Study on chemical components and sources of PM2.5 during heavy air pollution periods at a suburban site in Beijing of China

PM2.5 and its main chemical components have been investigated during six air pollution periods across four seasons from Jun. 16, 2016 to Jun. 15, 2017 at a suburban site in Beijing, China. Daily PM2.5 mass concentrations were measured with associated ions, OC, EC, 16 PAHs and seven heavy metals. Mass closure analyses were conducted using measured and reconstructed chemical components. Chemical ratios were used to identify possible pollution sources, while principal component analysis (PCA) coupled with multiple linear regression (MLR) method was used to estimate the source contribution to PM2.5 mass. The annual mean PM2.5 concentration (96.7 mu g m(-3)) is about 2.8 times the national level-2 standard limit, with apparent seasonal variation of daily exceedances as winter > spring > autumn > summer. Generally, higher levels are observed in winter and autumn than in spring and summer for NO3-, SO42- NH4+, Cl-, K+, OC, EC, PAHs and metals Zn, As, Cu, Pb, along with higher concentrations of NOx, CO and SO2. Secondary ions and carbon fractions are most abundant over all seasons, except levels of dust related species Ca2+, Mg2+, Ca, Fe and Mn are also high in spring. The reconstructed results show that PM2.5 consists of mostly organic compounds (26-38%) and ammonium salts (NH4NO3 + (NH4)(2)SO4) (43-53%) in winter and autumn, while summer is dominated mainly by ammonium salts (81%). In spring, very high PM2.5 episodes are occasionally composed of large amount of soil/road dust and gypsum particles (23-55%) due to floating dust events with drier weather conditions and human activities. Fuel combustions and secondary sources are responsible for similar to 70% of PM2.5 mass while mobile source is mostly a regional origin.

Automated summary

The study in suburban Beijing found that PM2.5 concentrations exceeded the national level-2 standard limit by 2.8 times, with higher levels observed in winter and autumn, dominated by organic compounds and ammonium salts. In spring, occasional high PM2.5 episodes were attributed to floating dust events caused by dry weather conditions and human activities.