Characteristics of PM_2.5 in Heavy Pollution Events in Beijing and Surrounding Areas from November to December in 2016

PM_2.5 and PM₁₀ mass as well as meteorological data at six stations in Beijing and Hebei Province are analyzed to investigate characteristics of heavy pollution processes from November to December of 2016. Results show that PM_2.5 concentrations are 73.1, 130.8 μg·m^-3 and 226.0 μg·m^-3 at Shangdianzi, Shunyi and Chaoyang stations in Beijing during the heavy pollution, which are lower than those measured at Baoding and Shijiazhuang stations in Hebei (357.8 μg·m^-3 and 346.9 μg·m^-3, respectively). The average concentration of PM_2.5 for the heavy haze days is 3-4 times of that in clean days at all six stations, with the haze accompanied by calm wind, high humidity, and other adverse weather conditions. Observations indicate most pollution cases last longer in Hebei than those in Beijing, which is probably caused by intensified emissions from industry in Shijiazhuang. In addition, the sulfur dioxide, nitrogen oxides and particulate matter discharged from Shijiazhuang and Baoding are blocked by the Taihang Mountains, which make pollutants easy to accumulate in this area.The daily average air quality index (AQI) during heavy pollution events has a complex relationship with the type, strength, duration and thickness of the inversion layer. Meanwhile, it has good consistency with the duration of the inversion both before and after the heating period in Beijing. The analysis of sounding data indicates that the atmospheric boundary layer also plays an important role in the accumulation of pollutants. Comparing with inversion at higher level, the inversion near the ground has significantly greater suppression effects on the diffusion. The pollution case from 17 December to 21 December lasts 5 days and PM_2.5 mass concentrations are higher than the case from 3 November to 5 November in 2016. It suggests that the vertical diffusion of pollutants is suppressed for longer time and contaminants accumulate on the ground with the temperature inversion. On the other hand, the horizontal wind speed is lower, and pollutants cannot spread horizontally which aggravate pollution. Concentrations of OC and EC in PM₁₀ at Gucheng in Hebei in two cases are also significantly different. Much higher OC, EC and OC/EC concentrations on 22 December are observed than on 3 November 2016, which may indicate more automobile exhaust and coal combustion in this heavy pollution event. The continuous appearance of the inversion layer, lower horizontal wind speed and more coal combustion and vehicle exhaust emissions are the main causes for this heavy pollution process.