Characteristics and emission-reduction measures evaluation of PM2.5 during the two major events: APEC and Parade.

The measurement of PM2.5 was conducted from 20th October to 25th November 2014, and 15th August to 15th September 2015 at Beijing, Shijiazhuang, and Tangshan, China, covering two important events of Asia-Pacific Economic Cooperation (APEC) and Grand Military Parade (Parade). A series of stringent emission-reduction measures were implemented in Beijing and neighbouring regions to ensure good air quality in Beijing. PM2.5 concentrations and major components (water-soluble ions and carbonaceous aerosol) were analyzed and compared between the two events. PM2.5 concentration during control demonstrated a decreasing trend with 51.6%-65.1% and 34.2%-64.7% compared to no control during APEC and Parade, respectively. The water-soluble ions and carbonaceous aerosol concentrations also decreased obviously. The lower secondary inorganic ions (SIA, including SO42-, NO3-, and NH4+) percentage (31.7%-38.3%) during control indicated the relative weak contribution of atmospheric chemical processes. Due to the unfavorable weather conditions and increased emissions during coal-fired heating, the relative higher organic carbon (OC), secondary organic carbon (SOC), secondary organic aerosol (SOA) concentrations during APEC was found. The WRF-CMAQ modeling system was also used to quantify the effect of emission-reduction and different sources controls on PM2.5. The results indicated PM2.5 decrease by 30.4% and 34.2% under control during APEC and Parade, respectively. We found that the local emission-reduction in Beijing contributes more to "APEC blue" (20.8%) and "Parade blue" (25.8%) in Beijing than those from neighbouring regions of Beijing, and meteorological condition was more beneficial for the improvement of the air quality during Parade than APEC. The emission source apportionment to PM2.5 in Beijing indicated that PM2.5 concentration increased by 13.5% and 14.7% during APEC and Parade, respectively if no vehicles emission-reduction measures were taken in Beijing.