The aerosol mixing state was investigated with an optical closure study at Xinken, Pearl River Delta of China in 2004. On the basis of in situ aerosol microphysical and chemical measurements and a two-component aerosol optical model an internal consistency algorithm was developed to model the mass ratio (r) of externally mixed elemental carbon (EC) to total EC, which minimized the discrepancies between measured and calculated optical properties. The rest of EC was assumed to be internally mixed. A time series of r was retrieved. Good agreement between model and observation was found, on the order of ±15% for total/back scattering coefficients and ±10% for absorption coefficient. The EC mixing state was strongly dependent on the local wind patterns. When north/northeasterly winds prevailed, the air came from the urban and industrial areas of mainland China, and EC was mainly externally mixed with an average r of 85 ± 12%. When the airflow was controlled by a weak local wind system, the mixing state showed a pronounced diurnal variation. During daytime the wind speed was nearly zero. This favored the increase of local pollution, and the average r was about 95%. However, during nighttime the EC mixing state transformed to be internally mixed apparently with an average r of 53 ± 15%, which can be explained by a more aged air mass. The south/southeasterly winds coming from the sea were found to have the most important effect on the transformation of EC mixing state in the night, but fairly rapid local aging processing was also observed. The uncertainties of the model were explored by a Monte Carlo simulation.