Seasonal and diurnal variation in particulate matter (PM10 and PM2.5) at an urban site of Beijing: analyses from a 9-year study.

Air pollution has become a top environmental concern in China. In the present study, 9 years of PM10 and PM2.5 data from an urban monitoring station in Beijing was analyzed relative to other gaseous pollutants and several meteorological parameters. The levels and characteristics of particulate matters as well as the sources and factors affecting them were provided in the present study. During the 9-year observation period, PM10 and PM2.5 showed high levels, with annual mean values of 138.5 ± 92.9 and 72.3 ± 54.4 μg/m(3), respectively. Slight decreasing trends of annual mean PM10 and PM2.5 were observed; autumn was the main season that contributed to the decrease in annual mean PM10 and PM2.5. Higher values for both PM fractions were generally observed in the spring and summer months, respectively, whereas lower values were found in the summer and spring months, respectively. Pronounced diurnal variations were found for PM10 and PM2.5, which both displayed a bimodal pattern with peaks between 7:00 and 8:00 a.m. as well as 7:00 and 11:00 p.m.; a minimum generally appeared at approximately noon. The seasonal and diurnal variations in particulate concentration are mostly dominated by the seasonal and diurnal variability of boundary layer and source emissions. A principal components analysis revealed that both the traffic-related emissions and combustion sources were major contributions to the particles; their contributions ranged between 35.5-75.1%. Furthermore, a directional analysis shows a stronger association between particles and the southerly winds, the PM derived from sources south of Beijing, most likely secondary PM, significantly affects concentrations at the sampling site. To improve air quality in Beijing, mitigation measures including phasing out high energy-consuming industries and prioritize the use of clean energy sources should be designed to reduce emissions from both local and regional sources.