The changes and long-range transport of PM2.5 in Beijing in the past decade

A sampling campaign of PM2.5 in Beijing from 2002 to 2004 and from 2011 to 2013 was performed to investigate the changes of typical characteristics of PM2.5 at the representative city over Northern China in the past decade. A spring PM2.5 monitoring network was operated to study the long-range transport of PM2.5 in five cities along the pathway of dust storm across Northern and Eastern China in spring 2004. PM2.5 concentrations gradually decreased, but PM2.5 pollution still retained a high level and has been changed from dust type to haze-fog pollution at Beijing in the past decade. The seasonal variation of PM2.5 was spring > winter > autumn > summer during 2002-2004, but winter > autumn > summer > spring in 2011-2013. The mass concentration of mineral aerosol was in the order of spring > winter > summer > autumn, and reached the highest level in dust storm episodes during the past decade. Secondary inorganic aerosol (SIA) followed the order of summer > autumn > winter > spring during 2002-2004, however winter > autumn > summer > spring during 2011-2013, and reached the highest level in haze events. The concentrations of SO42−, NO3− and NH4+ of SIA displayed the order of SO42− > NO3− > NH4+ in haze days. SO42− concentrations in different seasons during 2011-2013 were almost lower than those in corresponding period in 2002-2004 respectively. But NO3− and NH4+ concentrations in autumn and winter of 2011-2013 were higher than those in 2002-2004. The OA and SOA concentrations followed the order of winter > autumn > summer and spring, and SOA was one of the significant components in OA. The secondary chemical transformation was the most significant source for PM2.5 during 2011-2013, compared with that in 2002-2004. Atmospheric pollution has exhibited severe complicated pollution. Dust storm carried not only large amounts of mineral dust, but also some secondary inorganic aerosol (SIA) and organic aerosol (OA), which would have significant impact on the wide downwind areas. The mass fraction of mineral aerosol decreased from the dust source to the downwind coastal sites, while those of secondary and carbonaceous components increased.