Atmospheric PM_2.5 samples were collected by particulate matter samplers and the concentrations of water-soluble ions (F^-,Cl^-,NO₃^-,SO₄^2-,Na⁺,NH₄⁺,K⁺,Mg²⁺,and Ca²⁺) were analyzed in January 2018 in Wuhan city.The results show that NO₃^-,SO₄^2-,and NH₄⁺ are the major water-soluble inorganic ions in PM_2.5.Moreover,the correlation between PM_2.5 and water-soluble ions is significant,except for Mg²⁺ and Ca²⁺,and mobile sources play a dominant contributor role.The balance of anions and cations shows the PM_2.5 is neutral or weakly acidic.Four major back air mass trajectories and corresponding concentrations of PM_2.5 and water-soluble inorganic ions were obtained by a hybrid single-particle Lagrangian-integrated trajectory model and hierarchical clustering.The results indicate that regional transport had a considerable influence on this haze episode.