The Pearl River Delta(PRD)region in South China is one of the mosteconomically developed regions in China while also noted for its severe air pollution,especiallyduring the winter season.In order to characterize the features and the formation mechanism ofwintertime particulate pollution in the PRD region,continuous measurements of PM1 chemicalcompositions 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 massconcentration 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,indicatinghigh contribution from mobile emissions.In contrast,NO3-/SO42-had weak correlation withNOx/SO2,which suggested secondary inorganic aerosol contributed little to aerosol formation.Inthe absence of substantial secondary production,the high-level pollution was mostly affected bythe accumulation process associated with the stagnant meteorology.The ABL measurementshowed the decreasing wind speed,local circulation and thin surface inversion resulted in thehigh-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 diurnal variation of PM1showed minimum concentration in the afternoon was caused by dilution as a result of an increasein the ABL height.The maximum concentration of PM1 in the evening was the results of anincrease in traffic intensity during the evening rush hour together with a decrease in the mixinglayer height,wind speed and temperature.