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. To comprehensively evaluate the effects of the recent “ 2+26 ” regionalstrategy for air quality improvement, we compared the variations inPM 2.5 concentrations in Beijing during four pollution episodes withdifferent emission-reduction strategies. The 2+26 strategy implementedin March 2018 led to a mean PM 2.5 concentration 16.43 % lower thanthat during the pollution episode in March 2013, when no specificemission-reduction measures were in place. The same 2+26 strategyimplemented 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 resultssuggested that the effects of the 2+26 regional emission-reductionmeasures on PM 2.5 reductions were influenced by a diversity of factorsand could differ significantly during specific pollution episodes.Furthermore, we found the proportions of sulfate ions decreasedsignificantly, and nitrate ions were the dominant PM 2.5 componentsduring the two 2+26 orange alert periods. Meanwhile, the relativecontribution of coal combustion to PM 2.5 concentrations in Beijingduring the pollution episodes in March 2013, November 2016, November 2017and March 2018 was 40 %, 34 %, 28 % and 11 %, respectively,indicating that the recent “Coal to Gas” project and the contingent 2+26 strategy led to a dramatic decrease in coal combustion in theBeijing–Tianjin–Hebei region. On the other hand, the relative contributionof vehicle exhaust during the 2+26 orange alert periods inNovember 2017 and March 2018 reached 40 % and 54 %, respectively. The relativecontribution of local emissions to PM 2.5 concentrations in Beijing alsovaried significantly and ranged from 49.46 % to 89.35 % during the fourpollution episodes. These results suggested that the 2+26 regionalemission-reduction strategy should be implemented with red air pollutionalerts during heavy pollution episodes to intentionally reduce the dominantcontribution of vehicle exhausts to PM 2.5 concentrations in Beijing,while specific emission-reduction measures should be implemented accordinglyfor different cities within the 2+26 framework.

atmospheric chemistry and physics

Evaluating the “2+26” regional strategy for air quality improvement during two air pollution alerts in Beijing: variations in PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; concentrations, source apportionment, and the relative contribution of local emission and regional transport