Daily PM2.5 samples were collected in Guangzhou - the largest megacity in South China, for a period of one month in each season during 2009-2010. Mass concentrations of water-soluble inorganic ions, organic carbon (OC) and elemental carbon (EC) in PM2.5 were determined, and aerosol scattering coefficient (b(sp)) was synchronously measured. The daily PM2.5 mass concentrations ranged from 21.0 to 213.6 mu g m(-3) with an annual average of 76.8 +/- 41.5 mu g m(-3). The highest seasonal average PM2.5 was observed in winter (103.3 +/- 50.1 mu g m(-3)) and the lowest in summer (38.6 +/- 15.7 mu g m(-3)). Annual average PM2.5 mass scattering efficiency (MSE) was 3.5 +/- 0.9 m(2) g(-1), with obvious seasonal variations in sequence of autumn (4.5 +/- 0.2 m(2) g(-1)) > winter (3.9 +/- 0.5 m(2) g(-1)) > spring (3.0 +/- 0.4 m(2) g(-1)) > summer (2.3 +/- 0.3 m(2) g(-1)). To determine the relationship between b and the chemical components of PM2.5, b(sp) was reconstructed in each season using the original IMPROVE formula with a modification of including sea salt aerosols. The estimated bsp using this method was 22 +/- 28% smaller on annual average compared to the measurements. Multiple linear regression of measured b(sp) against (NH4)(2)SO4, NH4NO3, OM (Organic Mass), SS (Sea Salt), FS (Fine Soil), and CM (Coarse Mass) were also performed in all the four seasons. The estimated b(sp) from using the regression equation was 4 +/- 12% larger than the measured values. On average, (NH4)(2)SO4, NH4NO3, OM, SS, FS and CM accounted for 50 +/- 11%, 18 +/- 10%, 19 +/- 5%, 5 +/- 4%, 3 +/- 2% and 5 +/- 6%, respectively, of the estimated b(sp).