<p><strong>Abstract.</strong> With the completion of the Beijing Five-year Clean Air Action Plan by the end of 2017, the annual mean PM<sub>2.5</sub> concentrations in Beijing dropped dramatically to 58.0 μg/m<sup>3</sup> in 2017 from 89.5 μg/m<sup>3</sup> in 2013. However, controversies exist to argue that favorable meteorological conditions in 2017 that helped pollution dispersion were the major factor for such rapid decrease in PM<sub>2.5</sub> concentrations. To comprehensively evaluate this five-year plan, we employed Kolmogorov-Zurbenko (KZ) filtering and a WRF-CMAQ model to quantify the relative contribution of meteorological conditions and the control of anthropogenic emissions to PM<sub>2.5</sub> reduction in Beijing from 2013 to 2017. For these five years, the relative contribution of emission-reduction measures to the decrease of PM<sub>2.5</sub> concentrations in Beijing calculated by KZ filtering and the WRF-CMAQ model was 80.6 % and 78.6 % respectively. The WRF-CMAQ model further revealed that local and regional emission-reduction measures contributed to 53.7 % and 24.9 % of the PM<sub>2.5</sub> reduction respectively. For local emission-reduction measures, the regulation of coal boilers, increasing clean fuels for residential use, industrial restructuring, the regulation of raise dust and vehicle emissions contributed to 20.1 %, 17.4 %, 10.8 %, 3.0 % and 2.4 % of PM<sub>2.5</sub> reduction respectively. Both models suggested that the control of anthropogenic emissions contributed to around 80 % of the total decrease in PM<sub>2.5</sub> concentrations in Beijing, indicating that emission control was crucial for the notable improvement in air quality in Beijing from 2013 to 2017. Therefore, such long-term air quality clean plan should be continued for the future years to further reduce PM<sub>2.5</sub> concentrations in Beijing. Considering that different emission-reduction measures exert distinct effects on PM<sub>2.5</sub> reduction and existing emission-reduction measures work poorly to reduce ozone concentrations, future strategies for emission-reduction should be designed and implemented accordingly.</p>

<p><strong>Abstract.</strong> To comprehensively evaluate the effects of the recent <q>2+26</q> regional strategy for air quality improvement, we compared the variations in PM<sub>2.5</sub> concentrations in Beijing during four pollution episodes with different emission-reduction strategies. The <q>2+26</q> strategy implemented in March 2018 led to a mean PM<sub>2.5</sub> concentrations of 16.43 % lower than that during the pollution episode in March 2013, when no specific emission-reduction measures were in place. The same <q>2+26</q> strategy implemented in November 2017 led to a mean PM<sub>2.5</sub> concentrations of 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 <q>2+26</q> regional emission-reduction measures on PM<sub>2.5</sub> 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<sub>2.5</sub> components during the two <q>2+26</q> orange alert periods. Meanwhile, the relative contributions of coal combustion to PM<sub>2.5</sub> 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 <q>Coal to Gas</q> project and the contingent <q>2+26</q> 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 <q>2+26</q> orange alert periods in November 2017 and March 2018 reached 40 % and 54 % respectively. The relative contribution of local emission to PM<sub>2.5</sub> concentrations in Beijing also varied significantly and ranged from 49.46 % to 89.35 % during the four pollution episodes. These results suggested that the <q>2+26</q> regional emission-reduction strategy should be implemented with red air pollution alerts during heavy pollution episodes to intendedly reduce the dominant contribution of vehicle exhausts to PM<sub>2.5</sub> concentrations in Beijing, while specific emission-reduction measures should be implemented accordingly for different cities within the <q>2+26</q> framework.</p>

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