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The simultaneous observation and analysis of atmospheric fine particles(PM2.5) on a regional scale is an important approach to develop controlstrategies for haze pollution. In this study, samples of filtered PM2.5were collected simultaneously at three urban sites (Beijing, Tianjin, andShijiazhuang) and at a regional background site (Xinglong) in theBeijing–Tianjin–Hebei (BTH) region from June 2014 to April 2015. ThePM2.5 at the four sites was mainly comprised of organic matter, secondaryinorganic ions, and mineral dust. Positive matrix factorization (PMF)demonstrated that, on an annual basis, secondary inorganic aerosol was thelargest PM2.5 source in this region, accounting for 29.2–40.5 % ofthe PM2.5 mass at the urban sites; the second-largest PM2.5 source was motor vehicle exhaust,particularly in Beijing (24.9 %),whereas coal combustion was also a large source in Tianjin (12.4 %) andShijiazhuang (15.5 %), with particular dominance in winter. Secondaryinorganic aerosol plays a vital role in the haze process, with the exceptionof the spring haze in Shijiazhuang and Tianjin, for which the dust source wascrucial. In addition to secondary transformations, local direct emissions(coal combustion and motor vehicle exhaust) significantly contribute to thewinter haze at the urban sites. Moreover, with the aggravation of hazepollution, the OC ∕ EC mass ratio of PM2.5 decreased considerably and thenitrate-rich secondary aerosol increased during all four seasons in Beijing,both of which indicate that local motor vehicle emissions significantlycontribute to the severe haze episodes in Beijing. To assess the impacts ofregional transport on haze pollution, the PMF results were further processedwith backward-trajectory cluster analysis, revealing that haze pollutionusually occurred when air masses originating from polluted industrial regionsin the south prevailed and is characterized by high PM2.5 loadings withconsiderable contributions from secondary aerosols. This study suggests thatcontrol strategies to mitigate haze pollution in the BTH region shouldfocus on the reduction of gaseous precursor emissions from fossil fuelcombustion (motor vehicle emissions in Beijing and coal combustion inTianjin, Hebei, and nearby provinces)

atmospheric chemistry and physics

Chemical characterization and source identification of PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; at multiple sites in the Beijing–Tianjin–Hebei region, China