[1] 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.

A large data set including surface, aircraft, and laboratory observations of the atomic oxygen‐to‐carbon (O:C) and hydrogen‐to‐carbon (H:C) ratios of organic aerosol (OA) is synthesized and corrected using a recently reported method. The whole data set indicates a wide range of OA oxidation and a trajectory in the Van Krevelen diagram, characterized by a slope of −0.6, with variation across campaigns. We show that laboratory OA including both source and aged types explains some of the key differences in OA observed across different environments. However, the laboratory data typically fall below the mean line defined by ambient observations, and little laboratory data extend to the highest O:C ratios commonly observed in remote conditions. OA having both high O:C and high H:C are required to bridge the gaps. Aqueous‐phase oxidation may produce such OA, but experiments under realistic ambient conditions are needed to constrain the relative importance of this pathway.

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