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The Pearl River Delta (PRD) region in South China is one of the most economically developed regions in China while also noted for its severe air pollution,especially during the winter season.In order to characterize the features and the formation mechanism of wintertime particulate pollution in the PRD region, continuous measurements of PM1 chemical compositions were performed at an urban monitoring site,along with atmospheric boundary layer (ABL) measurement at a nearby site.During the campaign, the mean measured PM1 mass concentration was 56.1 μg m-3 which composed of organic matter (41.0%), sulfate (23.2%), nitrate (13.9%), ammonium (10.5%), black carbon (measured by aethalometer, 9.3%) and chloride (2.1%).Correlation analysis revealed a significant correlation between OM and NOx, indicating high contribution from mobile emissions.In contrast, NO3-/SO42-had weak correlation with NOx/SO2, which suggested secondary inorganic aerosol contributed little to aerosol formation.In the absence of substantial secondary production, the high-level pollution was mostly affected by the accumulation process associated with the stagnant meteorology.The ABL measurement showed the decreasing wind speed, local circulation and thin surface inversion resulted in the high-level accumulations of pollutants.Overall, the mean height of ABL in nighttime was ~150 m, while in daytime the maximal value was about 900m.Analysis of the diumal variation of PM1 showed minimum concentration in the afternoon was caused by dilution as a result of an increase in the ABL height.The maximum concentration of PM1 in the evening was the results of an increase in traffic intensity during the evening rush hour together with a decrease in the mixing layer height, wind speed and temperature.