Beijing aerosol: Atmospheric interactions and new trends

摘要

Beijing aerosols are scrutinized as a case study for atmospheric interactions in a complex multisource situation. For the first time, fine (2 μm) aerosols were continuously collected during a time period (20 months) long enough to capture seasonal trends of sources and interactions. Weekly samples were obtained from January 2003 to August 2004 downtown and during 9 months at two periurban sites. Aerosol samples were chemically characterized (black carbon (BC), organic carbon (OC), and major ions) and dust was obtained from mass closure. Concentration data were smoothed and boundary layer height (BLH) corrected in order to better identify sources and processes. All yearlong, the coarse aerosol is dominated by dust (75%) whereas the fine mode is dominated (46%) by carbonaceous particles. Photochemistry is an intense driving force for secondary aerosol formation including secondary organic aerosol (SOA). Dust particles present a reactive surface for secondary aerosol formation from the intense anthropogenic pool of acidic gaseous precursors (SO_2, HNO_3, and volatile organic compounds (VOCs)). These interactions favor the formation of a very significant coarse fraction for SO4, NO_3, and POM, a feature almost never encountered in developed countries. Surprisingly too is the presence of fine NH4NO_3 in summer. A new result is also that the winter “heating season” appears at present of minor importance with, however, a significant component from domestic heating as traced by BC/OC. In the future, traffic is likely to dominate downtown anthropogenic emissions. Year-to-year variability in meteorological conditions is likely to influence inputs from arid regions and from regional industrial and biomass burning sources.