Characteristics and sources of WSI in North China Plain: A simultaneous measurement at the summit and foot of Mount Tai.

To better understand the characteristics and sources of water soluble ions (WSI) in North China Plain (NCP), fine particles (PM 2.5) were simultaneously sampled at the summit (SM) and foot (FT) of Mount Tai during May 12th to June 24th, 2017. Ion chromatography analysis showed that concentration of WSI was lower at SM (22.26 ± 16.53 μg/m3) than that at FT (31.02 ± 21.92 μg/m3). The concentration and proportion of SO 4 2− in total WSI were both lower than the values reported in previous studies. Daytime WSI concentrations were higher than that at nighttime at SM, while the opposite results were obtained at FT, possibly associated with more anthropogenic activities and higher boundary layer height (BLH) during daytimes. A severe pollution event occurred during June 14th – June 16th was documented at both FT and SM. Regional transport and topography-forced vertical transport along the slope of the mountain could explain the higher concentrations of pollutants at SM. The analyses also indicated that NH 4 + existed mainly in the form of NH 4 HSO 4 and NH 4 NO 3 , but (NH 4) 2 SO 4 could also exist, especially when emissions of NH 4 + and NH 3 were increased during daytime at FT. The results of principal component analysis (PCA) illustrated that secondary aerosols, coal/biomass burnings, sea-salts and crustal/soil dusts were the main sources at SM, and secondary aerosols and crustal/soil dusts contributed most at FT. Backward air-mass trajectories were classified into four clusters, of which air masses with the highest frequency and WSI concentrations were originated from the southwest with secondary ions (SO 4 2-, NO 3 - and NH 4 +) as major pollutants. Image 1 [ABSTRACT FROM AUTHOR]