To commemorate the 70th anniversary of the victory of the Chinese people's Anti-Japanese War and the World AntiFascist War, an international parade was held in Beijing in September 2015. In order to ensure satisfactory air quality during this event, a phased emission control measures were taken in Beijing and its surrounding provinces. The 24-h PM 2.5 samples were collected in Beijing from August 1 to September 15, 2015 covering the period before, during and after this large-scale event. The observed PM 2.5 data, meteorological data, emission reduction measures, and air mass trajectory simulation results were systematically analyzed to understand the pollution characteristics and chemical compositions of PM 2.5 in Beijing. The results indicated that PM 2.5 concentration during the two emission control phases was reduced by 61.7% comparing to the non-control period, but the regional transport of pollutants and meteorological conditions had a more prominent impact on PM 2.5 than emission reduction during phase 2. The secondary water-soluble ions including SO 4 2-, NO 3 -, and NH 4 + were found as the main ions present in PM 2.5 . During the entire emission control period, organic carbon (OC) and elemental carbon (EC) mass concentrations were decreased by 53.1% and 57.9%. A PM 2.5 mass balance was analyzed, and it was found that the organic matter accounted for 29.3, 37.6 and 28.5% of the PM 2.5 mass before, during and after the emission control, while the contribution of mobile sources to PM 2.5 was relatively outstanding after a series of emission control measures.
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