Seasonal variation of carcinogenic heavy metals in PM_2.5 and source analysis in Beijing

During April, July, October 2009 and January 2010, daily (24-h average) PM_2.5 samples were collected at urban sites in Beijing and 29 metal elements were analyzed by the ICP-MS. The characteristics of 7 carcinogenic heavy metal mass concentrations, enrichment, and possible sources were discussed. The annual average concentrations of As, Cd, Co, Cr, Ni, Pb and Se were (11.6±14.0), (2.6±2.4), (1.0±0.7), (11.3±9.4), (4.0±2.4), (142.5±98.9) and (3.3±2.2) ng·m^-3, respectively. Only annual average concentration of As exceeded WHO standard by a factor of 0.8. Higher enrichment factors of As, Cd, Pb and Se were found and their enrichment factors exceeded 500. Their enrichment factors in summer were much higher than those in other seasons. The local coal combustion and vehicle exhaust should be the dominant sources for the above four carcinogenic heavy metals in spring, autumn and winter, while regional transportation contributed more in summer.