Highly time-resolved measurements of secondary ions in pm_2.5during the 2008 Beijing Olympics: The impacts of control measures and regional transport

Highly time-resolved measurements of SO42-, NO3-and NH4+in PM2.5were simultaneously performed at an urban site and downwind rural site in Beijing during the 2008 Olympics to investigate the impacts of control measures and regional transport. The mean concentrations (± standard deviations) of SO42-, NO3-and NH4+were 18.23 (± 19.96), 9.47 (± 11.41), and 9.70 (± 8.92) μg/m3, respectively, at the rural site. These concentrations were comparable to those of 20.74 (± 20.36), 8.83 (± 9.51), and 10.85 (± 8.99) μg/m3at the urban site. Clear diurnal variations of SO42-, NO3-and NH4+were observed at both sites, and were related to meteorological conditions, primary emissions, and regional transport. The effectiveness of the control measures on SO42-, NO3-and NH4+was evaluated by comparing the urban site concentrations during three periods: before the full-scale control, after the full-scale control but before the Olympics, and during the Olympics. The high pollution observed after the full-scale control was attributed to regional transport from the sector south of Beijing. The samples in the air masses from the northwest were selected to minimize the influences of meteorological factors and regional transport, and the results showed a clear reduction of SO42-, NO3-and NH4+concentrations (approximately 35%-69%) after the full-scale control began, suggesting the effectiveness of the control measures in reducing the local secondary inorganic aerosols. A widespread pollution episode was observed during August 3-10 at the rural site, with regional transport being identified as the main contributor. Secondary transformation evidently occurred during August 3-4 and contributed more than 50% of the rural secondary ion concentrations. During August 5-10, the whole region experienced a stable and well-developed plume.