Daily samples of ambient sub-micrometer particles (PM1, particles with an aerodynamic diameter ≤ 1.0 μm) were collected from July 2009 to April 2010 at an urban site over Guangzhou in southern China. Mass concentrations of water-soluble inorganic ions, organic carbon (OC) and elemental carbon (EC) were determined to characterize the chemical composition of PM1. The mass concentration of PM1 ranged from 14.6 μg m− 3 to 143.3 μg m− 3, with an annual mean value of 52.4 ± 27.3 μg m− 3. Seasonally-averaged PM1 concentrations decreased in the order winter > autumn > spring > summer. The annual mean concentrations of OC and EC were 6.2 ± 3.5 and 5.0 ± 2.9 μg m− 3, respectively. The OC and EC concentrations were measured following the IMPROVE_A thermal/optical reflectance (TOR) protocol. Total carbonaceous aerosol (the sum of organic matter and elemental carbon) accounted for 23.0 ± 4.4% of PM1 mass. Clear seasonal variations in OC and EC suggested sources of these two constituents were remarkable difference among the four seasons. Seasonally averaged OC/EC ratios were 1.2, 1.7, 1.4, and 1.5, from spring to winter respectively. Low OC/EC ratios in comparison with other cities in China revealed that vehicle emissions play an important role in carbonaceous aerosol levels in Guangzhou. SO42 −, NO3− and NH4+ were the three major inorganic ions in PM1, collectively contributing 30.0% ± 6.3% of the PM1 mass. SO42 − and NH4+ were both the highest in autumn and the lowest in summer. In contrast, NO3− was the highest in winter. Sulfur oxidation ratio was positively correlated with solar radiation and O3, but negatively correlated with SO2. Nitrogen oxidation ratio was positively correlated with NO2, NH4+ and Cl−, but showed a negative correlation with temperature. By applying the IMPROVE equation, PM1 mass was reconstructed and showed that (NH4)2SO4, NH4NO3, OM and EC accounted for (30.7 ± 11.4) %, (9.7 ± 5.2) %, (22.6 ± 5.0) % and (9.7 ± 2.3) % of PM1, respectively. Finally, source apportionment by positive matrix factorization revealed that (1) secondary aerosol and biomass burning, (2) diesel emissions, (3) gasoline emissions and sea salt, and (4) coal combustion were the greatest contributors to PM1.