio in PM10-2.5 was 7.8±14.2,suggesting the impact from combustion sources other than vehicular exhaust.The sources for PM10-2.5 comprised of ~20% vehicle-related resuspended dust (e.g.,tire dust/brake linear/petrol evaporation),~17% local resuspension road dust,~17% marine aerosol,~12% secondary aerosol/field burning,~11% vehicle emissions,and ~23% unidentified material.