The Nested Air Quality Prediction Model System (NAQPMS) was used to investigate the temporal and spatial variations of PM2.5 over tropospheric central eastern China in January 2013. The impact of regional transport and its implications on pollution prevention and control were also examined. Comparison between simulated and observed PM2.5 showed NAQPMS was able to reproduce the evolution of PM2.5 during heavy haze episodes. The results indicated that regional transport of PM2.5 played an important role in regional haze episodes in the city cluster including Hebei, Beijing and Tianjin (HBT). The cross-city clusters transport outside HBT and transport among cities inside HBT contributed 20%–35% and 26%–35% of PM2.5 as compared with local emission, in HBT respectively. To meet the Air Quality Standards for Grade II, 90%, 90% and 65% of emissions would have to be cut down in Hebei, Tianjin and Beijing, if non-control strategy was taken in the surrounding city clusters of HBT. This implicated that control of emissions in one city cluster is not sufficient to reduce regional haze events, and joint efforts among city clusters are essential. Besides regional transports, two-way feedback between boundary-layer evolution and PM2.5 also significantly contributed to the formation of heavy hazes, which contributed 30% of monthly average PM2.5 concentration in HBT.
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