[Concentration Characteristics of PM_2.5 in Beijing During Two Red Alert Periods]

Variations of PM_2.5 concentrations and effects of pollution control measures during two red alert periods in 2015 in Beijing were analyzed based on atmospheric pollutant monitoring data. The results showed that during the first red alert, the highest hourly-averaged PM_2.5 concentration occurred at 19:00 on 9th December with a value of 282 μg·m^-3 and the highest hourly PM_2.5 concentration appeared at Yongledian station which is near the southeast border of Beijing, with the peak concentration of 496 μg·m^-3. During the second red alert, the highest hourly-averaged concentration of PM_2.5 occurred at 20:00 on 22th with a value of 421 μg·m^-3. The highest hourly PM_2.5 concentration was monitored at Liulihe station which is near the southwest border of Beijing, with the peak concentration of 831 μg·m^-3. During the duration period of both red alerts, the concentrations at the southern stations were higher than those at downtown stations and the PM_2.5 concentrations at northern stations were found to be the smallest. The difference between these two red alerts was that during the second red alert, the PM_2.5 concentrations in southern Beijing were significantly higher than those in the northern area, while the magnitude of this south-to-north gradient was much smaller during the first one. During the second red alert, up to 93% of Beijing area showed an average PM_2.5 concentration of above 150 μg·m^-3, which was much larger than that in the first one. The meteorological conditions during the two red alerts were both not conducive to the spread of pollutants. Formation of secondary pollutants and regional pollutant transport existed as well. Though the stagnant weather conditions were in favor of the development of severe pollution, large regional-wide pollutant emission was the main reason for these two heavy air pollutions in Beijing. PM_2.5 concentrations were decreased by 20%-25% after the implementation of emergency response measures, which showed the significance of emission reduction in air pollution control.