Study of PM2.5 particles in the Gucheng, North China Plain: Morphology, mixing state, and source apportionment

The PM2.5 samples were collected at regular intervals from April to October 2017 at a meteorological station in Gucheng (GC) (39.08. N, 115.44.E), Hebei Province, China. An important site in the North China Plains (NCP). The internal composition, mixing state, and elemental mapping were determined by scanning electron microscope (SEM) and Transmission electron microscope in scanning mode (STEM). Several core -shell (C-S) structures had sulfate and nitrate shells with mineral and sea salt cores, indicating particle aging and distant transport. The major water-soluble ions (WSIs) found were NH4+, Na+, K+, Ca2+, Mg2+, Br-, Cl-, NO2-, NO3-, PO43- and SO42- . The three most dominant ions were NH4+, NO3-, and SO42- , with an average concentration of 16.02, 14.90, and 11.03 mu g/m3, respectively, accounting for more than 80% of the total WSIs in PM2.5. Ion balance measurements showed that the PM2.5 were slightly acidic. [NO3- /SO42- ] ratio of 1.11 shows a slightly high contribution from the mobile sources. The sea salt contribution was investigated by the molar ratio Cl-/Na+, which was slightly higher (1.9) than that for seawater (1.7), indicating an additional source, other than sea salt, for Cl- and Na+. The SOR was higher at all temperatures than NOR; SOR was strongly correlated with temperature, indicating gas -phase oxidation of SO2 to SO42- by OH radicle. The SO42- being positively correlated with NH4+ suggests NH4HSO4 and (NH4)2SO4 to be the dominant forms of SO42- present. Based on the internal structure, principal component analysis (PCA), and positive matrix factorization (PMF) analysis, the major sources of sulfate were industrial emissions from the surrounding cities, also confirmed by the Hysplit backward trajectory analysis. Nitrates arise mainly from local vehicular and industrial emissions and ammonia from fertilizers and animal manure. The local emissions and the regional transport from Shanxi, Shaanxi, Tianjin, Beijing, and Inner Mongolia were the main contributors to PM2.5 particles.