Evaluating the "2+26" regional strategy for air quality improvement during two air pollution alerts in Beijing: Variations in PM_2.5 concentrations, source apportionment, and the relative contribution of local emission and regional transport

To comprehensively evaluate the effects of the recent "2C26" regional strategy for air quality improvement, we compared the variations in PM_2.5 concentrations in Beijing during four pollution episodes with different emissionreduction strategies. The 2C26 strategy implemented in March 2018 led to a mean PM_2.5 concentration 16.43% lower than that during the pollution episode in March 2013, when no specific emission-reduction measures were in place. The same 2C26 strategy implemented in November 2017 led to a mean PM_2.5 concentration 32.70% lower than that during the pollution episode in November 2016, when local emission-reduction measures were implemented. The results suggested that the effects of the 2C26 regional emissionreduction measures on PM_2.5 reductions were influenced by a diversity of factors and could differ significantly during specific pollution episodes. Furthermore, we found the proportions of sulfate ions decreased significantly, and nitrate ions were the dominant PM_2.5 components during the two 2C26 orange alert periods. Meanwhile, the relative contribution of coal combustion to PM_2.5 concentrations in Beijing during the pollution episodes in March 2013, November 2016, November 2017 and March 2018 was 40 %, 34 %, 28% and 11 %, respectively, indicating that the recent "Coal to Gas" project and the contingent 2C26 strategy led to a dramatic decrease in coal combustion in the Beijing-Tianjin- Hebei region. On the other hand, the relative contribution of vehicle exhaust during the 2C26 orange alert periods in November 2017 and March 2018 reached 40% and 54 %, respectively. The relative contribution of local emissions to PM_2.5 concentrations in Beijing also varied significantly and ranged from 49.46% to 89.35% during the four pollution episodes. These results suggested that the 2C26 regional emission-reduction strategy should be implemented with red air pollution alerts during heavy pollution episodes to intentionally reduce the dominant contribution of vehicle exhausts to PM_2.5 concentrations in Beijing, while specific emissionreduction measures should be implemented accordingly for different cities within the 2C26 framework.