To gain a better understanding of the complex interactions between air pollutants and meteorological variables,and influence on the air quality of a polluted megacity,simultaneous measurements of near-surface PM2.5,O3 and meteorological parameters were conducted at 325-m meteorological tower in the urban areas of Beijing.Vertical profiles of PM2.5 and O3 were investigated at a 325-m meteorological tower from Sep.1,2004 to Feb.28,2005 and from Sep.1,2009 to Feb.28,2010.The results showed clearly defined vertical PM2.5 and O3 gradients were found at night.The vertical PM2.5 and O3 gradients were less evident or were negligible when a well-mixed convective layer developed.Overall,characteristics of these profiles were observed to be consistent with the formation of an upper residual layer during nighttime carrying over a fraction of the O3 from the previous day that contributes to the background concentration.In addition,the vertical distributions of PM2.5 and O3 are strongly affected by weather and meteorological conditions,and 4 types of vertical PM2.5 and O3 distributions corresponding to different weather systems,are considered in this study.A flat vertical gradient and low surface PM2.5 and O3 concentrations is found in type-1 (days with strong wind); a gradual increase of O3 with altitude and modest surface PM2.5 and O3 concentrations are found in type-2 (clear days); a weak vertical gradient with low surface PM2.5 and O3 concentrations is found in type-3 (rainy days); a sharp vertical gradient (strongly depressed PM2.5 and O3 in the PBL) with high surface PM2.5 and O3 concentrations is found in type-4 (hazy days).The weather conditions corresponding to the 4 PM2.5 and O3 types include the fronts associated with a high- pressure system (the passage of a cold front),a low-pressure (between the cold front and warm front),the center of a high-pressure system and the area between two low- pressure systems.The results of this study provide detailed vertical profiles of PM2.5 and O3,which are important for assessing the impacts of atmospheric stability on PM2.5 and O3 chemistry.In addition,knowledge of the vertical distribution of PM2.5 and ozone in the planetary boundary layer (PBL) may be essential for developing effective abatement strategies for ozone control.